Whether you refer to them as soils, root media, growing media or substrates, the material in which crops are grown and the roots develop, can have a profound effect on the development of the plant. Regardless of its composition, the substrate may fulfill up to five basic functions:

Provide physical support to the plant

 

Retain water in a form available to the plant

 

Provide for gas exchange between the roots and the atmosphere

 

Serve as a reservoir for plant nutrients

 

Sustain/support microorganism populations important in nutrient cycles and disease suppression

Under certain production situations, the substrate may fulfill each of these functions. However, in other situations, the substrate may not fulfill (and may not need to fulfill) all of these functions. For example, in hydroponic and pseudo-hydroponic production situations, the substrate (if one exists such as sand or rockwool) generally does not retain nutrients and does not encourage the development of microbial populations.

There are many components that may be used in the formulation of a substrate. Because these components rarely have optimal physical and chemical when used alone, they are usually combined to form suitable substrates. Additionally, the substrate is often amended with materials to adjust pH, fertility level and water-holding capacity.

Important Properties of Substrates

Rate of decomposition - All organic materials decompose over time. Eventually, all reduced carbon, stored as cellulose, hemicelluloses, lignin and other organic compounds, will be returned to the atmosphere as carbon dioxide. Microorganisms accomplish this process, and the rate of decomposition depends upon many variables including the chemical composition of the organic material, the microorganisms present, moisture and temperature. When used in the formulation of horticultural substrates, decomposition is generally undesirable because it reduces the volume of substrate in the container (referred to as "shrinkage") available to fulfill the functions listed above. Since all organic materials decompose, the objective is to select materials that have a slow rate of decomposition.

Carbon:nitrogen ratio - Organic matter serves as an energy source and provides the basic building blocks used by microorganisms in the substrate. As microorganisms break down organic matter, they require one (1) nitrogen for every 25 carbon atoms they utilize. If the organic matter being broken down has a carbon:nitrogen (C:N) of less than 25:1, enough nitrogen can be obtained from the organic matter to meet the microorganisms' need. However, if the C:N ratio is greater than 25:1, the microorganisms must obtain nitrogen from the surrounding environment. In these situations, the microorganisms will begin to use nitrogen from other sources including nitrogen being supplied to the crop through fertilization. If the carbon:nitrogen ratio is very high, large amounts of nitrogen may be taken from the surrounding environment. In these situations, the microorganisms may out-compete the plants for the available nitrogen (nitrogen tie-up or depletion) and nitrogen levels may drop below desirable levels and nitrogen deficiency may occur in the crop (unless additional nitrogen is supplied to compensate). However, a high C:N ratio alone will not result in nitrogen depletion. The organic matter must have a high C:N ratio and must decompose relatively quickly for significant nitrogen depletion to occur.

Bulk density - The dry weight of a specified volume of substrate is referred to as bulk density. It is most often expressed as g/cm³ in research or laboratory situations. It may also be expressed as lbs/ft³ or lbs/yd³ in commercial situations. Additionally, in commercial situations, bulk density may be measured at a specified moisture level that is usually the moisture level at which it is bagged or shipped. Bulk density is important for several direct and indirect reasons. A minimum bulk density may be desired in order to support tall plants and prevent them them from toppling. However, as bulk density increases, the weight (and thus cost) of the plant material to be shipped increases. In many cases, as bulk density increases, total pore space and air-filled pore space decrease. Therefore, substrates with high bulk densities may have suboptimal total pore space and air-filled pore space.

Total pore space - The majority of the volume of substrates is actually pore space. The distribution of pore sizes is important because the total volume of pore space and specifically the pore size distribution directly determine the substrate's water-holding capacity and air-filled pore space.

Air-filled pore space - During irrigation, all of the pores of a substrate in a container are filled with water (saturated condition). However, immediately after irrigation, the larger pores are unable to hold water against gravity and the water in these pores drain (container capacity). These pores become air-filled. Air-filled pores are important because they allow for gas exchange with the atmosphere out side of the container. Without this gas exchange, the roots would be deprived of oxygen required for respiration. Air-filled pore space is one of the substrate physical properties that change with container dimensions. For a given substrate, as the column height of the container increases, so does air-filled pore space. However, as a general rule, a substrate should have at least 10% air-filled pore space at container capacity when placed into a 10-cm (4-inch) container.

