Section 1: Introduction
Whether they are referred to as soils, root media, growing media, root substrates or substrates, the materials that are placed into containers and in which the plants' roots develop can have a profound effect on the development of the crop. This is because, regardless of its composition, the substrate may fulfill up to five basic functions:
Under certain production situations, the root substrate (substrate) might provide for all of these functions. However, in some production situations, the substrate may not be required to fulfill all of these functions, and as such may not be designed to fulfill all of these functions. For example, in hydroponic and some pseudo-hydroponic production systems, the substrate (if one exists at all) is generally not expected to retain nutrients for uptake by the plant. The substrate holds water and thus the nutrient solution that is applied during each irrigation (fertigation) cycle. However, this solution is replaced frequently and thus the root substrate is not specifically required to serve as a reservoir of nutrients for the plant. The substrate in this case is simply serving the functions of physical support and water retention. The importance of each of these functions may change depending upon the crop being grown and the cropping system being used.
There are many components that may be used in the formulation of a substrate. Because these individual components rarely have the optimal physical and chemical properties required when used alone, two or more components are often blended together to produce a composite material that will have the appropriate properties for the crop being grown and the specific cultural conditions under which the substrate will be used. In order for the substrate to fulfill the desired functions, it must have the appropriate physical and chemical properties. Therefore, understanding substrates begins with an understanding of their important physical and chemical properties.
Section 2: Important Physical Properties of Substrates
Bulk density
Bulk density is the dry weight of a specific volume of a moist substrate. It is determined by measuring a specific volume of a moist substrate (usually at 50% or 60% moisture by weight) and drying the substrate in an oven until no additional moisture is lost over a 24-hour period. Bulk density is therefore usually expressed as grams per cubic centimeter (g.cm-3). However, in the U.S. it is common in the greenhouse industry for bulk density to be expressed as a moist weight per volume. In these instances, bulk density is often expressed as lbs per cubic yard or lbs per cubic foot. In the European Union, bulk density of commercial substrates is sometimes expressed as kilograms per cubic meter. Although these are not proper expressions of bulk density in the truest sense, they do provide users with an understanding of the moist weight of the root medium, and therefore, the weight that must be supported by benches or that must be shipped. Most peat-based and peat-bark-based substrates have a bulk density of approximately 0.09 - 0.16 g.cm-3 (Table 1). Although a bulk density of 0.15 - 1.3 g.cm-3 has been recommended for greenhouse substrates, the required bulk density will depend upon the crop being grown in the substrate. A certain bulk density is desirable, especially with larger or taller plant materials since the weight of the substrate will support the plant and prevent the container from falling over. However, as bulk density increases, the cost of shipping also increases. Therefore, a substrate with a high bulk density will be desirable when growing a crop such as poinsettia trees or tall foliage plants, whereas a low bulk density root substrate would be desirable for growing bedding plants in small containers or in cell packs.
Total pore space
Most common substrates to be used for the production of containerized greenhouse crops will typically have a total pore space of 75% to nearly 90% (Table 1). These figures may vary depending upon the exact specifications and proportions of the components used to formulate the substrate. However, these numbers illustrate the important point that the majority of the volume of a substrate is actually comprised of the pore spaces that exist between the solid substrate particles. Although it has been recommended that total pore space of substrates should be 70% - 90% by volume, the total pore space itself is less important than the size of the pores that make up the total pore space. It is the size of these pores, or the abundance of different sizes of pores, that will impact the very important physical properties known as air-filled pore space and water-holding capacity.
Air-filled pore space
During irrigation, when a crop is top watered, 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 they drain and become air-filled. At this point, being that it is holding the maximum amount of water that it is capable of holding against gravity in the container in which it has been placed, the substrate is said to be at container capacity. These air-filled pores are important because they allow for gas exchange between the roots and the outside atmosphere. Without this gas exchange, the roots would be deprived of oxygen required for respiration. If air-filled pore space is too low, there is an increased probability of root rot occurring. Additionally, if oxygen in the substrate is too low, conditions may become anaerobic. Under anaerobic conditions ethanol and ethylene may build up in the substrate. Reduced forms of manganese and sulfur compounds may also result from these conditions. Although there are no absolute recommendations, an air-filled pore space of 10 to 20% has generally been recommended for most containerized greenhouse crops. As an example, a typical 80% sphagnum peat with 20% perlite or an 80% sphagnum peat with 20% composted pine bark (3/8 inch) will have an air-filled pore space of approximately 10% and 13% in a 4-inch container, respectively (Table 1). Increasing the amount of perlite or bark (or other large particles such as PBH or pumice) will generally increase the amount of air-filled pore space but only up to a maximum of the air-filled pore space of the aggregate being used (i.e. the perlite, pumice or bark).
As noted earlier, air-filled pore space and container capacity are defined as the condition of the substrate after drainage in a specific size container. This is because the container size, and specifically the height of the container, significantly affects the physical properties of air-filled pore space and water-holding capacity. Any substrate will have a different air-filled pore space depending upon the container size. Therefore, the air-filled pore space of a substrate should be discussed in relation to the container in which it is placed. The role of container size on air-filled pore space is explained further under water-holding capacity.
