Growing Media for Greenhouse Crops
Growing Media Components
Growing media for use in container production in greenhouses contain a variety of organic and inorganic ingredients. Organic ingredients include peat moss, bark, coconut coir, rice hulls, etc. Inorganic components include perlite, pumice, vermiculite, sand, rockwool, etc. Field soils are generally unsatisfactory for producing plants in containers because soils do not provide the aeration, drainage, and water-holding capacity required, and they need to be pasteurized or fumigated to prevent diseases and weeds. Except for organic growers, there are relatively few commercial operations that directly use soil. Some of these media components can be used alone to grow crops such as rockwool, 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 media will depend on the cost, availability, and plant requirements. Most commercial greenhouse media for container crop production contains 30 to 60 percent peat moss alone or in combination with composted pine bark. Other materials, such as vermiculite and perlite, are added to affect water retention and aeration. Mixtures of organic and inorganic components are popular because these materials have opposite, yet complementary, physical and chemical properties. Growing media are designed to achieve high porosity and water retention while providing adequate aeration. A nutrient charge is added, the pH is adjusted, and a wetting agent is generally added to the media to improve initial wetting.
Organic Media Components
Peat
Peat is a main component of most soilless substrate mixes used today. Peat consists of the remains of aquatic, marsh, bog, or swamp vegetation that has been preserved under water in a partially decomposed state. The composition of different peat deposits varies widely, depending upon the vegetation from which it originated, state of decomposition, mineral content, and degree of acidity. Most of the peat moss used for horticultural purposes in the U.S. is from Canada or the southeast United States.
Sphagnum Peat MossSphagnum peat moss (Figure 12.2) remains the premier component of greenhouse media because of its high-water holding capacity, adequate air space, high cation exchange capacity, and resistance to decay. Sphagnum peat moss is light brown in color (dark brown when wet), and the sphagnum plant structure is still visible. It has superior properties of stability over time, light weight, and high water and nutrient-holding capacities. Sphagnum peat moss is the most acid of the peats, with a pH level of 3.0 to 4.0, and requires 14 to 35 pounds of finely ground limestone (dolomite) per cubic yard (8–20kg/m3) to bring the pH up to the level that is best for most crops.
Shredded Bark
Shredded or pulverized softwood bark from redwood, cedar, fir, pine, hemlock, or various hardwood bark species, such as oaks and maples, can be used as an organic component in growing mixes and are frequently substituted for peat moss at a lower cost (Figure 12.4). Shredded bark improves aeration and reduces the cost of substrate.
Pine Bark. Pine bark 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 0.6 to 9.5 mm.
Hardwood Bark. Hardwood bark is commonly used and is an excellent substrate ingredient. Hardwood bark should be mechanically processed to small particles, which will pass through a 0.5-inch (12.7mm) mesh screen, with 10 percent of the particles larger than 0.125-inch (3.2mm) diameter and 35 percent less than 0.031-inch (0.8mm) diameter.
Coir
The substrate coir is derived from the husks of coconut fruit (Figure 12.5). 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 generally has a higher pH (4.9–6.8) than sphagnum peat moss, so it requires less limestone to adjust substrate pH.
Sawdust
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.
Rice Hulls
Rice hulls used in growing media are parboiled and then dried before use (Figure 12.6). 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. Rice hulls are naturally high in silicon, which decreases degradation. As a result, mixes that contain rice hulls will have a stable structure during production.
Soil
Soil is used as a part of a medium by some greenhouses. However, topsoil supply, uniformity, and quality are difficult to maintain, and soil must be pasteurized or fumigated. Pasteurization of some soils at high temperatures creates additional problems, such as manganese toxicity and an imbalance between ammonifying and nitrifying bacteria. The high bulk density of a soil medium increases handling labor and the cost of shipping plants.
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 (e.g., 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.
Inorganic Media Components
Vermiculite
Vermiculite, an aluminum-iron-magnesium silicate, is a mica-like mineral that provides spaces for air and water (Figure 12.7). When expanded, vermiculite is very light in weight, neutral in reaction with good buffering properties, and insoluble in water. It can absorb large quantities of water—3 to 4 gallons per cubic foot (40–54m3). Vermiculite has a relatively high cation-exchange capacity and thus can hold nutrients in reserve for later release. It contains magnesium and potassium, but supplementary amounts are needed from other fertilizer sources.
Perlite
Perlite is a volcanic rock that is crushed and heated rapidly to a high temperature, forming a white, lightweight aggregate with high pore space (Figure 12.8). Perlite is utilized extensively for its light weight, physical stability, and ability to improve drainage or aeration. The water-holding capacity of perlite is 3 to 4 times its own. It is chemically inert, with almost no CEC or nutrients and a neutral pH. Perlite, in combination with peat moss, is a very popular rooting medium for cuttings. It is much too light and causes too much of a reduction in bulk density to be used as a container mix component.
Calcined Clay
Aggregates of clay particles are heated to high temperatures (calcined) to form hardened particles, which make them desirable as potting substrates (Figure 12.9). Calcined clays are essentially indestructible particles that 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.
Sand
Sand is the most common addition to growing media. The composition of sand varies widely. Sands derived from calcareous sources (such as coral or limestone) are high in calcium carbonate (CaCO3); however, they 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.
Polystyrene Chips
This is a waste product created from polystyrene bead boards. 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
Rockwool, a lightweight hydroponic substrate, is made from spinning molten basaltic rock into fine fibers, which are then formed into a range of cubes, blocks, growing slabs, and granular products. Because rockwool and other stone or mineral wools used in horticulture originate from rock, they are considered by many to be a natural product. Rockwool is a widely utilized substrate in commercial horticulture for the production of crops as diverse as tomatoes, melons, cucumbers, peppers, strawberries, herbs, and cut flowers; however, it can also easily be used by smaller hydroponic growers wanting to take advantage of improved root zone technology.
Irrigation Management. A distinct characteristic of rockwool is its high air-holding capacity, even when fully saturated. One of the most important characteristics of rockwool is that plants are still able to extract water for growth at very low moisture tensions in the media. It is essential that the rockwool is allowed to drain completely so that excess nutrient solution, after being applied, leaves the slab or cube under the pull of gravity.
EC Management. The pH of rockwool is between 7.0 and 8.5 (often 8.0) but is not buffered. The pH level of the rockwool will adjust to the nutrient solution's pH level after one application. Checking the EC in the root zone is important with rockwool, just as it is with any other substrate.
Re-Usage of Rockwool. Rockwool, being essentially rock, does not decompose, fracture, or break down over time. Hence, growers can use it for many successive crops. However, it is recommended that rockwool is steamed or at least treated with boiling water before replanting to help prevent any carryover of root disease pathogens. A thorough leaching with clean water also helps remove any excess salts from the previous crop.
Forms of Rockwool. For this purpose, the material is almost exclusively formed into plugs, blocks, or slabs. Plants of most crops are propagated in small plugs or cubes (Figure 12.10) until the transplanting stage, when they are then transferred into blocks (Figure 12.11). These blocks are then placed onto slabs, where plants grow to maturity. A few crops, e.g., cucumber, are sown directly into blocks, which are later placed onto slabs (Figure 12.12). A wide range of blocks and slabs are currently manufactured, and they differ in density, fiber orientation, fiber diameter, size, and shape.
Click on the following topics for more information on growing media for greenhouse crops.
