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Section 5: Common Substrate Components

Peat
Peat is the partially decomposed remains of plant materials. The easily decomposable portions of the plant decomposed leaving primarily the lignified portions of the plant that decompose much more slowly. There are various types of peat and these may be classified based upon the plant origin of the peat, its degree of decomposition and its nutrient content.

Generally, the plant origin and the degree of decomposition are most commonly used when describing peat and these variables have a significant impact on the physical and chemical properties of a peat. The primary classifications of peats used in greenhouse crops production are sphagnum peat, hypnum peat and reed-sedge peat.

Sphagnum peat moss is typically light to medium brown in color and is formed primarily from various species of Sphagnum moss. It is generally the least decomposed of the major groupings of peat and usually the stems and leaves of the moss plant are still partially discernable. Sphagnum peat decomposes relatively slow so nitrogen depletion does not occur to an appreciable extent. It has a total pore space of up to 97% (although 82% - 85% is typical) and a high water-holding capacity; holding 60% to 68% of its volume in water. A typical sphagnum peat has an air-filled pore space of 10% to 15% and a bulk density of approximately 0.1 g.cm^-3. Unamended sphagnum peat has pH of 3.0 - 4.5 and thus must be amended with a material such as ground limestone before use as a root substrate for most crops. Sphagnum peat has a C.E.C. of 90 - 140 meq/100g so it is able to retain mineral nutrients for later release and uptake by the crop.

Hypnum peat moss is darker in color than sphagnum peat (usually black), and it is composed primarily of Hypnum moss. It has a finer texture than sphagnum peat and the parent plant material is not discernable. Its fine texture results in hypnum peat moss having a higher bulk density (approximately 0.26g•cm^-3), a lower total pore space (80% - 82%) and a lower air-filled pore space (6% to 7%) than sphagnum peat. A typical hypnum peat has water-holding capacity of 70% to 75% which is also higher than a typical sphagnum peat. Hypnum peat has an unamended pH of 5.0 - 6.5 and it therefore has a lower ground lime requirement than sphagnum peat (if it requires any limestone). Hypnum peat often has a higher E.C. than sphagnum peat; up to 3.0 mmho•cm^-1 is not uncommon. Hypnum peat often contains weed seeds. As with sphagnum peat, hypnum peat provides for water-holding capacity to the substrate and it is able to retain nutrients.

Reed-sedge peat tends to be variable in color with some being dark brown and others being brown to red in color. This peat is formed primarily from variety of plant materials including reeds, sedges, grasses and cattails. Although it can be obtained in different degrees of decomposition, it is nearly always more decomposed than sphagnum peat. Therefore, it has a finer texture, a higher bulk density, a lower total pore space and lower air-filled poor space than sphagnum peat. A typical reed-sedge peat will have a total pore space of approximately 80%, and air-filled pore space of 10% to 14%, a water-holding capacity of 65% to 67% and a bulk density of 0.14 to 0.16 g•cm^-3. Because of the variable nature of hypnum's plant origin it has a highly variable pH ranging from 4.0 to 7.5 but a pH of 5.0 to 5.5 is most common.

Peat humus is dark brown to black in color and is the most highly decomposed of all of the major types of peat. It is usually derived from hypnum or reed-sedge peat. It is decomposed to such a degree that the original plant remains are indistinguishable. Whereas sphagnum, hypnum and reed-sedge peat are usually greater than 90% organic matter, peat humus may contain significant amounts of mineral soil. The pH may range from 5.0 to 7.5 and it may have a moderate to high electrical conductivity and may contain moderate to high levels of nitrogen (including ammonium).

Any given type peat may also be classified according to a scale known as the Von Post Scale. This scale classifies peats based on their level of decomposition and color. The scale ranges from H1 to H10. The higher the number, the more decomposed the peat and generally the darker in color. As the number increases, the bulk density increases and the total pore space and air-filled pore space decreases. Although this scale may be used to classify peat, in practice most peats are based upon there plant origin and degree of decomposition as described above. Often the term "blonde" is used when referring to sphagnum peat. This term simply means that the peat is light in color and has undergone minimal decomposition so that it contains a high proportion of relatively large peat particles. Sphagnum peat is also commonly marketed as coarse (or medium coarse) and fine. Coarse is most commonly used for the formulation of root substrates for use in containers. A fine grade of sphagnum peat is most commonly used in the formulation of germination substrates.

Peat may be harvested using one of two basic methods. In the first, the upper surface of the bog is allowed to dry and is then tilled. Equipment with large vacuums then collects the peat for transport and additional processing. Peat may also be harvested using what is known as a block cut method. In this case, the peat is harvested from the bog in intact blocks, dried and then processed. Many greenhouse managers believe that block cut peat has a coarser structure with a higher total pore space and a higher air-filled pore space than vacuum-harvested peat. Although anecdotal reports support this belief, there is little independent research data to support this belief or to provide an understanding as to how great a difference exists between these two types of peat and what level of variability occurs.

Despite the different types of peat that are available, peat is often used as a base in the formulation of substrates. All types of peat provide for water-holding capacity as well as nutrient holding capacity. Depending upon the level of decomposition, peat may also provide for a portion of the required air-filled pore space of the substrate.

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Coconut coir
Coconut coir is a by-product of the coconut industry and is produced primarily in Sri Lanka, the Philippines, Indonesia and Mexico. Coconut coir is produced by grinding the coconut husk and screening out the long and medium length fibers. The long fibers are used for various purposes including the production of hanging basket liners. The remaining granular pith material is referred to as coir dust or more simply as coir. Coir is typically brown, reddish brown or dark brown to black in color depending upon its origin and its age. Fresher or younger coir tends to be lighter or redder in color. The physical properties of coir may vary depending upon its age and how it was processed and packaged. However, coir typically has a pH 5.8 to 6.9. Therefore, the addition of lime or ground limestone to coir is not required as is the case with sphagnum peat. The electrical conductivity of coir has been reported to range from 0.3 to 2.9 mmho/cm depending upon source. The primary ions contributing to the electrical conductivity of coir are of K⁺, Na⁺ and Cl^- and these ions are easily leached. The electrical conductivity is one of the most important quality aspects that producers and users of coir need to be aware. Many producers of coir will guarantee a product with a pH and electrical conductivity within a specific range.

Coir provides both nutrient-holding and watering-holding capacities to a substrate, and is therefore used in substrates for the same purposes as sphagnum peat. With a cation-exchange capacity of 39 to 60 meq/100g, coir does provide nutrient-holding capacity in the substrate, but its cation-exchange capacity is lower than that of a typical sphagnum peat. However, since most growers use a constant liquid fertilization program to provide for the nutritional needs of the crop (rather than relying on the substrate to retain nutrients for later use), the difference in the cation-exchange-capacity between coir and sphagnum peat rarely requires a change in the nutritional program when using coir-based substrates as compared to sphagnum peat-based substrates. Coir holds significant amounts of water and thus serves the primary function of holding water and/or nutrient solutions when used in the substrate. Typically coir will hold 73% to 80% of its volume in water which is slightly higher than a typical sphagnum peat. Coir readily absorbs water, and therefore, a wetting agent is not needed with coir as is the case with sphagnum peat. However, typical coir will have 9.5% to 12.5% air-filled pore space which is slightly lower than that of a typical sphagnum peat. Conversely, coir containing substrates will usually have a slightly higher water-holding capacity than equivalent sphagnum peat substrates (Table 1).

In addition to providing for the water-holding and nutrient-holding capacities in a substrate, coir has been shown to increase the disease suppressiveness of a substrate. Researchers demonstrated that when plants were grown in substrates composed entirely or predominantly of coir (at least 60% by volume), the incidence of damping-off of seedlings as well as root rot of transplanted plugs caused by several fungal pathogens was significantly reduced.