Water-holding capacity
Possibly the most important function of a substrate is to hold water (or the fertilizer solution when using a liquid fertilization program) in a form that is available for uptake by the plant. If the water-holding capacity is too low, frequent irrigation will be required to prevent water stress. However, if the water-holding capacity is too high (too many of the pores are filled with water at container capacity), the air-filled pore space may be too low. Additionally, if the water-holding capacity is too high, infrequent irrigations might be required, and in cases where a constant liquid fertilization program is being utilized, this results in infrequent fertilization and can lead to either nutrient deficiencies or the grower being forced to water too frequently in order to provide adequate fertilizer to the crop.
Water-holding capacity for substrates in containers is expressed as the percent of the volume of the substrate that is filled with water at container capacity and is expressed as a percentage of the total volume. Recommendations for the optimal water-holding capacity of a substrate vary depending upon the crop and growing conditions. However, a common recommendation has been that the substrate should have a water-holding capacity of 50% to 65%. However, many commonly utilized substrates may have higher water-holding capacities. A typical 80% sphagnum peat and 20% perlite substrate will hold approximately 68% water by volume while an 80% sphagnum peat and 20% composted pine bark substrate will hold approximately 70% water by volume (Table 1).
Not all of the water that is held by the substrate is available for uptake by plants. The water held by a substrate has traditionally been divided into easily available water, available water and unavailable water. Available water has generally been defined as water that is held at a tension of between 10 and 150 cm while easily available water has been defined as water held between 10 and 50 cm of tension. The important point from a crop production standpoint is that, although total water-holding capacity is usually what is reported for a substrate, not all water that a substrate retains is available for use by the plant.
In addition to the components and the ratio of the components, water-holding capacity, as well as the air-filled pore space of a substrate, is also a function of the container into which the substrate is placed. As the container height decreases, the water-filled pore space increases and the air-filled pore space decreases (as a percentage of the total volume of root substrate). By contrast, as the height of the container increases, the water-filled pore space decreases and the air-filled pore space increases. Thus, a given root substrate will hold more water and have less air-filled pore space when placed in a short container than when placed in a taller container. This is very similar to the phenomena seen when a sponge is saturated with water, picked up and allowed to drain. If the sponge is held so that its longest side is flat (horizontal), some of the pores of the sponge will drain and then drainage will cease. The pores that drain become air-filled. If the sponge is then turned with its longest side vertical, additional water will drain (and more of the pores will be air-filled). This is important because a root substrate that works well in a tall container, may have too high of a water-holding capacity and too low of an air-filled pore space when placed in a short container. Therefore, the container in which the substrate is being placed should be considered when designing a substrate.
Shrinkage
Shrinkage usually refers to the loss of substrate in the container over time due to decomposition of the organic components of the substrate. All organic materials used in substrates will decompose over time. However, the important issue is how fast the organic components decompose. If the organic components decompose slowly, the loss of the substrate is slow and the crop cycle may be complete before a significant reduction in the substrate volume occurs. If the substrate decomposes rapidly, the volume of the substrate will be reduced rapidly and before the crop cycle is completed, little of the organic components of the substrate might remain to fulfill the functions for which they were intended. With little substrate remaining in the container, very frequent irrigations become necessary to prevent water stress and without the weight of the substrate and the water that it holds, taller plants become prone to toppling.
Sometimes shrinkage has been used to refer to the volume loss that occurs when a substrate is allowed to dry. This is most prominent with peat-based substrates. As water is removed from the substrate through transpiration and evaporation the peat particles shrink and contract. If the substrate is allowed to become too dry, and especially if no wetting agent has been used, rewetting of the peat may be difficult.
Finally, shrinkage has occasionally been used to refer to the loss of substrate volume that occurs as a result of top watering. In this case, this loss of volume results simply from a settling of the substrate particles as water flows through the substrate. Most of this settling occurs during the first two or three irrigation cycles but it increases the bulk density and reduces the total pore space and air-filled pore space of the substrate.
Section 3: Important Chemical Properties of Substrates
pH
The pH impacts many aspects of the substrate environment. The pH affects the availability of mineral nutrients required by the plant. Some mineral elements such as iron, zinc, manganese and copper are more available at low pH while others such as molybdenum, calcium and magnesium become more available for uptake as the pH increases. The pH of the root substrate can also impact the incidence of some soil-borne diseases. Although the optimal pH for a new unused substrate varies depending upon the specific root substrate components, the quality of the irrigation water to be used, the fertilization program to be used and the specific crop being grown, an initial substrate pH within a range of 5.3 to 6.3 is generally desirable. The exception to this recommendation would be for ericaceous crops such as azalea that require a lower pH. Additional information regarding substrate pH is discussed under the "Mineral Nutrition" and "Irrigation Systems" learning units.
Electrical conductivity
The electrical conductivity (E.C.) is a measure of the total ions (K+, Ca++, NO3-, NH4+, Cl-, etc.) in the substrate solution (the liquid phase of the substrate). The E.C. is determined as a measure of the substrate solution's ability to conduct an electric current across a distance of one cm and is therefore commonly expressed as mmho.cm-1, mS.cm-1 or dS.m-1. These units of measure are equivalent to one another and require no conversion (1.0 mmho.cm-1 = 1.0 mS.cm-1 = 1.0 dS.m-1). The higher the ion concentration in the substrate solution, the higher the E.C. Because E.C. cannot be used to measure the level of any specific ions in the substrate solution, it can only be used as a measure of the total ion concentration. However, because many of these ions are plant mineral nutrients, E.C. may be used as an overall measure of the mineral nutrient content of the substrate.