Water-holding capacity - One of the functions of the substrate is to retain water for uptake by the plant. If the water-holding capacity is too low, frequent irrigation is required to prevent water stress. However, if water-holding capacity is too high, air-filled pore space may be below the desired levels. Additionally, if the water-holding capacity is too high, irrigation frequency will be low. In cases where liquid fertilization is being utilized, infrequent irrigation results in infrequent fertilization and mineral nutrient deficiencies may occur.

Cation-exchange capacity - Most components used to formulate horticultural substrates have negatively charged sites on their surfaces that allow them to retain cations. Many of the mineral nutrients used by plants are cations (i.e. Ca⁺⁺, K⁺). The negatively charged sites allow the substrate to retain nutrients for use by the plant and thus essentially provide a reservoir of nutrients. The negatively charges sites also retain protons (H⁺). These can be exchanged with the substrate solution and thus help buffer the substrate solution from rapid pH changes. Therefore, cation-exchange capacity (C.E.C.) is sometimes referred to as buffering capacity. Cation-exchange capacity is expressed as meq/100ml.

pH - The pH of the substrate (specifically the substrate solution) impacts many aspects of the substrate environment. The pH affects nutrient availability with certain mineral elements being more available as pH decreases (i.e. Fe, Mn, Cu) and others being more available as pH increases (i.e. Ca, Mg, Mo). The pH of the substrate also impacts incidence of some soil-borne diseases. For example incidence of Fusarium on Lisianthus is reduced as pH increases. Although optimal pH varies among crops and exceptions occur (i.e. azalea require a pH of 3.5 - 4.5), a pH of 5.2 - 6.5 is optimal for most greenhouse-grown crops.

Electrical conductivity - The electrical conductivity (E.C.) is a measure of total ions in the substrate solution. It is determined as a measure of the substrate solution's ability to conduct an electric current across one (1) cm and is expressed as mmho/cm (or its equivalent mS/cm). The higher the ion content of the substrate solution, the more effectively the substrate solution conducts the electric current and thus the higher the E.C. Optimal substrate solution E.C. level varies with crop, crop stage and time of year. However, for substrates to be used for well-rooted plants, an electrical conductivity of 3.0 mmho/cm or higher becomes problematic depending upon the crop. For young cuttings, the electrical conductivity should be 2.0 mmho/cm or less. For substrates to be used for seed germination, the electrical conductivity should be 1.0 mmho/cm or less. High E.C. levels are an indicator that the substrate contains high levels of ions. High E.C. levels can cause damage to root systems thus impairing function and making the plant more susceptible to attack from soil-borne diseases. Some commercial soil testing laboratories may determine the soluble salts concentration of a substrate solution. This is a measure of the total concentration of ions in the substrate solution and is expressed as parts-per-million.

Availability - Even if a material has suitable properties, if it is not readily available, it cannot be consistently used as a substrate component. The material must also be available in large enough quantities to impact the market. If only small volumes are available, it cannot be used consistently as a component. Availability also has a geographic consideration. A material that is available in only a specific geographic region may be suitable for use within that region, but shipping costs may prohibit its use outside of the geographic area.

Consistency - Substrate components must have consistent properties from batch to batch. If physical or chemical properties vary significantly, changes must be made in the blending of the substrate or crop production protocol in order to account for these differences. This increases costs of production. In many cases, consistency problems may not become evident until they manifest themselves as production problems in the greenhouse. As an example, vermiculite pH varies from source to source. The pH of some vermiculite sources can be very high. If a new vermiculite source with a higher pH is used in the substrate (and the amount of vermiculite used in the substrate is significant), the pH of the resulting substrate will be higher unless adjustments are made (i.e. reducing the amount of lime added to the substrate). If the resulting substrate pH is too high, micronutrient deficiencies (i.e. Mn, Cu, Fe, etc.) may occur.

Cost - Any potential substrate component must be economically compatible with horticultural markets. Even if a substrate component has exceptional physical and chemical properties, if it is too expensive for its intended use, it will not be used in commercial production situations. Essentially, the ideal substrate is one that just meets the needs of the crop (allows a crop of acceptable quality to be grown as quickly as possible) and is lowest in cost.

Chemical components - Substrates may contain numerous chemical components not directly related to pH, electrical conductivity or mineral nutrients. In some cases these components can be detrimental to plant growth (i.e. acetic acid from certain barks). In other cases these components may promote plant growth (i.e. humic and fulvic acids) or suppress soil-borne diseases (i.e. phenolic compounds and saponins).

