Growing Media for Greenhouse Crops
Types of Substrates
Today, growers can select from a wide range of materials, including soil, peat moss, coconut fiber (coir), composted bark, wood, perlite, vermiculite; and from various substrate additives that best fit the needs of their crops. Most substrates used today are soilless or contain only a small fraction of soil. Soil, if not pasteurized, will add weed seeds and pathogens to the substrate. Soil will also reduce the substrate’s drainage and will significantly increase its bulk density. For these reasons, soilless substrates are recommended for container production. Some of these materials can be used alone to grow crops, but more often, growing media are made with different portions of various materials, each contributing to the chemical and physical properties of the final product. The choice of substrates will depend on the location of the greenhouse, the resources available, and plant requirements.
The organic substrates improve the physical structure of the substrate by reducing weight and increasing its water-holding properties. They are also resilient to compaction. Organic matter has a high CEC and can store nutrients until needed by the plants. Some organic materials, such as compost, can contain considerable amounts of nutrients. Peat is the most popular organic component, but because of the destruction of valuable biotopes for the harvest of peat, alternative materials with similar physical and chemical properties are sought.
Soil is used as a substrate by some greenhouse operations but not as much in recent years for a number of reasons. The weight of soil increases handling and shipping costs in containerized plants. Topsoil supply, uniformity and quality are difficult to maintain. Soil invariably comes from an agricultural setting, and with it comes disease pathogens. Thus, all soil-based substrates need to be pasteurized while soilless substrates are not generally pasteurized.
Peat is a main component of most soilless substrate mixes used today. It is produced by the partial decomposition of plant material under low-oxygen conditions found in peat bogs. These anaerobic conditions slow down bacterial and chemical decomposition and often peat is many thousand years old. The majority of peat moss used for horticultural purposes in the U.S. is from Canada or the southeast United States. The terms peats generally refer to several materials that are similar in origin but distinctly different in their composition and physical and chemical properties. Peat is classified into four distinct types 1) sphagnum moss peat; 2) hypnum moss peat; 3) reed and sedge peat; and 4) humus peat. Differences in peat are related to the climate under which they are produced and the species of plant from which it is formed. All peats have good water-holding capacity, high CEC, low level of nutrients, and low pH (around 3 to 4.5).
Bark, a byproduct of saw mills, is used extensively as a substrate in greenhouses. It functions to improve aeration and reduce the cost of substrate. Bark variability stems from the species and age of tree, method of bark removal and degree of decomposition. When used in container production, bark can be used alone or amended with up to one-fourth (by volume) peat moss to help with water holding capacity (to hold water better). Bark particles of less than 3/8 inch (9.5 mm) in size are used in greenhouse media. In general, nutrient content and pH (3.5 to 6.5) of unprocessed bark are low. Normally, adding dolomitic lime is needed to bring the pH up to a desirable level.
Pine Bark. Pine bark (See Figure 9.2) is preferred over hardwood bark since it resists decomposition and contains fewer leachable organic acids. Pine bark is usually stripped from the trees, milled and then screened into various sizes. A good potting medium usually consists of 70 to 80 percent (by volume) of the particles in the 1/42- to 3/8-inch (0.6 to 9.5 mm) range with the remaining particles less than 1/42 inch. If pine bark is too coarse, water retention will not be adequate for plant growth.
Hardwood Bark. Hardwood bark is a commonly used, excellent substrate ingredient. Hardwood bark should be mechanically processed to small particles, which will pass through a 1/2-inch (12.7 mm) mesh screen, with 10 percent of the particles larger than 1/8-inch (3.2 mm) diameter and 35 percent less than 1/32-inch (0.8 mm) diameter. Hardwood bark differs greatly from pine bark in its chemical and physical characteristics.
