Reprinted from Grower Services Newsletter by Premier Tech Horticulture (June 2016)
Growing media are one of the most widely used materials for growing greenhouse crops. With many different formulations available to growers, it can be a challenge to choose the best blend to use. Understanding the composition, functions and intended use can make the selection process easier.
What are the functions of a growing medium? A growing medium holds water and nutrients, it is a place where gas and nutrient exchanges occur, and it also provides an anchor point for a plant's roots. These physical characteristics of a growing medium are determined by the components used and the proportions in which they are blended together. What is important to remember is that the resulting physical characteristics do not equal the sum of the ingredients.
First, let's look at the components used in formulating growing media. Then, we will focus on the characteristics of growing media, and finally on biological additives.
Components: Growing media components are either organic or inorganic. Organic components include peat moss, bark, coconut coir, rice hulls, etc. Inorganic components include perlite, pumice, vermiculite, sand, hydrogel, etc. Some of these components hold water on their surface, others hold water within their structure, while others hold little compared to other components. There are also components, such as perlite, that hold very little water, if any. Keep in mind that a specific type of ingredient can vary in its water holding capacity and physical structure, depending on its origin and how it is processed. For example, bark can vary greatly in its source and structure depending on how it is processed, aged, composted and screened. This is also true for peat moss. Light brown, fibrous peat moss has a porous structure and can hold up to 16 times its weight in water. However, if this same peat moss is processed into fine particles, the available water can be cut in half and the air porosity decreases dramatically. If you blend your own growing medium, your source materials should be consistent to produce a quality and predictable growing medium. It is important to know the structure as well as the chemical and physical properties of the ingredients you use to be sure that the growing medium blend you produce is the same, batch after batch.
"Common organic components used in growing media. Starting in the upper left, going clockwise: bark, sphagnum peat moss and coir. Source: Premier Tech Horticulture."
"Common inorganic components used in growing media. Starting in the upper left, going clockwise: vermiculite, perlite and rice hulls. Source: Premier Tech Horticulture."
Physical Characteristics: While there are a number of laboratory tests for the physical characterization of growing media, the three most familiar measurements are bulk density (weight per volume), water holding capacity and air porosity.
Water holding capacity is the volume percentage of water retained after a saturated growing medium is allowed to drain.
Air porosity is a measurement of the volume of pore space occupied by air after a saturated growing medium is allowed to drain. For the most part, packaged growing media products have a low bulk density, since the majority are made with a base of sphagnum peat moss and have a higher water holding capacity. Bark-based media are heavyweight products that are suitable when high drainage and container stability are required. Both products typically have good air porosity which is in a range of 10% - 18% by volume for most growing media.
Chemical Characteristics: Two important measurements for growing media are pH and E.C. (Electrical Conductivity). pH is a determination of how acidic or basic a substance or solution is. E.C. reading measures the ability of soil solution to carry an electrical current and is an indication of the amount of nutrients available for crops to take up. For general purpose growing media, the ideal pH range is between 5.2 - 6.2 with a target of 5.8 when saturated. Desirable E.C. for general purpose growing media is between 1.0-2.0 mmhos/cm. For seed germination and rooting of cuttings, the desired pH range will be slightly lower, between 5.0 - 6.0, with a target wet-out at 5.6. This pH range is slightly lower since pH can tend to rise during use from minimal fertilizer applications and water alkalinity of irrigation water from constant misting. Desirable E.C. for germination and propagation growing media is between 0 - 1.0 mmhos/cm.
"A combination E.C. and pH meter is used to measure these parameters of a growing medium sample. Source: Premier Tech Horticulture."
Most commercial growing media are pH-adjusted with either calcitic or dolomitic limestone and contain a balanced starter fertilizer to help plants acclimate after planting. It is generally recommended to begin fertilization once new plant leaves begin to emerge and new roots develop. The amount of fertilizer and frequency of applications will vary based on the crop type, stage of development, container size and frequency of plain water applications. Keep in mind that some ingredients used in formulating growing media may contain mineral salts, such as coir. It is recommended that coir should be leached thoroughly before use to reduce salt levels and potentially high nutrients (e.g.: potassium, chloride, sodium). The same is true for bark, since aging and composting can release undesirable elements. To be sure, laboratory tests are recommended to check pH, E.C. and individual nutrient levels.
Following are some of the physical and chemical properties of PRO-MIX products:
Active Ingredients: Something unique to the line of PRO-MIX products is that they are available as standard products or blended with active ingredients, which include BIOFUNGICIDE™*, MYCORRHIZAE or both.
BIOFUNGICIDE™* is a bacterium that suppresses plant root pathogens that cause damping off, such as Fusarium, Pythium and Rhizoctonia. This offers growers an alternative to applying chemical fungicides to control these root diseases.
MYCORRHIZAE is a beneficial fungus that attaches to plant root systems and acquires nutrients and water for the plant so that it grows healthier, faster and have a more prolific flower and fruit production. For benefits from both of these active ingredients, look for PRO-MIX products formulated with BIOFUNGICIDE™* + MYCORRHIZAE.
*All BIOFUNGICIDE™ products are available under BIOSTIMULANT in Canada and in Latin America.
What is the cost to mix your own?
Many growers buy a pre-formulated growing medium, but some choose to make their own. Purchasing a pre-formulated growing medium provides peace of mind because the manufacturer has done all the blending and consistency checks for you. If you make your own growing medium, have you considered the time and effort to make it? Does your blend deliver consistent and predictable results every time you make a batch? How much does it really cost to blend your own and what is the return on your investment (ROI)? If these questions raised your eyebrows a bit, simply follow this link for more information. This tool is designed to help growers determine the cost of making their own mix.