Biological components - Substrates contain a plethora of biological components (microorganisms). Some of these microorganisms are plant disease-causing fungi (i.e. Pythium and Phytophthora) and bacteria (i.e. Erwinia). Other microorganisms are fungi (i.e. Trichoderma) or bacteria (i.e. Pseudomonas) that suppress or parasitize plant disease-causing fungi and bacteria. Still other microorganisms (i.e. Nitrobacteria and Nitrosomonas) are important in mineral nutrient cycling and availability.

Sample Properties of Some Common Substrate Components and Substrates
	% Solids	% Water	% Air	Bulk density (g/cm3)
Sand	59	35	5	1400
Sandy clay	53	40	7	1364
Sphagnum peat	15	77	8	352
Vermiculite	25	54	20	497
Perlite	37	38	25	333
Pine bark	21	59	20	523
1 soil : 1 peat : 1 sand	46	49	6	1193
1 peat : 1 vermiculite	13	70	17	391
3 pine bark : 1 sand : 1 peat	30	53	17	623

Common Substrate Components

Field Soils

The physical and chemical properties of a field soil vary with soil type and texture. However, most field soils have relatively high water-holding capacities and nutrient retention characteristics (C.E.C). They have a relatively high bulk density, and often have low air-filled pore space and poor drainage when placed into a container. Their price may vary between $20.00 - 25.00 m^-3. Field soils may be highly variable even when obtained from a common source, and the potential for contamination with undesirable chemicals (i.e. salt, herbicides) and weed seed exists. Field soils are being used less today than in the past. However, some greenhouse growers use up to 20% (by volume) field soil in their substrates to increase bulk density, provide micronutrients and to increase the C.E.C. so the substrate is less susceptible to pH changes.

 

Peats

Different types of peat exist as a result of the plant source and the degree of decomposition of the peat. These two factors result in significantly different chemical and physical properties in different peats.

Sphagnum Peat Moss is light to medium brown in color, is formed primarily from Sphagnum peat, and is the least decomposed of the general categories of peat. It decomposes relatively slow so nitrogen tie-up does not occur. It has the highest water-holding capacity of the peats; holding up to 60% of its volume in water. Its pH of 3.0 - 4.0 is the lowest of all the peats and it has a C.E.C. of 90 - 140 meq/100g (7 - 13meq/100cm³). Sphagnum peat is the most common peat used in horticultural substrates and costs approximately $26.00 - 28.00 m^-3.

Hypnum Peat Moss is darker in color than Sphagnum peat, and it is composed primarily of Hypnum moss. It has a finer texture than Sphagnum peat, and it has a pH of 5.0 - 5.5. Hypnum peat moss has a C.E.C. of 100 - 200 meq/100g.

Reed-Sedge Peat is brown to red in color and is formed from a variety of plant materials (i.e. reeds, sedges, grasses and cattails). Although it can be obtained in different degrees of decomposition, it is usually more decomposed than Sphagnum and Hypnum peat. Therefore, it has a finer texture and lower ar-filled poor space than Sphagnum and Hypnum peat. Its water-holding capacity is lower than that of Sphagnum and Hypnum peats, and it has a pH that can range from 4.0 to 7.5, but it is most commonly approximately 7.0. Reed-Sedge peat has a C.E.C. of 80 - 100 meq/100g.

Peat Humus is dark brown to black in color and is the most highly decomposed of all of the peats. It is usually derived from Hypnum or reed-sedge peat. The original plant remains are indistinguishable. Whereas Sphagnum, Hypnum and reed-sedge peats are usually greater than 90% organic matter, peat humus may contain significant amounts of mineral soil. The pH level may range from 5.0 to 7.5. Because peat humus has moderately high levels of nitrogen, may have high levels of ammonium nitrogen and may have a high E.C. level, it is not recommended for seedlings or salt-sensitive plants. Peat humus has a C.E.C. of 160 - 200 meq/100g.

Composted barks

Composted barks used in horticultural substrates are usually derived from redwood, fir or pine. However, hardwood bark is occasionally used. Bark is relatively inexpensive ($15.00 - $20.00 m^-3), and because shipping constitutes a major portion of the cost of bark, having local sources are important.

Fresh barks can cause nitrogen-tie up. Therefore, barks should either be aged or preferably composted 90 to 120 days. The composting process reduces the C:N ratio, allows readily decomposable fractions of the bark to break down, and reduces particle size. The composting process also allows for the breakdown of potentially phytotoxic substances (i.e. acetic acid) that can occur in some hardwood barks. An additional benefit of composting the bark is that it increases its C.E.C. from approximately 8 meq/100g to 60 meq/100g. Therefore, in addition to providing for drainage and air-filled pore space in a substrate, composted bark provides significant nutrient retention capabilities. The pH of composted bark can vary significantly but is usually 5.0 - 6.5. Composted barks having an average particle size of 3/8 inch are preferred for use in greenhouse substrates.

