Soil-less potting mixes are widely used for the production of bedding plants and vegetable transplants. Since they don’t contain soil, there is a relatively low level of pathogenic microorganisms in this media, and that avoids problems with soil-borne diseases like damping-off. Soil-less mixes are also formulated to have physical and chemical properties that promote germination and rapid seedling growth.
Meeting the organic standards. Before the advent of the National Organic Standards, some certifiers allowed organic farms to use commercial soil-less mixes to start their transplants, so long as they were subsequently managed organically in the field. Other certifiers and organic growers shunned the use of commercial soil-less mixes because they contained small amounts of synthetic fertilizers and wetting agents. Today, all organic farmers must avoid using potting mixes that contain prohibited ingredients.
Years ago, this pretty much meant you had to make your own mix, but more and more commercial mixes have been developed in recent years that are acceptable in organic production. Such products often have “OMRI Listed” on their label. (OMRI is the Organic Materials Review Institute.) It is important that you confirm with your certification agent that any potting mix you are about to use is OK for organic production.
If you make your own organic potting mix, you still have to avoid using any prohibited ingredients, and that means checking out all the individual ingredients for their organic acceptability. It may surprise you to learn that products like peat moss or limestone are sometimes treated with prohibited materials such as wetting or anti-caking agents, so don’t rely on assumptions about purity. In addition to meeting certification requirements, your final product will also need to provide plant roots with the right amount of air, water and nutrients.
When you first start making your own potting mix, it’s a good idea to try several different recipes that have been proven to work for other growers, and compare how they do on your own farm. A publication that lists many organic potting mix recipes and provides information about mix ingredients can be obtained from ATTRA, the National Sustainable Agriculture Information Service at 800-346-9140, or see it on-line at http://attra.ncat.org/attra-pub/PDF/potmix.pdf.
Good compost is important. Compost is a key ingredient in most organic potting mixes, providing an organic source of nutrients, reducing reliance on the finite global supplies of peat, and supporting recycling of organic residues. In addition to using compost that meets organic standards, it is essential to use high quality compost that will result in good plant performance. Since compost can vary from batch to batch, there can also be variation in organic potting mix performance, even when it’s made using the same recipe. That’s one reason why conventional commercial potting mixes rarely contain compost.
When making or buying compost for your potting mix, try to have it ready at least six months before you’ll need it. For spring transplant production, compost should be made the previous summer and allowed to age through the fall and winter. That allows time for the compost to mature, which means that nutrients are stabilized, phytotoxic compounds have degraded, and in some cases, beneficial microbes have built up that can help suppress plant disease. It is important to test the compost you plan to use in a potting mix for nutrient content, pH, soluble salts, and indicators of maturity like C:N ratio. Compost tests and recommendations are widely available from soil testing labs. Several excellent guides to making and using compost can be obtained from NRAES, at (607) 255-7654 or www.nraes.org.
Compost is rarely used by itself as a potting medium. Compost alone does not have the optimal water holding characteristics, and soluble salt levels are often higher than optimal for potting mix. Plus, high quality compost can be relatively expensive so it makes sense to dilute it with other ingredients like peat. Organic potting mixes are typically made with 20% to 50% compost by volume, depending on the type of crop that will be grown in the mix, the container size, and the growing conditions.
Other ingredients. Sphagnum peat moss is a common ingredient in soil-less mixes because it is widely available, relatively inexpensive, and has desirable physical characteristics. It holds a lot of water and air and decomposes very slowly. Since peat is quite acidic, limestone is usually added to the mix to balance the pH. However, peat is not a source of plant nutrients in potting mixes. In some cases, compost can provide adequate amounts of phosphate, potash, and trace elements in the mix, but in other situations these nutrients will be need to be supplemented by adding bone meal, rock phosphate, sul-po-mag, greensand, kelp meal, and/or other approved organic fertilizers.
Nitrogen availability is usually the biggest challenge to organic potting mix formulation, as it can be quite variable, and N is often insufficient when compost is the sole nutrient source. Most organic potting mixes are supplemented with organic N sources such as alfalfa meal, blood meal, crab meal, etc. to assure vigorous seedling growth for several weeks or more. Some growers water with fish emulsion or other soluble organic N fertilizers to keep their transplants ‘growing on’ if a mix has run out of available nitrogen. Re-potting some plants, like tomato seedlings, is another way to keep them growing well.
When using blood meal, processed poultry manure fertilizer, or other relatively potent organic N sources, be aware that when first wetted and starting to break down, these materials may give off ammonia that can damage plants. It is best to moisten the potting mix at least a week before you plant into it, making sure it stays warm and well aerated during that time, allowing ammonia to be released and dissipate.
Avoiding performance problems. While many growers have had success with compost-based potting mixes, the performance of such mixes, whether commercially produced or homemade, has sometimes been inconsistent. In the worst cases, growers have experienced significant financial loss due to poor seedling growth associated with a problem mix. Poor seedling growth in compost-based mixes can result from low levels of available nutrients, high levels of soluble salts, excessive density of the mix, and/or the presence of compounds like volatile organic acids that can inhibit plant growth.
To prevent an unpleasant surprise, testing your organic potting mix well in advance or use is key. Send a sample to a soil test lab at a land grant university or at a private company that specializes in horticulture. I don’t recommend a regular field soil test, since potting mixes are very unlike soil in the field – they are much higher in organic matter and are also usually much higher in nutrient content. It’s better to get a greenhouse soil test that, at a minimum, measures pH, soluble salts (electrical conductivity), and all major elements, including nitrogen in the nitrate and ammonium forms.
A common test for media used in greenhouse production is the saturated media extract (SME) method. Unlike field soil tests that extract nutrients with weak acid solutions, in this procedure the sample is mixed with distilled water at a standard dilution and then analyzed. Since different labs may use different dilutions, stick with one lab.
Most university and commercial horticulture labs offer the SME test and provide information for interpreting the results, depending on the type of crop you are growing and their stage of growth. Turn around time ranges from 1 to 3 weeks, and your mix may need modification, so submit samples way before you need to start planting. It’s a good idea to have test results from batches of potting mix that performed well, in order to compare them to tests if you have a batch that doesn’t do so well. For a list of University Labs in the Northeast that perform greenhouse media test see the University of Vermont Agricultural Testing Lab web site, http://pss.uvm.edu/ag_testing/
‘Bioassays’ are also a great way to test the quality of a potting
mix. All you have to do is sow some fast-growing crops in the mix several
weeks before you plan to use it. Cress, oats, beans are just a few to consider.
Some growers also like to test their mixes with slower-growing crops. Onions
can be useful for bioassays as they seem to require a very high quality
mix for good germination and growth. It makes sense to include any key
crops that you grow as part of your bioassay.
revised 10-07