Chapter 11

Growing Media for Greenhouse Crops

Common Media Amendments

A variety of amendments may be added to growing media during the mixing process, including limestone, iron sulfate, wetting agents, fertilizers, and biological control agents for controlling diseases. If the decision is made to use amendments, uniform incorporation is important because plant roots have access to only a limited volume of growing media in the relatively small containers used in greenhouses. Uneven mixing of incorporated fertilizers is one of the major factors causing uneven growth in container greenhouse stock.


In most cases, greenhouse growers need to be concerned about raising the substrate’s pH since most of the organic substrates are acidic (e.g., sphagnum peat). The most commonly used material is either calcitic (CaCO3) or dolomitic limestone (mixture of CaCO3 and MgCO3). Generally, calcitic limestone is more reactive than dolomitic limestone, thus calcitic limestone will adjust (raise) substrate pH faster and may raise the substrate pH higher than the same amount of dolomitic limestone.

Application Rates

The amount of lime required to adjust the pH will depend on the starting pH; the desired pH, the particle size of the limestone (i.e., small particles faster acting than large ones); the type of substrate; and the alkalinity of irrigation water used. Agricultural and horticultural limestones are considered soft crystals that react quickly with acid. This is more desirable for adjusting media formulated with peat moss, pine bark, or coconut coir. Therefore, determining the rate of limestone to incorporate into a media to change its pH depends on the substrates used as well as the limestone’s properties (type, particle size, and hardness).

Iron Sulfate, Aluminum Sulfate, and Elemental Sulfur

There are some media components that have a high pH that usually require the addition of iron sulfate, aluminum sulfate, or elemental sulfur to lower the pH of the substrate. Iron sulfate and aluminum sulfate are relatively quick acting but have minimal residual effect. Elemental sulfur is slow-reacting but has more residual effect on substrate pH.

Wetting Agents

Wetting agents, also called surfactants, are used so that substrates 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.


Wetting agents are available in liquid and granular formulations. Liquid formulations mix readily with water to form true chemical solutions. Granular formulations use an inert “carrier” such as fine vermiculite to deliver the wetting agent to the substrate. When the medium is wet, the wetting agent readily dissolves off of the carrier and into the water. It then travels throughout the medium with the water.


Numerous components may be added to a substrate before use in order to increase the level of one or more mineral nutrients. These are usually added at rates that are designed to provide a low level of initial fertility for the developing crop (until the liquid fertilization program can take effect) and not to provide the entire nutritional need of the crop. For this reason, such amendments are referred to as starter charges. Some of the most common materials added as nutrient starter charges include calcium sulfate which is designed to provide calcium without causing an increase in substrate pH such as occurs with the addition of lime or ground limestone. Magnesium sulfate may be added to provide magnesium.

Biological Control Agents

There are numerous biological products that have been designed to be added to substrates for greenhouse crop production. These include such products as Rootshield, SoilGuard, Mycostop and Actino-Iron.

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