Sawdust

Sawdust is not commonly used as a substrate component. It must be composted or nitrogen tie-up can occur. The best sawdust has been composted for one year or more. Sawdust may also contain phytotoxic resins, tanins and turpentine even after aging or composting, and material from within composted piles of sawdust may be highly acidic. Unless abandoned piles can be found for free, sawdust may cost up to $21.00 yd^-3.

Rice hulls

Rice hulls are regionally available in the rice producing areas of the U.S. which include the southern gulf states north into Arkansas and Missouri and in California. There are several forms of rice hulls available and the form affects the properties. Unprocessed or parboiled whole rice hulls have a low bulk density, a near neutral pH depending upon contaminants and contribute little to the nutrient-holding ability of the substrate. They are added to the substrate primarily to improve drainage and provide for air-filled pore space. Whole rice hulls hold significantly less water than Sphagnum peat and plants grown in substrates containing significant proportions of whole rice hulls must be watered more frequently than plants grown in peat-based substrates. Most commonly whole rice hulls are added to a substrate at 20% to 50% of the total volume depending upon the crop and cultural conditions.

Parboiled rice hulls, or rice hulls sterilized through some other means, are preferred as the sterilization process eliminates viable weed seed that can occur in rice hulls. There are anecdotal reports of nitrogen tie-up as a result of the use of fresh rice hulls in substrates. However, under normal production conditions, incorporating of fresh rice hulls into the substrate does not result in nitrogen tie-up.

Rice hulls may also be carbonized by heating the hulls in an oven or over a fire. The pH of carbonized rice hulls was reported to be near neutral to slightly alkaline. As with unprocessed rice hulls, carbonized rice hulls have a low water-holding capacity and are added to substrates primarily to increase drainage and aeration. The carbonization process increases the availability of phosphorus and potassium from the rice hulls.

Aged rice hulls are usually brown in color and whole. These hulls have been allowed to sit for several months, but they have not undergone active composting. Since the hulls are primarily intact, aged hulls have properties similar to those of whole fresh hulls.

Composted rice hulls are also available. The composting process requires approximately 24 months depending upon the composting conditions. The composting process removes readily available carbon-containing compounds, lowers the C:N ratio and reduces particle size. Composted rice hulls hold approximately 50% of their weight in water, but have a lower air-filled pore space than fresh whole rice hulls. The pH of composted rice hulls has been reported to range from 5.7 to 6.2.

Coconut coir

Coir is a waste product of the coconut industry and is produced primarily in Sri Lanka, the Philippines, Indonesia, Mexico and parts of the Caribbean and South America. Coir is produced by grinding the coconut husk and screening the long and medium length fibers. The long fibers are used for various purposes including the production of hanging basket liners. The remaining coir is composed of brown or red colored granular pith with a small amount of short fibers. Coir provides both nutrient-holding and watering-holding capacities to a substrate, and therefore, it is used in substrates for the same purposes as Sphagnum peat and composted barks. The pH of coir may range from 5.6 to 6.9, but most commercially available sources range from 5.8 to 6.5. Therefore, liming is not required when using coir as it is when using Sphagnum peat. The electrical conductivity of coir has been reported to range from 0.3 to 2.9 mmho/cm. The electrical conductivity is probably the most important quality aspect that producers and users of coir need to be aware. With a cation-exchange capacity of 39 to 60 meq/100g, coir does provide for nutrient-holding capacity in the substrate, but its cation-exchange capacity is lower than that of Sphagnum peat. Coir typically contains higher levels of mineral elements than Sphagnum peat particularly P and K. Chloride levels of 400 to 700 parts-per-million are not uncommon in coir. However, these chloride levels typically do not present a problem so long as the electrical conductivity is in an acceptable range. Coir has a similar or slightly lower bulk density and air-filled pore space than most Sphagnum peats. However, at 1100% its weight in water, coir has a significantly higher water-holding capacity than Sphagnum peat. Additionally, coir readily absorbs water, and therefore, a wetting agent is not needed with coir as is the case with Sphagnum peat. Coconut husks may also be chopped to form small cubes that may be used as a substrate for such species as orchids and anthuriums.

