PLANT RESPONSES TO INSECTICIDES IN THE SOIL 



29 



Table 11. — DDT content of soil from the coldframes U years after adding 

 DDT to the soil, Beltsville, Md., January 19U9 X 



Rate of applying DDT 

 to soil in 1945 



Parts per million of DDT found in soil shown 





Sassafras 

 sandy 

 loam 



Chester 

 loam 



Evesboro 

 loamy 

 sand 



Mean of 



3 mineral 



soils 





Per 

 acre 



Per 

 millions 



New Jersey 

 muck 3 



Pounds 

 25 



100 



P.p.m. 



13.9 



55.4 



222.0 



554.0 



P.p.m. 



15.6 



54.1 



211.0 



551.2 



P.p.m. 



10.3 



55.6 



225.6 



555.5 



P.p.m. 



12.0 



53.3 



186.6 



487.4 



P.p.m. 



12.6 



54.3 



207.7 



531.4 



P.p.m. 



19.3 



117.3 



448.1 



1,430.6 



400 



1,000 





1 Analyst, L. Koblitsky. 



2 Calculated as parts per million in the surface 6 inches of an acre of soil, assuming a depth of 

 6% inches for the surface 2,000,000 pounds of typical mineral soil. 



3 Because a relatively dry muck soil is only y<i to % as heavy per unit volume as are the mineral 

 soils, the addition of 1 pound per acre of a substance will give parts per million equivalents that 

 are 2 to 3 times as large as for mineral soils. 



Table 10 shows the weights of the plants and pods of beans har- 

 vested in 1949 from the coldframes containing the four kinds of 

 soil that had been treated with the different amounts of DDT in 

 1945. It will be noted that the reductions in growth and yields 

 of snap beans on the several DDT treatments were even more 

 serious than the relative reductions in growth of young plants 

 observed in the greenhouse. The DDT had lost no noticeable 

 degree of its toxicity on these plots during a period of approxi 

 mately 4 years. It appears that the extreme reduction in early 

 growth of lima bean plants was, in part, only a retardation, and 

 that the reduction in yield was less severe. It is probable that, as 

 the deep-penetrating roots of the lima bean plants finally reached 

 through the soil containing DDT into uncontaminated earth be- 

 low, the retarded plants were able to recover to a large degree 

 Nevertheless, the 400 pounds of DDT produced significant reduc 

 tions in yield upon the Chester and the Evesboro soils, and upon 

 the mean yield for all three mineral soils. The growth of young 

 plants on the muck soil was significantly reduced at the 400-pound 

 rate, but the seed yield was not reduced. Injury was more per- 

 sistent and was significant at the 1,000-pound rate in the frames 

 where the pH of the mixture of the two mucks had been raised 

 to nearly 7.0. 



The Stringless Black Valentine variety of snap bean is highly 

 sensitive to DDT in the soil. The reductions in plant weight at 

 increasing dosages of DDT appeared to be relatively greater than 

 those observed for the young plants in the greenhouse tests. Ap- 

 plications of 100 pounds per acre, 4 years before, resulted in 

 significant reductions in weight of plant (without pods) on each 

 soil. Even 25 pounds of DDT appeared to depress plant growth 

 of this variety to a degree that bordered significance, on the 

 Chester and Evesboro soils. 



The effects of DDT upon weights of snap bean pods harvested 

 are rather startling. There were no pods to be harvested on 

 either the 400- or the 1,000-pound treatments on any of the min- 

 eral soils, and almost none on the 100-pound treatments of the 

 Chester and Evesboro soils. The nearly complete barrenness of 



