PLANT RESPONSES TO INSECTICIDES IN THE SOIL 



35 



under and between trees, and larger amounts of zinc were found 

 under the trees than in the controls. 



In the greenhouse test (table 14) the growth of rye was very 

 poor on the soil from under the trees, was good on that from 

 between the trees, and best from the same soil lying outside the 

 limits of the orchard. Not all the difference in response between 

 the controls and "between trees" and "under trees" plots is prop- 

 erly ascribable to DDT alone, or any other single known factor. 

 Wheat grew equally well on all three soils. Although there were 

 differences in arsenic content of the soil lots, they were not great 

 enough to affect the growth of wheat. Thus it appears that the 

 accumulations of chlorinated hydrocarbon insecticides DDT and 

 BHC, in a period as short as 4 years, seriously affected the capacity 

 of this particular orchard soil to produce a satisfactory cover 

 crop of rye. 



Table 14. — Plant response to soils from peach orchard, receiving arsenic, 

 DDT, and BHC sprays over a period of years, Beltsville, Md., 1949-50 



Crop and variety 



Plant weight per plot from soil 

 of source indicated 



Significance 





Under trees 



Between trees 



Control 





Rye, Abruzzi, test 1 



Grams 



621 



143 



584 



Grams 

 1,192 

 413 

 518 



Grams 



1,591 

 489 



577 



(1) 

 ( 2 ) 



Wheat, Tenmarq 



(3) 



1 Differences significant at the 5-percent level. 

 - Differences significant at the 1-percent level. 

 3 Differences not significant at the 5-percent level. 



DISCUSSION OF TOXICITY OF INSECTICIDES 

 General 



Each of several of the insecticides reported upon here has one 

 or more properties that make it very superior to insecticides used 

 before 1945. The small amounts required to control some pests 

 and the persistence of the effects of treatment make DDT, BHC, 

 chlordane, and perhaps others unusually effective and economical. 

 One or more of these substances has given remarkable control of 

 serious pests not satisfactorily or economically controllable before 

 1945. There is no doubt as to their effectiveness in the killing of 

 certain insects. 



For the present some phenomenal increases in yield or quality 

 of crop, or both, are being obtained with chlorinated hydrocarbon 

 insecticides that are persistent. Individual farmers have con- 

 tinued to use a particular chemical — when it was obtainable — 

 until it appeared to them that some other chemical or practice was 

 more profitable ; it is not improbable that such a human tendency 

 will continue. If DDT, BHC, or other persistent substance is 

 effective in such small amounts that it can be used at low levels 

 for many years without danger of accumulating a toxic level in 

 soil or affecting crop quality there is no reason why it should not 

 be used carefully within those safe limits of methods of use, rate, 