Composted manures

Composted manures were commonly used in substrates prior to the introduction of peat. However, now they are primarily used in container nursery stock production. They have a high cation-exchange capacity and provide significant levels of micronutrients. Nitrogen, phosphorus and potassium are provided at lower levels. Manures may be high in salts and ammonium nitrogen, especially after pasteurization. Salts and ammonium levels should be closely monitored, even during storage. Leaching may be necessary to reduce the electrical conductivity level. Cow manure is most commonly used. When used as a substrate component, cow manure is usually incorporated at a 10 - 15% rate (by volume). Composted manures cost approximately $21.00 m^-3.

Vermiculite

Vermiculite is mined and processed primarily in the United States, Africa and China. The ore itself is a mica-like silicate mineral. The ore is ground and then heated in a furnace to cause it to expand. The resulting product is a lightweight granule composed of numerous thin plates lying parallel to one another. Because of the extensive surface area created by these plates, vermiculite has a high water-holding capacity. Because of the large pores between the granules (for large-sized grades only), vermiculite provides for air-filled pore space and drainage in the substrate. Fine grades hold less water than coarse grades and do not provide for drainage and air-filled pore space. There are numerous negative charge sites on the plates resulting from the mineral origin of vermiculite. This gives vermiculite a cation-exchange capacity of 19.0 - 23.0 meq/100g. Vermiculite also provides some potassium, magnesium and calcium. The pH of vermiculite varies depending upon origin. Most U.S. vermiculite has a pH of 7.0 - 8.0. African vermiculite may have a pH of as high as 9.0. Vermiculite costs approximately $50.00 - $55.00 m^-3.

Vermiculite is generally not used with soil or for long-term crops because over time the physical structure compresses and the aeration and porosity decrease. There are several grades of vermiculite available and are based on particle size. However, no uniform grading system exists for vermiculite.

Perlite

Perlite is derived from siliceous volcanic rock that is crushed and heated in a furnace to 982° C until it expands to form the white particles that make up perlite. These expanded particles provide for air-filled pore space in a substrate, provide little water-holding capacity, have a negligible cation-exchange capacity (0.10 - 0.20 meq/100g) and have a pH of approximately 7.5. Perlite is considered chemically inert and has little effect on substrate pH. Perlite costs approximately $50.00 - $55.00 m^-3. As with vermiculite, perlite is available in different grades based upon particle size.

Sand

Sand is usually added to greenhouse substrates to increase the bulk density of the substrate. Concrete-grade (sharp, course sand) sand should be used. Washed sand is preferred to insure that it is free of other constituents and contaminants. Sand has a negligible cation-exchange capacity, a low water-holding capacity and a pH near 7.0. Sand also has little effect on substrate pH. Sand may cost up to $25.00 m^-3.

Calcined clay

Calcined clay is formed when aggregates of clay particles are heated to form hardened particles. The aggregates are large and irregularly shaped so they tend to form large pores between the particles. This gives calcined clay high a high air-filled pore space and excellent drainage. The clay particles themselves have many small pores and a large surface area that results in a moderate water-holding capacity. Because of its clay origin, calcined clay has a cation-exchange capacity of 6 - 21 meq/100g. The pH may vary between 5.0 and 9.0. Calcined clay may cost up to $80.00 per m^-3.

Polystyrene foam

This material is also referred to simply as "Styrofoam." It is lightweight and provides for drainage and air-filled pore space in the substrate, but it is otherwise inert will a negligible cation-exchange capacity. It may be purchased as beads or flakes in various sizes. Price can vary considerably, but generally these materials cost $17.00 to $20.00 m^-3. Many greenhouse operations are no longer using polystyrene because of environmental concerns and because the material floats and tends to rise to the surface of the substrate.

Rock wool

Rock wool is produced by burning a mixture of coke, basalt and limestone at a temperature of 1,600° C. The mixture liquefies, and the liquid is spun to form fibers. The process is similar to how cotton candy is produced. The resulting product is similar to a dense building insulation. Although commonly used to form slabs and propagation cubes, rock wool may also be obtained in a ground form that can be incorporated into substrates. Rockwool has a negligible C.E.C. and is usually slightly alkaline. However, it has little effect on substrate pH. Rock wool increases substrate aeration and drainage while also increasing the water-holding capacity. Rockwool cubes are used for propagation while rockwool slabs are used in hydroponic production .

Hydrophilic polymers

These materials are composed of acrylamide, starch or other polymers. They are used to increase water-holding capacity of the substrate. These materials often break down over time, particularly when fertilizer salts are present.

