ABSTRACT
Structure: The structure must be durable for at least one or more crops and not break down into small particles that will impair oxygenation to plant roots�
Composition: The particles of the substrate must not react with the nutrient solution or release elements into the water as that will upset the balance of the nutrient solution� For example, calcareous rock releases calcium and magnesium causing the pH to rise above optimal levels� Some substrates such as coco coir must be well washed to remove any residual sodium chloride as often the coconut husks are found near salt water where the palm trees are growing� Bark and sawdust must be sourced from Douglas Fir, Hemlock, or Redwood timber as they are known not to release any turpines or other resins found naturally in pine and cedar wood� Rice hulls have to be aged for some time in piles that are watered, to permit those with embryos to germinate� Composting will generate heat and kill the seedlings of the rice� Alternatively, the rice hulls can be burned under a smoldering fire to carbonize them� This process gives the rice hulls less smooth surface so that it will retain more water than raw husks� The burning process will also sterilize the rice hulls and kill any embryos present�
Sterility: Substrates for hydroponic culture must be free of pest and disease organisms� If you are uncertain as to the sterility of the medium, heat it to 160°F (71°C) for half an hour to kill the organisms� This can be done in a kitchen stove oven or if there is intense sunlight and high temperatures, place the substrate on a black polyethylene and also cover it with a black polyethylene� This process should be okay after about 1 week� Gravel and some coarse sands could be sterilized using a 10% bleach solution� Other finer substrates like perlite can be pre-treated with “Zerotol,” a hydrogen dioxide compound that is highly oxidizing� It must be soaked 2-3 days prior to sowing or transplanting�
Water Retention: The hydroponic substrate must not have properties of very high or very low water retention� However, the acceptable water retention will also be a function of the type of hydroponic system� Coarse gravel can be used with a subirrigation or ebb-and-flow watering system� The water (nutrient solution) will enter
the void spaces among the rock particles when the bed is being flooded and wets the particle surfaces� As the solution drains back out it will pull air into the substrate� This method of hydroponics functions well with coarse substrates�
With fine substrates like rockwool, sawdust, coco coir, and peatlite mixes, use a drip irrigation system that applies the solution on the top and spreads through the substrate by capillary action and drains out the bottom of the container�
Water retention must not be excessive causing lack of oxygenation or be insufficient to cause the substrate to dry quickly and starve the plant of both water and nutrients�
Root Support: The substrate must allow roots to easily penetrate between the particles and anchor the plant as the roots enter the void spaces seeking water� If the substrate is too fine, the roots cannot spread readily into the medium, whereas, if it is too coarse, the plant roots will not be able to hold onto the particles and the plant can easily fall over� Most long-term crops need a substrate to anchor their roots and take up oxygen, water, and nutrients�
Availability and Cost: For small hydroponic gardens, the cost is not an important factor as relatively small amounts of substrate are needed� Many types of substrates are available for hydroponics and may be purchased online or at hydroponic shops�
The following is a description of many of these suitable substrates for hydroponic growing� When we discuss the various hydroponic systems for different plants, specific substrates are recommended�
1� Gravel: This was one of the original substrates used in hydroponic culture from the 1940s through the 1960s� It was the substrate introduced by Dr� W�F� Gericke to establish outdoor hydroponic operations during World War II on non-arable islands of the Pacific� In many of those islands, volcanic rock was suitable for hydroponics and it was plentiful�
Choosing the most desirable rock takes into account a number of characteristics or properties� It should be irregular in shape (crushed is best), free of fine particles (fine sand to silt), and should be aggregate of ½–¾″ in diameter (Figure 11�1)� The particles must be of igneous (volcanic) origin and be structurally stable� They must retain adequate moisture in void spaces yet drain well to provide oxygen to the plant roots� Avoid the use of calcareous gravel (sedimentary origin-like limestone) as their release of calcium carbonate will continually alter the pH�
2� Pebbles (Bird’s Eye and Pea Gravel): These gravels have round, smooth surfaces with size ranging from a 1
8 ″ to ¼″ diameter (bird’s eye vs� pea
gravel) (Figure 11�1)� Since their surface is less irregular, the smaller size is critical to their suitability� If you select gravel larger than pea gravel size, it must be with a sub-irrigation system as described earlier for gravel� Again, this material must be of igneous origin� With this substrate, I have used drip irrigation or soaker hoses to distribute the nutrient solution to the base of the plants� However, due to the rapid percolation, it is best to use a recycle system of hydroponic culture and more frequent irrigation cycles than for
other finer materials� These media, like gravel, can be sterilized with a 10% bleach solution between crops� Always remember to flush the substrate with raw water after using a bleach solution to remove any residual chlorine�
3� Leca (Expanded Clay): This fired clay (Figure 11�1) is sometimes called “Haydite” or “Herculite” as it is light weight and is used in construction� It has good water retention from its irregular surface and is especially suitable for small hobby units and indoor gardening� It does fracture with time resulting in the release of fine sand and silt, but with a hobby unit it can be replaced when this build-up of fines occurs�
4� Scoria: Crushed rock from volcanic origin is highly vesicular (full of holes from escaping gases during cooling from molten magma) (Figure 11�1)� It is usually dark brown, black, or purplish red and light in weight� It can also be called cinder� It is fairly good in retaining water and at the same time gives good oxygenation� Various particle sizes can be used for hydroponics, ranging from ½″ to less than 1 8″ in diameter�
5� Sand: The best sand is river sand of igneous origin� It must be well washed by the quarry operator� This washed river sand is available from most aggregate suppliers� Do not use mortar sand as its fine particles cause puddling, water coming to the surface when vibrated� The settling of the very fine particles will reduce any void spaces and eliminate the available oxygen� If properly screened to eliminate any particles of diameter over 2 mm (0�0625″) and under 0�6 mm or 0�025″, the sand will drain freely and provide adequate oxygen to plant roots�
6� Sawdust: Where large forest industries are present, sawdust is a good medium provided the source is from Douglas fir or western hemlock trees�
Western red cedar and pines should not be used as their resins are toxic to plants� In many locations, logs are floated in barges on the ocean and they collect sodium chloride from the water� Test the sawdust for sodium chloride content and leach it thoroughly with pure raw water before planting�
7� Peat: Peat is partially decomposed freshwater marsh or swamp vegetation� Use peat from sphagnum moss, as some of the other types are very fine and hold too much water� Other peats from sedges, reeds, and hypnum moss decompose rapidly upsetting the structural integrity of the medium� Peat is readily available in 3�8 cubic foot bales in compressed form� There are many blends available mixed with various percentages of perlite, vermiculite, or Styrofoam particles to add aeration to the medium� It also comes with the beneficial microorganism, Mycorrhizae� For example, Premier Tech Horticulture offers a line of peat-based substrates� Their “Pro-Mix BX Mycorrhizae” is a general purpose peat-based medium designed for greenhouse/indoor growing and contains a mycorrhizal inoculum� The symbiotic fungi colonize the root systems of plants and increase water and nutrient acquisition� The ingredients of this mix include perlite, vermiculite, dolomite lime to adjust the pH, and a wetting agent to assist in absorption of water during its initial dry state�
8� Peatlite Mixes: There are various mixtures of peat-based media that have been developed with extensive research by various universities� The two most popular ones are the UC mix (University of California) and the Cornell “Peat-Lite” mixes (Cornell University, New York)� These mixtures are combinations of peat, sand, perlite, pumice, and vermiculite with added nutrients and dolomite lime to adjust the pH� The pH of peat is very low so a base, such as dolomite lime, is needed to increase the pH� You may refer to these and other mixes on the Internet or in my book Hydroponic Food Production� It is easiest to simply purchase a complete mix such as the “Pro-Mix�”
9� Redwood Bark: As the name suggests this is bark from redwood trees� It is much coarser than sawdust with particle diameters from 1 8″ to 1 3″ or larger� This is usually the preferred medium for the growing of orchids, but not commonly used in other hydroponic cultures�
10� Rice Hulls: This is the outer husk or shell of rice (Figure 11�1)� This is a by-product of rice milling and is a waste product� They will last from 3 to 5 years without decomposing� They have a smooth surface so do not retain water readily and have poor capillary (lateral) movement of water� If they are burned by a smoldering fire, the surface is improved to give more water retention� Rice hulls are best mixed with peat or coco coir, usually at 20% rice hulls�
11� Vermiculite: Vermiculite is expanded mica through a heating process to form spongy particles (Figure 11�1)� Heating also insures that the medium is sterile� With the layers (cleavage) formed in each particle water retention is high� The irregular shape of the particles also creates ample void spaces to retain moisture and make oxygen available to the plants� The horticultural vermiculite comes in four grades from the coarse material
of particle size from 0�2″ to 0�3″ in diameter to the fine material with particle size of 0�04″ (1 mm)� Use the coarse grade for hydroponic growing and the fine to medium grade for seed germination� Be careful not to press the vermiculite together when adding water as that will destroy its structure�
12� Perlite: This comes from volcanic pumice� It is crushed and heated to expand the particles (Figure 11�1)� It is very light-weight and is sterile due to the process of heating� A coarse particle size between 2 mm and 3 mm (0�065-0�13″) is best for hydroponics� Particles are irregular, but more structurally stable than that of vermiculite� It contains no nutrients and has a pH of 6�0-8�0� A fine grade is used for seed germination while the coarse grade is best to use in plant growth� It may be used by itself or in mixtures with peat as explained earlier with peatlite mixtures�
13� Coco Coir: Coco coir is ground-up dried coconut palm husk� The processed material includes coir fibers and pulp� Most comes from Indonesia, India, Sri Lanka, Thailand, and Brazil� This substrate is now becoming the principal one used in large greenhouse operations basing their production on “sustainable” agricultural technology� The reason for that is that coconut husks are a renewable product�
A number of different forms of coco coir are available in the market� There is a compressed bale or block that expands upon the addition of water� The blocks will expand to double their size within 15 min of soaking in water� A product created especially for greenhouse hydroponics is the slab form where the coco coir is encased in a long plastic bag (Figure 11�2)� It measures about 1 m (39�4″) long by 6-8″ wide by 3-4″ thick� These slabs are aimed at growing vine crops such as tomatoes, cucumbers, peppers, and eggplants� They are available in different grades of coco coir with varying percentages of the coir pulp and fibers� This imparts a series of air-holding capacities from 20% to 40% air capacity at saturation� The choice depends upon the crop to be grown� Slabs of half husk chips and half coco pith are recommended for tomatoes and peppers while those of 100% husk chips are for cucumbers� Coco coir plugs and blocks are also available to start and transplant seedlings�
Coco coir can be mixed with perlite, rice hulls, or vermiculite similar to the peatlite mixtures described earlier� One fact to consider is the source of the coco coir as that from coastal areas may contain sodium chloride� In that case, it is necessary to flush the coco coir with pure raw water to remove the sodium chloride� Most manufacturers do that during processing so their products will not require initial flushing�
14� Rockwool: Rockwool is made from basaltic rock (solidified lava) that is liquefied at 1500°C (2732°F)� It is spun and then pressed into sheets that are cut into slabs, blocks, and cubes (Figure 11�3)� The slabs have similar measurements to coco coir slabs� Rockwool is slightly alkaline but inert and does not decompose� With 95% pore spaces, it has good water-holding capacity� The pH between 7 and 8�5 must be adjusted before seeding or
planting by saturation with an acid nutrient solution to reach an optimum pH of 6�0-6�5 for most vine crops� A drip irrigation system is used as is the case with coco coir, peatlite, perlite, and other fine mixtures� Managing the irrigation cycles is crucial to success as rockwool needs a 20%–30% leachate (drainage) to prevent any mineral build-up in the substrate� Emphasis now is on recycling the leachate back to a tank where its pH and electrical conductivity are adjusted with an injection system before its reuse in the crop� Special trays are now on the market to collect the leachate from the
slabs and return it to a central batch adjustment tank� The same principle of recirculation of spent nutrient solution with rockwool applies to home gardening� These details are described in Section V�
Rockwool cubes and blocks are the standard method of starting your plants for hydroponic culture� Even if you do not use rockwool as a final growing medium, it is still best to start your seedlings in rockwool cubes that are transplanted to larger rockwool blocks before their final transplanting to the growing area� Rockwool cubes are available in 1½″ × 1½″ × 1½″ size for tomatoes, cucumbers, peppers, and eggplants, and 1″ × 1″ × 1½″ deep smaller ones for lettuce and herbs� The blocks come in several sizes as well, with larger ones for plants that you wish to keep in the seedling area longer� They are available as 3″ × 3″ × 2�5″, 3″ × 3″ × 4″, 4″ × 4″ × 2�5″, and 4″ × 4″ × 3″� The blocks have round holes of 1½″ diameter by 1½″ deep to fit the cubes during transplanting�
Loose rockwool granules can be used in containers and/or mixed with other media of larger aggregates such as scoria to improve moisture retention�
Do not confuse rockwool with insulating material, such as the pink or sandy color bats for your home, since it is not suitable for growing plants� It absorbs a lot of water and remains saturated excluding oxygen from plant roots� It also collapses in on itself structurally when squeezed�
15� Water: The use of water alone is the choice of medium for growing lettuce, basil, arugula, and some herbs� The systems discussed later are those of the raft culture and nutrient film technique (NFT)� In these systems, the plant roots are suspended directly into the nutrient solution� A supporting cover of Styrofoam boards (raft culture) and channels or gutters in the case of NFT keep the plants above the nutrient solution (Figure 11�4)�
Water, of course, is the medium for the essential elements to form the nutrient solution� Water must be very pure with few or no extraneous elements� It must not have sodium chloride present in excess of 50 ppm� If your source of water is from a city reservoir, it will be safe to use in hydroponics� If it comes from well water, you will need to have it tested to determine what elements are present and at what levels� Once you know the levels of the elements, you can determine if it is necessary to adjust the nutrient formulation to take these into account� You simply subtract the concentration of each of the elements present from the total you wish to add and calculate the balance of each you must add to get your optimum formulation� Often raw water is “hard�” That means that there is calcium and magnesium carbonates present, so the pH will have to be lowered with an acid� Remember that both calcium and magnesium are essential macro-elements; therefore, you will adjust the levels to add by those levels present in the raw water� In effect, this will save on the amount of calcium and magnesium used in the nutrient solution� Be particularly aware of any micronutrients that are present in raw water, especially boron as sometimes there is sufficient in the raw water that it will not have to be added to the formulation�
If the water is just hard, you can purchase a prepared nutrient formulation specifically for hard waters assuming that you may not wish to make up your own formulation�
Water analyses are offered by a number of laboratories that also do soil and tissue analyses� Please refer to the reference section for specific websites� Whether growing in soil or hydroponics, it is important to know the elements and their concentrations in the raw water in order to adjust your fertilization program to avoid excess levels of any element� With hydroponics, it is much easier to make these adjustments to your nutritional program compared to soil growing�
