A Primer on Formulating Horticultural Container Media from Sustainable Substrates

Dr. Richard Freeman

This primer introduces the audience to a framework for understanding and formulating horticultural container media (HCM) from sustainable substrates. The intended audience includes horticulturists, orchardists and farmers who are interested in sustainable practices. Please see the author’s note at the end of this guide.


Awareness of declining resource availability, climate change and other forms of environmental degradation is generating demand for sustainable production in all sectors. Meanwhile, increasing interest in urban agriculture, horticulture and gardening is stimulating interest in container-based cultivation, especially where sites have limited growing space or severely compromised field soils and in operations growing high-value crops At the intersection of these trends, demand is growing for a sustainable, intensive container-based horticulture for gardening, subsistence farming and small-scale commercial farming.

In most urban contexts, building a soil profile is not remotely possible, so organic media is necessary for cultivating plants. In this scenario, the best solution is using media based on making media from local resources. The next best solution beyond that is importing agricultural by-products.

Growing vigorous, resilient plants requires a container environment that provides all the functions they require to maintain full production. Thus, formulating or obtaining productive, high-quality horticultural container media (HCM) is fundamentally important to a horticultural operation. Likewise formulating a high-quality, productive and sustainably-produced HCM is fundamentally important to an ecological horticulture.

This manual provides the necessary background for formulating high-production horticultural container media (HCM) based on principles of function and using substrates sourced from agricultural byproducts.

Section 1. A Brief Overview of Horticultural Container Media

Container-based horticulture as we know it emerged during the early 1950s, following advances in greenhouse and nursery technology. Technology such as plastic containers, drip irrigation, fertigation and liquid fertilization, plastic coverings for greenhouses and hoophouses, heating, ventilation and air-conditioning advances, and mass propagation techniques led to rapid expansion and standardization of the greenhouse and container horticulture industry. Likewise, industry and university research made significant advancements in HCMs.

At the advent of these advancements in controlled-environment horticulture, practitioners continued to use field soil as a major component of growing media. Due to their biological, chemical and physical attributes, these media presented a multitude of problems to growers, especially in the moist, protected environments associated with greenhouses and nurseries. Problems ranged from a plethora of pest problems to increased labor and machine costs associated with handling heavy materials.

1.1. U.C. Type Soil Mixes

In response to the problems with using field soil in container horticulture, researchers and practitioners experimented with a variety of substrates, eventually replacing soil with HCMs. In 1957, the University of California System for Producing Healthy Container-Grown Plants introduced their framework for developing the U.C.-Type Soil Mixes (UCMs). The authors described four basic functions a media should supply in terms of “support,” “moisture,” “aeration” and “mineral nutrients.” “Reliability” was a prominent concern, specifically in terms of uniformity of components within a mix and uniformity in “physical and chemical properties” between batches. Operational concerns such as weight for ease of handling and shipping in flats and containers, sterility, storage life, easy of mixing and shrinkage were also important considerations. The UCMs combined fine sand with peat at various ratios depending upon plant needs, though the authors emphasized that any aggregate that would meet the requirements would work. Examples include composted redwood shavings and rice hulls. Many of the substrates tried and adopted during these years came from regional and local sources and are still used today. Peat and pine-bark chips remain common, though pine-bark chips are on the decline due to limited available quantity.

1.2. Cornell Peat-lite

In 1967, Cornell University researchers published a bulletin describing the Cornell Peat-Lite mix (Peat-lite), which soon became the industry standard and remains the basis for most HCMs in use. Similar to the UCMs, Peat-lite is sterile media consisting of equal volumes of peat moss and perlite or vermiculite. Cornell researchers developed the peat-lite family of HCMs to provide greenhouse growers with horticultural media that: a) is consistent, reliable and stable; b) minimizes risks from pathogens; c) provides a uniform texture for irrigation; d) provides a physical matrix on which to apply fertilizers (mostly salts); e) is easy to make and handle; f) consists of readily available materials; and, g) is low-cost. Since the advent of the Cornell peat-lite, the original ratio and variations have become the horticultural standard internationally. Common ratios between peat and perlite or vermiculite range from 50:50 to 85:15.

By the early 1980s, at least 35 commercial HCMs based on Peat-lite were on the market and it remains the basis for most “potting soils,” which generally consist of a peat-lite mixed with some amount of compost – usually less than ten percent. Some companies are also starting to add biochar to potting soils.

1.3. Super soils

Peat-lite has also functioned as the basic medium from which “super soils” are made. Super soils are HCM formulated to grow heavy-feeding, high-value plants from vegetative stage through flowering without the need to add fertilizers. These media rely on a significant biological component, and are loaded to the maximum feasible limit with nutrients, in order to feed a robust microbial ecology that in turn will support a rhizosphere rich with biologically-active nutrient pathways. Super soils emerged from the illegal Cannabis community in 2009 when High Times magazine published the article, “Subcool’s Super Soil Step-by-Step.” In 2012 The Rev published his book True Living Organics, compiling several articles he had written and describing his own supersoil-like HCM and introducing a system of spikes and layers to assure an abundant nutrient supply through the first stages of flowering. Since the advent of super soils, several commercial vendors have offered products claiming to be super soils, with variable efficacy and quality and countless growers are mixing their own variations. These commercially available HCMs vary widely in quality and price.

Section 2. Overview of the Horticultural Container Ecosystem

The container environment is dynamic and fast-changing and the farmer must remain highly aware of the many relevant environmental and nutritional indicators to avoid problems. It is a drastically simplified version of a natural soil ecosystem, and natural buffering systems are absent or highly constrained. Containerized plants commonly face fluctuations in soil water, humidity, soil and aerial temperature, light and other factors, and they are especially susceptible to pests. In addition, HCM formulation, irrigation and fertilization, choice of containers, handling, and other operational activities influence HCM behavior.

2.1. Container Dynamics

The container environment goes through several notable short-term and long-term changes during the growing cycle:

2.1.1. Phase changes

Every watering cycle, daily or more frequently, the HCM moves through solid, liquid and gas phases as water enters and empties from water-filled pores. Likewise, during an irrigation event, depending upon evenness of coverage and infiltration, the HCM undergoes rapid phase changes as the waterfront moves downward through the container.

2.1.2. pH Swings

HCMs tend towards significant pH swings during the growing cycle, depending mostly upon substrate properties, irrigation and fertilization. These swings can affect plant growth in several ways and they can affect cation exchange capacity, which is pH dependent. Swings towards acidity through the growing cycle are due to: i) hydrogen being released by plants during metabolism of ammonium; ii) a build-up of carbon acid as a result of water running through pores filled with gaseous carbon dioxide; iii) uptake of cations, build up of acids from acid-based fertilizers; iv) release and accumulation of root exudates; and, v) contributions of hydrogen by substrates (notably peat moss).

Swings towards higher pH (alkalinity) result from: i) release of bicarbonate compounds from plant roots metabolizing anions like NO3-1 and SO4; ii) increased salt content due to fertilizer residues; and, iii) shrinkage of organic matter.

2.1.3. Physical properties deterioration

HCM physical properties change significantly through a growing cycle due to root displacement, biological decomposition, physical root action, irrigation, handling, settling, shrink-and-swelling and disturbance. Particle size often decreases and roots displace porosity, requiring more frequent watering – in addition to the increased watering demands of growing plant mass.

2.2. Properties of HCMs contrasted with field soils

HCMs are significantly different from field soils in several ways including:

2.2.1. Small Volumes

Container systems have a relatively smaller volume of substrate and available water per plant than with field soils. An important outcome of this constraint is that “the volume of medium in which container plants are grown is usually small in relation to the potential loss of water by evapotranspiration […].”

2.2.2. High root density

Container-grown plants have a relatively higher root density than field-grown plants. Because of high root density, root interception replaces diffusion of minerals in media solution.

2.2.3. High nutrient concentrations

Container-grown plants have relatively higher nutrient concentrations in the rhizosphere.

2.2.4. Water-soluble nutrients

Nutrients in HCMs are largely water soluble compared to nutrients in field soil. For HCMs, soil laboratories usually use water extractions instead of weak acid or salt solutions. In the United States, saturated media extract (SME) is preferred over suspension extractions.

2.2.5. Isolation

Usually, the rhizosphere of each plant is isolated from other plants.

2.2.6. High moisture & perched water table

Plastic containers provide a much wetter environment than is common in field conditions with pronounced differences in distribution. Horticultural containers at container capacity always retain a saturated perched water table at the bottom of the HCM. In a shallow drained soil, such as found in a horticultural container, water retention pressure balances and exceeds drainage pressure, creating a wetting front that backs up from the bottom of the container to a level called the capillary rise. The saturated zone between the container bottom and the capillary rise is named the perched water table. Even small volumes of persistently saturated HCM increase risks related to anoxic or anaerobic conditions, which is sub-optimal for plant growth. This sub-optimal condition creates costs at larger scales.

2.2.7. Variable air-filled porosity

Container media display wide variation in air-filled space within the container, depending upon moisture stratification.

2.2.8. Dynamic physical properties

HCM physical properties change significantly through a growing cycle due to root displacement, biological decomposition, physical root action, irrigation, handling, settling, shrink-and-swelling and disturbance.

2.2.9. Acidic environment

HCMs usually function at lower pH levels (more acidic) than field soil. HCM will operate at 5-6.5 – one to 1-1/2 points lower on the pH scale than a typical field soil, which is 6.5-7. As noted above, during the growing cycle, pH can plunge, sometimes to sub-optimal levels; low pH makes several metals available at toxic levels (including aluminum, manganese, and iron). Cation exchange capacity is pH dependent, decreasing with increasing acidity (decreasing pH).

2.2.10. Cation-exchange structures unique

HCM have different cation-exchange dynamics than do field soils because they are organic media rather than mineral media. Strictly speaking, the conventional cation-exchange capacity (CEC) does not apply to HCM, though the term is still used. The CEC concept depends on the colloidal model, based on the outer-sphere complex of a clay colloid, and HCMs do not include clay. In that regard, the model is not strictly reliable for the container ecosystems, with predominantly organic cation exchange surfaces. Rather than using CEC for analyzing HCMs, some labs use total exchangeable cations which relies on surface analysis of all the substrates in a mix.

2.2.11. Mass metals balance altered

Because colloid properties have a small role in determining HCM properties, nutrient plant availability will change in reference to the properties of the organic cation exchange surfaces. Mass balance of calcium, magnesium, potassium and sodium will interact with pH and other elements differently than with soil, so the effects of a given proportion will change, much like it does between a sandy soil and a clay soil. Potassium takes a more prominent role in HCMs than it does in soils.

2.2.12. Increased nutrient demands

HCM require regular feeding because they lack mass flow, mycelial networking, and other natural forms of nutrient travel common to field soils.

2.2.13. Natural pH buffering lacking

Container environments lack the natural buffering systems (especially pH buffering) found in healthy field soils, which have to do with the sheer volume of soil and the extensive nutrient cycling capacity of the native soil food web.

2.2.14. Vulnerable to root disease

HCM are increasingly vulnerable to root disease contamination due to incomplete soil food webs or their absence.

2.2.15. No soil profile

Horticultural containers lack soil profiles with horizons developed from rock materials over time and used as the basis for soil taxonomy.

Section 3. The Functions that Horticultural Container Media support

Robust HCM will support several ecological functions crucial to optimal plant growth. This section briefly discusses six of these functions with reference to media physical properties, chemistry and biology.

3.1. Provide plants with physical support

Lateral and vertical physical support of plant mass against mechanical disturbance and gravity is an important function. Roots require mass for leverage versus lateral and vertical forces, from the mass of coarse (aggregate) substrates or compressed, adequately cohesive fine substrates.

3.2. Hold available water

Providing plenty of available water to plants without holding unavailable water is key to rhizosphere health and plant production. Excess, unavailable water can nurture pathogens, while water deficiencies create plant stress and compromise production as plants restrain transpiration to save water, thereby slowing photosynthesis and cell growth. Long before visible wilting, “growth rates of plants in small containers of fertile media are significantly reduced.” An HCM must be able to absorb and distribute water rapidly to meet operational needs.

Fiber content of substrates introduces another factor, because fibers redistribute water more evenly through the media by capillary action which operates along matric potentials. One outcome of this redistribution is that fiber-based HCM (coir or peat) will lose 25-30% irrigated water through evaporation from the top surface.

3.3. Promote Aeration

Aeration is gas exchange between the rhizosphere and the atmosphere, which is required for maintaining metabolic processes that require oxygen and for removing CO2 and other gases from the rhizosphere. Plant roots need access to air and oxygen for photosynthesis and respiration to diffuse gases like CO2. In field soil, “roots can consume up to nine times their volume of gaseous oxygen each day.” A temporary lack of oxygen can reduce root and shoot growth, while anaerobic conditions for a few days can promote ethylene and ethanol production, reduce (redox) and release manganese, eventually killing roots. Increases in temperature also increase the demand for oxygen because microbes increase metabolic rates and less oxygen can dissolve in the soil solution. In addition, beneficial microbes require oxygen, and many pathogenic microbes are anaerobic.

Aeration, is “a direct function of air-filled porosity, the gas diffusivity in free air, and the degree of linkage between pores, known as pore effectiveness coefficient.” Thus, aeration requires provision of oxygen and air as well as soil-gas diffusivity. Soil-gas diffusivity is the measurable ability for a gas to diffuse through HCM between air-filled pores and the atmosphere. Diffusivity depends on both the volumetric quantity of air filled pores and the characteristics of their linear connectivity, also briefly discussed below. The oxygen diffusion rate is “the rate at which oxygen diffuses through media to the roots of plants.”

3.4. Hold exchangeable nutrients

Holding exchangeable nutrients for microbial and plant update is a vital function of a good HCM. This function requires abundant cation and anion exchange sites, which in turn depends upon the surface area and electrostatic properties of the substrates. Chemical stability, especially pH, is important to plant production, as are sufficient levels of available nutrients (SLAN) and adequate nutrients to occupy the total exchange capacity of the HCM. Another key factor of this function is the availability and diversity of microbes that participate in the nutrient pathways making up the rhizosphere.

3.5. Support a robust microbial beneficial ecology

Providing a robust biology enhances pest management and and promotes nutrient uptake. Generally, beneficial organisms significantly outnumber and outweigh pathogens in biodiverse rhizosphere, and an active, benefical soil biology can stimulate systemic acquired resistance (SAR) or induced systemic resistance (ISR). In addition, a biodiverse microbial ecology maintains diverse nutrient pathways in the rhizosphere, vastly increasing potential nutrient uptake.

This function involves physical, chemical and biological factors. In reference to physical properties, complex surfaces rich with ion-exchange sites are beneficial to microbes and promote biofilms – especially on surfaces with high electrical conductivity. Likewise, a beneficial biology requires a balance of air-filled and water-filled pore space, as do fine root hairs, making those structural qualities important factors for roots and soil organisms. In reference to chemical qualities, chemical stability, the correct pH range and a suitable range of moisture levels are necessary to maintain a robust rhizosphere biology.

3.6. Protect roots from disturbance, light and stress

HCM should protect roots from several forms of disturbance, including mechanical disturbance (for example, due to rough handling and improper watering techniques), exposure to sun and to temperature extremes, chemical exposure via spills, and some biological pathogens by directly inhibiting airborne spores from landing on roots.

Section 4. HCM properties and their effect on function

Formulating an HCM to promote ecological functions in a container environment requires mixing diverse substrates in optimal proportion to create the right physical, chemical and biological environment. The resulting “aerated growing media” should strike an optimal balance between available water and gas diffusion, offering the grower flexibility in irrigation and fertigation.

