36 CIRCULAR 862, U. S. DEPARTMENT OF AGRICULTURE 



and time. It should be remembered, however, that there are 

 limits. 



There is no reason at this time to suppose that a toxic amount of 

 DDT, BHC, or chlordane, as examples, will be accumulated in the 

 soil any sooner than suggested by the studies reported in this 

 circular. If only 2 to 3 pounds of DDT per acre are applied per 

 year it will take 10 to 15 years or more to accumulate enough to 

 be detected in a sensitive crop such as Stringless Black Valentine 

 bean. Many crops, however, are treated at rates of 5 to 10 

 pounds of DDT annually, suggesting that sensitive crops on land 

 receiving such amounts might be definitely depressed in yield after 

 5 to 10 years. A crop as tolerant as corn, however, might remain 

 unaffected following such applications for 100 years. But 15-, 

 20-, and 30-pound annual applications can be expected to cause 

 trouble in 3 to 5 years on some crops. 



Substances less persistent than DDT, such as BHC or chlordane, 

 will, of course, accumulate less rapidly than implied above if equal 

 amounts are applied. Too few years have elapsed since these new 

 toxic substances have been used in common practice to know how 

 long a time will be required for a harmful residue of the more or 

 less persistent ones to accumulate. 



Excessive or other improper use of such substances is to be 

 strongly discouraged in the interest of economical and efficient 

 crop production. If the most efficient methods of application are 

 used with the best dosages known, strictly in accordance with 

 sound recommendations, less material of a possibly harmful 

 nature will be accumulated in the soil over a period of years than 

 if it is used carelessly. 



DDT 



Because of the great persistence of the toxicity of DDT and its 

 relatively high toxicity to many crops on the mineral soils on 

 which it has been tested, it appears that harmful residues will 

 accumulate under some conditions of use in as short a time as 3 

 to 5 years. It is probable that residues harmful to some cover 

 crops have been developed already in certain orchards in which 

 30 to 40 pounds or more have been applied per acre per year. In 

 orchards the toxic effect should be first noticeable on the cover 

 plants under the trees where the DDT has accumulated from drip, 

 and fall of leaves. In areas where the DDT has been applied uni- 

 formly to crops over a field there will be no localized concentra- 

 tion, as in orchards, to emphasize or make clear its toxic effects. 

 In the absence of any specific symptoms a more or less uniform 

 reduction in growth over the field may thus escape detection until 

 it has gradually progressed to a serious degree. The lack of clear- 

 cut symptoms of injury and the lack of control, or untreated 

 areas, within single fields probably will delay discovery of injury 

 until considerable damage has been done to the productivity of 

 fields that regularly receive substantial dosages. 



Although certain crops commonly treated with DDT appear 

 highly tolerant to it, as many grains, sweet corn, and Irish pota- 

 toes, some other crops commonly grown in rotations with them 

 are highly sensitive, as Abruzzi rye, tomatoes, squash, and snap 