PVF/PVC foam

These are foams (i.e. Oasis) with high water-holding capacities that are most often used in propagation for the rooting of vegetative cuttings.

Miscellaneous agricultural, municipal and industrial byproducts

These materials are experiencing increased usage. These materials may offer significant cost savings, particularly if local sources are available. However, in many cases, little information is available. Some of these materials are ground straw, peanut hulls, bagasse, waste paper sludge and ground rubber tires. If the material decomposes rapidly, it may result in nitrogen tie-up, and it therefore may need to be composted before being used or supplemental nitrogen must be provided during production. Some of these materials contain phytotoxic levels of mineral elements. For example, ground rubber tires contain very high levels of zinc.

Formulating Substrates

Several substrate components may need to be mixed together in order to formulate a substrate with the appropriate physical and chemical properties. Both the components and the ratios of the components may be varied in order to obtain a substrate with the desired properties.

There are no absolutes when designing substrates. The general approach is to select a component with a good C.E.C. and water-holding capacity and to combine this (if necessary) with a material that increases the air-filled pore space of the substrate. If the substrate requires a higher bulk density, a component with a high bulk density (i.e. sand) may be added. In some cases, multiple components may be used for each purpose. It is important to remember that when a component is added, it can affect the properties of the other components.

The individual components, and the final product, should be low in undesirable elements (i.e. Na, Cl, F, etc.) and the electrical conductivity should not be too high for the intended use. For substrates to be used for well-rooted plants, an electrical conductivity of 3.0 mmho/cm or higher becomes problematic depending upon the crop. For young cuttings, the electrical conductivity should be 2.0 mmho/cm or less. For substrates to be used for seed germination, the electrical conductivity should be 1.0 mmho/cm or less.

After the components are selected, amendments may be included to adjust the physical and chemical properties of the substrate. Usually pH needs to be adjusted upwards (reduce acidity) because of the use of acidic materials such as Sphagnum peat. The desired pH depends upon the crop and production conditions, but usually a pH of 5.0 - 6.0 is a desirable starting pH (before use) for a substrate. Usually some form of lime is used to raise pH and at the same time to supply calcium and possibly magnesium (if dolomitic limestone is used). The amount of lime required depends upon the substrate pH, the C.E.C. (buffering capacity) of the substrate and the desired pH. Generally, for peat-based media, 10 - 15 lbs of lime is incorporated per cubic yard (6 kg/m³). Several types of liming materials may be used. The reaction rate of the lime is dependent upon type of lime, grind size, hardness, moisture and temperature. Lime reacts very slowly (or nearly not at all) in a dry substrate. As temperature increases, the reaction rate increases. As hardness increases, reaction rate decreases, and as the grind size decreases, reaction rate increases. Typically, a fine grind of calcitic limestone will react and cause the pH of a Sphagnum peat-based substrate to increase to acceptable levels within 48 hours. After that time, pH will continue to increase slowly for several days at which time pH will stabilize (excluding other factors such as effects of fertilizer, decomposition of the peat and water quality).

This material is similar to calcium carbonate except that dolomitic limestone contains magnesium. For this reason, it is often used, at least in part, to increase substrate pH so that a source of magnesium is added to the substrate.

This material is quick acting and is used at approximately 2/3 the rate of ground limestone. It may be used in combination with calcium carbonate or dolomitic limestone. The hydrated lime results in a very rapid increase in pH and the other liming materials provide for long-term pH control. This material should be avoided where significant levels of urea or ammoniacal nitrogen are being applied to the soil. This material may also be added to water to create an alkaline solution that can be applied to substrates to increase pH.

Often a phosphorus source is added to the substrate to supply the phosphorus requirement of the crop. This is usually accomplished by using either superphosphate or triple superphosphate.

Superphosphate (0-20-0) - Ca(H₂PO₄)*2H₂O + CaSO₄ (8% P)

Superphosphate is usually incorporated into substrates as a source of phosphorus. A rate of 4.5 lbs/yd³ (2.7 kg/m³) is common. Superphosphate also provides Ca and S.

Triple superphosphate is similar to superphosphate except that it does not contain S and Ca and is 45% P instead of 20%

In some cases a microelement package (i.e. Micromax) may also be incorporated into the mix. These are often incorporated at 50% the label rate and additional an additional microelement application is made during production if necessary.

Calcium nitrate [Ca(NO₃)₂] or potassium nitrate (KNO₃) may be added to the substrate to provide a "nutrient starter charge". However, slow-release fertilizers, manures, ammonium nitrogen sources and urea should not be added prior to substrate pasteurization.