Coconut husks may also be chopped to form small cubes that may be used as a substrate for such species as orchids and anthuriums. Shredded coconut husk or chopped husk may also be placed in grow tubes and used as a hydroponics substrate. The large particle sizes create air-filled pores while the pith of the coconut husk holds water.

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Composted and aged barks
Various types of hardwood and softwood barks may be aged or composted and used in substrates. Sometimes the terms "aged" and "composted" are used interchangeably with respect to bark. However, aged bark has typically been placed out of doors in piles and allowed to passively decompose for many months. Composted bark is typically ground and screened before composting. A nitrogen source is added and the bark is placed in windrows and a typical thermophilic composting process is followed. The aging or composting process serves several functions. The readily decomposable portions of the bark are decomposed by microorganisms. The remaining bark components are more resistant to decomposition so the rate of shrinkage from decomposition is reduced and the potential for nitrogen depletion is greatly reduced. Additionally, composting reduces the particle size of the bark and increases the cation-exchange capacity. Finally, aging and composting reduces the level of any potential phytotoxic compounds that might exist in the bark.

After composting, the bark may be used immediately or it may be screened prior to use to remove fine particles and provide a more uniform product containing primarily 3/8 inch particles. The exact physical properties of aged or composted bark will depend upon the size of the particles initially used to create the compost, the degree of decomposition and the particle sizes (large particles versus fine size particles) that make up the final composted product. Bulk densities for milled composted pine barks have ranged from 0.2 to 0.5 g•cm^-3. The total pore space, air-filled pore space and water-holding capacity again depend on the distribution of particle sizes in the composted product. However, most composted barks contain a significant proportion of large particle sizes that produce air-filled pore space. Bark also holds water in small pores that occur between particles as well as within the structure of the bark particles. The finer particles in the bark, the higher the bulk density, the lower the total pore space and the lower the air-filled pore space. Screened 3/8 inch (5 mm) composted bark product was reported to have a bulk density of approximately 0.12 - 0.2 g•cm^-3, total pore space approaching 80%, a water-holding capacity of approximately 60% and air-filled pore space of 20%. Therefore, depending upon the specific product, composed barks may provide for both water-holding capacity and air-filled pore space in the root substrate.

Softwood barks are commonly used in the southern and southeastern U.S. These typically are derived from various species of Pinus. Hardwood barks are more common in the northern and central portions of the U.S. Fir and Redwood barks are common in the northwest U.S. Hardwood barks contain more cellulose than softwood barks and thus decompose more quickly and to a greater degree than softwood barks. Most hardwood barks are phytotoxic in their raw state (as are some softwood species) and must be composted to eliminate the phytotoxic compounds. Therefore, the appropriate composting is critical to ensure the reduction of phytotoxic compounds in hardwood barks.

Composted hardwood barks also will typically have a higher pH (usually above 7.0) than composted softwood barks (6.0 - 7.0). When using a high proportion of an aged or composted hardwood bark in a root substrate, amendments may need to be added to reduce the root substrate pH. It has been demonstrated that the chemical make up of softwood barks depends on the tree species involved as well as the nature of the soil on which the tree was grown and that composted hardwood bark may have high manganese levels. Where crops susceptible to manganese toxicity are being grown, proper substrate selection and preparation and pH management will help to prevent the occurrence of manganese toxicity.

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Perlite
Perlite is derived from siliceous volcanic rock that is crushed and heated in a furnace to a very high temperature until it expands to form the light-weight white particles that make up perlite. Perlite is generally added to the root substrate to increase the proportion of large pores and thus to reduce water-holding capacity and increase the air-filled pore space of the root substrate. Perlite is produced in various grades but even a given grade will contain a range of particle sizes. Typically, the larger the perlite particles, the lower the total pore space and water-holding capacity and the higher the relative air-filled pore space. Nelson reported that a horticultural grade of perlite had a total pore space of 63%, a water-holding capacity of 38% and an air-filled pore space of 25% at container capacity. Perlite has a negligible cation-exchange capacity. It typically has a pH of 7.0 to 7.5, but it is considered chemically inert and has little effect on the overall substrate pH.  Although perlite contains only trace concentrations of mineral nutrients, it may contain fluoride. For certain sensitive crops, it has often been recommended that perlite either not be used in the substrate or used in only small amounts to reduce the potential for fluoride phytotoxicity. However, several researchers have noted that fluoride toxicity is not likely to occur unless a low substrate pH occurs.