Recommendations for optimal E.C. of unused substrates can be difficult because the desired level will depend upon the crop, stage of the crop, production conditions, and the specific ions that make up the E.C. In cases where the substrate will be used for germination, a very low E.C. is generally desirable. When transplanting rooted cuttings or well developed plugs, a higher E.C. may be tolerated or desired. However, this is dependent upon the crop. For example, the desired E.C. of an unused substrate in which New Guinea impatiens will be transplanted will be lower than the desired substrate E.C. in which a large well-rooted geranium cutting will be transplanted. As a general rule of thumb, for rooting of cuttings and germinating seed, an E.C. of less than 2.0 is desirable. For well rooted crops that are not sensitive to E.C., an initial E.C. of 2.5 or less is desirable.
In some cases, a nutrient starter charge may be added to an unused substrate to increase the mineral element content of the substrate (and thus the E.C.). This starter charge is designed to provide a source of rapidly available nutrients for the developing new crop and to allow the crop to more quickly become established. More information is provided about nutrient starter charges under "Common Substrate Amendments" in the "Mineral Nutrition" learning unit.
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 1 nitrogen for every 25 carbons that they utilize. If the organic matter being broken down has a carbon:nitrogen ratio of greater than 25:1, the microorganisms must obtain nitrogen from another source in the surrounding substrate environment. Under such conditions, the microorganisms will utilize nitrogen in the substrate supplied by the fertilizer and intended for use by the plant. If the carbon:nitrogen ratio is very high, significant amounts of nitrogen must be taken from the fertilizer sources. In these situations, microorganisms may out-compete the plant for the available nitrogen and induce a nitrogen deficiency (often referred to as nitrogen tie up). However, a high carbon:nitrogen ratio alone is not sufficient to induce a nitrogen deficiency in a crop. The organic matter must have a high carbon:nitrogen ratio and must decompose relatively quickly for significant nitrogen depletion to occur. Therefore, organic materials that have a high C:N ratio and decompose quickly (i.e. sawdust, cotton gin trash, bark) are generally not used in substrates, or they are composted before use to lower the C:N ratio and allow for the readily available carbon to be utilized by microorganisms during the composting process.
Cation-exchange capacity
Many components used in the formulation of substrates have negatively charged sites on their surfaces that allow them to retain cations. Many of the mineral nutrients required by plants are cations (Ca++, K+, NH4+, Mg++, etc.). These negatively charged sites allow the substrate components to retain mineral nutrients for uptake by the plant and thus provide a reservoir of nutrients. The negatively charged sites also retain protons (H+). These protons can be exchanged with the substrate solution and thus help to buffer the substrate solution from rapid pH changes. Therefore, the C.E.C. provides the substrate with a certain buffering capacity and is sometimes referred to as the substrate buffering capacity. The higher the substrate buffering capacity, the more cations the substrate can retain and the more resistant the substrate is to pH changes. Cation-exhange capacity is expressed as meq.100ml-1.
Specifications
Mineral ions may occur in phytotoxic concentrations. This can be an issue with some potential substrate components and may make a component unsuitable for use in greenhouse substrates. In these cases, the overall E.C. may not be too high, but the level of a specific ion may occur at a concentration that can be phytotoxic to plants.
For example, ground rubber tires often contain phytotoxic levels of zinc. Some composts may contain phytotoxic levels of ammonium. When using any new or unfamiliar material in substrates, the mineral element content should be determined prior to use.
Section 4: Common Substrate Amendments
Lime and ground limestone
Substrates containing significant amounts of sphagnum peat or other acidic components usually require the addition of some type of lime or ground limestone to increase the substrate pH to a desirable level. Several types of pre-plant amendments may be used to increase substrate pH.
Hydrated lime [Ca(OH)2] is typically available as a fine powder and causes a rapid increase in substrate pH, but it has minimal residual effect on pH. Therefore, it is primarily used as a substrate amendment when a rapid pH adjustment is required. Where residual pH control is desired, hydrated lime must be used in combination with other slower-reacting components. Hydrated lime should be used with caution as large quantities can be caustic to plants. Furthermore, hydrated lime should be avoided where significant levels of urea or ammoniacal (NH4+) nitrogen have been included in the substrate as the rapid pH increase brought about may result in the evolution of phytotoxic ammonia (NH3) in the substrate.
Ground calcitic limestone (CaCO3) contains 36% - 39% calcium. It reacts more slowly in the substrate than hydrated lime, but it has a longer residual effect on substrate pH than hydrated lime. Dolomitic limestone (CaCO3*MgCO3) typically contains up to 12% magnesium depending on its specific chemical makeup. It is often preferred to calcitic limestone because in addition to increasing pH and providing a source of calcium, it provides magnesium.
When discussing the reaction of a lime or ground limestone in a substrate, there are three aspects of the reaction (and thus the pH increase) that are of interest. The first aspect is the reaction rate. This generally refers to how fast a limestone increases the pH of the substrate up to the point of equilibrium (where the pH no longer increases as a result of the lime or limestone). Another factor is at what pH does pH equilibrium occur. Finally, the residual effect of the limestone is important. This is essentially how much of a reservoir of calcium carbonate exists (once equilibrium has been reached) to maintain the pH in a desirable range, particularly if low alkalinity water and acidic fertilizers (which tend to push pH down over time) are used.