Dry peat normally repels water. Therefore, a wetting agent (surfactant) may be added to the peat or the substrate to improve wettability. Some common wetting agents are AquaGro 2000, Hydro-Wet, Surfside, Triton B-1956 and Ethomid 0/15. In some cases, peat and pre-mixed soil mixes have wetting agents already added. Always use wetting agents according to label directions.

Mixing of Substrate Components

Small-sized batches of substrate may be mixed by hand or with a small rotary drum mixer. If done by hand, the major concern is the thorough mixing of both the components and the amendments.

Intermediate-sized batches of substrate may be mixed using tractors with scoops or augers, automated mixing equipment, or converted cement mixers. These systems may be combined with container-filling or flat-filling machines and automated seeders or transplanters.

Large-sized batches require automated mixing equipment. These systems usually have several hoppers that contain the components and amendments. The materials are released at the desired proportions into a central mixer or onto a conveyor belt ("ribbon") that transports the materials to a mixer. After mixing, the materials may be transferred to a storage unit or supplied to a filling machine. Some systems are designed to allow continuous mixing of substrate components so that an uninterrupted supply is available to the production line.

Selection of the appropriate components, the proper ratios of components and inclusion of appropriate amendments are important first steps in designing a substrate that can be successfully used to produce greenhouse crops. However, these decisions are only a part of the process. Mixing and handling of a substrate can have a significant impact on its physical and chemical properties. Even a well-designed substrate may perform poorly if it is mixed or handled improperly.

Substrate components should be mixed thoroughly. Otherwise, containers filled with a poorly mixed substrate may have a different mixture in them, and the physical and chemical properties may vary among containers. Total pore space, air-filled pore space, drainage and water-holding capacities can vary significantly among containers when filled with a poorly mixed substrate. Variation in these properties results in variation in the water-holding capacity and drainage which in turn results in uneven drying of the medium once the crop is placed in the greenhouse. This can make it difficult for the grower to manage the watering regime of a crop. Growers should, however, be careful not to over-mix a substrate because the excessive mixing may break down the component particles. This results in the production a "fine" particles that increase bulk density and reduce air-filled pore space.

Complete mixing is also important because amendments added to substrate need to be mixed well throughout or chemical properties may vary among the containers. For example, if limestone added to a medium to increase its pH is not mixed thoroughly, some areas of the substrate will have more limestone than desired while others will have less. This can result in variable pH levels among containers filled with the substrate.

Adding small amounts of a material (i.e. microelements) into a large volume of substrate can be problematic since it is very difficult to get thorough mixing. If a small amount of a material needs to be added to a large volume of substrate, the material should be mixed in a carrier such as sand or vermiculite before then adding it to the substrate.

Water should be added to the substrate during mixing. This allows the substrate components to expand, maximizing the medium's pore space. If the substrate is put into containers dry and watered in, the substrate will not be able to expand to its fullest extent. Furthermore, when water is added to a dry substrate in a container, the surface may wet, but the water will often channel through the substrate and out of the container. The substrate thus looks wet, but it may be dry below the surface. It may take numerous irrigations to get the entire volume of the substrate wet. Additionally, moisture is required to allow the amendments (i.e. lime) to begin reacting in the substrate.

The substrate should be stored moist and in a clean location. Avoid contamination of the substrate with potential pathogenic bacteria or fungi by placing it on bare ground or in areas where it may mix with contaminated substrate or plant materials.

Do not allow the growing medium to become too dry. The substrate should contain approximately 60% moisture when filling containers. Do not pre-fill containers and allow the substrate to sit in the containers for an extended period of time and become dry before use.

Avoid stacking pots so that the weight of the containers compresses the substrate. Not only does the compression reduce air-filled pore space and drainage, but the amount of weight placed on each container is not consistent. This introduces variability in drainage, air-filled pore space and water-holding capacity and will be seen on the bench as uneven drying of the substrate. If stacking is necessary, stack the containers so that the container rims are supporting the weight rather than the substrate.

Some growers purchase pre-mixed substrate. In some cases, the substrate is a commonly available commercial one while in other cases it may be a substrate formulated to the specifications of the grower (custom blended). This option allows the grower to avoid the need for mixing of substrates. The premixed substrate may be received bagged or in bulk. In some situations companies will provide substrate on a "just-in-time" basis that reduces the need for storage.

