4 CIRCULAR 862, U. S. DEPARTMENT OF AGRICULTURE 



THE PROBLEM OF CHEMICAL RESIDUES IN SOILS 



The remarkable new insecticide DDT (clichlorodiphenyl tri- 

 chloroethane) first became available for agricultural use in 1945. 

 Because of its apparent stability and its unusual potency as an 

 insecticide the question immediately arose as to whether its pro- 

 longed and repeated use on crops might result in the accumula- 

 tion of harmful residues in the soils of fields receiving DDT dusts 

 or sprays for insect control. 



The same question soon arose about technical benzene hexa- 

 chloride, hereinafter called BHC, of which the insecticidal frac- 

 tion usually consists of 10 to 12 percent of the gamma isomer 

 of 1,2,3,4,5,6-hexachlorocyclohexane. Because of the strong and 

 persistent odor of technical BHC a further problem of the impair- 

 ment of food properties through "off" flavors of some commodi- 

 ties was presented. In the 5 years after World War II numerous 

 other new and potent insecticides, fungicides, and herbicides 

 rapidly appeared in commerce. Some of these are known to be 

 relatively unstable, and they are therefore not expected to create 

 a soil-residue problem. Others, such as TDE, are claimed to 

 have persistent insecticidal qualities in the soil and are there- 

 fore presumed relatively stable and likely to accumulate upon 

 repeated use. 



The harmful effects of arsenical residues in the soils of certain 

 orchards in Washington (23, 2U, 43) 2 and cottonfielcls in South 

 Carolina (2, 11, 12, 33), after many years' use of lead arsenate 

 or calcium arsenate, suggested that similar troubles might be 

 encountered sooner or later if DDT, BHC, chlordane, or some 

 toxic decomposition products of them, or some impurity, should 

 prove to be highly stable in the soil. Plantsmen, entomologists, 

 and manufacturers of insecticides all have been, therefore, inter- 

 ested in several practical questions, some of which follow. 



Is DDT toxic to certain plants when mixed with the soil in 

 which the plants are to be grown? If it is toxic, how much can 

 different kinds of plants tolerate in the root zone without showing 

 any measurable effect? Once DDT reaches the soil how long will 

 it persist? Is it so stable that small annual residues will accumu- 

 late to produce a harmful effect after a number of years? If it 

 will accumulate to a harmful degree, will a given accumulation be 

 more harmful in some soils than in other soils? Will it accumu- 

 late faster in some soils than in others, assuming that equal 

 amounts reach the specified soils each year ? If a residue that is 

 harmful to certain crops should accumulate, can anything be done 

 to correct the trouble? What component part of the DDT mix- 

 ture of different chemical substances is most toxic to plants ? 



For each of the new chemicals having established value as in- 

 secticides, fungicides, herbicides, or other agricultural use it is 

 important to know whether or not they are toxic to plants, either 

 when applied to the plant or when incorporated into the soil. It 

 is also important to know whether or not they persist in the soil, 

 whether they are broken down by organisms, or whether they are 

 leached from the soil. Research on such questions is important 



- Italic numbers in parentheses refer to Literature Cited, p. 39. 