Many factors affect the specific reaction of a ground limestone when incorporated into a substrate. One factor is the grind size of the limestone. Ground limestone is classified based upon its particle size distribution. Pulverized limestone is classified as having a particle size that allows 60% of the particles to pass through a 100 mesh (150 µm) screen. A superfine limestone has a particle size that allows 60% of its particles to pass through a 200 mesh (75 µm) screen. Microfine limestone has a particle size that allows 95% of its particles to pass through a 325 mesh (45 µm) screen. The general conclusion from research that has been conducted on particle size was that the finer the particle size, the more rapid the reaction rate of a limestone. Likewise, the finer the particle size, the less residual effect of the limestone. More specifically, in research trials, a coarse particle with a size greater than 20 mesh (particle size of greater than 0.850 mm collected on a 20 mesh screen) was very slow to react and had little residual pH buffering capacity (ability to neutral acids over time and prevent pH decreased in the substrate). A limestone particle size of 20 - 100 mesh (particle size of less than 0.850 mm and larger than 0.150 mm) reacted and increased substrate pH and had a significant residual pH buffering capacity over the crop production period. A limestone particle size of 100 - 200 mesh (smaller than 0.150 mm and larger than 0.075 mm) was very reactive, rapidly increased pH levels to high levels and had little residual pH buffering capacity. Thus, the conclusion was that limestone with particle sizes within a 20 - 100 mesh range was best for greenhouse substrates.
The specific chemical makeup of the limestone and its hardness also affects its reaction rate. Typically, the harder the limestone, the slower the reaction rate and the greater its residual effect on the pH of the substrate.
In addition to grind size, chemical makeup and hardness of the limestone affecting the reaction of a limestone when added to a substrate, the substrate itself will have an important impact. Different substrates will have different initial pH and different cation-exchange-capacities which will result in different buffering capacities. Even sphagnum peat from different sources may have different initial pH and buffering capacities, and thus a given amount of limestone can result in different increases in pH with different peat types or sources.
Recommendations for the amount of a ground limestone to add to a substrate to achieve a desired pH vary. One recommendation for peat-based substrates was to incorporate 10 - 15 lbs of calcitic or dolomitic limestone per cubic yard with 2.5 to 3.0 lbs of ground calcitic or dolomitic resulting in approximately a 0.5 unit increase in pH. Another recommendation has been 8-10 lbs of dolomitic limestone per cubic yard for peat-based substrates and 5-6 lbs of dolomitic limestone per cubic yard for bark-based substrates. In published research trials, dolomitic limestone incorporated into a peat and perlite substrate at a rate of 3, 6, or 9 kg•m-3 increased the pH of the substrate from 3.9 to 5.8, 5.8 and 5.9, respectively. However, the residual activity varied based on the amount of limestone added. The pH of the substrate receiving only 3 kg•m-3 decreased over the next 16 weeks from 5.8. The pH of the substrate receiving 6 kg•m-3 increased to 6.0 by week 6 and then decreased during the following 10 weeks. The pH of the substrate receiving only 9 kg.m-3 increased to 6.2 after 6 weeks and then decreased to 6.0 for the remaining 10 weeks that the substrate was monitored. When a dolomitic hydrated limestone was incorporated at a rate of 0.9, 1.2 and 1.5 kg•m-3 into a 70% sphagnum peat and 30% perlite substrate, the pH increased to 5.0, 5.7 and 6.4, respectively, and remained relatively constant and those pH levels for 28 days.
Because of all of the factors that can affect the reaction of limestone when added to a substrate, it can be difficult to make specific recommendations regarding the amount of limestone to add to a substrate to achieve a specific pH. All of the studies that have been conducted on the effect of various limes and limestones on the pH of root substrates have used different substrates as well as different sources and types of lime and limestone. Although the rates discussed above serve as a general starting point or guide, greenhouse managers should experiment to determine exactly how a given amount of lime or limestone will affect the pH of the intended substrates. If substrates, substrate components, or limestone source is changed, evaluations with small test batches of substrate with varying amounts of limestone added should be conducted to determine what pH changes might occur. The greatest increase in pH usually occurs within the first two days of mixing. However, the pH will continue to increase and will equilibrate after 7 to 14 days. Therefore, it is generally recommended that the pH of the substrate be tested after 7 days. When conducting tests, it is important that the substrate be moist in order for the limestone to react and increase the pH.
Iron sulfate, aluminum sulfate and elemental sulfur
Because most greenhouse substrates are based on sphagnum peat which is acidic, it is most common that the pH of the substrate must be increased using lime or a ground limestone. However, in some cases components that have a pH that is higher than desired may be used to formulate a substrate. In these situations, iron sulfate, aluminum sulfate or elemental sulfur may be added as a pre-plant amendment to lower the pH of the substrate. Iron sulfate and aluminum sulfate are relatively quick acting but have minimal residual effect. Elemental sulfur is slow-reacting but has more residual effect on substrate pH. Elemental sulfur at a rate of 0.25 to 0.5 lbs•yd-3 or iron sulfate at 0.5 to 1.0 lbs•yd-3 were reported to lower substrate pH by 0.5 units. However, as with using limestone to increase pH, the reaction will vary depending on the specific substrate and the environmental conditions. Therefore, it is best to experiment to determine exactly how a given amount of a sulfur compound will affect the chemistry of the substrate.