Pumice
Pumice is a light weight silicate mineral of volcanic origin. The particles are very porous and come in a range of particle sizes. The particles hold a small amount of water but because of the relatively large particle size, pumice provides for drainage and air-filled pore space when used in a containerized root substrate. The amount of pumice included in a substrate depends upon the desired physical properties, but using 30% pumice in peat or compost-based root substrates has been demonstrated to be a suitable root substrate for containerized crops. Pumice has a pH near neutral but has a low buffering capacity and has little effect on the root substrate pH. It may contain potassium and sodium, but generally only trace levels of other mineral elements. Pumice is more commonly used in substrates in southern Europe and western Canada than in the U.S.

Arkalyte
Arkalyte is a porous expanded clay material that resembles pumice but because of its clay origin it is usually red or red-brown in color and has a higher bulk density than pumice. Arkalyte provides for drainage and air-filled pore space and it also provides for increased bulk density. Arkalyte holds little water and has a low CEC. It is most commonly used in green roof installations (usually in combination with 20% - 30% organic material) where plant support and high drainage levels are desired.

Sand
Sand is usually added to greenhouse substrates to increase the bulk density of the substrate. In containerized peat- and bark-based substrates, the addition of sand does not increase the air-filled pore space since the sand tends to fill in the large pore spaces that drain after irrigation and become air-filled. Air-filled pore space is thus usually reduced with the addition of sand. Sharp coarse concrete grade sand free of clay, organic matter and calcium carbonate should be used. The sand should also be free of contaminants such as herbicide residue or salts. Sand has a negligible cation-exchange capacity and has little effect of the chemistry of the substrate unless it is contaminated with other materials.

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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. It has a very low bulk density and a negligible cation-exchange capacity. It is considered to be chemically inert in the substrate and has no significant effect on substrate pH. It may be purchased as beads or flakes in various sizes. Many greenhouse operations no longer use polystyrene because of environmental concerns and because the material floats and tends to rise to the surface of the substrate. Additionally, some municipalities have banned this material from being placed in landfills and others have placed restrictions on its use to prevent the material from entering sewage treatment facilities. 

Sawdust
Sawdust is not commonly used as a substrate component for containerized greenhouse crop production. It must be thoroughly composted for at least a year or nitrogen depletion can occur. 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.

Parboiled fresh rice hulls
Rice hulls are a byproduct of the rice milling industry and are primarily produced in Arkansas, Texas, Louisiana, Mississippi and California. Parboiled fresh rice hulls (PBH) are rice hulls that are removed from the grain after being exposed to boiling water for a specific period of time. Therefore, PBH is sterile and free of viable weed seed when initially produced.

Parboiled rice hulls have a pH near neutral to slightly alkaline. However, because of their low cation-exchange capacity, they are poorly buffered and have minimal effect of the substrate pH. When used in the range of 20% to 40% of the total substrate volume, a PBH-containing substrate has a similar pH as equivalent perlite-containing substrates. Parboiled fresh rice hulls have a low electrical conductivity, and although not at concentrations that would fulfill crop requirements, PBH does contain significant levels of phosphorus and potassium. Parboiled fresh rice hulls also contain significant levels of manganese. When used in the range of 20% to 40% of the total volume of root substrate, the manganese concentrations are usually within recommended ranges for greenhouse crops.

