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PLANT RESPONSES TO INSECTICIDES IN THE SOIL H 



Figure 1. — Plot arrangement of tests of toxicity of certain chemicals in the 

 greenhouse, Beltsville, Md. 



large compartments of the greenhouse. Additional untreated 

 plots were on other beds in the same greenhouse section. 



As the benches were filled with soil the amount of DDT required 

 for each plot was weighed in the form of a 25 percent pyro- 

 phyllite powder. The powder was dusted evenly over the soil of 

 the plot and mixed with it by cutting and turning with hand 

 shovels 20 times. 3 After each crop was removed the soil was 

 again thoroughly mixed and turned, but no more DDT was added. 

 At the time of applying the DDT all plots received a 5-10-5 

 commercial fertilizer at the rate of 1,000 pounds per acre. The 

 fertilizer was thoroughly mixed with the soil incidental to the 

 mixing of the DDT. The check plot received pyrophyllite at 

 800 pounds per acre. 



Single plots usually consisted of 1 row each of about a half 

 dozen species or varieties, the number of rows differing some- 

 what among crops, depending upon the space requirements of 

 the plants to be grown. Depending also upon the size of the 

 species to be grown, either 50 or 100 seeds were planted per 

 row across the bench. The plots were watered, weeded, and 

 otherwise cared for as uniformly as possible, in accordance with 

 ordinary greenhouse practice. No DDT or other chlorinated 



3 For some of the later established plots the DDT or other chemical was 

 mixed with the soil in a rotating drum before the soil was put into the green- 

 house benches. 



