Several years ago, in connection with experimental work on calcium arsenate as a long-term crabgrass control, I went back over the literature to discover what harmful effects calcium arsenate might have had on grasses when used in heavy applications. I found that grass was amazingly tolerant, except on sandy soils. On clays and clay loams, cereal-type grasses and others were not injured at rates that ran into thousands of pounds of calcium arsenate per acre, applied over a two- or three-year period. Soy beans withstood doses of as high as 30 pounds of actual D.D.T. per thousand square feet on clay, but were injured on sandy soils by doses as low as 8 pounds per thousand square feet. If available iron was added to the soil, injury on sandy loam did not show up at calcium arsenate rates lower than IS pounds per thousand square feet. In New Jersey, lima bean, snap bean and turnip seedlings were killed by applications of 30 pounds per thousand square feet. The few seedlings that did survive, however, were able to grow into normal plants once their roots penetrated past the soil layer on which the calcium arsenate had become fixed.
Today, of older materials used for soil treatment, only calcium arsenate and lead arsenate are of much importance. Both are being
used as pre-emergence crabgrass controls at rates of about 10 pounds to a thousand square feet of lawn area. When applied to lawns, these two chemicals form a layer of treated soil in which germinating crabgrass seedlings cannot survive. They work by substituting for phosphorus, starving the seedling for that element.
The correction of arsenate toxicity is obvious: heavy applications of superphosphate will undo practically any case of toxic soil due to either of these chemicals.
Each year sees a decrease in reports of lead and copper toxicity as these older insecticides are dropped from modern spray schedules. Today, our concern is with residues of chlorinated hydrocarbons such as D.D.T. and chlordane. A further problem is that of residues from chemical weed killers, which is covered separately in Chapter Sixteen.
THE BIG ONE
The most serious residual problem likely to be experienced by home gardeners is from D.D.T. used for soil treatment. Grasses are highly tolerant: doses of as high as 30 pounds to a thousand square feet have shown no injury. Theoretically, since D.D.T. is used largely to grub-proof lawns, there should be no human hazards from overdoses. But, of course, grass plots may not always remain as grass plots; if they are converted vegetable gardens, the D.D.T. overdose hazard becomes real.