PLANT RESPONSES TO INSECTICIDES IN THE SOIL 



19 



Table 3. — Effect of kind of soil on plant response to DDT in soil in the 

 greenhouse, Beltsville, Md., 19^5-4-6 



SPECIES, OF WHICH VARIETIES TESTED WERE DEFINITELY SENSITIVE i 



DDT 



Percent stand and plant weight on kind of soil shown 



applied 



Chester 



Sassafras 



Evesboro 



Muck 



(pounds) 



Stand 



Weight 



Stand 



Weight 



Stand 



Weight 



Stand 



Weight 



_ 



Percent 

 58.3 

 56.5 

 59.3 

 53.0 

 49.8 



Grama 

 9.86 

 8.08 



7.34 

 4.47 

 3.32 



Percent 

 53.0 

 50.5 

 54.8 

 43.5 

 40.1 



Grams 



13.16 



11.64 



8.71 



6.91 



5.63 



Percent 

 66.3 

 71.0 

 67.5 

 60.0 

 48.2 



Grams 

 10.74 

 8.92 

 8.53 

 4.16 

 3.44 



Percent 

 63.2 

 62.5 

 64.9 

 62.5 

 60.6 



Grams 

 12.92 



25 



100 



400 



15.42 

 14.73 

 13.57 



1,000 



10.35 







SPECIES, OF WHICH VARIETIES TESTED WERE LESS SENSITIVE OR TOLERANT 2 



___ 

 25 __ 

 100 _ 

 400 _ 

 1,000 



62.1 

 56.8 

 58.8 

 54.1 

 55.1 



4.46 



53.9 



13.64 



62.6 



10.15 



63.9 



4.40 



55.0 



10.13 



64.1 



8.90 



62.0 



5.80 



55.3 



9.80 



62.6 



9.28 



67.9 



4.99 



49.9 



12.23 



60.1 



9.09 



64.5 



3.73 



46.8 



11.26 



53.5 



6.93 



64.8 



11.01 

 10.46 

 11.39 

 11.54 

 12.49 



1 Means based on 92 tests involving lima bean, snap bean, beet, cucumber, pumpkin, summer 

 squash, spinach, tomato, soybean, and cotton. 



2 Means based on 70 tests involving broccoli, cabbage, carrot, muskmelon, onion, turnip, and 



mineral soils the growth on the Chester loam and the Evesboro 

 loamy sand was similar for any one treatment. The Sassafras 

 sandy loam, however, not only produced larger plants on the con- 

 trol plots than did the other two mineral soils, but also on the 

 respective treated plots. The successive reductions in growth 

 accompanying the increasing amounts of DDT were slightly less 

 severe for the species tested on the Sassafras than those on the 

 Chester and Evesboro soils. This difference in response on the 

 mineral soils is not explained. It seems reasonable to suggest, 

 however, that the much lower toxicity of DDT on the muck soil 

 may be due to the adsorption of the DDT by the organic colloids of 

 which the soil is largely composed. Soil acidity may also be a 

 factor. 



RESPONSES TO COMPONENTS OF DDT 



Table 4 shows that among those species classified as less sensi- 

 tive to DDT none showed significant responses to the amounts of 

 o,p' DDT, p,p' DDT, p,p' TDE, or bis(p-chlorophenyl) sulfone 

 added to the soil in these tests. Among the sensitive species soy- 

 beans, cotton, and peanuts showed only slight and nonsignificant 

 results. Reference to table 1 shows that, although these three 

 plants are listed among the sensitive species, their responses to 

 technical DDT were less marked and consistent than the other 

 sensitive ones. 



In general, it appears that the sensitive species are depressed as 

 much or slightly more by the 20 percent of o,p' DDT present in 

 technical DDT as by the 75 percent of p,p' DDT that it contains. 

 The p,p' TDE present in 400 pounds per acre or less of the tech- 

 nical material appeared to have no effect whatever, but of course 

 larger amounts might be toxic. The results of sulfone plus o,p' 



