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IS GYPSUM A GYP? by Rommy Lopat Hey Rommy: On the inside back cover of TWG’s Summer 98 issue, it talks about ground preparation. I just asked a landscape professional how to soften my clay soil, and he said adding gypsum was the best. The article says not “to add sand or gypsum unless you know the big drawbacks”. What are they? —Shirley Remes, via e-mail, South Elgin, IL. TWG Editor: Well, Shirl girl, your question demonstrates one of the great truths of gardening: if there’s one truth for sure, there’s a different truth right behind it. I spent one whole day trying to get the low-down on this gypsum thing. I started to answer your question by speaking with three credible sources: a prestigious North Shore landscaping company, a writer with many gardening books to his credit, and a soil scientist on the “plant hotline” at a major Chicago public garden. I also read through my entire soils’ file. The literature was useless and often contradictory, and the three professionals were clueless (two admitted they didn’t know; one made up a screwy answer). This is the claim on the back of a bag of Faesy & Beshoff gypsum: “Gypsum is a safe and easy-to-use soil conditioner. It gradually opens up the soil, promoting good drainage and aeration. It neither raises nor lowers the pH and is non-burning. Gypsum will loosen clay soil to 3-6” depth simply by making at least 2 applications a year, in both vegetable and flower gardens. The calcium tends to make the fine clay particles stick together as aggregates, improving the draining capacity of the soil...”. U.S. Gypsum Corp.’s [USG’s] literature echoes this, adding that gypsum is of special benefit to legumes; reduces the odor of compost and manure and conserves nitrogen; eliminates surface crusting; clears muddy farm ponds; and can be combined with phosphate fertilizer to make instant bone meal.” In a call to USG representative Joe Furco, he added that “Ben Franklin discovered gypsum’s effect on clay. While I can’t cite a reference proving what I say, gardeners will see a big difference in their soils if they buy a coarse ground gypsum such as “Sof’n-Soil”. Talc-like gypsum is too powdery for the garden and pelletized gypsum is needlessly expensive. I recommend applying gypsum every two weeks since it’s water soluble and will leach. I use it at home and it really works.” However, even USG’s brochures caution that gypsum is not a complete or permanent fix to heavy soils: “...to achieve greater depth and faster action, gypsum, in conjunction with organic material, must be cultivated into the soil. Gypsum does not permanently loosen clay soil: it keeps soil conditioned only as long as enough soluble calcium is available to keep the clay ‘flocculated’. For more permanent loosening, organic matter must be added along with the gypsum.” H’mmm... Next I read an article by Dr. David Minner, professor of horticulture at Iowa State University [(515) 294-5726], who writes a regular column for “sportsTURF” magazine, and the true story began to be revealed. He wrote: “Gypsum, or calcium sulfate, is often applied, but seldom needed, due to a classic misunderstanding arising from its association with improving water movement and soil structure on sodic (high sodium) soils that are typically found only in semi-arid climates [TWG Editor: such as California]. The calcium in gypsum displaces salt in the soil. The salt then reacts with sulfate to form sodium sulfate, which is highly water soluble and easily leaches away. Removing the salt from the soil helps to aggregate soil particles, which eventually restores the soil structure. But this situation only occurs when the soil structure deteriorates because of high salt. These wet, sodic soils are slick, sticky, and have poor drainage...very similar to heavily trafficked clay soils that are not affected by excess salt. Both situations create hard soil with poor structure and drainage. Only a soil test will determine if there is a true need for gypsum application.” Following the theory that gypsum will help leach salt out of soil, The Morton Arboretum recommended to the City of Chicago that it add gypsum to the soil mixes used in the planter boxes that line the city’s streets. So, for example, if you have an area near a road where vegetation is suffering because of winter salt spray, gypsum will help. Call The Morton Arboretum Plant Clinic to obtain the application rates, which they’ve modified for the Chicago area. Another important point! Adding gypsum—a combination of calcium and sulfur—to a soil can have a nutritional benefit if you live somewhere in the world where the soils lack calcium, which is essential for good cell wall formation, especially at the growing tips of roots and shoots. Calcium also speeds up the decomposition of organic matter (which is why you must add more compost to your gardens all the time). Here in the upper Midwest, however, many soils are built primarily from dolomitic limestone containing calcium and magnesium made from zillions of prehistoric clamshells. In fact, our soils often contain boatloads of calcium, since our concrete walkways and foundations are constantly leaching lime. So you don’t need gypsum as a means of adding calcium. So Shirley, where does this leave us? Those that sell gypsum sound incredibly believeable and one would hope that the country’s largest gypsum manufacturer is not full of well, gypsum, so to speak. No one showed me any definitive studies for “typical” Chicago soils contradicting their claims of effectiveness. But back to basic truths: is your clay so horrible that you’d want to add gypsum to it no less than twice a year and as often as every two weeks, plus the work of adding organic matter too? I’m way too lazy—my garden’s lucky to get just the compost and sometimes a little fertilizer. Besides, Shirley, recall that if your soil’s compacted, gypsum won’t do a thing for you. The only cure is to apply some muscle and compost to it or build a raised bed. If the soil is just tough-to-dig, then you are already obliged to dig in 2-3” of compost every year. By the way, I like the compost made from yard wastes much better than mushroom compost. Most mushroom compost that I’ve ever bought is simply mucky manure with old shoes and beer cans added, which is why I have advertisements in the back of this magazine for a really great compost that I use myself. In addition, when I can afford it, I add “Profile” or “Turface” calcined clay to my gardens. [You could add 8” of sand to every 2” of clay (do not add less, or you will create concrete), but that seems pretty ridiculous.] Holy cow! I just this minute saw Jerry Baker on TV sprinkling pelletized gypsum laced with talc and bone meal (for a double calcium whammy) into the soil he was using to plant a tree. And the beat goes on... |
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