30 CIRCULAR 862, U. S. DEPARTMENT OF AGRICULTURE 



the plants on the 400-pound muck plots was also striking, in view 

 of the amount of plant growth made in those plots and in view 

 of the early plant growth in earlier greenhouse trials. The data 

 in table 10 give no indication that the toxicity of DDT in the soil 

 has decreased noticeably during a period of 4 years, at least to 

 sensitive crops. 



It is planned to keep these soils in place in the coldframes for 

 many years, cropping them annually to a range of species of dif- 

 fering sensitivity to DDT. It is hoped that some of the moder- 

 ately sensitive or slightly sensitive crops may later reveal a de- 

 crease in toxicity that may be still too high to be revealed by a 

 very sensitive indicator crop like Stringless Black Valentine bean 



FIELD EXPERIMENTS 



Materials and Methods 



With the tentative information afforded by the earlier phases 

 cf the greenhouse work upon the probable crop response to DDT 

 in certain soils, it appeared feasible to design a comprehensive 

 study having potentialities for answering several practical ques- 

 tions under field conditions. The difference in response of sensr 

 five crops on muck versus mineral soils suggested that increasing 

 the organic-matter content of the soil might tend to lessen the 

 toxicity of DDT present in the soil. The results also suggested 

 that lime content of pH of the soil may be important. Therefore 

 a split-block design was planned to include four replicates with 

 two sub-blocks receiving manure and no manure, and each sub' 

 block containing five levels of DDT : 0, 25, 50, 100, and 200 pounds 

 per acre. These 40 plots, each 40 by 44 feet, were split in two for 

 treatment with limestone at a low and a high rate, respectively 



Through an error in machine operation in preparing the field- 

 the manure for the sub-blocks was not placed as shown in the 

 design. The result is that instead of having four true replicates 

 for manure the field consists of two contiguous blocks within each 

 of two replicates. While this does not upset the precision of the 

 measurements planned for DDT effects and lime effects, it serL 

 ously reduces the precision of measuring the single effect of 

 manure. 



The manure, DDT, and lime treatments were applied to a field 

 of moderately heavy bottom land soil near a small creek in the 

 early summer of 1947, and the plots were deeply cultivated re 

 peatedly through the summer to mix the added materials thor- 

 oughly with the soil. The required amount of DDT for each plot 

 was weighed out as a 50-percent dry talc powder. It was mixed 

 with fertilizer and limestone and applied to the surface of the 

 well-marked plots by means of a small row-type fertilizer distrib- 

 utor. Care was taken to avoid drift, to apply the mixture uni- 

 formly, and to deliver the entire weighed quantity to the appro- 

 priate plot. The plots were disked eight times, including cross 

 disking, before planting. 



Winter grains were sown in the fall of 1947 and harvested in 

 the early summer of 1948, followed by simultaneous crops of lima 

 beans, snap beans, and potatoes. In 1949 spring crops of peas 