The substrate coir is derived from the husks of coconut fruit (See Figure 9.3). After most of the fibers are removed, the remaining coir, or coir dust, is marketed for substrate. Chemical and physical properties of the coir are variable, depending largely on the amount of fiber remaining in the material. Its physical and chemical properties are probably closer to peat moss than any other existing substrate material. Coir has been used as a partial substitute for peat moss because of environmental concerns. Coir holds moderately less water than peat moss but is better aerated. Due to less shrinkage of coir compared to peat moss during crop production, pots of coir and peat moss hold a similar amount of available water.
Sawdust, wood shavings, and wood chips constitute a rather broad category of wood particles generated by sawmills and other wood processing industries, often involving a wide range of particle sizes and several tree species. Sawdust has characteristics that make it desirable for use in a growing mix. It has a bulk density slightly less than sphagnum peat moss, has similar water retention but greater air space after drainage than pine bark.
Animal Manure Composts
Non-composted animal manure is rarely used in greenhouse production today. Although manures do contain most essential nutrients for plant growth, the concentration of elements varies considerably with the animal, mulching material used (straw, etc.), the technique of manure collection and storage, and manure age. In addition, fresh manures are not pleasant to handle, considering the odor and the high water content. Some potential dangers in using raw manure include: soluble salt damage from high nutrient content, ammonia damage to roots and foliage, and weed seeds, insects, pathogens and nematodes contained in non-pasteurized or non-fumigated manures. However, considerable effort is being put forth to develop methods for using composted animal manure as a component in soilless media. This is driven first by the need to provide organic certified media to growers of organic floral and vegetable products and second by the need to reduce the use of peat moss in media in general. Composts of manures from dairy cows, horses, and poultry show promise for use in growing media.
Composting. Composting is the physical and chemical decomposition of organic materials caused by the digestive activities of insects, fungi, and bacteria. Ideally, compost is made from a mixture of animal manures. Mature compost should not produce an unpleasant odor. The compost should be dark in color and have a rich, earthy smell. The texture should be friable and crumbly; the original organic materials should not be recognizable.
Carbon to Nitrogen Ratio. C:N is a good indicator of whether nitrogen will be limiting or excessive (Landis and Morgan 2009). The higher C:N is, the higher the risk of nitrogen being unavailable to plants.
Rice Hulls. Rice hulls used in growing media are parboiled and then dried before use (See Figure 9.4). This heat process kills weed and viable rice seeds. The primary function of rice hulls is that of drainage and aeration. Large particle sizes of whole parboiled fresh rice hulls can increase the drainage and air-filled pore space in peat-based substrates without causing significant nitrogen immobilization. Rice hulls provide a less expensive substitute for perlite and add a higher level of aeration than gained by an equal amount of perlite in the substrate.
Cotton Gin Trash. Cotton gin trash contains mainly the cottonseed hulls after the linters and the kernels inside of the hulls are removed. Composted gin trash can increase the water and nutrient-holding properties of the media. High soluble salts and high pH are potential concerns but, this can be corrected through leaching with water and less addition of lime.
Inorganic components improve the physical structure of a substrate by increasing the aeration pore space and the drainage properties. Many inorganic materials have a low CEC and provide a chemically inert base for the substrate. Inorganic materials with high bulk densities provide stability to large, freestanding containers.
Vermiculite, an aluminum-iron-magnesium silicate, is a mica-like mineral which, when heated above 1,400 degrees F, expands to an open-flake structure that provides spaces for air and water (See Figure 9.5). Vermiculite particle size is determined by the particle size of the ore, prior to heating. There are several grades or particle sizes of vermiculite used by greenhouse operations. The finer grades are generally used in mixes formulated for small pots and plug tray applications, while coarser grades are usually found in mixes designed for larger containers. Due to the range of pore spaces of processed vermiculite, it retains considerable moisture upon wetting. The pH of most of the vermiculite used in horticulture falls within a range of 6.0 to 8.9. In some ways vermiculite is similar to perlite in that they both originate as mined minerals that are then heated to produce a finished product.
Many calcined clays have properties, which make them desirable as potting substrates (See Figure 9.6). Calcined clays are essentially indestructible particles, which provide non-capillary pore space to a mix due to the large spaces created between particles, and hold water internally within their open-pore particle structure. Most calcined clays have good cation exchange capacity (6 to 21 meq/100 g), which helps in the retention of nutrients but have no nutrient value of their own.
