PLANT RESPONSES TO INSECTICIDES IN THE SOIL 37 



beans. Well-established orchard trees have tolerated as much as 

 3,000 pounds per acre of DDT applied on top of the soil beneath 

 the spread of the branches, with no evident effect on the tree, but 

 as little as 100 pounds per acre has seriously interfered with the 

 growth of rye, a popular winter cover crop in orchards and truck 

 crop fields in general. Thus, although there is no fear that the 

 productivity of a field will be seriously impaired for all crops 

 within a few years' time, it appears that DDT may interfere with 

 established rotations, cover crop systems, or culture of the more 

 profitable crops normally grown in a particular situation. 



Kinds of crops, varieties, kinds of soil, and doubtless other fac- 

 tors have a bearing upon how toxic any stated amount of DDT 

 might prove to be in a field soil. Meager evidence, however, 

 suggests that it accumulates almost as fast as it is applied to the 

 crops. If this is borne out by further work it will mean that it is 

 necessary to find some less persistent, relatively unstable com- 

 pounds to use in place of DDT for a large number of crops. 



BHC 



One determination of BHC in a greenhouse soil 3 years after 

 treatment with 100 pounds per acre showed about half of it re- 

 maining. This does not mean, of course, that all of it will have 

 disappeared in 6 years. It may take longer or less than 6 years 

 for all of it to disappear from the soil, but how long is unknown. 



Suppose, however, that about 15 percent of an amount added, 

 within rather wide limits, shall be destroyed per year in the soil. 

 Where as much as 40 to 50 pounds of technical BHC (about 5 to 6 

 pounds of gamma BHC) is added per acre per year, a residue of 

 about 165 pounds will have accumulated in 6 years. If 40 pounds 

 is added every 2 years, more than 100 pounds will have accumu- 

 lated in 6 years. Reference to table 5 shows that amounts of 

 technical BHC as high as 100 pounds may be expected to produce 

 a reduction in growth of 25 percent or more in many crops. 

 Although it is not proved in actual practice that this is happening 

 now (1950), or will happen in the next 3 to 5 years, the results 

 of these investigations strongly suggest potentialities for serious 

 losses in productivity following continued heavy annual use of 

 technical BHC. A less persistent substance is needed to replace it. 



Technical BHC in the soil poses another problem that is of 

 immediate importance. The objectionable odor and flavor of 

 technical BHC are imparted to potatoes and other crops, the edible 

 parts of which develop in the soil to which it has been applied. 

 Losses to potato growers in some areas have been very heavy be- 

 cause of their inability to sell bad-flavored potatoes from soils 

 treated with BHC. Some State agencies have warned growers 

 and members of the peanut and fertilizer trades against the appli- 

 cation of BHC to the soils, in fertilizer or otherwise, in which 

 peanuts are to be grown. It has been known for several years 

 that BHC in the soil will give a bad flavor to root crops grown 

 in such treated soil. It has never been recommended as a soil 

 treatment for insect control immediately before growing any root 

 or tuber crop. 



