By Troy Buechel
Formulating a growing medium that works for organic growers can be a challenge depending on what a grower is looking for. Organic growers often choose to use composts, peat, bark and other materials to make an organic growing medium. Some components are acidic and will require pH adjustment, and most organic components are hard to wet with water.
So, which components are the best and why?
Concerns with Compost
Compost can work well in a growing medium (Figure 1). Most provide plant nutrients in varying levels, but have poor physical properties. Properly composted materials consist of fine particles that plug up air-filled pores in the growing medium, thereby stressing plant roots due to lack of aeration in the growing medium (Table 1). Watering is critical as overwatering can quickly make a compost-based growing medium water-logged with a very slow dry-out rate. This is especially a problem when starting vegetables or herbs in the winter/early spring when slow drying of the growing medium can increase the potential for root disease, not to mention there is greater potential to attract fungus gnats which feed on rich organic material, such as compost, or shore flies that are looking for wet growing media to reproduce.
Another major concern with compost is inconsistency. Unless there is a reliable and reproducible way to produce compost each year, it often has variability that can lead to unpredictable growth or varying availability of nutrients. The materials used to make the compost (leaves, grass clippings, food scraps, manures, etc.) must be the same each time as each has different nutrient composition, texture, pH and soluble salts.
Ideal Media Components
For most growers, eliminating variables is a key to successful growing. Traditional organic materials such as sphagnum peat moss, softwood bark (Figure 2), coir (Figure 3), perlite (Figure 4), vermiculite (Figure 5), expanded clays, etc. have been and continue to be used to create predictable, consistent growing media from year to year. These materials have better physical properties because they are coarser than compost, soil or sand, so they can create larger, air-filled pores, reducing potential water logging (Table 1). They work together to allow for faster dry-out between waterings, so they are more forgiving when overwatered, especially during the cloudy winter and early spring months. The chemical properties of these materials are consistent because, unlike compost and coir, they provide few mineral nutrients that can fluctuate, so there are no “hot or cold” pockets of nutrients in the growing medium.
One of the preferred properties of compost is its biological properties. Compost is teaming with various beneficial microorganisms that can convert complex organic molecules into useable plant nutrients. Soilless media formulated with sphagnum peat moss, coir, bark, perlite, etc., have low populations of these microorganisms. Many growing media companies are adding biological additives to their organic products to improve biological diversity.
One such active ingredient is MYCORRHIZAE (mycorrhizal fungi), a natural fungus that comes standard with most PRO-MIX growing media. Mycorrhizal fungi colonize plant roots to obtain sugars from the plant and in turn acquire nutrients and water for the plant where its roots are not present. Mycorrhizal fungi have been shown to secrete enzymes that aid in the “release” of useable plant nutrients from organic molecules.
Another group of active ingredients that are often incorporated into growing media are biofungicides*. Biofungicides* can be natural fungi, bacteria or actinomycetes that colonize plant roots and then suppress root disease pathogens. They reduce root disease and plant loss, making disease management less laborious.
To introduce microorganisms that can help break down complex organic molecules, the growing medium can be drenched with compost tea. Compost teas provide nearly the same microorganisms as those found in compost.
Since sphagnum peat moss and bark are acidic, limestone is typically added to increase the pH of a growing medium. Limestone can be used in certified organic growing and is still the best material for pH adjustment of growing media.
Sphagnum peat moss, bark and possibly compost can be hard to wet with water. Traditional growing media have wetting agents incorporated to help hard-to-wet components absorb water, but they cannot be used for certified organic growing. Yucca-based wetting agents are used as many are approved for certified organic growing. Since they are plant-derived, microorganisms in the growing medium use them as food, causing rapid degradation of the wetting agent. It is best to use an organic growing medium within 2 months (during the summer months) to 4 months (during the winter) after manufacturing to ensure that it properly absorbs water.