Superphosphates
Superphosphate [Ca(H2PO4)*2H2O + CaSO4] or triple superphosphate [Ca(H2PO4)2*2H2O] are sometimes incorporated into the substrate as a pre-plant amendment as a phosphorus source. A typical superphosphate contains 7% to 9.5% phosphorus while a typical triple superphosphate contains 17% to 23% phosphorus. Both contain calcium and have been reported to contain varying amounts of boron, copper, manganese, molybdenum and zinc. A typical recommended rate of incorporation of superphosphate has been 4.5 lbs•yd-3 while 2.0 lbs•yd-3 has often been recommended for triple superphosphate. The actual desired incorporation rate will depend on the crop and the desired P concentration. Superphosphate and triple superphosphate have been reported to contain fluoride and for this reason it has often (although not universally agreed upon) been recommended that they not be included in substrates to be used to produce lilies, certain foliage plants or other monocotyledonous crops that are sensitive to fluoride. Reports regarding the effect of superphosphate and triple superphosphate on substrate pH have been contradictory. Superphosphate and triple superphosphate were reported by one researcher to have a neutral effect on substrate pH. However, other researchers reported that both superphosphate and triple superphosphate reduced the pH of a peat and sand substrate. The difference in these results may have been a function of the specific substrates used in the respective trials and the amount of sulfate in the substrate.
Superphosphate and triple superphosphate are no longer as commonly used as a substrate amendment as they once were used. There are two primary reasons for their reduced usage. The first is the increase in the use of complete water soluble fertilizers which are more often used as the sole phosphorus source. These fertilizers most often use ammonium phosphate or diammonium phosphate as the phosphorus source. The second was the discovery that high phosphorus levels resulted in elongation of young plants and thus greenhouse managers began to reduce phosphorus levels supplied to greenhouse crops.
Wetting agents
Wetting agents are a class of surfactants that are used to break surface tension and allow substrates to more readily absorb water. Wetting agents are particularly important for sphagnum peat-based substrates because dry sphagnum peat is very hydrophobic (repels water). Being surfactants, wetting agents varying significantly in their chemistry. Anionic surfactants tend to be used as foaming agents and not as wetting agents. Cationic surfactants are often highly toxic to plants. Most surfactants used as wetting agents for horticultural substrates tend to be nonionic. Many older wetting agents are based upon ethylene oxide polymers or propylene oxide polymers. Many newer wetting agents are polyoxyalkylene glycols. The later are less phytotoxic to plants than the ethylene oxide or propylene oxide polymers, but they also tend to be broken down by microbial activity faster and thus have a shorter life span than the ethylene oxide and propylene oxide polymers. Wetting agents are broken down over time by microbial activity and therefore their effectiveness diminishes over time. Researchers reported that a wetting agent added to a peat-based substrate lost only 30% of its effectiveness over a 40 week period. However, some commercial product labels caution that the wetting agent effectiveness will be reduced after 4 months. The exact life span of a given wetting agent will be determined by the specific chemistry of the wetting agent and the conditions under which the substrate is stored after the wetting agent is incorporated. Growers should consult with the manufacturer of a specific wetting agent to determine how long the wetting agent is effective after being blended into the substrate.
Wetting agents are available as liquids or as granular materials. Liquid formulations are generally blended with a specified amount of water and applied to the substrate while mixing to insure uniform distribution. Granular formulations which have the wetting agent applied to a carrier such as fine vermiculite also may be added to the substrate while mixing. The primary importance is to specifically follow the label for the specific wetting agent being used and to be sure to obtain uniformity when mixing.
Many wetting agents may also be applied as drenches to the substrate of a crop after planting and/or during production. This is particularly useful for long-term crops where the wetting agent may begin to loose its effectiveness before the crop production cycle is complete. Some wetting agents are designed to be included in liquid fertilization programs and therefore, the wetting agent may be applied periodically or with each irrigation. Typically, the more frequently the wetting agent is applied, the lower the rate that is used and product labels list the rate recommended for each use and application method. Greenhouse managers should check the product label to determine if the specific wetting agent may be applied as drenches to actively growing crops or through the fertilization program and, if so, at what rate and under what environmental conditions. Failure to specifically follow label recommendations can result in plant damage.
Although there are numerous wetting agents available, the three that are most commonly used in conjunction with substrates are Suffusion®, Soax®and Aquagro®. Suffusion® is available in liquid, granular and tablet forms. Soax® and Aquagro® are available in liquid and granular forms.
The primary function of a wetting agent is to reduce surface tension and allow the substrate to fully wet up and absorb water to its fullest potential. Wetting agents do not change the physical properties of the substrate per se. However, by allowing the substrate to fully wet up, they in essence increase the water-holding capacity by allowing the substrate to absorb more water than it might without a wetting agent. In other words, the wetting agent allows the substrate to reach its full water-holding potential. Wetting agents have also been reported to increase drainage and air-filled pore space of some substrates. The information to support this claim is very limited and the data reported does not show dramatic increases in drainage and air-filled pore space with the use of a wetting agent. However, because substrates expand when they absorb water, and as they expand total pore space increases, it is reasonable that some of the increase in total pore space might be air-filled pore space. Regardless, many benefits may be obtained through the incorporation of an approved wetting agent in horticultural substrates.
Biological amendments
There are numerous biological products that have been designed to be added to substrates for greenhouse crop production. These include such products as Rootshield, SoilGuard, Mycostop and Actino-Iron. The majority of these biological amendments are incorporated into the substrate to suppress soil-born diseases caused by organisms such as species of Pythium, Phytophtora and Thielaviopsis. Some of these biological agents are specific strains of species of bacteria such as Bacillus, Streptomyces or Pseudomonas. Others are fungi such as Trichoderma or Gliocladium. These beneficial organisms may suppress disease development in one or more ways. They might cause induced plant resistance, they might directly parasitize the disease-causing organism, they might compete for space or food sources important to the disease-causing organism or they might produce antibiotics that inhibit development or kill the disease-causing organism.