The objective of creating the aerated HCM requires identifying substrate properties that will support these functions, then balancing and optimizing an effective mix of these substrates to achieve optimal growth given management objectives. In many cases, substrates will meet more than one function.

Chemical properties also depend upon substrate properties, and they reflect horticultural practices as well. Substrate chemical properties have a strong influence on plant growth and are an area of primary concern.

Horticulturists and HCM manufacturers use a variety of substrates in their mixes, each with unique physical, chemical and biological properties. Each substrate exhibits significant variations and behaves differently in the presence of other substrates and substrate combinations. In addition, each crop plant species responds differently to each substrate and each combination, horticultural cultural practices aside. “For these reasons, the principles of formulating mixes are now considered rather than the descriptions of specific formulae.”

Standard soil labs can determine these values for any of these parameters for a substrate or an HCM, however, different lab techniques will yield differing values, and the values will yield different interpretations. In general, recommended values for these properties are merely guides. The particular factors of the operation will determine the optimal ranges for all these parameters.

4.1. Physical Properties

The HCM formulator has influence over several important physical properties through the choice of materials, most notably gas diffusivity and available water. Diffusivity depends upon the quantity of air-filled pore space (as percentage of container volume) and the tortuosity, or non-linearity of its path to an air source outside the container. Available water depends upon pore size, geometry and distribution, which depend upon particle geometry and surface, size distribution and shape. This section briefly explains the physical properties that are important to formulating an HCM.

Formulating a high-quality HCM requires balancing and optimizing these physical properties, which is a complex process because so many factors influence HCM behavior. “Although a certain formula may have achieved the desired results in one locality, it does not follow what this would apply universally. Physical properties of potting mixes are the produce of interactions between all the materials used and there will be considerable local variation in materials.” In general, the physical properties of an HCM depend upon substrate properties, particle size distribution and the “packing characteristics” of the particles, the latter being determined by shape.

To note, the variation between analytical lab techniques used to determine and report the same parameters is substantial, making any comparisons inconclusive, especially in regard to porosity. Thus, the information and data in this book can only serve as a guide for any horticultural operation to develop optimal HCMs through formulation, testing and analysis and adjustment.

4.1.1. Porosity and total pore space

Porosity refers to the size and distribution of pores in an HCM. Pore space is the volume of HCM occupied by air, in contrast to the total volume occupied by mineral or organic particles. Porosity, expressed as ɸ, is the fraction of medium taken up by total pore space. Total pore space equals the volume of medium that water will occupy at full container saturation. Porosity data is highly variable due to difference in lab technique and environmental conditions (container size, etc.)

A note of caution: some researchers use the term effective porosity to refer to total air space minus internal pore space (internal to materials like perlite). This book uses porosity to refer to pore space that excludes internal porosity.

Porosity is fundamentally important because along with particle size, it determines “the balance between water retention and aeration in a potting mix.” Likewise, “[t]he basic character of the pore space affects and is affected by critical aspects of almost everything that occurs in the soil: the movement of water, air, and other fluids; the transport and the reaction of chemicals; and the residence of roots and other biota.” Pore space, by convention, “excludes fluid pockets that are totally enclosed within solid material – vesicles or vugs, for example, that have no exchange with the pore space that has continuity to the boundaries of the medium.”

Pore space is contiguous with complex, variably-sized and shaped, tortuous and highly-connected fluid pathways, “a collection of channels through which fluid can flow. The effective width of such a channel varies along its length. Pore bodies are the relatively wide portions and pore openings are the relatively narrow portions that separate the pore bodies.”6 However, for quantification, analytical models commonly depict pore space as discreet, individual pores within the medium.

Particle shape affects pore space because “[p]articles more irregular in shape tend to have larger gaps between their nontouching surfaces.” Pores can originate from from the cell structures of plant tissues within the substrates themselves (intragranular) or from the spaces between overlapping particles (intergranular).

As a proportion of total media volume, porosity is usually between 0.3 and 0.7, depending on packing density, particle size distribution, particle shape(s), and cementing (infilling with fine particles). In reference to packing density and particle size distribution, polydisperse media (versus monodisperse) often exhibit less porosity because finer particles nest between larger particles. Thus, adding a substrate with larger particles to a substrate with smaller particles decreases total pore space until the proportion of larger particles reaches a threshold proportion. After reaching the threshold proportion, total pore space increases until the larger substrate constitutes the total mix.2 For example, One researcher reports that “the addition of perlite into peat and coir decreases the total porosity (water content at saturation) and water retention capacity of the peat–perlite and coir–perlite mixtures compared to the pure substrates.”

Some researchers write that TPS is optimal at 85% of total volume, while others suggest a range between 50%-90%.

At container capacity, total pore space includes water-filled pore space and air-filled space, which are inversely related. Air-filled pore space (AFPS)

Air-filled pore space at container capacity, which is inversely related to WFPS, determines the properties and behavior of HCMs in production and is key for providing a plant with atmospheric oxygen and for diffusing gasses released by plant roots (CO2 and others). Several factors determine a substrate’s AFPS, including particle shapes, sizes, surface geometry and distribution. Air-filled space and the presence of oxygen and CO2 is stratified in the container, increasing and decreasing in inverse proportion to water-filled space at that depth. Total air space is inversely related to dry bulk density and positively related to coarseness, even with substrates having high internal (material) porosity, like perlite.

However, though air-filled pore space is so important to plant function and vitality, measuring this parameter presents complications. “[M]ost potting media are composed of polydispersed materials having a range of particle sizes. Because the shape, texture and internal porosity of the particles vary as well as their size, it is usually accepted that the AFP of a mix cannot be predicted with accuracy; it must be determine empirically.”

Generally, air-filled pores will measure greater than 0.3 mm in diameter. A general target for average AFPS in a 15-CM-tall container at capacity is 30% of total pore space, or 25% total volume (TPS 85%) though some suggest that a range of 10% to 30% is acceptable with some variation in opinion. Recommendations can vary widely, for example from “7-50% volume depending on species and situation.”

AFPS is inversely related to soil bulk density. Water-filled pore space (WFPS)

The volume and properties of water-filled pore space at container capacity, which is inversely related to air-filled pore space, determine the properties and behavior of HCMs in production. Container capacity is “the total amount of water present after the medium in a container has been saturated and allowed to drain” and is equal to total porosity minus air-filled pore space. Water-holding capacity or water-filled porosity are synonymous to container capacity in reference to container environments. It likewise refers to the ability of an HCM to hold available and unavailable water at container capacity. It is not a reliable indicator of available water. Total water holding capacity is inversely proportionate to aeration (described below). (The volume of container capacity equals the volume of water-filled pore space.)

Smaller pores hold water at higher tension (negative pressure), depending upon their shape, surrounding pressure gradients, and the electrostatic properties of the media substrates. Generally, spaces between overlapping particles form water-filled pore spaces, or capillary pores. In a 15-cm container, capillary pores measure between 0.03-0.3 mm in diameter, corresponding to water-holding tensions ranging from 10 kPa (small pores) to 1 kPa (large pores). Large pores, or macropores, generally include pores larger than 75 µm diameter (0.075 mm), mesopores between 30-75 µm and micropores under 30 µm. Easily available water and buffering capacity are held in pores with 60-300 µm and 30-60 µm diameters, respectively.

A general target for optimal water-filled pore space in a 15-CM-tall container is 70% of TPS, or 60% total volume, given total pore space (TPS) is 85% of the total volume. An acceptable range is 45-65% of total volume. Thirty to forty-five percent (30-45%) of water-filled space should hold easily available water, or at least 18%-24% total volume. Water availability also depends upon shape of pores.

4.1.2. Soil gas diffusivity

Soil-gas diffusivity is the measurable ability for a gas to diffuse through HCM between air-filled pores and the atmosphere, which depends on adequate air-filled space and the pore effectiveness coefficient, the degree of linear connectivity of air-filled pores, also discussed below.

As noted, soil-gas diffusivity is a function of pore effectiveness coefficient and pore size distribution. Pore effectiveness is the ability of a pore to transport oxygen to and gases from the rhizosphere to an outside air source. This ability is tied to the linearity of the continuous pores and is the inverse of tortuosity, the non-linearity or lack of continuity between pores. Diffusivity can decrease with increased air-filled space if pore-effectiveness (connectivity) is low.

Likewise, the oxygen diffusion rate is “the rate at which oxygen diffuses through media to the roots of plants.” Oxygen diffuses more slowly through thicker films of water, and a film of varying thickness always covers a live root. Because of the perched water table, the lower depths of containers have markedly less air-filled porosity than upper depths. Large containers exaggerate this tendency. In addition, “[t]he temperature of the medium also affects the oxygen status in two ways: an increase of 10 degC in the temperature doubles the respiration rate; at the same time an increase in the temperature of the soil water from 20-30 degC reduces the amount of oxygen that can be dissolved […].” .

Diffusivity is related to hydraulic conductivity, as well, which is inversely related to pore tortuosity.

Generally, as HCMs dry or re-wet, water-filled pores pass between liquid and gas phases, with transitions often characterized by Haine’s jumps – sudden emptying or filling events caused by the collapse of menisci, in turn resulting from pressure dynamics as water fills and empties from pore bodies. As water volume decreases in an HCM, tension retaining water increases, making water gradually unavailable.

4.1.3. Available water

In the conventional framework, an optimal HCM retains plenty of easily available water and water buffering capacity, which describe the range of tensions at which water is held and what range of pressure the plant root must exert to attain water. Easily available water is “the amount of water held by a medium after it has been saturated with water and allowed to drain, minus the amount of water present at some defined water tension. This is often taken as the volume of water between 10 and 50 cm tensions” (c. 1-5 kPa). Generally, water buffering capacity is held at tensions between 5 and 10 kPa. However, recent research demonstrates that plants can obtain water to photosynthesize fully at retention pressures much greater than those conventionally attributed to readily available water, depending upon the plant and the HCM.

The the sum of easily available water and water buffering capacity will occupy 30-45% of water held at container capacity in an optimal HCM. The volume of easily available water, specifically, should exceed 24% of water held though early researchers reported that easily available water should range between 20%-30% volume at container capacity. Water buffer capacity should equal 4-10% of water held.

As a general reference, most plants growing in peat-based HCMs will reach permanent wilting point between 7.9% and 20.9% of total substrate volume – in the range of -1,000 to -2,000 kPa. However, in practice, plants will reach wilting point after different time lengths in different substrates, and each plant species and variety has a different wilting point for any given substrate.

HCMs hold water at tensions at pressure potentials determined by gravitational, matric and osmotic gradients. Gravitational pressures are determined by gravitational force. Matric pressures are held due to capillary forces, in turn determined by HCM and substrate pore size and electrostatic properties. Osmotic pressures are determined by osmotic gradients resulting from salts in the soil solution and exerting pressure against root uptake.

1 10 100 1000 15000 0 1 2 3 4.17 0.098 0.978 9.78 97.8 1467
Units of Measurement
for Soil Tension and Available Water

To note, laboratory methods under-estimate available water for coarse-texture substrates relative to the volume of water that plants can extract from the medium, because water and particles break contact as the first water is removed by bottom suction, allowing water to escape.

4.1.4. Hydraulic conductivity

Hydraulic conductivity “is the substrate’s ability to transmit water […] and is a function of substrate water content as well as the structure of porosity. Conductivity of a substrate depends on the geometry of the pores and the properties of the fluid in them. In saturated conditions, water movement is predominantly through large pores. As water content decreases, the large pores drain, tortuosity of the flow path increases, and water movement is mainly through smaller pores.” In other words, hydraulic conductivity is the capacity of water to move through the medium along a potential gradient, generally including gravity gradients and matric gradients linked to the capillary properties of the medium in the context of container environments.

Hydraulic conductivity can vary significantly within an HCM. The hydraulic conductivity of the HCM in close proximity to the rhizosphere, the substrate hydraulic transport is especially important as it determines water availability, nutrient availability and gas diffusivity for plant roots.

4.1.5. Particle properties

Particle properties like shape (geometry), size (especially screen size or second-largest diameter) and density determine the functionality of HCM substrates. Particle size, distribution and structure

Particle size distribution and particle shapes of HCM substrates determine most of the above properties – pore size and size distribution, soil-gas diffusivity, hydraulic conductivity and pore tortuosity. The horticulturist seeks a blend of particle sizes that will optimize gas diffusion (air porosity plus minimal tortuosity) with water availability (plenty of water available at low tensions). The specific optimal diameter size will depend upon particle shape as well as size.

However, as a rule, particle size distribution is not a reliable predictor of easily available water and diffusivity, so HCM formulation requires testing and adjusting mixes to achieve desired values for these parameters. Known, general relationships between particle size distribution and these HCM properties can only serve as a guide to begin the formulation process. “It is impossible to know what the physical properties of a particular mixture of components will be; they must be measured.” Lab analysis and live plant testing are indispensable in formulating a successful mix.

Generally speaking, in standard working conditions, coarser substrates hold relatively more air and less water, while finer substrates hold less air and more water. However, while the coarsest substrates hold the most air, gas diffusion decreases substantially as diameter size increases, while air porosity increases only slightly with size increases. In regard to finer substrates, while the finest substrates hold the most water, easily available and total available water decrease substantially with decreasing particle size.

Thus, research strongly suggests that fine to medium-sized particles tend to generate the best optimal gas diffusion properties (low tortuosity and adequate air-filled porosity), while fine particles (distinct from very fine particles) hold most easily and totally available water. The fine and medium-sized particle ranges overlap significantly, so that available water and air will share space during liquid and gas phases respectively.

With these general relationships as a basis of understanding, formulating an optimal mix will depend upon horticultural objectives and will require lab analysis, growth and yield testing and adjustment. For example, some research indicates that an HCM should include a majority of particles between 0.2-3.0 mm, with less than 25% by volume between 0.1-0.5mm and less than 10% less than 0.1%. In the case of the fibrous substrates, some researchers recommend that 89% of coir particles and 60% of peat by weight be within the range of 0.25-2.5 mm.

In regard to providing available water and easily available water in an HCM, some research suggests that the “fraction of particles in the size range 0.1-0.5mm is responsible for the plant-available water holding capacity of the media. These particles are expected to produce pores in the range of 30-300µm between them, being optimal for water retention in growing media.” Other research suggests that particles sized between 0.1-0.25 mm will hold the largest share of easily available water, except for coir, which will hold “readily available water” (easily available plus buffering capacity) with particle distributions up to 1 mm due to its internal porosity.”

Substrates with coarse particles form larger pores and more air-filled pore space that generally hold little or no water, with exceptions. Some research indicates that particles with diameters larger than 0.8 mm predominantly form non-capillary, air-filled pores, which increase in proportion to increases in particle size distribution. However, some research has demonstrated that coarse particles (>0.8 mm) of some substrates in mixed (polydisperse) media, for example mixes with peat and composted wood bark, do retain available water (32-36% by volume) and easily available water (23-26%).

Significantly, increasing particle size of materials over 0.5-1.0 mm diameter has diminishing effect on air-filled porosity, while tortuosity substantially increases and pore effectiveness and gas and oxygen diffusion decrease. For example, in regard to composted wood bark chips, research indicates that tortuosity increases “linearly with increasing wood bark particle size.” Particles ranging between 2-4 mm diameter displayed as much air-filled porosity and much better diffusion than particles ranging from 8-25 mm; likewise, particles 1-2 mm diameter displayed as much air-filled porosity as particles 8-16 mm. The coarseness index is the percentage of media volume >1 mm (2nd longest vertex).