Parboiled fresh rice hulls are canoe-shaped and approximately 6 to 10 mm in length. Because of their relatively large size and length, parboiled fresh rice hulls have a low bulk density (0.1 g•cm^-3), a high total pore space of approximately 90% and a high air-filled pore space of approximately 69%. When blended into a substrate, PBH provides for drainage and air-filled pore space. Substrates containing up to 20% PBH had similar physical properties as sphagnum peat-based substrates containing 20% perlite. However, as the amount of PBH increased, the air-filled pore space increases at a greater rate than for perlite so that peat-based substrates containing at least 30% PBH have a higher air-filled pore space and a lower water-holding capacity than sphagnum peat-based substrates containing at least 30% perlite.

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Vermiculite

Vermiculite is produced in different grades. However, there is not a consistent grading system for horticultural vermiculite. Often the terms fine, coarse and extra coarse are used to describe vermiculite. In other cases, producers use a numbering system to describe vermiculite grades. The exact physical properties of vermiculite will vary depending upon the average particle size. However, all vermiculite sources will hold significant amounts of water and will provide for nutrient-holding capacity. Smaller grades of vermiculite will not significantly contribute to the air-filled pore space of a substrate. Large particles of vermiculite will initially create air-filled pore space in a substrate. However, the vermiculite particle is fragile and will be compressed in the substrate over time. As the particle is compressed, it losses its ability to provide for air-filled pore space. For this reason, vermiculite is typically not used to provide drainage and air-filled pore space. Its primary purpose is to provide for water-holding and nutrient-holding capacities in a substrate. If vermiculite is mixed into a substrate, it is especially important not to over mix since vermiculite is fragile and may be broken down into small particles with excessive mixing. Vermiculite should not be used in substrates that contain significant amounts of sand or field soil as the weight of these mixes will compress the vermiculite. Additionally, because the vermiculite particle tends to break down over time, vermiculite is not recommended for use in substrates used to grow long-term crops.

In addition to being blended into substrates to increase water-holding capacity, vermiculite is sometimes added to the surface of plug trays to cover seed (i.e. as a top coat). The layer of vermiculite holds water and helps to maintain an environment of constant moisture ideal for germination of many seeds.

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Calcined clay
Calcined clay is formed when aggregates of clay particles are heated at high temperatures to form hardened particles. Most commercial cat liters are calcined clays that have dyes and perfumes added to them. Because calcined clay aggregates are large and irregularly shaped, they tend to form large pores between the particles. This gives calcined clay a high air-filled pore space and good 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 3.4 - 11.8 meq/100g depending upon the specific clay used to make the aggregates. Additionally, because of its clay origin, calcined clay has a very high bulk density. Some calcined clay sources have been reported to contain significant amounts of calcium and sulfur. Although the pH of calcined clay may vary from 4.5 to 9.0, depending upon its clay origin, some researchers have reported that it has only a minimal impact on the pH of a substrate. However, another researcher noted that when 20% calcined clay was added to peat the resulting substrate had a pH of 5.03. When the calcined clay was increased to 50%, the pH increased to 5.78. Therefore, calcined clay resulted in a significant increase in the substrate pH. How much a given calcined clay will increase the substrate pH will depend on the characteristics of the clay from which is made. Therefore, experimentation would be required to determine the exact pH changes resulting from calcined clay incorporation into a substrate.

Calcined clay is generally used as a substrate component to specifically obtain both air-filled pore space and a high bulk density. When used as a substrate component, it has been recommended that calcined clay be used at 25% to 33% of the total volume of the substrate. However, the exact amount to be used would depend on the desired bulk density. Because calcined clay is expensive, perlite, composted bark or PBH would be used to provide for air-filled pore space when an increase in bulk density is not also desired.

