The Q&A Archives: How to make your own potting soil mix

Question: I use my own potting soil mix (leaf mold, garden soil, peat moss, vermiculite and alfalfa meal) to start seedings. Last year, though, the soil smelled of ammonia and formed a thick crust on top after a few days. What went wrong? Nauman Dowdy Mineral, VA

Answer: It sounds as if some of your ingredients are still undergoing rapid decay. The ammonia indicates an overabundance of nitrogen rich material. The alfalfa meal is the likely culprit, though undigested leaves could cause a similar odor, says John Trew, horticulturist with Necessary Trading Company, an organic garden supply company in New Castle, Virginia. Leave the mix outside to continue decomposing for a few days before using it. If the ammonia smell is still present, your best solution is to make anew mix (or purchase a commercial seed starting medium) in order to get your plants up and growing in time for spring. Id follow the Natural Organic Farmers Association (NOFA) guidelines for organic potting soil mix, says Trew. To make one bushel of mix, combine 16 quarts each of sphagnum peat moss and vermiculite. For nutrients add 3 1/2 ounces of dried blood, seven ounces of bonemeal and 3 1/2 ounces of ground limestone. Avoid alfalfa meal it makes the mix too hot, explains Trew. If you wantto make a mix closer in texture to the one you were using, try the old 'thirds' ratio: 1/3 mature compost or well rotted leaf old, 1/3 fine garden loam and 1/3 course builder's sand.

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