Growing Media for Greenhouse Crops
Properties of Growing Media
Successful greenhouse production of container-grown plants is largely dependent on the physical and chemical properties of the growing media. A number of critical physical and chemical properties need to be evaluated before making a final media decision.
Growing Media Physical Properties
Bulk density is the mass per unit volume of a substrate and is expressed as grams per cubic centimeters (g/cm3), kilograms per cubic meter (kg/m3), pounds per cubic foot (lbs/ft3), pounds per cubic yard (lbs/yd) or by any other weight to volume units. In other words, it is the dry weight of the substrate that occupies a certain volume when the medium is moist. In practical situations, bulk density is important since wet, dense potting mixes are heavier that wet light mixes.
Water-holding capacity is the proportion or percentage of a substrate’s pore space that remains filled with water after gravity drainage. A good growing medium has a high water-holding capacity but also contains enough macropores to allow excess water to drain away and prevent waterlogging. Water-holding capacity varies by the types and sizes of the substrate ingredients. If the growing medium in the container is made up of materials which create very small pores or air spaces, the growing medium retains large quantities of water.
When we fill a container with media, the total volume of space in that container is filled with two things: the solid media components and the spaces or voids between all the solids. Ideally the total volume of empty pores should be in the range of 50 to 70 percent. This is referred to as total porosity. The total porosity of a media is further composed of two parts: air and water. Both components are critical for good plant growth, but not enough of either can limit growth.
The stability of a planting substrate needs to be considered. It is undesirable to use a substrate that decomposes quickly, since this will result in a dramatic reduction in substrate porosity. The need for stable substrates is especially important for larger containers, which require more than a growing season for the plant to reach a marketable size.
Growing Media Chemical Properties
Cation Exchange Capacity
Cation exchange capacity (CEC) is the total capacity of a soil to hold exchangeable cations (CEC). Substrates such as clay, silt, organic matter, and vermiculite have fixed negative electrical charges that attract and hold positively charged nutrient ions, known as cations. CEC can vary widely depending on the type of component. Cations associated with plant nutrition are calcium (Ca2+), magnesium (Mg2+), potassium (K+), and ammonium (NH4+), listed in order of decreasing retention in the substrate. In addition to cations, plants also require negatively charged anions such as nitrate (NO3-), chloride (Cl-), sulphate (S04-), and phosphate (HPO4).
Media pH is a measurement of the acidity or alkalinity of a substrate. Substrate pH is a critical issue because it plays a major role in determining the availability of many nutrients. In an acid substrate, calcium and magnesium, nitrate-nitrogen, phosphorus, boron, and molybdenum are deficient, whereas aluminum and manganese are abundant, sometimes at levels toxic to some plants. Phosphorus, iron, copper, zinc, and boron are frequently deficient in very alkaline substrate. Most greenhouse crops grow best in a slightly acid pH range of 6.2 to 6.8 in soil-based substrate and 5.4 to 6.6 in soilless substrate.
Electrical conductivity (EC) is used to measure total dissolved salts (free ions) in the substrate and is commonly expressed in units of deciSiemens per meter (dS/m). High salt concentrations cause problems by changing the osmotic potential of the substrate, causing water to leave plant roots and flow into the medium. Therefore, affected plants will grow poorly since they are in essence suffering from drought or water stress.
Most greenhouse media are formulated primarily from organic materials, such as saw dust, coir, compost, bark, and peat moss. The C:N ration of the organic materials is a good indicator of whether nitrogen will be limiting or excessive; the higher the C:N, the higher the risk of nitrogen being unavailable to plants. If the amount of carbon exceeds the amount of nitrogen from the organic materials, microorganisms use the nitrogen intended for the plant in the container.
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