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Kenaf
Kenaf is the ground or chopped non-composted woody stem core of the Hibiscus cannabinus plant. The plant is grown for its long stem fibers. The woody central pith is a by-product and may be ground or chopped into various particle sizes. Having a bulk density (for a small particle size) of approximately 0.11 g.cm-3, kenaf is light in weight and generally tan or brown in color. The pithy particles are able to absorb water or nutrient solution when used as a substrate component. However, it holds less water than a typical sphagnum peat. Therefore, when substrates containing significant amounts of kenaf as an alternative to sphagnum peat are used, more frequent irrigations may be required than would have been required had only sphagnum peat been used. A serious limitation to using kenaf in a greenhouse root substrate is that it rapidly decomposes so shrinkage is a significant problem. Kenaf has a pH near neutral and inclusion of significant amounts of kenaf in a peat-based substrate results in an increase in the substrate pH.

Plant growth trials using kenaf as a substrate component have provided conflicting results. It has been demonstrated that several plant species could be successfully grown in substrates containing up to 70% kenaf. However, one group of researchers reported poor growth of celosia, viola and impatiens when up to 30% kenaf was included in the substrate. These differences in plant performance may have been a result of how the kenaf was processed, the plant species evaluated or the specific cultural conditions under which the plants were grown.

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Rockwool
Rockwool 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 fibers are then bonded together to form dense slabs that may be cut into various dimensions. The material may also be formed into small granules that may be used as a substrate placed into containers or that may be mixed with peat. Granular rockwool may be used alone as a substrate for container-grown crops. It has a high total pore space, retains water and nutrient solution for uptake by the plant and has a significant air-filled pore space. A typical medium grade granular rockwool product had 91% total pore space, 65% water-holding capacity and 25% air-filled pore space. More commonly a mixture of granulated rockwool and sphagnum peat has been used for containerized crop production. A 50:50 mixture of peat and granulated rockwool was shown to be a good root substrate for containerized greenhouse crop production. However, this root substrate was shown to have a potential drawback in that significant settling occurred with irrigation. In fact, the volume of the substrate was reduced by 20% after a single irrigation. Over several irrigation cycles the volume was reduced by more than 30% due to settling. Applying a light compaction to the substrate during container filling significantly reduced the amount of settling that occurred. A 30 rockwool:70 peat ratio has also been reported to serve as a good root substrate. This substrate was reported to have a total pore space of 80%, a water-holding capacity of 58% and an air-filled pore space of approximately 22% when placed in a 1.6 liter container.

Rockwool slabs are most often used for hydroponic production of crops such as greenhouse-grown vegetables, cut gerbera and roses. Slabs come in various dimensions as well as densities. The density of the fibers (how closely the fibers are packed together) and the dimensions of the slab affect its physical properties. A typical rockwool slab used for hydroponics production has a bulk density of 0.16 g.cm^-3, a total pore space of 82% and air-filled pore space of 11% (although the actual values depend upon the specific density of the slab). Of the water retained by the rockwool, nearly 67% is considered easily available water. This is typical of findings related to rockwool in which rockwool holds approximately the same volume of water as a coarse sphagnum peat, but it holds that water at a lower tension and thus releases it for uptake by the plant more readily. Rockwool may also be formed or cut to form cubes that are used for propagation.

Although the pH of rockwool is very alkaline, usually being 8.0 or higher, it has little or no buffering capacity and will assume the pH of the nutrient solution applied to it or that of the peat or bark with which it is blended. Rockwool has a negligible cation-exchange capacity and thus only retains nutrients in that it is able to retain an applied nutrient solution. The rockwool fibers do contain mineral nutrients, but these are only very slowly released into the substrate solution as the rockwool weathers. Rockwool may also have an electrical conductivity that is higher than optimal for certain crops. In such a case, leaching is recommended before planting. This serves to reduce the electrical conductivity and to wet up the rockwool before use.

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Hydrophilic polymers
Hydrophilic polymers or gels may be divided into three basic categories including starch-based gels, polyacrylamide gels and propenoate-propenamide copolymers. They form a lattice structure that can hold large amounts of water and thus these materials are generally added to substrates to increase the water-holding capacity of the substrate. The exact amount of water held depends upon the chemical structure of the specific polymer. However, most have been reported to hold 300% to 1,500% of their weight in water when exposed to deionized or distilled water.