Significant evidence has been generated by private companies as well as by universities that support the efficacy of such biological amendments. However, there has also been significant research published that has raised doubts regarding the efficacy of such biological amendments. In fact, in a review in 2005, Daughtrey and Benson reported that incorporation of biological components was effective at reducing disease incidence less than 20% of the time.
In 2006, Hoitink and Lewandowski published a review article and clearly pointed out that many factors can affect the efficacy of biological components added to substrates. Some of these factors include the nature of the substrate, the environmental conditions, and the crop. Considering the information published, the best course of action for growers interested in such amendments is to experiment with them on a small scale to determine their efficacy under the specific growing conditions and with the specific crops being grown. When conducting trials, control groups without the biological components should be included so that the specific effect of the biological agent may be determined.
Micronutrients
In some production situations, a micronutrient fertilizer package may be incorporated into the substrate to supply the micronutrient needs of the crop. These may be mixtures of sulfate salts (i.e. iron sulfate, zinc sulfate, copper sulfate, etc.) that are water soluble and readily available for uptake by the plant such as in the case of Promax® or S.T.E.M®. Although readily available for uptake by the plant, these fertilizers supply micronutrients for a relatively short period of time; generally 6 to 8 weeks. This time span may be adequate for some crops, such as bedding plants, but not for other longer term crops. Other types of micronutrient fertilizers embed the microelements in very small pieces of glass (FTE No. 555®) and are known as fritted microelements. Still others embed the micronutrients on clay particles (Esmigran®). These types of micronutrient fertilizers slowly release the microelements into the substrate and thus have a longer life span. Newer formulations of micronutrient packages such as Micromax® and Micromax Plus® may also include the macronutrients calcium, magnesium, and/or phosphorus.
Nutrient starter charges
Numerous components may be added to a substrate before use in order to increase the level of one or more mineral nutrients. These are usually added at rates that are designed to provide a low level of initial fertility for the developing crop (until the liquid fertilization program can take effect) and not to provide the entire nutritional need of the crop. For this reason, such amendments are referred to as starter charges. Some of the most common materials added as nutrient starter charges include calcium sulfate which is designed to provide calcium without causing an increase in substrate pH such as occurs with the addition of lime or ground limestone. Magnesium sulfate may be added to provide magnesium. Calcium nitrate provides both calcium and nitrogen while potassium nitrate provides potassium and nitrogen. Ammonium nitrate and urea may be added to provide nitrogen. Commonly recommended rates for these starter charges are listed in Table 1. However, the exact concentration of a specific starter charge component will depend on the desired concentration, environmental conditions and the crop to be grown.
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.
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.
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.
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.
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.
Vermiculite
Vermiculite is produced from a mica-like silicate ore that is mined primarily in the United States, Africa and China. The ore is ground and then heated in a furnace to cause it to expand to many times its original volume. The resulting product is a lightweight accordion-like particle composed of numerous thin plates lying parallel to one another. Because of its mineral origin and the extensive surface area created by these plates, vermiculite has a high cation-exchange capacity of 10 - 23 meq/100g. Also because of its mineral origin, vermiculite contains significant levels of potassium, magnesium and calcium. Different ores may have different chemical constituents which results in vermiculites from different origins having a different pH. Typically, most U.S. vermiculite has a pH of 7.0 - 8.0 while African vermiculite may have a pH of as high as 9.0. Vermiculite produced from ore from China may have a variable pH depending on the specific ore source. If growers change vermiculite sources, they should experiment to see how the pH changes and how using the new vermiculite changes the pH of any substrate into which it is incorporated.
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.
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.
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.
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.
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.
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.
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.
Section 6: Designing Growing Media
One of the most common questions asked by greenhouse managers is what substrate should be used. This is a daunting question to attempt to answer because there are so many possibilities. For a given crop and specific production situation, there are many potential substrates that may be used successfully. It is most important to understand how different substrate components and amendments function in a substrate and to understand the basic philosophy behind designing a substrate.
In designing a substrate for most containerized greenhouse crop production situations, water-holding capacity is usually the first consideration. A component should be included that will serve as the foundation of the substrate and provide for, at least to a large extent, water-holding capacity. Although many substrate components hold water to some degree, peat, composted organic matter, rockwool, coir, composted rice hulls, and vermiculite are most commonly used to provide for water-holding capacity. Which material or materials are used depends upon the production system being used, the crops being grown and cost. In some cases more than one material might be used to provide for the water-holding capacity. For example, peat and composted rice hulls or peat and composted bark might be blended together. Or, especially for seed germination or plug production situations, peat and medium or fine grade of vermiculite might be blended together. In this case vermiculite is added to further increase the water-holding capacity of the substrate as compared to what the water-holding capacity would be if peat were used alone. Another example would be to use a granulated rockwool alone or in combination with other substrate components. Depending upon its density, rockwool can hold significant amounts of water.
