PLANT RESPONSES TO INSECTICIDES IN THE SOIL 3 



more toxic to later growth than DDT, pound for pound, and no 

 highly tolerant crops were noted, as were observed with DDT. 

 Two hundred pounds per acre of technical BHC was highly toxic 

 to subsequent growth of all crops tested, although it was more 

 toxic to some than others. One hundred pounds was harmful to 

 a majority of crops tested. 



Highly purified gamma isomer of BHC, more than 99 percent 

 pure, applied at the same rates as the technical grade, had no 

 effect on germination and stand. It did, however, appear to 

 depress growth of most crops tested as seriously as did the tech- 

 nical grade, pound for pound. In practice only about one-eighth 

 as much pure gamma isomer needs to be used as if technical 

 BHC is applied for insect control. 



Less extensive tests indicated that chlordane was even more 

 harmful to germination and stand than BHC soon after putting 

 it into the soil in amounts of 100 pounds per acre or more. Most 

 vegetable crops tested were significantly depressed in stand by 

 as little as 25 pounds per acre. Corn appeared tolerant. Honey 

 Dew melon appeared especially sensitive. In subsequent growth, 

 100 pounds per acre or less appeared to be without consistent 

 effect, but 400 pounds reduced growth of most cucurbits, tomato, 

 and beet markedly. Beans, members of the cabbage family, sweet 

 corn, and cotton appeared somewhat tolerant. 



Limited tests on toxaphene, all within 4 months of its mixing 

 into the soil, indicated that amounts of 100 pounds or more per 

 acre depressed somewhat the germination of all vegetable crops 

 tested except sweet corn. Only tomato and watermelon were 

 affected significantly. In subsequent growth cucurbits and tomato 

 were significantly depressed in growth while corn and beans were 

 not at 400 pounds per acre of toxaphene. 



Also in rather limited tests, relatively heavy doses of parathion 

 appeared to have a slight but temporary depressing effect on 

 germination and stand. In a few tests run soon after application, 

 tomato, muskmelon, and snap bean were depressed in growth by 

 50 or 100 pounds per acre. Crops planted some months after soil 

 treatment were quite unharmed. 



Extensive data on persistence of these substances in the soil 

 are presented only for DDT. DDT was found to be very highly 

 persistent in its toxic effects on plants, no measurable decrease 

 in toxicity occurring after 4 years. BHC is less persistent than 

 DDT, but it is believed to be persistent enough to accumulate 

 temporarily to a harmful degree in the soil under some conditions 

 of practical use. After 3 years about half of a 100-pound appli- 

 cation had disappeared. 



Chlordane is said to be less persistent than DDT. Its per- 

 sistence as an insecticide suggests, however, that it is persistent 

 enough to accumulate to some extent in the soil. Toxaphene has 

 been shown to be susceptible to breakdown by soil organisms, 

 and is believed to be relatively unstable in the soil, therefore, it 

 is less likely than some other insecticides to accumulate harmful 

 residues. Parathion is evidently unstable and is used in such 

 small amounts per acre that it is not now an object of concern 

 as a potential harmful residue in the soil. 