Hydrophilic gels composed of starch are rapidly broken down by microorganisms in the substrate and have a life span of a few days to a few months, whereas synthetic gels have been reported to have a lifespan of up to two years. Researchers have reported that after 200 days in the substrate, the water-holding capacity of a synthetic hydrophilic gel began to decline, and exposure to fertilizer solutions and repeated drying and rehydration cycles reduce the water-holding capacity of hydrophilic gels. Fertilizer salts also greatly reduced the water absorption of hydrophilic gels. In particular, the ions Ca⁺⁺, Mg⁺⁺, K⁺ and NH₄⁺ supplied in the fertilizer solution reduce the amount of water absorbed by hydrophilic gels to only 10% to 20% of their maximum possible water absorption as compared to when deionized water is used. Additionally, cations in fertilizer solutions or water have been reported to greatly inhibit the ability of the gels to expand and contract.

Hydrophilic gels have been shown to retain ammonium in the substrate but not nitrate.  The use of a hydrophilic gel was shown to increase the concentration of nitrogen and potassium in plant shoots but reduced the uptake of calcium and magnesium from the substrate. Presumably NH₄⁺ and K⁺ were retained but released for uptake by the plant, but divalent cations (i.e. Ca⁺⁺, Mg⁺⁺) were retained by the hydrophilic gel and not released for uptake by the plant. Thus, the effect of hydrophilic gels on a mineral nutrition program is more complicated than simply retaining nutrients for use by the plant.

Some hydrophilic gel products also claim increased plant growth when incorporating the product into the substrate. Some studies have supported such claims while others have found no significant increase in plant growth as a result of the use of hydrophilic gels in the substrate. Most likely cultural conditions and plant species have a significant impact on whether incorporation of such gels impact plant growth.

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Aged and composted rice hulls
What are often referred to in the industry as composted hulls may in fact be composted or aged hulls. Aged hulls have been allowed to passively decompose to a point where they are brown in color but the hulls are still largely intact.

Composted hulls are dark brown in color and most of the rice hull particles have been broken into smaller pieces as they have composted. Therefore, composted rice hulls have a much smaller particle size distribution and thus different properties from aged hulls. Typically composted rice hulls have a total pore space of 85%, and air-filled pores space of 18% and water-holding capacity of 67% and a bulk density of 0.2 g•cm^-3. Composted hulls generally have a pH in the range of 5.0 to 5.6 and an E.C. of 0.4 to 0.6 mmho.cm^-1. However, it is important to note that even with composted hulls, different methods of composting are often used and no specific composting standard has been developed. Therefore, the exact properties of composted hulls may change from supplier to supplier and batch to batch.

Because they are still intact and have undergone minimal decomposition, the properties of aged hulls are usually very similar to fresh parboiled hulls. Aged hulls are added to a root substrate primarily to increase drainage and air-filled pore space. Composted rice hulls, with their much smaller particle size, are primarily used to provide water-holding capacity in a substrate.

Composted peanut hulls
Composted peanut hulls are most common in areas of the southeast U.S. The hulls are typically composted which breaks down the hull into smaller particles and reduces the C:N ratio. As with other composted organic materials, the properties may vary between sources and compost batches. Heavily composted peanut hulls are composed almost entirely off very small particles and humus. This type of material is used primarily to provide water-holding capacity and increased bulk density to the root substrate. Where the peanut hulls have been composted to a lesser degree, and medium and large particle exist in the compost, they serve to provide water-holding capacity and air-filled pore space. Peanut hulls have been reported to suffer significant shrinkage and thus were not recommended for containerized crops with production times greater than one year. Peanuts hulls have also been reported to have a potentially high electrical conductivity.