Sand is the most common addition to growing media. Sand is nothing more the finely ground stones. It has the same chemical content as the parent material. The composition of sand varies widely. Sands derived from calcareous sources (such as coral or limestone) are high in calcium carbonate (CaCO3), however, and can have dangerously high pH values. Some plants grown in the greenhouse may be adapted to local calcareous soil conditions and may not suffer from the increased pH if the sand is used sparingly. Deep-mined, white mountain sands are mainly silica. They are usually free disease organisms, insects, and weed seeds.
Perlite is a volcanic rock that is crushed and heated rapidly to a high temperature (See Figure 9.7). The material expands to form a white, light-weight aggregate with high pore space. Perlite is utilized extensively for its light weight, physical stability and ability to improve the drainage or aeration. Water-holding capacity is fairly low, as water is retained only on the surface and in the pores between particles, but it is not absorbed into the perlite aggregates. It is chemically inert with almost no CEC or nutrients, and a neutral pH.
Polystyrene foam is known more commonly by trade names such as Styrofoam, Styropor, and Styromull. Like perlite, it constitutes a good substitute for sand, bringing improved aeration and light weight to growing media. It is a white, synthetic product containing numerous closed cells filled with air. It is extremely light. Like sand, it does not absorb water and has no appreciable CEC. It is neutral and thus does not affect growing media pH levels.
Rockwool is manufactured from a mineral called basalt through a heating and fiber extrusion process (See Figure 9.8). Although rockwool is utilized primarily for insulation, it can be utilized as a rooting medium by itself or in combination with other ingredients, such as peat, bark, and perlite to make a soilless growth medium. A distinct characteristic of rock wool is it high air-holding capacity even when fully saturated.
Growing Media Additives
Two types of limestone are commonly available to growers used to raise the pH of greenhouse media: calcitic limestone, which contains calcium carbonate (CaCO3) and dolomitic limestone, which contains calcium magnesium carbonate [CaMg(CO3)]. Generally, calcitic limestone is more reactive than dolomitic limestone, thus calcitic limestone will adjust (raise) substrate pH faster and may raise the media pH higher than the same amount of dolomitic limestone.
Wetting agents, also called surfactants, are used so that media will wet out (“hydrate”) uniformly when watered. This is very important when the medium is relatively dry, as it often is when brand new, or when it fails to get watered on a regular schedule. Many times when a containerized (potted) plant is watered, the water will “pond” or “puddle” on top or channel down the insides of the container without readily infiltrating the medium. Both of these situations illustrate common examples of the need for a wetting agent. By adding a small amount of wetting agent to the medium or irrigation water, “ponding” on top or “channeling” down the insides of the container can be rapidly and effectively eliminated for several weeks.
Commercial Media Formulations
Premixed media is a common sight in the greenhouse industry. Suppliers offer a diversity of mixes in either pre-packed (bags, bales, super sacks) or bulk forms. Recipes are specially formulated for propagation, specific crops (e.g., Poinsettia mix) or general crops. In addition to the major substrate components (peat moss, bark, perlite, and vermiculite), starter nutrients (sometimes including micronutrients), lime (dolomitic, calcitic lime, or both), and wetting agents are common components of the premixed media recipes.
Self-blended Media Formulations
The decision for a greenhouse operation to mix its own media, as opposed to purchasing commercially blended formulations, is basically an economic one. Costs include mixing equipment, raw materials, skilled labor, and quality control testing. In general, mixing media from scratch is not considered economical for small growers. Generally, 50 to 75 percent of the media will be peat or coir. Bark, perlite, vermiculite, rock wool and polystyrene may add to the remaining 25 to 50 percent. Lime and fertilizer amendments (including micronutrients) should be selected on the basis of the chemical properties of the substrate. Wetting agents should be added by following the manufacturer’s directions, since high concentrations may be toxic to plants.
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