Conversely, air space in an HCM significantly decreases at thresholds of decreasing particle size, which a major break ranging from 0.1-0.25 mm (depending on material and geometry). For example, “pine bark particles in the 0.1-0.25-mm range decreased air-filled porosity to a larger extent than did either those in the 0.25- to 0.5-mm range or those smaller than 0.1 mm.” Likewise, according to some research, composts should contain less than 15% particles under 0.1 mm. Consistent with these findings, particles smaller than 0.25 mm diameter decrease air-filled porosity more than particles 0.25-0.5 mm. Others note a threshold effect with particles less than 0.8-1.0 mm diameter.

Substrates with very fine particles form small pores with little or no air-filled space, little available water and excess unavailable water. HCMs with a large proportion of finely-size particles tend to decrease available water, as fine pores will retain water at tensions unavailable to plant roots. Generally, particles sized smaller than 0.01-0.05 mm in diameter will retain excess water that is unavailable to plants. Particles with diameters between 0.01-0.1 mm will hold available water but not easily available water. Particle shape and geometry

In addition to particle size distribution, substrate particle shapes determine the specific diffusion and water availability properties in a given mix. “Therefore, the information on shape on top of size of particle may be necessary to choose components with respect to aeration purposes.” Disk shaped particles and round particles, characterized by radial symmetry, increase HCM tortuosity and decrease pore-effectiveness, thereby decreasing gas diffusion. Threadlike particles decrease tortuosity and increase pore effectiveness, increasing gas diffusion: “There was a distinctive effect between large porous fragments (peat nuggets) or threadlike bodies (wood fibre) and impermeable, disk-shaped (coconut, bark) or spherical bodies (perlite). The large, impermeable fragments (bark, perlite, coconut shell) caused a significant decrease in the pore effectiveness coefficient.” To note, tortuosity, in addition to being a factor in and analytical parameter for gas and oxygen diffusivity, affects root growth geometry and quantity. Mass is more evenly distributed in less tortuous HCMs. Roots are shorter and fatter in more tortuous media and compaction increases the effect.

Likewise, particle shape is a key factor in determining water availability insofar as it structures pore space and geometry. Particle shape affects pore space because “[p]articles more irregular in shape tend to have larger gaps between their nontouching surfaces.” Pores can originate from the cell structures of plant tissues within the substrates themselves (intragranular) or from the spaces between overlapping particles (intergranular). The term material pore size and shape refers to characteristics of the pores in the substrate solid, in contrast to the pores created between particles.

Particle size also affects nutrient exchange capacity insofar as it affects surface area (smaller particles corresponding to larger surface area). Thus, a small-particle substrate with high nutrient exchange capacity would also benefit a rich biological component, though it will not be as effective in distributing water as a more fibrous substrate. Increased surface area also increases negative effects of a substrate. For example, fine, raw (unrinsed) pine biochar can contribute significant quantities of calcium carbonate while adsorbing and immobilizing nitrogen, thereby causing nutrient depletion and salt burning, especially in seedlings and transplants. Particle density

Particle density is the material density – the density of the dry particle solid with no air-filled space. Particle density is always much higher than dry bulk density.

4.2. Chemical Properties

A substrate’s electrostatic and chemical properties play big roles in nutrient exchange and biological nutrient cycling. In some cases, minerals attached to substrates can cause unexpected effects on media chemistry. In reference to the biochar example, unwashed biochar often carries a salt coat with a significant calcium-carbonate presence, which will increase pH levels and electrical conductivity in the HCM solution. In other cases, a substrate’s electrostatic qualities can confer functional benefits; biochar, for example, will often absorb toxic compounds and will buffer against over-fertilization by extending and lowering nitrogen and phosphorous release rates (“flattening the curve”).

Surface structure and complexity also play important roles in nutrient exchange and biological metabolism, by creating more space and active exchange sites per volume. Biochar provides a good example, because its surface area is known to be highly complex (with a high surface to volume ratio compared to other forms of stabilized carbon).

Because adding organic fertilizers to established potted plants is difficult (costly), formulators should load available nutrients into the HCM insofar as possible. However, excess nutrients can cause HCM decomposition, anaerobic conditions and salt burn.

In regard to measuring chemical properties, as with physical properties, lab procedures play a significant role in determining the value of common parameters like pH and cation exchange capacity, as well as nutrient levels. “Some laboratories use weak acids or salt solutions as extractants and consequently larger amounts of plant nutrients are removed. The extent to which this occurs depends upon several factors including the strength and type of extractant, chemical characteristics of the medium, i.e., its exchange capacity and ability to fix phosphorus, and also the length of the extraction time.” When translating and applying lab data, horticulturists need to calibrate their conclusions to the standards and recommendations relevant to the lab technique used to generate the data.

Further, optimal ranges for chemical parameters vary between substrates and between plant species and variants. The horticulturist or formulator needs to conduct plant trials to determine optimum nutrient nutrient needs for the target plant species and variants and correlate this data to leaf-nutrient content for vigorous plants, and nutrient cycling in the HCM (addition and loss to growth and leaching). This information provides the basis for nutrient management and diagnosing problems.

In regard to the following HCM properties, any recommendations are for general targets and will not apply to a specific operation.

4.2.1. pH and pH trends

pH is a logarithmic measure of acidity, with 7.0 considered neutral. pH has a major impact on nutrient availability and cation exchange capacity, especially in HCMs. In the container, iron is unavailable at pH levels above 6.5. As pH drops, potassium increasingly becomes available and calcium less available. As pH continues to drop, manganese can reach toxic levels. Cation exchange capacity lowers significantly as pH declines.

Optimal pH range in a container can range from 4.5-6.5, depending on plants, nutrient management regime and other factors. A general recommended range is 5.5-6.5.

4.2.2. Electrical conductivity (EC)

EC is a measure of salinity and usually indicates levels of sodium or potassium that at excess can hurt production. Recommended upper ranges for container grown plants range with a general target of 1.8-2.5 mS/cm1 (1:1.5 v/v SME).

4.2.3. Cation exchange capacity (CEC)

Cation exchange capacity is “the sum of the exchangeable cations or bases that a soil can absorb per unit weight,” often expressed as milligram equivalents per 100 grams, or meq 100g. However, CEC is also expressed in milligram equivalents per /volume, often per liter (meq/L), and some labs report parts per million (ppm). Currently the trend is towards using cmol+/kg (centimoles of positive charge per kilogram).

As noted, cation exchange capacity is pH dependent, especially in HCMs, decreasing with increasing acidity (decreasing pH).

Recommended ranges for container grown plants vary by plant and species, but a general target is 50-200 meq/L

Some labs prefer to report Total Exchangeable Cations rather than CEC, because the CEC analysis is built on the model of the outer-sphere complex of the clay colloid. Since modern organic HCMs exclude clay, the colloid model does not strictly apply.

4.2.4. Nitrogen drawdown and nitrogen drawdown index (NDI)

Nitrogen drawdown refers to the nitrogen fixed from the soil solution into microbial tissue. NDI measures microbial use soluble nitrogen and estimates nitrogen needs for plant growth. “The ability of a potting mix to consume soluble N is indicated by its nitrogen drawdown index. The test method measures the rate of disappearance of nitrate-nitrogen added to the mix from a solution containing either 75 or 150 ppm N to give drawdown indexes designated as NDI75 and NDI150, respectively. If all the added nitrogen remains after four days of incubation, the mix is said to have an NDI of 1. If there is no nitrogen left, the mix is said to have an NDI of 0.” NDI75 is the most commonly use NDI.

4.2.5. C:N ratio

The C:N ratio is the ratio of carbon to nitrogen in a substrate. High C:N values indicate unstable substrates, which microbes will decompose until they have respired adequate CO2 to lower the C:N ratio. C:N ratios will vary widely between substrates.

4.2.6. Available nutrients

Formulators usually supplement HCMs with nutrients to compensate for substrate chemical properties or deficiencies, and generally should supply HCMs with the minimum necessary nutrients to sustain life for the shelf-life of commercial plants. As with other substrate properties, sufficient and optimal nutrient loading will vary with management patterns and plant choices.

Plants depend upon at least sixteen essential elements to sustain life. Three of these, carbon (C), oxygen (O) and hydrogen (H), plants attain in non-mineral form. Carbon constitutes roughly 45 percent of total dry plant mass. Plants attain C by transforming atmospheric CO2 (drawn through stomata) into starches and sucrose. Oxygen and hydrogen constitute, respectively, 45% and 6% of total dry plant mass. Plants attain most O and H by transforming water, though they also transform some atmospheric O, which they obtain in O2 form through stomata. Plants also obtain a small amount of H through metabolizing ammonium and other nutrients in the soil solution.

Plants obtain the remaining thirteen essential elements and beneficial elements through the soil solution. Convention divides these elements into macronutrients and micronutrients according to the quantity found in total dry plant mass. This convention reports macronutrient levels as percent of total dry plant mass and micronutrients as parts per million (ppm). One percent equals 10,000 ppm. The six essential macronutrients include those nutrients most abundant in plant tissue, including nitrogen, potassium, calcium, magnesium, phosphorus, and sulfur. The seven essential micronutrients that plants require include iron, zinc, copper, manganese, boron, molybdenum, and chlorine. Silicon and nickel are considered beneficial macro- and micronutrients, respectively.

Generally, plants will uptake nutrients as they need them. Thus, given a fertilizer salt addition, the plant is likely to use one ion at much greater concentrations than the other. In addition to supplying container systems with nutrients in available form, balancing the proportional balance between nutrients is fundamentally important. Further, some nutrients increase uptake of others, and some nutrients substitute for others in the absence of an adequate supply of the first choice.

The brief discussions below of the essential mineral nutrients refer to plant nutrients in general terms as they concern HCM formulation and basic plant physiology. For more specific information regarding deficiency symptoms and deficiencies of these elements, please refer to one or more of the several reference texts in print. Nitrogen (N)

Nitrogen is the most crucial mineral element to manage in HCMs and is a primary consideration when determining elemental nutrient levels for an HCM. In terms of plant nutrition, N is the most abundant mineral element in plant tissue, composing roughly 1.5 percent of total dry plant mass. It is mobile in plant physiology, and plants move it from older tissue to growing tissue when a soil or HCM is deficient. Nitrogen is the first of two elements in the “Nutrients that are part of carbon compounds,” Group 1 in the Classification of plant mineral nutrients according to biochemical function. It is part of every amino acid, and therefore it is part of every protein and genetic molecule in every plant cell and is thus fundamental to every plant function. Because N is mobile, deficiency symptoms usually first appear in older leaves, since plants translocate N to younger leaves.

Nitrogen is mostly available to plants in two forms, ammonium (NH4+1) and nitrate (NO3-1). In the absence of fertilizers, nitrogen is largely made available from decomposing organic matter at optimal temperature (68 – 86 deg. f) and moisture levels similar to those providing readily available water for plants. In aerated soils and media, microbes can readily oxidize NH4+1 to NO3-1 with NO2 as an intermediary, releasing H in the reaction, which consequently decreases pH in the rhizosphere solution. Generally, NO3-1 is not toxic to plants, but excess NH4+1 is toxic to plant roots. Plants transform NO3-1 into NH4+1 if uptake exceeds the plant’s ability to metabolize the nitrates due to environmental conditions or to deficiencies in other minerals, so excess NH4+1 can cause toxic levels of NH4+1 in plant tissue. In combination with microbial decomposition of organic materials and transformation of NH4+1 to NO3-1, plant transformation of NO3-1 can release toxic levels of NH4+1 into the rhizosphere.

Because it is so fundamental to basic biological growth and so dynamic, N plays a primary role in HCM ecology and microbiology, which has significant outcomes for plant management. A key ecological outcome of plant NH4+1 uptake is the release of H atoms and subsequent decrease in pH in the rhizosphere. Further, water readily leaches available N, and microbes in the HCM commonly transform it between available forms depending upon pH, moisture levels, temperature, substrate biology and other factors. In addition, NO3-1 uptake increases Ca2+1 and K+1 uptake, and reduces SO4-2 uptake.

Formulators should begin the chemical formulation process with establishing N-levels. Potassium (K)

Potassium is the second most abundant mineral element in plant tissue at roughly 1.0% and is mobile in plants so that most K is located in the symplast. It is the first element in Group 3, “Nutrients that remain in ionic form.” It is central to maintaining turgidity, photonastic movement (changes in plant position in response to light), thigmonastic movement (changes in plant position in response to touch) and electrostatic balancing due to its role in osmotic function as a primary cation in osmoticum, in which it balances and co-functions with the Cl-1 anion. In addition, it has multiple roles in protein synthesis and in regulatory functions as a co-factor for over 60 enzymes. Deficiency symptoms generally first appear in older leaves.

Plants uptake K in ionic form (K+1). Potassium is often adsorbed onto surfaces of organic compounds in ionic bonds, and its uptake is increased in the presence of NO3-1 uptake. Several substrates contribute generous quantities of K, including poorly-rinsed coir, pine bark, green waste compost and vermicompost. Calcium (Ca)

Calcium is the third most abundant mineral element in plant tissue at 0.5% of dry matter and is immobile in plants so that most Ca is located in the apoplast. It is the second element in “Nutrients that remain in ionic form.” It is fundamental in several plant structural properties, including strengthening and stabilizing cell walls. Furthermore, it has important roles in cell division, it activates key enzymes and it has numerous roles related to plant signaling in response to environmental stimuli. Deficiency symptoms usually first appear in younger leaves.

Plants uptake calcium in ionic form (Ca+2) from the soil solution. It is frequently present in ionic form as a salt with carbonates, sulfur or other anions or is adsorbed to organic materials, becoming readily available in solution. Magnesium (Mg)

Mg is the fourth most abundant mineral element in plant tissue at 0.2% of dry matter and is mobile in plants with the largest concentrations in the symplast. It is the third element in “Nutrients that remain in ionic form,” and it has roles in enzyme activation, including facilitating rubisco processes, and it is a key metal in the chlorophyll molecule. Deficiency symptoms usually first appear in older leaves.

Plants uptake Mg as Mg+2. Like Ca, Mg is frequently present in ionic form as a salt with carbonates, sulfur or other anions or is adsorbed to organic materials becoming readily available in solution.

Formulators should mix Mg in relationship to Ca in a Ca/Mg ratio of 1.5-10. Phosphorus (P)

Phosphorus is the fifth most abundant mineral in plant tissue at 0.2% of dry matter and is mobile in the plant. It is the first element in Group 2, “Minerals important to energy storage or structural integrity,” taking a multitude of forms based on the orthophosphate (Pi) structure with structural and metabolic functions. In its various Pi forms, phosphorus is central to energy storage, balance and transfer and enzymatic function and it plays a structural role in cells as part of most genetic molecules and a component of phospholipids. Deficiency symptoms generally first appear in older leaves.

Plants uptake P in one of two Pi forms – HPO4-2 in soils with pH >7.2 and H2PO4-1 in other soils. Phosphorus becomes available for plant uptake through microbial mineralization of organic and inorganic molecules under the right temperature (68 – 86 deg. f) and moisture conditions (similar to plant needs). Though P is usually present in soils or media in large quantities, available P is usually low and ephemeral because it is highly reactive.

Phosphorus is the only macro-element that moves through HCMs and soil only through diffusion and not mass flow. Formulators should set P levels in ratio to N levels. Sulfur (S)

Sulfur is the sixth most abundant mineral element in plant tissue at roughly 0.1% of dry matter and is immobile in plants. Sulfur is the second of two elements in the “Nutrients that are part of carbon compounds,” group. In plant physiology, sulfur plays several roles: it plays structural roles in cellular membranes; it constitutes part of the genetic material, particularly molecules containing the amino acid cysteine; it is part of key electron-transfer mechanisms (with iron); and, it is essential to several growth-related and metabolic compounds. Deficiency symptoms usually first appear in younger leaves.

Plants obtain S as sulfate anions (SO4-2), which microbes release from organic and inorganic molecules. In aerated media, microbes readily oxidize elemental S to SO4-2, consequently decreasing pH in the rhizosphere soil solution. Furthermore, NO3-1 uptake decreases SO4-2 uptake.

Formulators should set S levels in ratio to N levels. Though it constitutes less percentage of total dry plant mass than N, S is immobile so it requires an adequate soil reserve. Iron (Fe)

Iron is the first most abundant essential micronutrient uptaken in cationic form, constituting roughly 100 ppm of total dry plant mass. Because iron easily changes valencies in redox transformation, it is central to energy transfer and enzymatic function due to its role in electron transport systems.

Plant root uptake Fe in Fe+2 form, which it obtains by reducing chelated Fe+3, its predominant form in soil solution, at the root tip. Microbial decomposition produces the soluble organic molecules in coordination complexes with Fe+3, which they obtain from mineral form. Thus microbes transform mineral iron to available iron. Iron concentrations in plant tissue is considerably higher than its concentration in the soil solution, and it is insoluble in high-pH solutions, because chelates become unstable, severely limiting root uptake. Manganese (Mn)

Manganese is the second most abundant essential micronutrient uptaken as a cation, constituting roughly 50 ppm of total dry plant mass. Manganese readily undergoes redox transformations and as such activates or is a co-factor in dozens of enzymatic reactions.

Plants uptake Mn as Mn+2. Manganese usually derives from primary rocks, often paired with Fe. But, it easily chelates and precipitates in solution, so it is available under specific conditions related to pH, redox potential, and chelation conditions (complexation). Zinc (Zn)

Zinc is the 3rd most abundant essential micronutrient uptaken as a cation, constituting roughly 20 ppm of total dry plant mass. Zinc catalyzes enzymes and has structural roles in protein synthesis. Plants uptake zinc in divalent form (Zn+2). Copper (Cu)

Copper is the 4th most abundant essential micronutrient uptaken as a cation at roughly 6 ppm of total dry plant mass. Cu has roles in energy transfer and enzymatic function in several metabolic pathways due to its ability readily to change valencies in redox reactions.

Plants uptake Cu in Cu+2 form, likely reducing a chelated trivalent form to available divalent form in soil solution at the root tip. Concentrations in plant tissue are much higher than in soil solution. Copper easily complexes with organic molecules and becomes unavailable to plant roots. Copper adsorption increases with pH. Copper is often present as an impurity in calcium carbonates. Chlorine (Cl)

Chlorine is the most abundant essential micronutrient uptaken as an anion, at roughly 100 ppm of total dry plant mass. It has roles in enzymatic reactions and as an osmoticum, paired with K, to regulate water status and plant movement. Plants uptake Cl in solution as the anion Cl-1. Chlorine is usually ubiquitous in the soil solution and deficiencies are rare. Boron (Bo)

Boron is the fourth most abundant essential micronutrient at roughly 20 ppm of total dry plant mass. Though Bo has a large representation in plant matter, its roles are still unclear in conventional science as of this writing. Plants uptake Bo as boric acid H3BO3. HCMs it will be present in rock or H3BO3 form or bound to organic molecules, from which it will become available as boric acid. Availability is sensitive to pH changes, decreasing in availability with increasing pH. Molybdenum (Mo)

Molybdenum is the third most abundant essential micronutrient uptaken as an anion (though it is a metal), at roughly 0.1 ppm of total dry plant mass. It is a key co-factor for several enzymatic reactions, including the reduction of NO3-1 to NH4+. It is uptaken in anionic form as Mo4-2), its predominant form in solution. It becomes less available as pH decreases.

Micronutrients are fundamentally important to plant growth though they appear in relatively small quantities. Horticulturists can use mineral dusts as sources of available micronutrients for an HCM.

4.2.7. Nutrient leaching

In general, nutrient leaching can be a substantial problem with container horticulture, raising inefficiencies and contributing to nutrient loading in runoff water or public sewage systems. Soluble fertilizers leach more than controlled release fertilizers.

Nitrogen and phosphorus leach out of HCMs rapidly because they remain in the media solution, with a small minority adhering to positively-charged surfaces or immobilized into organic form by microbes. Some studies indicate losses from a peat-lite mix from one flush at 33% for NH4, 75% for NO3-1, 43% for P and 45% for K. In some studies, N and P anions leach rapidly in the first two weeks. Some studies indicate that nitrates leach substantially and rapidly with losses decreases over time, while phosphate losses gained over time. In solution, they precipitate with cation nutrients, leaching them away as well. In one study, P losses were above 20% for a single drainage. In all cases, leaching increases as fertilizing increases.

Potassium leaches as well, though peat-based substrates tend to leach less than composted pine bark (discussed below). CaSOH additions to HCMs will leach within 50 irrigation days.

4.4. Biological Properties

The biological properties of HCM formulations can vary widely, and an individual HCM’s biological properties can vary widely depending on several factors, substrate physical and chemical properties being a primary factor. (Other factors include container properties, atmospheric heat and moisture, overall sanitation and pest management, irrigation and fertilization, ventilation, and exposure to infection among other factors.) An HCM’s biological properties can contribute benefit or cost to an operation.

4.4.1. Beneficial biology

On the benefit side, HCM substrates can benefit horticulture through propagation of beneficial microbes and suppression of pathogens. Suppressive substrates suppress pathogens using a number of mechanisms. Known mechanisms include: a) “Competition for nutrients, space and occupation of infection sites by other micro-organisms […] for a variety of pathosystems”; “hyperparasitism followed by lysis”; b) “antibiosis [… the] production of antibiotics”; “futile pathogen germination”; c) “adsorption of chemical signals that enhance the germination and root colonization by pathogens”; d) “induced systemic resistance […] or systemic acquired resistance.”

Likewise, beneficial microbes can sequester nutrients, especially nitrogen, which other beneficial microbes release as ammonium when they consume the original microbes. Roots are covered with biofilms, and soil microbes, mostly beneficial, congregate in the rhizosphere, providing a vast ecological food web that feeds off plant exudates and make elements and compounds available to the fine root hairs.

4.4.2. Problem Biology

Pathogenic biological factors can: a) release toxic by-products into the rhizosphere due to substrate decomposition; b) propagate pathogens; c) shrink substrate particles due to decomposition; d) suppress or interrupt plant functions and signaling (to microbes); and, e) suppress beneficial microbial functions and signaling.

The formulator has some influence on plant and microbe solution by choosing or omitting certain substrates, for example biochar, which interacts with plant and microbe signaling. The formulator also has influence over release of toxic by-products, pathogen infection and substrate shrinkage by choosing high-quality stable, mature substrates. Substrate stability, which in turn is linked to substrate maturity, are especially important with composted substrates (discussed below).

By definition, HCM stability refers to its ability to maintain its original physical and chemical properties during the growing cycle, which is predominantly a function of its biological environment. Green, undecomposed and/or poor-quality substrates can decompose in the container leading to shrinkage and decreased particle-size distributions.

A simple method for assessing biological stability involves the biostability index. Determining the biostability index involves wetting and maintaining a moisture level in a volume of HCM for a period comparable to the growing cycle of a chosen plant variety the dividing the final dry weight by the initial dry weight. For example, if no HCM has decomposed, final dry weight will equal initial dry weight and the index will be 1.0. If only 70% remains, the index is 0.7. Other methods include NDI tests, oxygen uptake tests, and CO2 production tests.

Manufacturing, shipping, handling and storing processes as well as plant management can substantially determine the biological status of an HCM.

Section 5. Operational and Sustainability Considerations in Formulating an HCM

In addition to the biological and ecological factors to consider in formulating HCM, the operator must show a return on investment (ROI). Concerns for ROI for agricultural factors necessitates careful attention to their operational costs. For the mixing facility, HCM costs include procuring, handling, storing and processing substrates, mixing them into the HCM, storing the HCM, preparing the HCM for shipment, shipping and administering the whole process. For the horticulture operation, costs include procuring, handling and storing the HCM, containerizing it, managing plants growing in it and disposing of it. Some horticulture operations might mix part or all of their HCMs, combining associated costs. Labor and safety costs dominate these cost structures. Long-term operational concerns for the horticultural operation and mixing facility include availability of substrates, the reliability of sources with reference quality, price and supply, and the sustainability of producing the substrates and HCMs.

Sustainability concerns include effects on environmental quality and functionality (atmospheric carbon loading, water quality, air quality, ecological health and other concerns) as well as concerns regarding social costs (community and regional health, political violence and other concerns).

This section will first consider operational concerns of producing and using an HCM and then will discuss the long term costs vis-a-vis sustainability of producing substrates.

5.1. Operational Factors

Producing an HCM will reflect all these cost areas and material and processing choices will determine the specific costs. Several substrate properties will determine costs in each of these areas, with each substrate involving cost increases and reductions in each cost area. This section will briefly describe some important substrate properties that affect costs associated with producing and using an HCM.

5.1.1. Dry bulk density

Dry bulk density (DBD) is “the dry mass per unit volume of moist medium.” DBD is an important operational concern because materials with high bulk densities require more work to handle, which increases raising labor, equipment, and transportation costs. (However, Sometimes HCM volume is the limiting factor in shipping, because DBDs are so light. personal experience) In addition, accurate DBD values are necessary for interpreting lab test results and applying recommendations from labs that report using per weight basis. Finally, DBD can roughly indicate the watering needs associated with a substrate or HCM, because DBD is inversely related to total porosity in most substrates (like peat), so lower bulk densities generally indicate high air-filled pore space and hence more watering. Even with perlite, which has high internal porosity that cannot contribute to pore space, the inverse relationship holds.

Suggested DBD values range from around 420-840 lbs/yd3.

5.1.2. Physical stability

Media particle size and shape stability through the growing cycle is highly-important for ensuring uniform chemical and physical properties and maintaining a stable plant environment. In addition to biological processes, root growth, physical handling (mechanical damage), shrinking and swelling, and displacement and compaction from chemical processes all degrade particle-size.

Root growth causes significant changes in HCMs by compacting them and by realigning and displacing particles, thereby altering pore space and diffusivity, often with increased gas exchange in peat-based HCMs. Watering and fertigation operations alter HCMs, also, through their contribution to cementing, packing, and swelling and shrinking.

Changes in HCM properties due to biological decomposition of unstable substrates include volume loss, compaction, decrease in porosity, air content and easily available water, decrease in particle size, change in “gaseous phase composition due to carbon dioxide production,” increase of pH, water content, CEC, and EC (due to mineralization of cations) and the synthesis of phytotoxins and phytostimulants. In addition to presenting problems and risks to cultivation, biologically unstable substrates cause operational problems because they decompose in storage as well as in use.

5.1.3. Wettability and hydration efficiency

Wettability, or the ability of an organic substrate to capture and retain water is a significant operational concern. Hydration efficiency is a proportion (values between 0 and 1) that equals 1.0 divided by the number of hydration events required to fill a container to container capacity. Generally, as substrates dry, especially peat, composted pine bark, coco coir and biochar, they retain less water per irrigation event and less overall water after a series of 10 irrigation events; once a substrate becomes too dry, it will rarely hydrate to its original potential maximum moisture level under irrigation. Another important parameter is water drop penetration time, the time that water remains on the surface before infiltrating, which generally should not exceed 5 seconds for standard irrigation.

5.1.4. Chemical stability

Changes in chemistry generally are due to decomposing substrates and can cause significant variation in chemical properties and other problems including concentrating salts and toxic levels of nutrients, acidifying media, and nurturing pathogen build-ups, among other effects. Exposure to moisture can intensify chemical degradation, as well.

5.1.5. Blending and mixing efficiency

Within an HCM batch, thorough blending and mixing is important to assure even distribution of chemical and physical properties, especially nutrients which can cause toxic effects at excess levels. However, mixing can also reduce particle size substantially, so efficient mixing is important. Some substrates mix more easily than others. This book will address blending efficiency in the next edition.

5.1.6. Workplace hazards

In horticulture operations, working with substrates can present several hazards, including, among others, hazardous dust, equipment-related injuries, lifting injuries, repetitive movement injuries and toxic exposure.

5.1.7. Substrate availability

Substrate availability will vary regionally, with transportation costs rising in proportion to distance from manufacturer.

5.1.8. Substrate variability

Consistency in substrate and HCM properties is important, yet substrates can vary substantially in all properties from batch to batch and from producer to producer. Likewise, HCMs commonly vary in every important property from batch to batch and producer to producer. Testing basic physical and chemical properties is fundamentally important for maintaining a consistent HCM from batch to batch.

5.2. Sustainability of Substrate Production

Sustainability in the most basic terms requires that production of a substrate be feasible into perpetuity. However, in practice, applying this principle will involve gradations of sustainability, beginning with managers choosing between available substrates by phasing out the least sustainable as an ongoing policy. This decision-making requires knowledge of the sustainability of producing these substrates, which involves a complex analysis because of all the associated productive factors. This book only superficially comments on the sustainability of substrates. A practical analysis would include the following considerations and more.

5.2.1. Sustainability indexing

Sustainability indexing requires identifying empirically verifiable indicators based on a goal-programming process, assigning weighted value to the indicators, and transforming them into an index that reflects and sums all the indicators. In regard to substrate production, several factors lend themselves to assigning indicators, from mining or harvesting and all associated environmental costs, to transportation.

5.2.2. Precautionary principle

The precautionary principle is fundamental to sustainability. It states that before undertaking any action that will affect public health or the environment, in the absence of scientific knowledge on the effects, the manager should adopt full precautionary measures. It is based on the principle, “First, do no harm.”

5.2.3. Renewable resources

Renewable resources can be renewed on the productive site within a generation or some similar time window relevant to those who must live with the degradation of their environment based on ecological and biological prerogatives.

5.2.4. Sustained yield

Sustained yield is a principle from forestry that requires that a periodic yield of trees from a forest be matched by growth in that period continuing into perpetuity so that feasible harvest levels never decline from period to period.

Section 6. Some Common Substrates for HCM formulation

Because sustainably formulating and blending high-production HCMs requires obtaining substrates that are reliably available without compromising long-term environmental values, researchers continue to address the issues of regional availability and sustainability of substrates. In this context, several researchers have worked to develop HCM substrates from agricultural waste streams, yielding with some notable successes. However, until horticultural industries can produce effective substrates to meet regional demands, formulators necessarily will continue importing substrates. Horticulturists and HCM manufacturers use a variety of substrates in their mixes, each with unique physical, chemical and biological properties.

Understanding the basic properties of each substrate is fundamental to understanding the overall properties of an HCM. This chapter describes some commonly available substrates with an emphasis on sustainable substrates in terms of their physical, chemical and biological properties, as well as operational considerations and sustainability. The chapter describes using the same format some alternative substrates readily available in the United States and Mexico: coconut coir, biochar, composted wood bark, green waste compost and vermicompost. It ends with descriptions of the properties of peat and perlite for background reference, since they are currently the most common substrates, especially in context of the peat-lite mixes.

6.1. Coconut Coir (coir)

Horticultural coconut coir, also known as coir dust, is a fibrous substrate that is increasingly gaining use as a horticultural substrate and is now widely used in place of peat. Because it contains fibers, coir distributes water through capillary action. However, to note, particle size distribution, fiber size and quality vary significantly between coir – within facilities and between facilities.

