Chapter 10

Growing Media for Greenhouse Crops

Properties of Growing Media

The success of a greenhouse growing media is mainly based on the behavior of the plants grown in them; high quality substrate with the proper physical and chemical properties can result in high yields and excellent plant quality. When plants are grown in containers, their roots are restricted to a small volume, consequently the demands made on the substrate for water, air, nutrients, and support are much more intense than those made by field-grown plants which have an infinitely greater volume of soil in which to facilitate root growth. Interactions between individual substrate properties, irrigation water applied, the environment the substrate is subjected to, and the cultural practices they undergo, influence substrate structure (settling, decomposing, etc.) through time and in turn, plant performance.

Growing Media Chemical Properties

In general, a good growing media must be chemically stable, which avoids any release of elements that can generate problems of salinity or phytotoxicity, or induce undesired precipitations in the solution.

Cation Exchange Capacity

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. The capacity of a substrate’s negative electrical charges to hold positively charged ions is known as cation exchange capacity (CEC). Cation exchange capacity is defined as the sum of exchangeable cations or bases that a medium can absorb per unit weight and is expressed as milliequivalents per 100 g (meq/100 g) substrate. Cations associated with plant nutrition are calcium (Ca2+), magnesium (Mg2+), potassium (K+) and ammonium (NH4+), listed in order of decreasing retention in the substrate. Many micronutrients are also adsorbed, such as iron, manganese, zinc, and copper.

pH

Substrate pH is a measurement of the acidity or alkalinity of a substrate. The pH of growing medium is a measure of its relative acidity or alkalinity. pH values range from 0 to 14; those below 7 are acidic and those above 7 are alkaline with a pH of 7.0 being neutral. A pH range of 6.8 to 7.2 is termed near neutral.  Substrate pH is a critical issue because it plays a major role in determining the availability of many nutrients. 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. 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.

Raising Substrate pH. In most cases, greenhouse growers need to be concerned about raising the substrate’s pH since most of the organic substrates are acidic. The most commonly used material is either calcitic (CaCO3) or dolomitic limestone (mixture of CaCO3 and MgCO3).

Lowering Substrate pH. Generally, growers are not required to lower the pH for organic substrates. If a high pH is an issue the problem is typically due to using an irrigation water source that has high alkalinity (>100 ppm CaCO3).

Carbon-to-nitrogen Ratio

Most greenhouse media are formulated primarily from organic materials, such as saw dust, coir, compost, bark, and peat moss, which mainly consist of carbon. The carbon-to-nitrogen ratio in a growing media is important when growing greenhouse crops. Microorganisms use the carbon from these organic materials to grow. Microorganisms require 1 pound of available nitrogen for every 24 pounds of available carbon from organic matter.

Electrical Conductivity

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. High substrate-EC levels in can be a sign of too much sodium and chloride in the substrate, the substrate itself, or it can result from over-fertilization.

Growing Media Physical Properties

Growing media is engineered to provide ideal physical properties for crops by looking at the substrate particle sizes, water holding capacity, air porosity, solid content, and the height and shape of the containers used.

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 (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.

Porosity

The total porosity of a growing medium is the sum of the space in the macropores and micropores; plants need both. A growing medium composed primarily of large particles will have more aeration and less water-holding capacity than a medium of smaller particles, which will have less aeration and more water-holding capacity.

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. A general range for water-holding capacity is between 20 and 60 percent. Specifying a narrower range depends on the plant species being grown.

Substrate 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.

Substrate Column Height and Containers

Another factor relating substrate to air/water relations in the root zone is the size of the growing container. In all containers, there will be a certain amount of saturated medium at the bottom of the container after drainage. This is due to what is called a perched water table. This perched water table is an area where all the pore spaces in the substrate are filled with water and occurs no matter how many drainage holes are in the container.

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