Once a material or materials have been selected to provide for water-holding capacity, further adjustments may need to be made to the physical properties of the substrate. For example, if peat is used as a base and to serve the primary function of water holding, the physical properties may be excellent for seed germination without additional components being added. However, if the substrate is to be used to grow 4-inch poinsettias, the air-filled pore space is likely to be too low and an additional component will need to be added to increase air-filled pore space (and thus drainage). Materials such as perlite, PBH, 3/8 inch composted bark, pumice or calcined clay would typically be used as components to increase air-filled pore space. Which of these components should be used depends upon availability, cost and what other physical properties are desirable. Often, more than one of these might be used in a substrate. For example, PBH will add air-filled pore space, but it is light in weight and will not provide for an increase in bulk density. Therefore, PBH may be a good choice for increasing air-filled pore space of a substrate to be used for bedding plant production. However, if the substrate is to be used to grow poinsettia trees, an increase in the bulk density would be desirable to provide additional weight to support what will become a top heavy plant. In this case, calcined clay or large gravel aggregates would be a good choice since these large particles will provide for air-filled pore space and will also increase the bulk density of the substrate. In a situation where increased bulk density is desired, and air-filled pore space is more than adequate or even higher than desired, coarse sand or field soil may be added to the substrate. This will increase bulk density, but also decrease total pore space and air-filled pore space. Of course if too much sand or field soil is added, the total pore space and air-filled pore space may be reduced to undesirable levels. Sometimes, multiple components may be used to formulate a substrate with some of the components serving more than one role. An example would be a peat, perlite and composted bark substrate. The primary purpose of the peat would be to provide for water-holding capacity. The primary purpose of the perlite and screened 3/8 inch composted bark would be to provide for air-filled pore space, but perlite and screened 3/8 inch composted bark will also hold varying amounts of water (although less than the peat).
In some instances the design of a substrate may proceed in the reverse direction. A material that is readily available and economical may serve as the base. Additional components are then added to adjust the physical properties to the desired levels. For example, a composted screened 3/8 pine bark or composted peanut hulls may be readily available and economical based upon the location of the greenhouse. The bark may provide excellent air-filled pore space, but it may not hold enough water for the desired use. In this case, additional components such as peat, coir or composted organic materials may be added to increase the water-holding capacity. Regardless, the goal is to mix the components in such a way as to provide a substrate with the desired physical properties. The components may be added or deleted, or the ratio of the components changed, to make the necessary changes.
There are unique production situations where substrate components are utilized differently from what has been described above. One such scenario is in seed germination. Common germination substrates may be composed of 100% of a fine grade of sphagnum peat or a combination of peat and vermiculite. In other cases, plug trays may be filled with 100% sphagnum peat, the seeds sown and then the seed covered (given a top coating) with a fine vermiculite. In such as situation, the peat and the vermiculite are not mixed together. The peat holds water and serves as a substrate in which the developing root can grow. The vermiculite covering allows oxygen to reach the developing seed, but also holds significant amounts of water so that the developing seed does not dry out (maintains a high relative humidity around the seed).
Rockwool is a unique material in horticultural substrates. As previously discussed it may be produced in different densities (with different properties). It may be produced as a granulated material to be placed in containers alone or with other components. It may also be formed into blocks to be used for seed germination or for hydroponic production. In these later uses, rockwool is used as a sole component and not blended with other components. Its physical properties are designed into the product in the manufacturing stage.
Like rockwool, coir comes in many forms. Coir pith or dust is often used as a component in substrates as it holds significant amounts of water. However, coir is also produced as small chunks or blocks that may be used directly as a sole substrate component. Most commonly this type of material is used for the production of crops that require excellent drainage and high levels of aeration such as orchids. Coir slabs are also available that are designed to be used as a sole substrate component for hydroponic production much the same way that rockwool slabs are used.
Because so many possibilities are available regarding root substrates, greenhouse managers need to experiment to find the best substrates that work for their crops and cultural conditions. Components should be adjusted as required to obtain the desired physical properties. An alternative to mixing and experimenting is to purchase premixed substrates from a commercial company. These companies offer numerous types of substrates with varying physical properties designed for various uses. The different companies can make recommendations regarding the best substrate based upon the crops to be grown and the cultural conditions to be utilized during production. If large volumes of substrate are required, custom blends can be developed to meet specific needs.
Once the components and ratio of components has been selected, amendments may be required to alter the properties of the substrate. A wetting agent may be added to improve wettability. Lime may be added to increase the pH to a desirable level, especially if the substrate is a peat-based substrate. In cases such as coir-based or composted rice hull-based substrates, little or no lime is typically needed. Additional fertilizer components such as superphosphate, calcium nitrate, potassium nitrate, ammonium nitrate or microelements might be added to provide a nutrient starter charge as desired. All of the amendments should be uniformly blended into the substrate so that a uniform substrate is produced.
Section 7: Mixing and Handling of Growing Media
Selection of the appropriate components, the proper ratio of the components and the proper amendments are important first steps in designing a substrate that can be successfully used in the production of greenhouse crops. However, these decisions are only part of the process. Proper mixing and handling of the substrate can have a significant impact on the substrate's physical and chemical properties and its ultimate performance in the greenhouse. Even a well designed substrate may perform poorly if not mixed and handled properly.
Small batches of root substrate may be mixed by hand or by using a small-volume rotary drum mixer. If mixed by hand, the primary concern is that the substrate be thoroughly mixed and that any amendments are also evenly distributed throughout the substrate. Intermediate-volume batches of substrate may be mixed using a front end loader, rotary drum mixers, converted cement mixers or various types of automated mixing equipment. Where large volumes of substrate are required, they may be mixed as batches using large rotary mixers or mixers utilizing an auger apparatus that turns the components thereby mixing them. A continuous feed system may also be used. In this system, a series of hoppers containing substrate components and amendments are aligned over a conveyor belt. The materials are released onto the conveyor belt ("ribbon") that transports the materials. As the ribbon moves along, different components are placed one on top of the other until finally the components are dropped into a bin where an augur turns the products and transports the substrate to a temporary holding bin or directly to a pot-filling machine.