Researchers have demonstrated that coir is roughly equivalent to peat – the industry standard – for using in an HCM for greenhouse container agriculture. While a few studies indicate that plants do not grow as well in pure coir as in pure peat (using Mexican coir), many studies indicate that coir can be equal to or superior to peat, depending upon the qualities of a given substrate product. Management factors, including coir source and quality, plant species, HCM contents and proportions, watering and fertilizing regimes, and such, play a large role in the success of coir or any other substrate.

6.1.1. Production

Coir is made from shredded coconut (Cocos nucifera) husks, the mesocarp. The coconut mesocarp is a combination of pith and fiber, parenchyma cells and schlerenchya cells, respectively, pith being the largest component. Coir consists of the dust with “sponge-like structure” and “short to medium-length fibers” remaining after long fibers have been removed for other uses, such as brushes, doormats and other durable goods.

Coir producers remove the coconut mesocarp using one of two processes. One process uses an exclusively mechanical process, leaving more fiber to the advantage of an HCM formulation. The other process involves retting the husks in water or saline water for six months to loosen the mesocarp. After retting and shredding the husks, producers will typically rinse the shredded coir in water with or without calcium (often calcium nitrate) and magnesium (dolomite) to buffer against and partially remove sodium and chloride from the retting process. After washing and drying, processors compress and bail the coir for shipping. Growers or HCM manufacturers soak and decompress these bails using specialized equipment that will minimize fiber damage.

Coir quality is highly variable. The coir industry lacks a uniform characterization or grading standard, and properties, especially physical and chemical properties, vary considerably between regions and manufacturers. One significant distinction is between the retted southeast Asian coirs, dominantly those from Sri Lanka and the Philipines, in contrast with Mexican unretted coirs. Aside from generally showing different physical and chemical properties, as discussed below, Sri Lankan and Philipine coir is less variable in these qualities, possibly due to strict manufacturer quality controls.

HCM formulators rely on coir from both regions, but this Primer will refer mostly to Sri Lankan and Philipine coir.

6.1.2. Physical Properties – Coir

The physical properties of coir make it an attractive fiber component of an HCM.

However, coir properties are highly variable, depending upon the processing facility and regional customs. So, though general characteristics and properties are helpful, a horticultural operation should be careful when purchasing coir in order to maintain consistent quality in the HCM. Total porosity

Generally, coir that has a typical range of particle sizes provides generous total pore space. One survey of Philippine coir from different sources ranged in total pore space from 85.5 – 89.5% of total volume, while another study of Malaysian coir reported 91.5% and 94.1% in a sample of Sri Lankan coir.

In one research study, coir blended into pine bark increased total porosity compared to 100% pine bark.

Water-filled pore space. WFPS is equal to the volume of water in the HCM at container capacity – the volume of water retained 30 minutes after draining a saturated container. Researchers have determined that increasingly smaller-diameter particle fractions displayed increasing bulk densities and thus water-holding capacity. Coarse fractions released most water volume at low tensions (<0.5kP) and, conversely, fine fraction curves held much of its water at low tensions. At particle sizes above 0.5 mm diameter, easily available water and water buffering capacity steeply decline. In one study of SE Asian coirs, WFPS ranged between 73.0 – 80.0% total volume, while another study reported 53%.

Air-filled pore space.

At particle sizes above 0.5 mm diameter, AFPS increases dramatically. In one study of SE Asian coirs, AFPS ranged between 9.5 – 12.5% total vol,1 while another study reported 31.7% AFPS in Sri Lankan coir samples.2

In a research study, coir blended with pine bark decreased air-filled pore space. (See table below.) Soil gas diffusivity (and Hydraulic conductivity)

One study reports saturated hydraulic conductivity of a coir substrate to be 0.9 cm/minute while another reports saturated hydraulic conductivity at 7.8 cm/minute.

The first study also reported a value of 2.8 cm/min for a 50:50 blend of coir:perlite. Available water

Researchers report a range of values for easily available water and water buffering capacity in coir, including, respectively, 22.5% v/v and 5.3% v/v, 32.09% v/v and 7.05% v/v. Other research on on coir-pine bark blends reported that available water increased as the percentage of coir increased to a maximum of 65%. Readily available water (EAW plus WBC) was 14%, 14.1%, 15%, 16.9%, and 16.4% of total volume for blends with coir at 0%, 10%, 25%, 40% and 65%, respectively. The same study reported that with “the 65% coir treatment, 60% of total water holding capacity can be extracted without decreases in photosynthetic rate.” Particle properties

Particle size and size distribution. Most coir particles are within 0.5-4.0 mm, though coir often has more fines at diameters less than 1.0 mm than does peat.

According to common practice, coir particles with screen diameters greater than 8-mm are considered fiber. The remaining particles are mostly pith, which are categorized in these size classes: 4-8 mm, 2-4 mm, 1-2 mm, 0.5-1 mm, 0.25-0.5 mm, 0.125-0.25 mm and <0.125 mm. The coarseness index is often applied to coir, which reports coarseness as percentage of the substrate over 1 mm (2nd largest diameter). One study reported a coarseness index of Sri Lankan coir ranging from 31-36, while another reports a CI of 38.

Coir processing, including “degree of grinding, screen size, or screening time” determines particle size distribution and fiber content, which vary significantly between facilities and between regions. Generally, Mexican coirs (no retting) are coarser than Sri Lankan, with larger particles and more fiber, though these qualities vary dramatically in Mexican coir; unretted Mexican coirs have much wider variation than Sri Lankan. On the other hand, Sri Lankan coir contains more pith but varies far less in particle size. See below table. (Note, discrepancies are the result of the original report.)

Particle size classes of typical Mexican & Sri Lankan Coir

Sri Lanka*> 8.02.5%

4 – 83.5%

2 – 46%

1 – 218%

0.5 – 1.028%

0.25 – 0.521%

0.125 – 0.257%

< 0.1252%


2.37-2.51St. dev.
Mexico> 8.011%

4 – 85%

2 – 423%

1 – 225%

0.5 – 1.018%

0.25 – 0.511%

0.125 – 0.256%

< 0.1252%


2.53 – 5.05St. dev.

Another study of coir (of unreported origin) reports the particle size-class distribution in the following table.

Particle-size of a typical coir

< 0.07Negligible
0.07 – 0.1252
0.125 – 0.2510
0.25 – 0.528
0.5 – 1.034
1.0 – 2.022
2.0 – 4.03
4.0 – 8.0< 1

Particle geometry. Pores on the surfaces of Sri Lankan and Mexican coir particles greater than 2 mm generally consisted of cell lumina and the intercellular spaces in the pith tissue. The pithy cells were cylindrical with round to oval cross-section, 30-80 um in diameter and 200-300 um in length with average with exterior openings roughly 44.3 um diameter. Internal porosity is 41% by volume and surface porosity is roughly the same. Particles less than 1.0 mm had few pores, while particles less than 0.5 mm were almost without pores. Pores between coarse particles will be larger than with finer coir “due to microstructure and porosity properties.”1

Mexican coir has higher surface porosity and larger openings in the coarse pithy tissue particles, which facilitates water penetration and drainage. The retted Sri Lankan coir has greater tissue decomposition but displays the same pore structure, with the cross section of the fiber cells oval 50-300 um diameter. These fibers are mostly “strands of long schlerenchyma cells” with diameters 5-15 um. In addition to size degradation, retting removes silica bodies on coir surface.

Particle density.

6.1.3. Chemical Properties – Coir

The chemical and physico-chemical properties of coir vary significantly from source to source and region to region due to fertilization of the coconut crop, processing technique and age of stockpiled coir.1 The rest of this primer considers coir from Mexico and Sri Lanka, two of the largest producers pH and pH trends

Coir tends to become more acidic through the cultivation period, with one study reporting a shift from pH values of 6.6 to 4.4.

One study of Philipine and Sri Lankan coir measured pH values between 5.6 – 6.9, while another study measured Sri Lankan coir pH values between 4.9 – 5.1. Electrical conductivity (EC)

One study of Philipine and Sri Lankan coir measured EC values between 30 – 290 mS/m, while another study measured Sri Lankan coir EC values between 70-240 mS/m, and another at 0.5 mS*cm. A study of Malaysian coir reported an EC value of 160 and 210 mS/m for 100% coir and a blend of 70/30% coir/perlite, respectively. High EC values in coir result from poor rinsing of retting solutions. Cation exchange capacity (CEC)

One study of Philipine and Sri Lankan coir reported CEC of 39.0 – 60.0 cmol+/kg1,1 while another study measured Sri Lankan coir CEC values between 92.9 – 95.4 cmol+/kg1. Nitrogen drawdown and nitrogen drawdown index (NDI) C:N ratio

Coir is a good medium for holding available fertilizers and nutrients due to its absorbency (with water) and adsorbancy. As with peat, smaller-particle grades hold more cations than larger particle grades. Available nutrients

Coir is a good medium for holding available fertilizers and nutrients due to its absorbency (with water) and adsorbancy. As with peat, smaller-particle grades hold more cations than larger particle grades.

Coir without amendment does not contain any nutrients in correct quantities to support plant production. However, all coir contributes potassium, and washed buffered coir contains contributes calcium and phosphorus. Retted coir that is not thoroughly rinsed contains high amounts of sodium-chloride.

Coirs that have not been adequately rinsed may contain substantial potassium (19 – 950 mg/lit) and chloride (26 – 1,636 mg/lit) as residuals from the retting solution Possible chemistry problems

Three mechanisms explain most problems associated with coir: high nitrogen draw-down, high EC from potassium content and sodium-chloride NaCl from retting residue and possibly toxicity from phenolic compounds. Formulators and growers can easily manage nitrogen drawdown through fertilization and EC through the use of buffered coir or Mexican coir. Addition of gypsum will buffer with calcium while contributing sulfur, found only at low levels in coir. Buffering eliminates the potassium excess, which can be beneficial with many plants. Others have indicated the presence of phenolic compounds can explain observed plant growth inhibition, though problems from phenolic compounds seem infrequent in the research. These compounds are also found in bark chips (less so when composted) and peat and are known to thwart some root pathogens.

6.1.4. Biological Properties – Coir

Coir, like peat, is a sterile medium for all practical purposes, if processed and handled correctly. As an organic material that will hold nutrients and water, it works well with biologically-rich HCM, however for the same reasons, it is vulnerable to carrying pathogens if handled incorrectly.

6.1.5. Operational Considerations – Coir

Coir displays “good shipping, storing and handling qualities (light-weight, durable, somewhat compressible), maintaining consistency in storage,” and generally has “greater physical resiliency (withstands compression of baling better) than S. peat.” Though coir demonstrates minimal biological degradation during processing, as a fibrous material, rough physical handling, common in mechanical container-filling, can degrade particle size. Dry bulk density

Coir DBD values vary widely. While coir DBD values commonly ranges from 110-250 lbs/yd3, they can range as far as 70-400 lbs/yd3. Physical stability

Generally, coir is a stable, sterile medium, though during a grow cycle, it shrinks in containers. Likewise, coir maintains its physical consistency through a growing cycle better than peat.

The biostability index for coir (of uncertain origin) is 100. Wettability and Hydration efficiency

Coir generally has better wettability properties than other organic substrates. However, as with other substrates, wettability and hydration efficiency decline as coir dries. With moisture content at 45% of container capacity, coir without wetting agent will retain 89% container capacity for the first irrigation and 99% for the 3rd. Once coir has dried beyond 15% its container capacitiy, coir will only retain 21% container capacity for the first irrigation, 46% for the 3rd and 85% for the 10th. Container capacity will range from c. 64-72% of HCM volume. In addition, some research correlates coir with slow water drop penetration time, an indicator of water infiltration capacity. Chemical stability Blending efficiency Workplace hazards Substrate availability

Coir is readily available for purchase in all parts of North America, Europe and other parts of the world, though prices will continue to increase transportation costs rise in respect to other costs. Substrate variability

Physical properties of coir vary considerably between sources, much of this variation resulting from differences in particle size.

Chemical properties vary significantly, also, especially betweenretted and unretted coir and buffered and unbuffered coir.

6.1.6. Sustainability

Coir is relatively environmentally sustainable for two reasons following from its status as a waste product. First, because coir is a waste product, its redirection towards a commercial product reduces landfill impact – a huge issue, globally. Second, also because it is a waste product, it requires relatively few resources and energy to process. However, coir is an unsustainable product insofar as it is shipped thousands of miles from its origin.

6.2. Composted Pine Bark

Composted pine bark (CPB) is a byproduct of lumber milling and has been a common HCM substrate since the 1970s in Europe and the United States, especially in the southeastern US and Pacific northwest. It is highly effective as an HCM substrate, especially for use as an aggregate substate, given the right screen size range.

Producers grind the fresh bark, often with a hammer-mill, and/or pile it in high piles, wet it and maintain moisture, often add nitrogen supplement and store for at least a year with occasional turning. Composted wood bark properties vary depending upon type of bark, age, the composting process, and handling.

6.2.1. Physical Properties – CPB

CPB physical properties are highly variable, depending upon particle-size distribution, which in turn, largely depends upon degree of decomposition and processing. Generally, pine bark that are in the size-class distributions of interest to This chapter and are aged from six to 12 months have suitable physical properties for horticultural use. This chapter will generally consider CPBs with the major proportion of diameter size values ranging between 2.0-5.0 mm as the data is available. Total porosity Water-filled pore space

Particle size distribution of the CPB has a substantial influence on water-holding characteristics. Air-filled pore space Soil gas diffusivity

Particle size distribution of the CPB has a substantial influence on gas and oxygen diffusion. Container capacity

Particle size distribution of the CPB has a substantial influence on water-holding characteristics. Available water Hydraulic conductivity Particle size and distribution Particle shape and geometry

CPB particles have 45% internal porosity, some of which holds water available to plants with developed root structures. Furthermore, “structural examination of milled pine bark particles showed that there are openings of 5-60 Um diameter with interconnecting channels which allow water and nutrient storage within the particles.”

6.2.2 Chemical Properties – CPB

CPB chemistry shifts with age as a result of aerobic activity common in normal pine bark processing operations. pH and pH trends

CPB tends to increase in pH during the growing cycle, from 4.1-4.2 to 4.5-5.0. EC and EC trends

EC decreases, typically from 0.06 to 0.03. Cation exchange capacity (CEC) Nitrogen drawdown and nitrogen drawdown index (NDI) C:N ratio Available nutrients Possible chemistry problems

CPB can increase nitrogen draw-down, through biological immobilization, adsorption, and storage in internal pores. Low HCM pH levels can release manganese at levels toxic to plant roots.

These compounds are also found in bark chips (less so when composted) and peat and are known to thwart some root pathogens.

6.2.3. Biological Properties – CPB

Generally, horticulturists manage CPB as a sterile medium, though it does have suppressive characteristics, including suppression of Phytophthora root rot. It can also take inoculation of “bacteria (Bacillus spp.) and fungi (Trichoderma spp.) to enhance suppression of root disease organisms.”

As an organic material that will hold nutrients and water, it works well with biologically-rich HCM, however for the same reasons, it is vulnerable to carrying pathogens if handled incorrectly.

6.2.4. Operational concerns – CPB Dry bulk density

CPB generally ranges in DBD around 260-285 lbs/yd3. CPB particles between 2-5 mm diameter average 200 lbs/yd3, while CPB particles between 0.5-1.0 mm diameter average 350 lbs/yd3. Physical stability

Composted bark loses little if any mass during a growing cycle year due to aerobic decomposition. In the balance, gas diffusivity . Saturated hydraulic conductivity The biostability index for one “pine bark compost” was 97.4. Wettability and Hydration efficiency

Composted bark generally has high wettability and hydration efficiency, which declines as it dries. With moisture content at 45% of container capacity, peat without wetting agent will retain 93% container capacity for the first irrigation and 100% for the 3rd. Once composted bark has dried beyond 15% its maximum hydration level, it will only retain 41% container capacity for the first irrigation, 78% for the 3rd and 88% for the 10th. Container capacity is low with composted bark is low, ranging from c. 37-39% of HCM volume. Chemical stability Blending efficiency Workplace hazards Substrate availability Substrate variability

Particle size distribution, fiber size and quality vary significantly between CPBs – within facilities and between facilities.

