Answer: When selecting a commercial potting mix, it is important that you choose a mix that is appropriate or your intended use. You will be able to find commercial blends formulated for seed starting, growing transplants, and maintaining larger containerized plants, with each blend differing in significant ways.
Mixes designed specifically for seed starting are usually made from screened peat moss so that they are fine in texture and have a very low fertilizer concentration, if any. A seed-starting mix is ideal for germinating seeds in flats or plug trays, but seedlings will need to be potted up into a growing mix soon after true leaves develop. A germination mix can also be used for propagating cuttings.
Growing mixes or "all-purpose mixes" usually contain peat moss, perlite, and/or vermiculite, an organic wetting agent, a liming agent, an organic fertilizer, and an array of organic amendments including compost, worm castings, organic fertilizers, and microbial inoculants. Many growers choose to use a standard growing mix to germinate seeds. This helps reduce the need to pot up young seedlings, but it is important to note that a high fertilizer concentration in the potting mix can inhibit seed germination. If you are selecting a growing mix for germination, choose a blend with a lower fertilizer concentration and plan to add supplemental fertilizer as needed during transplant production.
Potting mixes designed for larger containerized plants are usually coarser in texture and include composted pine bark or other woody material, in addition to the standard components of a potting mix, to provide better drainage and increase porosity. These mixes can also have a higher nutrient concentration, to feed the potted plant for a longer duration than a standard growing mix for transplants. Bulk discounts are available from some suppliers, which can help you save money if you purchase a large quantity of mix in cooperation with other growers in your area. In addition, if you are buying in large quantities, you may be able to work with the manufacturer to have them create a custom blend. You may want to check with your local farming organizations, conservation district, and farming neighbors to see if any groups are already participating in a bulk-buying program.
Learn more in the newly updated ATTRA publication Potting Mixes for Certified Organic Production. This publication addresses considerations for an organic grower selecting a potting mix to use for transplant or containerized plant production or for someone wishing to blend organic media. It also discusses individual components of potting media in addition to organic and biological amendments to improve plant performance. Several organic potting mix recipes are included in the appendix, in addition to suppliers of organic media and amendments.
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