tt-135 BY DON MEEHAN

BAGGED POTTING SOILS VARY WIDELY

My coworker, George Pinyuh, from King County wrote an interesting piece on bagged potting soils. Since many of you enjoy growing plants inside and outside the home I felt this might be a good one to share. Here is what George has to say. Some weeks ago an article appeared in a locally available national newspaper dealing with the topic of bagged potting soils. Based on a great deal of the research and experience of Dr. Frank Gouin, a horticulturist at the University of Maryland, it warned purchasers about the lack of information on the label of the various potting media available and the too frequent lack of quality of the stuff inside. Because there are no standards, regulated either by the industry or government agencies for potting soils, manufacturers can say just about anything they want to on the outside of the bags. Most often they don't say much so it's absolutely impossible to tell anything about the physical or chemical makeup of the mixes. According to Dr. Gouin, many of the packaged potting media he analyzed were not appropriate to grow plants in. Many had no fertilizer in them at all, so unless one started fertilizing right away, the plants would begin to starve immediately. On the other side of the coin, Dr. Gouin found that one brand of mix had too many soluble salts (fertilizer) in it and it would likely burn up any plant or seeds put in it. Another's pH was so low nothing much would be able to survive in it either. In 1986 a researcher at the University of California, Riverside, carried out a similar analytical project on 15 different potting soils available in our region, and his results turned out to be pretty much the same as the information recently reported in Maryland. Besides finding many of them chemically inadequate, the California research found quite a few deficient in their ability to provide air to the roots of plants in them. In other words, they would drain poorly and retain too much water in the pore spaces. After a pot is watered and drains away excess water, a considerable number of a soil's pore spaces should be filled with air, not water. Most plants tend to rot at the roots and die when kept in water-logged potting media. A potting soil must contain a high proportion of bulky organic materials like bark, wood chips, or peat moss as well as nonorganic things like sand, perlite, vermiculite, pumice or calcine clay (cat box litter). These materials will assure that the mix contains plenty of pockets or pores that can fill with air once gravity has pulled the excess water from the pot. Many of them also hold enough moisture for good plant growth. There are a wide range in chemical properties for the potting mixes analyzed in California. A number of them had pH's quite a bit below the accepted range for a container soil of 5.0 to 6.5. A couple had pH readings of above 7.0. Salt levels were much too high in some of the mixes, high enough that plants put into them would suffer serious injury if the levels remained high. The upshot of all this is that you've got to be careful when you buy a prepackaged potting soil. Information on the label is often lacking or does not accurately state the quality of the contents inside. Dennis Pittenger, the California researcher, suggests that when buying bagged potting soil, choose a mix that is high in bark or other wood products, sphagnum peat and inorganic materials like perlite or vermiculite. If you can't see into the bag, squeeze some of the stuff at one of the corners. Knead it a bit with your fingers; if it feels too light, or soft, look for another brand. He also suggests that any packaged soil be thoroughly leached three or four times before putting any seeds or plants in it. This will get rid of excess soluble salts which may be in it. Finally, it's necessary to begin to fertilize very soon after the plants are growing in the mix, say within about two weeks. This fertilizing will replace the leached minerals as well as what has been taken up by the plants. Of course, you can avoid any of the possible problems associated with packaged potting mixes by preparing your own. An easy one to put together consists of equal parts of sandy loam garden soil, clean builders sand, and horticultural or sphagnum peat moss. To each bushel (8 gallons) of this mixture, add one quart of steamed bone meal and one pint of dolomite lime. You could substitute for the bone meal one cup of 5-10-10 fertilizer and half cup of 0-20-2 singer superphosphate. Mix all this stuff together thoroughly by shoveling it form one pile to another at least three times. There should be no lumps or clods. With proper management this basic mix should be able to grow just about any kind of plant. You can also amend the basic mix with more sand to grow cacti and other desert plants, or with peat to grow foliage plants or African violets. A good potting medium can also be made up without using garden soil. Mix up 14 cubic feet of peat moss and 14 cubic feet of vermiculite, perlite or calcine clay, add to this 5 pounds of 5-10-10 fertilizer, 5 pounds of dolomite lime and one pound of ferrous sulphate. This should grow most things well, but of course, as the incorporated nutrients are leached away, fertilizer applications will have to be made. The above figures will give you a cubic yard of potting mix.




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