6.2.5. Sustainablity

6.3. Biochar

Biochar, a form of charcoal made for agricultural uses, is growing in use internationally as an HCM substrate. It can serve as a coarse aggregate substrate, a fine substrate or a combination. However, biochar should serve as a minority substrate, generally in proportions 30% or less of the HCM, though some researchers suggest much higher proportions.

Several researchers have demonstrated that biochar is effective as an HCM substrate, depending upon its characterization and properties, how it is used, the properties of other substrates in the mix, and the needs of the specific plant crop.  However, because biochar is extremely diverse in all important properties and in its effects on horticultural plants, the formulator must use it with great care.  Generally, biochar should be accompanied by compost rather than being applied in raw form.  Given correct precautions, biochar is a valuable substrate for HCMs when used as a co-substrate.

Biochar properties vary widely depending upon several factors having to do with the feedstock and process used to make it.  This subsection discusses biochar in general, then discusses physical, chemical and biological properties in reference to biochar made by slow pyrolysis from pine wood (pinaceae family) at medium temperatures (550-650°C), henceforth “pine biochar.” Pine forestry residues (slash) are abundant and reliable in the western mountainous and forested regions of the United States, where this primer originates. Further, a backyard practitioner can make biochar of this quality using a simple, open-burning kon-tiki cone kiln – or even a pit dug with sides at the same angle (63°). The primer makes secondary reference to hardwood urban forestry residues and a few other sources of ligno-cellulosic biomass that one can procure in the neighborhood.

6.3.1. Production

Biochar is biomass that has been thermally treated with a relative absence of oxygen, or pyrolized, to create a geometrical jumble of carbon allotropes mostly devoid of other elements. According to one production consultant, “[t]he resulting charcoal resembles a blackened, shrunken version of the original biomass. But it now has very little hydrogen and oxygen. Microscopically, it inherits much of the structure of the original biomass. The only difference is the material now has been converted from lignin, cellulose and hemicellulose to many of the allotropes of carbon […] a collection of disjointed graphite crystals based on hexagonally-shaped carbon rings, with some leftover hydrogen and oxygen attached, along with minerals (ash) that were in the original feedstock. These hexagonal carbon compounds are fused carbon rings […] also called “aromatic” carbon.”

Biochar comes in a wide range of forms, which display a wide range of properties, depending upon feedstock properties, pyrolysis method, especially highest treatment temperature (HTT), processing and handling. Forms of biomass used for biochar feedstock include a wide variety of materials, such as poultry manure, switch grass, rice hulls, peanut shells, cotton gin wastes, wood and bark in a variety of forms. Typically, pyrolysis occurs at temperatures between 350-600°C, though sometimes pyrolysis occurs at temperatures as high as 750°C. Generally, biochars produced at HTTs below 500° C vary significantly from those produced at HTTs above 550°C. Likewise, biochars produced at HTTs below 700° C vary significantly from those produced at HTTs higher than 750°C.

Generally, biochar falls into one of three categories: low-temperature, with HTTs <550° C, medium-temperature, with HTTs 550-750° C, and high-temperature, with HTTs >750° C. Low temperature biochar is not as porous as medium- and high-temperature biochar, but it retains more OFGs, which are important for adsorbing cations in an HCM context. Medium-HTT biochars display more porosity and surface area as well as being highly electrically active (discussed below). Generally speaking, low temperature biochars can contribute more nutrients than high temperature biochars, which in turn can form higher cation exchange properties. Typically, HTTs less than 450° C produce biochar with a distinct odor much life creosote, due to the condensation of volatile oils that have not completely combusted. These condensates clog up pores, contain toxic aromatic compounds, and do not promote plant growth. Optimal blends of biochar will depend upon the horticultural objectives.

This range of biochars can derive from a number of processes. Primitive pyrolysis methods for creating charcoal produce chars at temperatures between 450-600° C. These methods typically cover a fire pit with spoil and allow the feed stock to smolder for days, releasing volatile gasses into the air as toxic smoke. Slightly more sophisticated methods recover volatile gasses for fueling the charring process or for manufacturing acetic acid, acetone and methanol. Likewise, making biochar in a properly proportioned open-fire kiln can produce a high-quality blend of medium-temperature or low-temperature biochars (depending upon the process) at small or large scales.

In contrast to slow pyrolysis, fast pyrolysis, a process for manufacturing fuel oil at HTTs between 350-600° C using finely ground feed stock, requires a fairly short residence time and is highly efficient. Gasification, the most sophisticated method, transforms woody materials to manufacture syngas, similar to natural gas, from woody feed stocks. This range of biochars can derive from a number of processes.

Biochars show significant diversity in physical and chemical properties, and as its use is nascent, the HCM industry lacks grading standards for this substrate. The biochar industry has only voluntary standards, established by the International Biochar Initiative. These standards are minimal and mainly pertain to toxicity and declaration (labeling) of values for commonly measured physical and chemical properties. In regard to toxicity, the standards detail thresholds for various contaminants (cyclical carbon allotropes), many of which originate during pyrolysis. The standards also prescribe testing and assessment procedures appropriate to a variety of biochar products to determine contamination levels.

In relation to other physical and chemical properties relevant to HCM formulation, the IBI standards require declaration of values for only a few key properties, including moisture content, particle size distribution, pH, EC, organic carbon, and total nitrogen. From the standpoint of formulating HCMs, most of these standards are too inclusive and imprecise as to be useful. For example, biochar is classed into three classes based on carbon content: ≥ 60% (Class 1), < 60% and ≥ 30% (Class 2), and <30% and ≥10% (Class 3).

The main focus of this primer is medium-temperature biochar made from pine forestry residues (slash). These materials are the most abundant and reliable substrates in the western mountainous and forested regions of the United States, where this primer originates. Also, a backyard practitioner can make biochar of this quality using a simple kon-tiki cone kiln. The primer makes secondary reference to hardwood urban forestry residues and a few other sources of ligno-cellulosic biomass that one can procure in the neighborhood.

6.3.2. Physical Properties – Biochar

The physical properties of biochar vary depending on feedstock, HTT, treatment duration, other manufacturing factors, processing and handling. Feedstock size, shape and composition and HTT largely determine texture, particle size, geometry and surface area, which determine the physical properties important for horticulture. For example, feedstock made from large diameter cells in stem tissue can transform to large pores in biochar. (As a side note, the ability to determine these properties introduces opportunities for creating chars with desired attributes and optimizing HCMs.) HTT largely governs porosity “due to degassing of volatiles and fracturing through subsequent cooling and shrinkage). Porosity

Porosity, gas diffusion, container capacity, available water, and hydraulic conductivity in biochar are highly dependent on particle-size distribution, and somewhat dependent on feedstock and manufacturing parameters. Some research indicates that biochar of any size decreases total porosity of a mix. The same research indicates that between two mixes with the same proportion of biochar, the mix with smaller-sized biochar had greater total porosity than the mix with larger-sized biochar. However, the same research indicates that biochar sized 1.19-3.36 mm and 0.238-0.595 mm increased air-filled porosity in a mix in proportion to its representation in the mix. In this study biochar-containing substrates all exceeded a target minimum of 85% total pore space and 45% water-filled pore space, except for a mix with 40% biochar at 1.19-3.36 mm, which had 82% total porosity. Likewise, mixes met the target range of 10% to 20% air-filled pore space.

Total porosity. One research study compared a commercial peat-light mix (control) with samples of peat-lite mixed with incremental percentages (20%, 30%, 40% and 100%) of biochar in three particle-size classes (0.420–0.841 mm, 0.595–2.38 mm and 1.19–3.36 mm). Additions of 20% and 30% of the smallest two size grades increased total porosity compared to control, but decreased porosity at 40% and 100% percent. Adding 20% of the second largest grade (0.595–2.38 mm) increased total porosity, but larger additions decreased it compared to the control. Total porosity for all samples varied between 79–89%; total porosity for the control was 86% and 70-80% for biochar samples in the smallest particle-size classes.

Air-filled porosity (AFP). The same research indicates that biochar samples from the middle size-classes (1.19-3.36 mm and 0.595-0.238 mm) increased AFP in a mix in proportion to representation in the mix. The smallest sized biochar slightly decreased AFP with 20% and 40% additions and slightly increased air-filled space at 30%. The second-to-smallest biochar grade increased AFP proportionate to the percentage added. AFP for the control sample was 14.5%. AFP was 14.2% for the biochar in the smallest particle-size class and 30% for the next largest size class.

Water-filled porosity (WFP). The smallest sized biochar added at 20% and 30% increase WFP from the control (71.8% of total volume) and had no net affect at 40%. The second-to-smallest sized biochar 20% slightly raised WFP and decreased WFP at higher percentages. Samples in the two largest size classes lowered WFP at all percentages. The 100% biochar samples had substantially less WFP than the control sample. (The volume of WFP equals the volume of container capacity.)





Peat-lite control

TABLE: Porosity of HCM samples with Biochar Additions

(PCS = Particle-size class; TP = Total porosity (container capacity); WFP = Water-filled porosity.)

Other functional parameters. Gas diffusivity and hydraulic conductivity in biochar are highly dependent on particle-size distribution, and somewhat dependent on feedstock and manufacturing parameters. Particle-size distribution and shape

Particle size distribution and quality varies significantly between biochars, depending on feedstock, HTT, treatment duration, other manufacturing factors, processing and handling. Feedstock size, shape and composition and HTT largely determine texture, particle size, geometry and surface area, which determine the physical properties important for horticulture. For example, feedstock made from large diameter cells in stem tissue can transform to large pores in biochar. (As a side note, the ability to determine these properties introduces opportunities for creating chars with desired attributes and optimizing HCMs.)

According to IBI standards, particle size classes include: <0.5 mm, 0.5-1.0 mm, 1-2 mm, 2-4 mm, 4-8 mm, 8-16 mm, 16-25 mm, 25-50 mm and >50 mm. Particle size is an important property in biochar, because finer particles have greater surface area in bulk, and surface area determines nutrient exchange to a great extent, especially in the context of CaCO3 and its role in raising pH.

Generally, biochar particles are highly porous because pyrolysis retains the feedstock cell wall structure, resulting in a wide distribution of pore sizes, resulting in large surface area. Feedstock made from large diameter cells in stem tissue can transform to large pores in biochar. In one study, internal porosity (microporosity) of pine biochar was c. 45% of total volume. In another study, estimated inner surface area of char formed between 400 and 1000°C ranged between 200–400m2/g, while others have observed BET surface areas of pine biochars pyrolized at 600°C to be 196 and 127 at 10 and 60 minutes duration, respectively, and others have indicated much less for pine biochar pyrolized at 600°C for 6 hours. Generally, surface area begins to decline rapidly at HTTs higher than 600-700°C. (As a side note, the ability to determine these properties introduces opportunities for creating chars with desired attributes and optimizing HCMs.) As a result of its porosity, biochar has a low material density.

Pine biochar particle-size distribution will vary depending upon HTT, residence time, pyrolysis technique, handling and more. A study of pine biochar pyrolized for six hours at 350–600°C and screened at 3.0 mm found this breakdown (values are approximate).

Particle Size (mm)Percentage
2.0 – 3.03%
1.0 – 2.031%
0.5 – 1.030%
0.2 – 0.516%
0.1 – 0.25%
< 0.115%
Particle-size breakdown for pine biochar Particle shape and geometry

Estimated inner surface area of char formed between 400 and 1000°C ranged between 200 – 400m2/g. Generally, surface area begins to decline rapidly at HTTs higher than 600°C. Generally, biochar has a low material density. (As a side note, the ability to determine these properties introduces opportunities for creating chars with desired attributes and optimizing HCMs.)

6.3.3. Chemical Properties – Biochar

Biochar’s unique physical structures, organic chemistry and ash-based nutrients make it a valuable addition to HCM. Organic chemistry

The organic chemistry around biochar is plays a big role in its functionality. Biochar attaches a wide assortment of organic functional groups (OFGs) during pyrolysis, which in turn form abundant cation and anion exchange sites. These exchange sites and the pore structures of biochar moderate release of nitrogen, phosphorous and potassium fertilizer inputs, reducing initial release while extending overall release. Total release is equal to substrates containing no biochar, but leaching losses decrease and nutrients are more evenly available. As a result of this organic chemistry and its moderating effect, “biochar may enhance plant growth via improved nutrient availability under suboptimal nutrient conditions.”

HTT largely governs the presence of OFGs, and medium- and high-temperature pyrolysis degrade most OFGs. With increasing HTT levels, biochar progressively loses its OFGs, but it gains in surface complexity and surface area, increasing its ability to adsorb NH4 and NH3 (until HTTs rise beyond 600°C).

Low HTT biochars include a variety of “organic compounds belonging to various chemical classes, including n-alkanoicacids, hydroxy and acetoxy acids, benzoic acids, diols, triols, and phenols” and more. In small amounts as displayed in biochar in HCM, these compounds can stimulate plant growth, plant resistance to pathogens, and development of robust populations of beneficial microbes, in addition to providing sites for cation exchange. In large concentrations, these organic compounds can have the opposite, toxic effect, a relationship known as hormesis. Electrical activation and conductance

Due to the its structure, biochar can act as an electrical conductor at micro-scales, as described in this excerpt:

“The fused carbon rings are also responsible for the electrical activation of the biochar carbon sponge. Fused carbon rings form a special bond with each other that allows electrons to move around the molecule producing electrical properties like those that are found in engineered carbon materials such as graphene sheets and carbon nanotubes. Depending on the pyrolysis temperature and resulting arrangement of atoms, biochar can be an insulator, a semiconductor or a conductor of electricity. Electrically active fused carbon rings also support “redox” or oxidation and reduction reactions that are important to soil biochemistry, by acting as both a source and sink of electrons. In soils, microorganisms use aromatic carbon both as an electron donor and as an electron acceptor during metabolic chemical reactions.”

“With its pores and its electrical charges, biochar is capable of both absorption and adsorption. Absorption is a function of pore volume. The larger pores absorb water, air and soluble nutrients like a normal sponge. Adsorption depends on surface area and charge. The surfaces of biochar, both internal and external, adsorb materials by electro-chemical bonds, working like an electric sponge.” pH and pH trends

In general, pH values are highly variable among raw biochars, ranging according to feedstock and manufacturing processes. For example, a study on pine biochar pyrolized at 600°C for 6 hours reported a pH value of 8.5, while another on pine pyrolized at 600°C for 10 and 60 minutes reported values of 196 and 127, respectively. Due to the CaCO3 and other salts, raw pine biochar has high pH and has an immediate liming effect in HCMs. In the study mentioned above with a commercial peat-lite HCM as control, pH increased as particle size decreased or as percentage of biochar increased, though with higher percentages biochar increases plateaued. In this study, the raw pine biochar in the smallest two size-classes (see table above) started at a pH of 7.5 and the control mix started at a pH of 3.5. In the same mix, over 16 weeks with constant moisture levels, pH levels rose on all samples.

Other research suggests that H:C and O:C ratios have a larger role determining pH in biochars. This research demonstrates a strong negative (inverse) correlation between H:C ratio pH and between O:C ratio and pH to a lesser extent and a weaker correlation between ash and pH.

