Growing Media for Greenhouse Crops
Properties of Growing Media
Most growing media are composed of a combination of one or more components, such as sphagnum peat moss, perlite or pumice-type materials, coir, bark or wood products, vermiculite, and other materials. The physical and chemical properties of the media need to be considered in order to provide the plant with the best possible environment for greenhouse plant production.
Growing Media Physical Properties
Bulk Density
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 (lb/ft3), pounds per cubic yard (lb/yd3), 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 than wet light mixes.
Water-Holding Capacity
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 that create very small pores or air spaces, the growing medium retains large quantities of water.
Air-Filled Porosity
Total porosity is the measurement of the growing medium’s total volume of these pore spaces. Air porosity refers to the measurement of the volume of pore space in a growing medium occupied by air after it is saturated and allowed to drain. Total porosity and air porosity are important for the movement of water and nutrients and for gaseous exchanges within and throughout the root zone. Plant roots need proper air porosity so that the transfer of oxygen and other gases can take place. Coarse particles in the growing medium create larger pores that maintain air space and facilitate drainage.
Media Stability
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. 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. Cation exchange capacity 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¯), sulfate (S04¯), and phosphate (PO43¯).
pH
Media pH is a measurement of the acidity or alkalinity 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
Electrical conductivity (EC) is used to measure total dissolved salts (K+, Ca2+, NO3¯, NH4⁺, Cl¯, etc.) in the substrate and is commonly expressed in units of deciSiemens per meter (dS/m). High substrate EC levels can be a sign of too much sodium and chloride in the substrate or the substrate itself, or it can result from over-fertilization. The higher the ion concentration in the substrate solution, the higher the EC.
Carbon-to-Nitrogen Ratio
Most greenhouse media are formulated primarily from organic materials, such as sawdust, coir, compost, bark, and peat moss. The C:N ratio 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.
Click on the following topics for more information on growing media for greenhouse crops.