Foam cubes
There are various types of foam cubes, but those most commonly used in horticulture are formed from phenol formaldehyde or polyurethane. They are light weight and hold large amounts of water. They are most commonly formed into blocks or strips that are used for either rooting vegetative cuttings or for germinating seeds. Some of these materials may have a high E.C. and may need to be leached prior to use. There are also rooting cubes made from peat mixed with a polymer.

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Miscellaneous municipal and agricultural composted materials
There are various types of composted organic wastes. These include such things as animal manures, municipal green wastes, sewage sludge, paper products and mushroom composts. All of these will have different properties and many have been shown to be suitable as substrate components although in many cases they may be used in only small amounts in the substrate. All of these also tend to have limitations that make their use problematic.

Composted sludges often have high concentrations of heavy metals and some of these may be phytotoxic to plants. These materials tend to be composed of very fine particles and thus have a very low air-filled pore space and reduce the air-filled pore space of substrates to which they are added. It has been recommended that this type of material not exceed 20% of the total volume of the substrate.

Many municipal waste composts may contain glass and metal fragments. Some municipal composts may have high levels of boron if they were derived from a high percentage of cardboard cartons. These materials have been used in substrates at up to 30% of the total volume, but some researchers have noted that mineral element toxicities could occur if used at levels higher than 20% of the total substrate volume.

Spent mushroom compost often has a high pH and a high E.C. level. It usually contains high levels of N, P and K. Uncomposted spent mushroom waste tends to be a very coarse material that holds significantly less water than a typical sphagnum peat. However, after it is allowed to compost, the particle size is reduced, the bulk density increases, water-holding capacity increases and air-filled pore space decreases. Because it often has a high pH, some type of sulfur compound might need to be added to reduce the pH to an acceptable level. Spent mushroom composts have been used in substrates at 25% to 50% of the total volume.

Composted waste paper pulp has also been evaluated for use as a substrate component with some degree of success. However, Most of the research related to these materials has been for use as a substrate for the production of containerized woody plant materials. Cotton gin trash and ground corn cobs may have a high E.C. and the possibility exists that they could contain herbicide residues.

Most composted manures have a high cation-exchange-capacity and are good sources of micronutrients. They typically have a high water-holding capacity but usually a low air-filled pore space. Composted dairy manures have generally been considered to be the best types of composted manure for use in substrates. Most recommendations have been to include up to 20% of the total substrate volume. Composted horse manure and turkey manure have been successfully used in substrates when relatively small amounts where included. Composted poultry litter often has a high E.C. and high ammonium. For these reasons it is only used in very limited amounts in substrates. Composted swine manure had a high E.C. and a high pH. All manures may contain significant levels of ammoniacal nitrogen. Therefore, if pasteurized, sufficient time should be allowed between pasteurization and use to allow the microorganism population to recover and to convert much of the ammonium to nitrate. All composted manures may also have a high E.C. and testing should be conducted to determine if E.C. is within acceptable levels.

There is a large volume of research that has been conducted on the use of different types of organic waste products as substrates. However, it is very difficult to make specific recommendations to growers because of the high level of variability that occurs in these materials. Even in situations where research has been conducted on a composted material, it is nearly impossible to give sound recommendations because the same material from different sources (or between batches from the same source) may be different. Additionally, whether a material is suitable as a substrate component will depend to some degree on the specific composition of the substrate, how much of the composted material that is to be used, the crops to be grown and the cultural conditions. Greenhouse managers should be aware that variability may be a serious issue and that on-site trials should be conducted to determine if a specific composted product is appropriate for the intended use.

Field soils
The physical and chemical properties of a field soil vary with soil type and texture (sand vs. clay vs. silt). However, most field soils have relatively high water-holding capacities and cation exchange capacities. They have high bulk density, and often have low air-filled pore space and poor drainage when placed into a container. 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. In some cases field soil may be added to a substrate to increase the bulk density of the substrate or to increase the cation-exchange-capacity thus making the substrate more resistant to changes in pH.

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