In an HCM, biochar pH decreases significantly during a plant growing cycle. A mix with 70% softwood biochar (HTT 800° C) and 30% peat began with an initial 10.4 pH and ended with pH at 7.5 after nine weeks. Conversely, the same biochar at 10% raised the mix from 4.4 to 5.6 during the same period. Electrical conductivity (EC)

EC values are highly variable among raw biochar, depending on feedstock and manufacturing processes. For example, a study on pine biochar (600 C) reported an EC value of 0.04. Cation-exchange capacity (CEC)

Biochar oxidizes in soil and increases CEC with time, making all key cations available in the HCM. C:N ratio Available nutrients

Raw biochar contributes important macro and micronutrients to an HCM, including significant levels of phosphorous and potassium, calcium carbonates found in the ash adsorbed to the complex surfaces and a variety of micronutrients. Ash content, notably calcium carbonates, tend to increase with HTT. Nitrogen drawdown and nitrogen drawdown index (NDI)

Raw biochar has substantial NDI. Possible chemistry problems


ue to the CaCO3 and other salts, raw biochar has high pH and often has an immediate liming effect in substrates and sometimes contributes unwanted salinity (EC) if used in raw form.

In raw form, biochar can substantially draw down nitrogen, phosphorous and potassium in an HCM.

6.3.4. Biological Properties – Biochar

Biochar can exhibit beneficial and detrimental horticultural effects. In its raw state, biochar is a sterile medium, however, it begins oxygenating almost immediately and because of its organic chemistry, it is readily inhabited by a variety of organisms. Further, due to its high porosity, surface area and adsorption qualities, biochar provides a robust habitat for microorganisms. Beneficial biology

Biochar, in general, increases microbial mass and taxonomic diversity. Many beneficial organisms seem to benefit from biochar, including several plant growth-promoting rhizobacteria (PGPR) and fungi (PGPF), which, in many plant species, can induce systemic acquired resistance (SAR), as well as “induced systemic resistance (ISR) to foliar pathogens, including Botrytis cinerea (gray mold) and Leveillula taurica (powdery mildew) on sweet pepper and tomato and Podosphaera aphanis (powdery mildew) on strawberry plants”.

Research is mixed on biochar’s influence on mycorrhizal fungi. While some researchers have found that biochar increases mycorrhizal presense, other researchers have found no evidence of this effect. For example, research provided no evidence that biochar benefits the effects of mycorrhizal fungi Glomus mosseae.

Insofar as biochar boosts mycorrhizal activities, several mechanisms seem to be at work. One mechanism that may be at work is that biochar changes soil nutrient availability, which has been the focus of most research. Another mechanism may be that “biochar alters the activity of other micro-organisms that have effects on mycorrhizae,” including mycorrhization helper bacteria and phosphate solubilizing bacteria. Likewise biochar can interfere with signaling between plants and mycorrhizal fungi and can detoxify allelochemicals. Biochar might provide refuge for colonizing fungi and bacteria and can “serve as an electron buffer for redox reactions, [which] helps bacteria swap electrons among themselves, improving their metabolic efficiency as a microbial community.” Detrimental biology

Biochar effects on biology can be detrimental to plants. Some research has demonstrated that biochar “offers a good environment for Pythium ultimum.”

In addition, in reference to raw biochar, “substrates with a high proportion of BC such as in this study could have detrimental effects on biological processes that support plant productivity, largely due to interference (e.g., sorption) of chemical signals between beneficial microorganisms and host plants. As a result, biochar could lessen establishment of rhizobial and mychorrizal associations and reduce nodulation in leguminous species.”

6.3.5. Operational Considerations – Biochar Dry bulk density

Typically, biochar has a low material density. For example, pine biochar (600°C) were 0.73 grams/cm3. Physical stability

Biochar produced from wood and other nutrient poor feedstock has “exceptional structural stability and is extremely recalcitrant against microbial decay.” Research indicates that “because of its macromolecular structure dominated by aromatic C, biochar is more recalcitrant to microbial decomposition than uncharred organic matter.” Wettability and Hydration efficiency

Raw pine biochar generally exhibits poor wettability. Chemical stability Blending efficiency Workplace hazards

Biochar presents dust exposure and related problems. Substrate availability Substrate variability
6.3.6. Sustainablity and Carbon Sequestration

6.4. Parboiled Rice Hulls

Parboiled rice hulls, or PBH, constitute an effective sterile aggregate HCM substrate that is gaining in popularity in the horticulture business. PBH is best used as a co-substrate due to its physical and chemical properties (discussed below), with a maximum proportion of 25-40% of the HCM volume.

Rice millers produce PBH as a byproduct of removing rice from the husks by soaking the rice grain, steaming it, and drying it – roughly 20% of the mass of the entire grain. Because farmers grow rice around the world, roughly 600 million tons annually, PBH is readily available at 120 million tons per year, but is used for other agricultural purposes as well, for example as a bulking agent for seeds in seed spreaders.

6.4.1. Physical Properties — PBH

Parboiled rice hull physical properties make it useful as a porous aggregate. Total porosity Water-filled pore space Air-filled pore space

Generally, PBH-substrates have higher air-filled porosity than growstone, sphagnum peat and perlite. “The air-filled pore space was not different between substrates containing 20% perlite or PBH. However, the air-filled pore space was higher in PBH-containing root substrates than in equivalent perlite-containing substrates when the amount of PBH or perlite was at least 30%.” Furthermore, “the air- filled pore space of PBH-containing substrates also increased at a higher rate […] compared with perlite-containing substrates.”

Substrates with at least 25% parboiled rice hulls have more air-filled pores than substrates with growstone, sphagnum peat and perlite, attributed to “the long and angular shape of the PBH vs. the more rounded shape of the perlite.” Furthermore, “air-filled pore space ranged from 9.5% to 12.7% for perlite-containing root substrates and 11.5% to 37.8% for PBH-containing substrates.”

“Perlite had an air-filled pore space of 54% whereas PBH had an air-filled pore space of 69%.” The higher air-filled pore space of PBH could at least partially account for the higher air-filled pore space of PBH-containing substrates overall, and specifically in substrates containing at least 30% PBH.

Furthermore, “The difference in the rate of change in air-filled pore space may have been partially the result of the higher air-filled pore space of PBH, but also the result of the elongated shape of PBH (in contrast to perlite granules, which are generally spherical), which allowed the individual hulls to cross connect and create more and larger pores in substrates containing high concentrations of PBH. Soil gas diffusivity Container capacity Available water Hydraulic conductivity Particle size and distribution Particle shape and geometry

Elongated particles that will cross-connect in sufficient quantity.

6.4.2. Chemical Properties – PBH pH and pH trends Electrical conductivity (EC) Cation exchange capacity (CEC) Nitrogen drawdown and nitrogen drawdown index (NDI) C:N ratio Available nutrients

Rice hulls will contribute potasium in an acidic environment. Possible chemistry problems

PBH does not deplete nitrogen, but excess potassium could be a problem with 100% PBH, as could manganese under acidic conditions.

6.4.3. Biological Properties – PHB Beneficial biology
6.4.4. Operational Considerations – PBH Dry bulk density Physical stability
6.4.4. Operational Considerations – PHB Dry bulk density Physical stability

PBH does not lose significant volume or total porosity in a typical growing period, however it loses substantial air-filled porosity. Wettability and hydration efficiency Chemical stability Blending efficiency Workplace hazards Substrate availability Substrate variability
6.4.5. Sustainablity

6.5. Green Waste Compost (GWC)

Composts are biologically-active, nutrient-rich bulk substrates with highly variable chemical and physical properties. They have gained use in recent decades as a substitute for or addition to peat in HCMs, and compost is the defining substrate of commercial “potting soil” when added to a sterile medium such as peat-lite.

This chapter refers strictly to mature and stable green waste composts made by composting plant wastes using thermophilic processes. Green waste composts are suitable as a minority substrate in HCMs due to their chemical properties (discussed below) Recommendations for maximum quantities of compost in an HCM range from 15-40%. Likewise, because compost is biologically robust, it is not suitable as an germination HCM.

Composts have always had an important role in agriculture, and in the 19th and early 20th centuries, “compost” generally referred to a variety of materials used as HCM in greenhouses, most notably the “John Innes Compost,” a mixture of mineral loam and decomposed organic materials.

Modern horticulture relies primarily on composts manufactured from municipal and agricultural wastes, such as human waste, kitchen waste, plant (green) wastes, animal wastes and animal remains. These facilities use thermophilic composting to produce a commercial produce while significantly reducing waste storage and removal costs. During this composting process, micro-organisms, mainly bacteria, decompose organic materials (made of carbohydrates, proteins, lipids and lignin) for energy and nutrients. Bacteria and fungi create enzymes to decompose tissue. Decomposing complex and tenacious tissue structures requires creating complex enzymes and thus requires more energy.

Roughly one-third the original material mass will remain after the composting process, the rest released as CO2. “Of the carbon in each ‘bite’ taken from the materials by microbes, about 60% is converted to CO2, about 35% is made into microbial cells, and the rest is discarded as solid wastes. When these microbes are used by others as food, about 60% of their carbon is converted to CO2, and so on.”

Thermophilic composting is crucial for creating sanitary, pathogenic compost that will mature and stabilize (discussed below). Different manufacturers using different processes and various feedstocks will produce variations in time and temperature patterns, creating different qualities of compost.

To make green waste compost, producers shred, mix and pile various green wastes to achieve a starting C:N ratio of 25-30, thoroughly wetting the piles as they proceed. During the first stage, mesophilic bacteria and fungi decompose the easily-digestible carbohydrates and lipids, raising the temperature of the pile to 105 deg. F, 40 deg. C. This phase lasts roughly one or two days.

Stage 2 begins as thermophilic bacteria (mostly actinobacteria and gram-positive species) become predominant, decomposing proteins, fats and complex carbohydrates. During this stage, which takes roughly four weeks, temperatures rise over 130 deg. F to as high as 160 deg. F . Producers turn the pile regularly to provide oxygen and maintain temperatures below 160 deg. F. By the end of the thermophilic stage, the pile volume has markedly decreased and most nitrogen is in nitrate form. During this stage the compost turns highly acidic due to synthesis of ammonium and subsequent release of hydrogen atoms and then turns back to basic as microbes transform ammonium to nitrates.

During the third stage, mesophilic bacteria and fungi, having survived in spore form, continue to decompose the most recalcitrant materials and eat other microorganisms. This stage, which lasts roughly four weeks, further reduces volume while increasing C:N ratio to a stable ratio. At the end of this period, C:N ratio should be 15-18 and most nitrogen should be in nitrate form.

Quality-conscious producers will then allow the compost to mature and become colonized by fungi, but some commercial producers will sell compost after the second mesophilic stage.

The most significant source of variation in green-waste compost properties is feedstock, with lignin content and plant tissue structure being major factors. For example, hemp fibers create physical properties that compost well into physical properties beneficial to HCM. Producers can supplement nitrogen inputs at the beginning and after the thermophilic stage to boost available nitrogen in the final compost.

Due to scarcity and lack of oversight, the majority of commercial compost is not optimal and sometimes not suitable for HCM and toxic contamination is a common problem.

The HCM manufacturer should be very careful about sourcing compost. If and when buying commercial media, the mixer should ask for product specifications on the compost and make assurances that these standards reflect the standards actually represented in the product. Vendors of high-quality compost will always provide lab tests for their compost, and should be able to provide these tests with little effort.

6.5.1. Physical Properties – GWC

Compost physical properties are highly variable, depending upon particle-size distribution. Commonly, pore space shrinks in HCMs with compost. Total porosity Water-filled pore space Air-filled pore space Soil gas diffusivity Container capacity Available water Hydraulic conductivity Particle size and distribution

Particle size distribution can vary between green waste composts – within facilities and between facilities – depending upon manufacturing processes and age. A good-quality green waste compost should display an even diameter distribution between 0.1 and 5 mm. Particle shape and geometry
6.5.2. Chemical Properties – GWC

Generally, commercial compost varies from source to source and batch to batch in almost every important detail: pH, C: N ratio, ammonium: nitrate ratio, nutrient content, micronutrient content, biological content and other parameters pertaining to maturity (stability) and available nutrient content.

“Organic matter is the original slow-release fertilizer.” Compost has exceptionally high pH and EC values and is suitable only as a minority bulk component, less than 15-20%. Chemical properties of green waste composts are highly variable from source to source and batch to batch. pH and pH trends

Generally, GWC have high pH values — greater than 7.5. Electrical conductivity (EC)

Generally GWC have high EC values. Cation exchange capacity (CEC) Nitrogen drawdown and nitrogen drawdown index (NDI) C:N ratio Available nutrients

High-quality compost contributes substantial amounts of nutrients and is a good medium for holding available fertilizers and nutrients due to its cation exchange capacity and absorbency (with water). As with other substrates, smaller-particle grades hold more cations than larger particle grades.

While overall nitrogen content is high in composts, mineralised N is low due to microbial immobilization. However, manufacturers can boost mineralised N by adding N-rich materials to the compost at the beginning of the compost cycle or directly part-way through the thermophilic stage. Nutrient poor composts demonstrate the most improvements to adding N-rich components during the thermophilic stage, however the compost loses stability and will lose mass. Possible chemistry problems

GCW is highly variable in chemical properties and can present serious risks if not sourced carefully. Some of the problem areas to be aware of include:

a. Heavy metals contamination — primarily the EPA’s 10 most wanted list.
b. Pesticides content.
c. Pathogen content, including Escherichia coli and Salmonella enterica and the fungi Plasmodiophora brassicae (causal agent of clubroot of Brassicas), and Fusarium oxysporum f. sp. Lycopersici, which are more heat tolerant.
d. Immaturity and high NDI.
e. High salt content (CaCO3, Ca5(PO4)3, NaCl
f. Physical content: junk and rocks.
g. Phytotoxicity.

Compost tends to leach Phosphorus, especially under high-frequency irrigation regimes.

6.5.3. Biological Properties – GWC

High-quality, stable, mature compost is key to good HCM, as it maintains physical and chemical properties over the growing cycle without contributing phytotoxic compounds. In a stable, mature compost, little or no decomposable organic matter remains to promote microbial activity. Decomposition of immature compost in a horticultural container will produce nitrogen draw-down, oxygen depletion and often phytotoxic compounds. Conversely, a stable, mature compost will promote plant growth.

“Maturity” and “stability” are usually used synonymously, though they are distinct in technical detail. Compost maturity refers specifically to the substrate and the “decomposition of phytotoxic organic compounds produced during the active composting phases.” The horticulturist can determine the level of maturity through “biological methods such as seed germination index, by physical properties such as odour and colour and by chemical parameters such as C/N< 12, NH4+/NO3-1– < 0.16 and water soluble carbon < 1.7%.” Beneficial biology

High-quality compost is rich in beneficial soil microbes, which significantly add microbial diversity, enrich microbial nutrient-cycling, and continue to cycle nutrients in an HCM during the planting cycle, especially if stimulated by biological fertigation. Because of this microbial diversity, compost is known to suppress several pathogens and is widely used as suppressive media. Detrimental biology

Compost can also favor pathogenic microbes if not produced correctly and with high-quality substrates. However, a well-regulated composting process will produce a wide variety of beneficial microbes that will suppress if not eliminate most pathogenic microbes.

6.5.4. Operational Considerations – GWC Dry bulk density Physical stability

Compost stability “refers to the level of microbial activity in the compost” and is “generally determined by O2 uptake rate, CO2 production rate or the heat released due to microbial activity.” The biostability index for mature green waste composts ranges from 89.7 to 96.4.

