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.