Substrate components should be mixed thoroughly. Otherwise, containers filled with a poorly mixed substrate may contain different combinations of components and amendments 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 if the root substrate is not uniform. Variations in these properties can result in some areas of the crop drying out more rapidly than other areas while some areas may remain wet for undesirably long periods of time. This uneven drying makes water management difficult. Areas that dry out too quickly may suffer water stress while areas that remain wet may serve as areas where soil-borne diseases are more likely to occur. Additionally, if amendments are not thoroughly mixed, the resulting chemical properties of the substrate may vary among containers. For example, if the lime is not evenly distributed throughout the substrate, the pH will vary among containers. Some may have unacceptably high pH while others may have a low pH.
Although uniform mixing is critical, the substrate should not be over-mixed. This is because as the substrate is turned or tumbled, the particles may be broken down. This creates smaller particle (i.e. "fines") sizes. These smaller particles result in a higher bulk density, reduced total pore space and reduced air-filled pore space. Therefore, although high quality components were selected and mixed at the appropriate ratios, the excessive mixing changed the physical properties of the components and the resulting substrate. No absolute rule can be provided regarding the amount of time required for proper mixing of a substrate. However, the goal is to mix only as much as is required to produce a uniform product.
Water should be added to the substrate during mixing. Water allows the substrate components to expand, thus maximizing the volume of the substrate and its total pore space. If the substrate is placed into containers in a dry condition and then watered, the substrate will not expand to its full potential. Furthermore, when water is added to a dry substrate in a container, the surface may appear wet, but the water will often channel through the substrate and out of the container. The substrate may appear to be wet on the surface, but may be dry below the surface of the container. It may make take many irrigation cycles to wet up the entire volume of the substrate and usually the substrate will never expand in volume to its full potential. In addition to its effect on the physical properties of a substrate, water is required for many amendments such a lime to begin to react when incorporated into the substrate.
Substrate that has been in compressed bales must be broken apart and fluffed to maximize the volume before it is placed into pots, flats, or plug trays. This is best accomplished using a bale buster and then fluffing the mix using a falling or tumbling motion (preferably over a ½" hardware cloth). Loose-fill substrate that has become compressed in bags due to shipping and storage may also need to be fluffed before use. Be careful not to over mix the substrate.
If not used immediately, the substrate should be stored moist and in a clean location. If the substrate is not in sealed bags or totes, it should be placed in bins, hoppers or other clean storage containers and not on bare ground or unclean surfaces in order to avoid contamination of the substrate with potentially harmful pathogenic bacteria or fungi or weed seeds. Ideally, the substrate should be covered to prevent contamination and to minimize moisture loss. If containers are pre-filled with substrate, they should not be allowed to dry out before use. Otherwise, rewetting may be a problem when plants are finally planted into the containers. Additionally, if containers are filled with substrate for later use, the containers should not be stacked on top of one another in such as way that the container weight is supported by the substrate. The weight of the containers and substrate compresses the substrate. This compression increases bulk density while reducing total pore space and air-filled pore space. Additionally, in a stack of containers or flats, not all of the containers are compressed equally. Therefore, in addition to compacting the substrate, a significant level of variability in the physical properties can be introduced and this, as with uneven mixing, may result in significant variability in the rate of drying once placed in the greenhouse. If containers or flats are stacked, containers should be staggered or boards used to separate layers of containers so that the weight is supported by the containers and not the substrate. If not used soon after filling, the stacks of substrate-filled pots should also be covered with plastic to minimize water loss and contamination.
Many greenhouse managers choose to purchase pre-mixed substrates that are ready for use. This option allows greenhouse managers to avoid the need for mixing equipment, reduces the required storage space and eliminates the time and expertise needed to appropriately mix the substrates. There are numerous commercial substrates on the market from which greenhouse managers may choose based upon their specific desired properties. If large enough volumes are required, custom-blended substrates may be ordered. Substrates may be purchased in small 2.8 cubic foot bags, loose bails, large totes or when very large volumes are required, substrates may be provided loose in semi-trucks that may be used to feed container-filling lines using an augur system.
As previously discussed, wetting agents improve the wettability of substrates. Most commercial mixes contain a wetting agent. However, wetting agents are broken down over time by microbial activity. The lifespan of a wetting agent depends upon the specific wetting agents and the storage conditions of the substrate. Greenhouse managers should be careful when using old substrate as the wetting agent activity may be greatly reduced over time and the substrate difficult to wet. Additionally when growing long-term crops, a maintenance application (applied as a substrate drench or through the irrigation system) of wetting agent might be necessary.
Substrates containing organic materials such as sphagnum peat and composted bark generally should not be pasteurized. This is because pasteurization will eliminate or reduce the populations of beneficial microorganisms. Where sand or field soils are being added to a substrate, and there is concern that disease organisms might be introduced into the substrate, it is preferable to pasteurize the mineral component and then add it to the organic components. Components such as perlite, vermiculite and PBH do not require pasteurization because they are exposed to sterilizing temperatures during production. When pasteurization is required for field soils or sand, the temperature of the material should be increased to 160°F (throughout the volume of material) for 30 minutes and then allowed to cool before use.
© M.R. Evans, 2008, 2009, 2011, 2014