HCMs with composts at quantities greater than 20% can shrink significantly, with 12% shrinkage noted for mixtures with 40% compost, possibly due to compaction and changes in physical structure. Wettability and Hydration efficiency Chemical stability Blending efficiency Workplace hazards Substrate availability Substrate variability

Generally, commercial compost varies from source to source and batch to batch in almost every important detail: pH, C: N ratio, ammonium: nitrate ratio, nutrient content, micronutrient content, biological content and other parameters pertaining to maturity (stability) and available nutrient content. However, a good composting operator can minimize these ranges by careful attention to detail, and good compost is commercially available.

6.5.5. Sustainablity

6.6. Vermicompost (VC)

Vermicompost, or “worm compost, or “worm castings,” is a nutrient rich, physically dense, finely textured substrate typically used as a fertilizer and source of beneficial microbial inoculation and as a substitute for peat. Vermicompost is best used as a minority substrate because of its physical properties and the density of salts.

Vermicompost is a product of composting worms passing organic materials through their digestive systems where microbes decompose the materials into castings and feed nutrients to the worms. Most commonly, producers use “red wrigglers,” Eisenia foetida, though other species are common as well, including, for example, tiger earthworm,” Eisenia jetida. Producers generally use shallow, well-aerated bins (roughly 2-feet deep) filled with a lower layer of high-C:N, absorbent (“brown”) materials as a bedding on which they place a top layer of low C:N (“green”) materials, most commonly animal manures. After 1-2 months, producers separate the finished castings from the worms, screen for large particles and package the castings. Some operations use continuous-operation bins with a screened bottom which they constantly feed from the top and collect compost on the bottom.

6.6. Physical Properties – VC

Vermicompost physical properties vary widely due to the wide variation in materials that operators use for feed. Total porosity Water-filled pore space Air-filled pore space Soil gas diffusivity Container capacity Available water Hydraulic conductivity Particle size and distribution Particle shape and geometry
6.6.2. Chemical Properties – PBH pH and pH trends Electrical conductivity (EC) Cation exchange capacity (CEC) Nitrogen drawdown and nitrogen drawdown index (NDI) C:N ratio Available nutrients

High-quality vermicompost contributes substantial amounts of nutrients and is a good medium for holding available fertilizers and nutrients due to its cation exchange capacity and absorbency (with water). Though they only contain minute amounts of nitrogen, they contribute enough phosphorus and potassium for months of growth. As with other substrates, smaller-particle grades hold more cations than larger particle grades. Possible chemistry problems

Vermicompost can contain toxic levels of zinc.

6.6.3. Biological Properties – Vermicuilture

VC is biologically robust and is not suitable for germination. Beneficial biology

High-quality VC is rich in beneficial soil microbes, which significantly add microbial diversity, enrich microbial nutrient-cycling, and continue to cycle nutrients in an HCM during the planting cycle. Detrimental biology
6.6.4. Operational Considerations – Biochar Dry bulk density Physical stability
6.6.4. Operational Considerations – Biochar Dry bulk density Physical stability Wettability and Hydration efficiency Chemical stability Blending efficiency Workplace hazards Substrate availability Substrate variability
6.6.5. Sustainablity
6.7. Peat Moss

Peat moss (or peat) a fibrous substrate made semi-decomposed plant matter, is the prevalent horticultural substrate and is found in virtually every commercial HCM.1 Peat includes a variety of plant materials that have been partially decomposed to varying degrees in saturated, acidic, anaerobic bog ecosystems.2 Generally, Sphagnum peat is the highest quality peat for horticulture and is the focus of this review.

6.7.1. Production

Sphagnum peat is the product of peat bogs, which consist of a thin layer (millimeters) of Bryophyte mosses from the Sphagnum family growing on a wet substrate of dead plant matter in deposits as deep as six meters. These deposits accumulate over millenia, because the Sphagnum plants grow and die at a faster rate than bacteria consume the dead and dying plant tissue and organic residues. Thus, the bottom layers of the deposits are older and more decomposed than the top areas, yielding significant variation in peat properties as a horticultural substrate. Harvesting and manufacturing generally involves clearing trees and other vegetation, clearing the Sphagnum live tissue, harvesting and drying the peat substrate (using various techniques and machiney), sifting, packaging and distributing the peat substrate.3

The HCM industry grades peat according to level of decomposition and according to particle-size (as run through a screen), with significant differences in substrate properties and behaviors and a variety of horticultural applications using a variety of frameworks.4 As peat decomposes in bogs, color darkens, particle size, pore size, total pore space and fiber content decrease. Darker, finer peats have higher bulk densities, and more surface area per volume. Consequently, chemical and physical stability, water-retention and cation adsorption increase, while hydraulic conductivity, capillary action and air-filled pore space decrease.5

The von Post scale, widely adopted after its establishment in 1922, remains the common framework for grading peat. The von Post method grades peat according to decomposition on a scale, H1 through H10, with H1 representing the least decomposed peats. Within the scale, the method divides peat into three categories: young, minimally decomposed, known as white peat (H1-H3); partly decomposed (H4-H6) and older, highly-decomposed, known as black peat (H7-H10). Other methods for determining decomposition include rubbed fiber content and unrubbed fiber content tests, pyrophosphate-soluble organic matter tests, color-based tests and others.6

HCM formulations generally rely on grades H1-H3 in the von Post scale. Unless noted, This chapter refers to the properties of these grades.

6.7.2. Physical Properties – Peat Porosity

Total pore space of two typical peat were 82.0 and 94.2.

Air-filled Pore Space. Horticultural Peat also has relatively high air porosity and diffusivity and low tortuosity and therefore it has good gas exchange properties. Diffusivity increases as peat ages in the container. On the other hand, peat loses moisture from surface evaporation more rapidly than do other substrates. One study reported AFPS for peat at 41.2% v/v.

6.7.2.X. Available water

One study reported easily available water at 22.5% v/v and water buffering capacity at 4.4%v/v for peat.

6.7.2.X. Gas diffusivity and hydraulic conductivity

Peat typically displays relatively high gas exchange and high diffusivity (low tortuosity). In white peats, diffusivity increases as peat ages in the container, because tortuosity decreases in coarse peat due to particle re-alignment by root growth, physical wearing due to root action, handling and water-movement all in conjunction with chemical and biological weathering. In the darkest peats, diffusivity decreases as the peat ages in the container. H4-H6 peats also lose some diffusivity.

6.7.2.X. Particle properties

Particle size distribution, fiber size and quality vary significantly between peats – within facilities and between facilities. White peats have are coarser, with larger particles with larger pores.

Particle-size distribution. According to American Standards for Testing and Materials (ASTM) standards, particle sizes fall into these classes (sift size): dust, 0-0.149 mm; fine, 0.149-0.849 mm; medium, 0.849-2.38 mm; coarse, greater than 2.38 mm. European researchers report that particles sizes (second longest diameter) fall in these classes: super fine, 0-3 mm; fine, 0-5 mm; medium, 5-10 mm; coarse, 10-20 mm; and super coarse, larger than 20 mm. The Norwegian standard defines fines as less than 6.0 mm with 30% particles greater than 1.0 mm, medium as less than 15.0 mm with 60% particles greater than 1.0 mm, and coarse as less than 40.0 mm with 70% particles greater than 1.0 mm.

A typical horticultural peat has a courseness index of 63 and breaks down with roughly 10% of volume made up by particles < 0.25 mm, 40% by particles 0.25-2.0 mm, 40% between 2-4 mm and >5% over 4 mm in diameter.

Particle size classes of typical peat

> 8.015%
4 – 816%
2 – 416%
1 – 213%
0.5 – 1.015%
0.25 – 0.512%
0.125 – 0.257%
< 0.1255%
2.37-2.51St. dev.
> 8.011%
4 – 85%
2 – 423%
1 – 225%
0.5 – 1.018%
0.25 – 0.511%
0.125 – 0.256%
< 0.1252%
2.53 – 5.05St. dev.

Particle Geometry. Horticultural peat material pores are oval, measuring 11.5 µm (+-0.6) by 20.2 µm (+-1.3), and cover 12% of the material surface, with 51% internal porosity. These pores allow water exchange by ellipsoid water-holding cells, 110 um x 15 um, with an internal porosity of 51%. When dry, “thickened bands of cell wall material” prevent the cell structures from collapsing.

6.7.3. Chemical Properties – PBH pH and pH trends Electrical conductivity (EC) Cation exchange capacity (CEC) Nitrogen drawdown and nitrogen drawdown index (NDI) C:N ratio Available nutrients

Though peat does not contribute substantial nutrients, it is a good medium for holding available fertilizers and nutrients due to its absorbency (with water). As with peat, smaller-particle grades hold more cations than larger particle grades. Generally, HCMs using peat require some sort of liming to raise pH to optimal levels, and of course, plant culture will require fertilization. Possible chemistry problems

Peat tends to start the growing cycle with a low pH, which commonly decreases through the growing cycle, sometimes to suboptimal or dangerous levels. Peat also binds copper.

6.7.4. Biological Properties – Peat

Generally, horticulturists manage peat as a sterile medium, and it is for all practical purposes, if processed and handled correctly. As an organic material that will hold nutrients and water, peat works well with biologically-rich HCM. Beneficial biology Detrimental biology

Peat is vulnerable to carrying pathogens, especially if handled incorrectly, and several pathogens will readily infect this substrate. For example, peat is associated with susceptibility to fusarium root rot and may predispose some species to susceptibility.

6.7.5. Operational Considerations – Peat Dry bulk density

The highest grades of peat (H1-H3) range in DBD from 67-168 lbs/yd3. Commercially available peats range in DBD up to 370 lbs/yd3. Particle density is 2,545 lbs/yd3. Physical stability

Peat is relatively resistant to decomposition – especially the older, darker peats – due to the presence of phenols and sphagnans, a pectin-like compound found in cell walls that deters microbes. Prior to harvesting, decomposition determines particle size distribution and fiber content. However, with rough handling such as mechanical container filling, peat loses particle size and quality. In addition, peat particle structure changes over the course of a growing cycle, with pore size and total pore space decreasing, especially in zones occupied by roots, which fill the macropores. The result is decreased water retention and decreased air filled pores.

The biostability index for European peat (of uncertain von Post decomposition rating) ranges from 85.0 to 93.4. By some estimations, peat loses at least 5% mass per year due to aerobic decomposition, which “under high N additions, can compromise physical and chemical properties.” In the balance, gas diffusivity increases for coarse peat and decreases for fine peat. Saturated hydraulic conductivity remains about the same in coarse peat and decreases slightly in fine peat. Wettability and Hydration efficiency

Peat generally has poor wettability and low hydration efficiency, which get worse as it dries. With moisture content at 45% of container capacity, peat without wetting agent will only retain 23% container capacity for the first irrigation, 39% for the 3rd and 81% for the 10th. Once peat has dried beyond 30% its maximum hydration level, peat will only retain 19% container capacity for the first irrigation, 27% for the 3rd and 56% for the 10th. In addition, peat shrinks substantially as it dries and loses moisture from surface evaporation more rapidly than do other substrates. Chemical stability

Because peat trends towards acidity, periodic monitoring of leachates is necessary. Blending efficiency Workplace hazards Substrate availability

Peat is readily available for purchase in all parts of North America, Europe and other parts of the world, though prices will continue to increase in respect to other substrates as the easily extractable peat reserves are exhausted and as transportation costs rise in respect to other costs. Substrate variability

Peat can vary from brand to brand and batch to batch, so testing AFPS is highly important for the operator.

6.7.6. Sustainability

Peat use is not environmentally sustainable, except under very restrictive conditions. The European horticulture industry has decreased its use in HCMs in the last two decades with the European Union’s encouragement. Peat is unsustainable for at least four reasons. First, harvesting peat involves severely disrupting the ecosystem, which bestows several environmental services including groundwater cleansing, providing wildlife habitat, mitigating flooding and sequestering carbon. Second, peat is not produced according to a legitimate sustained-yield management plan, nor could it be, because peat grows at a small fraction of what is harvested in any given area. Though Sphagnum farming is yet to be demonstrated, one paper asserts that it is theoretically feasible. Third, peat harvesting directly releases sequestered carbon to be oxidized as it destroys the ability to sequester atmospheric carbon, directly contributing to atmospheric carbon. Finally, peat is shipped long distances from producer to consumer.

6.8. Perlite

Perlite, in combination with peat, is the most commonly used sterile aggregate substrate in the United States, Europe and other countries. It is a fundamental component of peat-lite and similar mixes and is a significant part of every HCM on the market (personal observation). However, perlite is best used as a co-substrate due to its physical and chemical properties (discussed below).

Perlite manufacturers mine aluminosilicate ore (73% silicon dioxide, 13% aluminum oxide) and grind it to a specified particle size. Then, they heat it to temperatures up to 1,000 deg. C. (1,832 deg. F), expanding it 4 to 20 times its original size.

6.8. Physical Properties – Perlite

Perlite is the lightest of all the aggregates commonly used in horticulture and is most commonly used in “peat-lite” HCMs as described above. Total porosity Water-filled pore space Air-filled pore space Soil gas diffusivity Container capacity Available water

Perlite by itself will hold little available water – mostly between particles and on particle surfaces. Thus, the volume of water held between container capacity and wilting point for some ornamental flowers is around 7-34% of container capacity.

Adding perlite to peat or coco coir generally decreases total porosity and water retention, but increases water held at tensions around 0.5kPa due to increasing macropores and pore-size distribution. Hydraulic conductivity Particle size and distribution

Particle size distribution varies with perlite.

Common size grades: Particle shape and geometry

Perlite has a closed cellular structure with high internal porosity, so particle density is low.

6.8.2. Chemical Properties – PBH

Minimal contribution to medium chemical content. pH and pH trends Electrical conductivity (EC) Cation exchange capacity (CEC) Nitrogen drawdown and nitrogen drawdown index (NDI) C:N ratio Available nutrients Possible chemistry problems

Perlite can leach aluminum and fluoride to plant-toxic levels in the rhizosphere under acidic conditions (pH < 5.0).

6.8.3. Biological Properties – Perlite Beneficial biology Detrimental biology
6.8.4. Operational Considerations – Biochar Dry bulk density Physical stability
6.8.4. Operational Considerations – Biochar Dry bulk density

DBD depends upon particle size, however of all the sterile aggregates, perlite by far has the lowest dry bulk density. Typical DBD values range from 215- 355 lbs/yd3. Physical stability

Perlite is completely resistant to microbial decay, though rough physical handling will degrade particle size. Wettability and Hydration efficiency Chemical stability

Perlite does not degrade chemically under optimal conditions, though it can leach aluminum and fluoride to plant-toxic levels in the rhizosphere under acidic conditions (pH < 5.0). Blending efficiency Workplace hazards

Dry perlite “produces a siliceous dust classified as an eye and lung irritant.” Substrate availability

Perlite is readily available for purchase in all parts of North America, Europe and other parts of the world, though prices will continue to increase in respect to other substrates as the easily extractable ores are exhausted and as energy and transportation costs rise in respect to other costs. Substrate variability
6.8.5. Sustainablity

Perlite use is not environmentally sustainable. Perlite production requires mining (large carbon/energy footprint and ecosystem destruction) and production is energy-intensive.

Author’s Note

This primer is in draft form. It includes no foot notes or references and it contains significant omissions. Anyone interested in republishing using the content in this manual should first contact me, the author, Richard Freeman, to gain permission.

This paper makes reference to the scientific research of several authors. For those interested in obtaining a final version with footnotes, please contact me. I am also interested in co-authoring a final version of this paper.


The information I present is for educational purposes and I am not liable or responsible for anyone’s use or misuse of this information.

I will periodically post updates to this blog.