Substrate Pasteurization

Sterilization is the killing of all living organisms on or in a material. Pasteurization is the killing of most living organisms on or in a substance. In the past, the general dogma was that pasteurization killed the harmful organisms (i.e. disease-causing fungi) in a substrate but allowed most of the beneficial organisms to live. However, pasteurization does kill many of the beneficial organisms (i.e. Trichoderma, Gliocladium, nitrifying bacteria) in the substrate (or substantially population) and may actually increase the incidence of soil-borne diseases. Unless field soil is added to a substrate, or there is some reason to believe the substrate may be contaminated with weed seed, nematodes or a high level of disease-causing organisms (i.e. with repeated monoculture in ground beds), substrates should not generally be pasteurized.

Methods of Substrate Pasteurization

Steam:

Steaming is the best methods for pasteurization of substrates. The basic objective is to apply the steam until the coldest area of the substrate reaches 160° F (71° C) for 30 minutes. Of course, other areas of the substrate will be at a higher temperature. The substrate should be moist and loose prior to steaming and a system that allows the steam to move into the substrate should be used.

Lime, superphosphate, inorganic fertilizers and microelements can be incorporated prior to steam pasteurization. However, slow-release fertilizers (unless otherwise indicated), manures and urea-based fertilizers should not be incorporated prior to pasteurization.

Several problems can occur as a result of steam pasteurization of substrates. Manganese toxicity can occur if a field soil containing large amounts of manganese is included in the substrate. Toxic levels of ammonium can occur when substrates with high volumes of organic matter, manures or ammoniacal nitrogen are pasteurized. This often occurs because the microorganisms involved in converting organic nitrogen to nitrate are killed by pasteurization. The ammonifying bacteria recover first followed 3 - 6 weeks later by the nitrifying bacteria. During this time ammonium can build up to toxic levels. If slow release fertilizers are added prior to steam pasteurization, the coating may be damaged and fertilizer salts rapidly released. This can result in a high E.C. levels and subsequent damage to the plant root system.

Chemicals:

• Methyl Bromide is scheduled to be removed from the market. It is highly toxic and is most often used for field situations. When being treated with methyl bromide, the soil should be loose and moist and must be covered tightly prior to the injection of the gas. Since methyl bromide is odorless, chloropicrin is usually added as a safety precaution. Methyl bromide effectively kills soil-borne fungal and bacterial pathogens, nematodes and weed seed.
• Chloropicrin is also known as tear gas. It is used in a manner similar to methyl bromide.
• Vapam is similar to chloropicrin. It is often used in the nursery industry to pasterurize soil beds. It is most effective against weed seed.

Growers should always exercise caution when using chemicals and follow the label for the specific chemical being used. After chemical sterilization, the medium should be allowed to sit for up to 14 days before planting to allow any phytotoxic chemical residue to dissipate.

Container Effects on Media Properties

The substrate physical properties of water-filled pores, water-holding capacity and air-filled pores space, are a function of both the substrate and the container in which it is placed. As the container height decreases, the water-filled pore space and the water-holding capacity (as a function of volume) increases and the air-filled pore space decreases. A medium that has 67% water-filled pores and 20% air-filled pores (by volume) in a 6-inch container may have 84% water-filled pore space and only 3% air-filled pore space in a small plug cell. By contrast, as the column height increases, the water-filled pores and the water-holding capacity decreases and the air-filled pore space increases.

This occurs because when a substrate is placed in a container, a perched water table is created (unlike the continuous soil column found in a field situation). As gravity acts on the water in the substrate, water drains from large pores, but water in small pores is held with enough tension to be held against gravity and remain in the pores. As the column height increases, the weight of the water in the column forces more water from the substrate thus resulting in a reduced volume of water-filled pores and an increases volume of air-filled pores. Additionally, all of the pores of the medium near the bottom of the container will remain filled with water after drainage (even the large pores). Thus there is a flooded zone near the bottom of the container. The height of this flooded zone is a function of the forces of gravity, weight of water above and attraction of water to soil particles. As the column height is reduced, the proportion of the medium remaining flooded increases and the proportion of pores filled with air is reduced.

This phenomenon is essentially the same as occurs with a water-saturated sponge. When held flat, the sponge drains for a period and then ceases drainage. However, if the sponge is turned so as to increase column height, additional drainage will occur. The sponge now holds less water and has a higher air-filled pore space than when it was held flat. This occurs even though the makeup of pores in the sponge has not changed.

Enzyme Essentials

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© 2003, M.R. Evans