PLANT RESPONSES TO INSECTICIDES IN THE SOIL 17 



average the plant weight of those sensitive crops was reduced 

 about 25 percent by the mixture of 100 pounds of DDT per acre 

 in the soil. 



Examination of the roots of seriously affected plants in the 

 several experiments with any form of DDT showed them to be 

 short, often thickened and stubby, and abnormally numerous. 

 It appeared that toxic concentrations of DDT tended to stop root 

 growth and stimulated the starting of new roots that in turn 

 were retarded, whereupon still others made their short excursions 

 into the toxic soil. 



It should be pointed out that these results are probably more 

 extreme than usually will be found under field conditions with 

 equal amounts of DDT per acre in the soil. In these greenhouse 

 studies the soil was only about 6 inches deep in the benches* 

 Repeated working of the soil in the benches could not, therefore, 

 further dilute the concentration of the DDT by mixing it with 

 still deeper layers or larger volumes of soil as would be done in 

 plowing and working a field soil to a depth of 8 inches or more, 

 Furthermore, under field conditions, deep-rooted plants probably 

 would grow through the DDT-soil mixture into soil without DDT 

 and be less affected. It is also recognized that harvesting these 

 plants at an early stage of growth affords only a rough index 

 of the effects of DDT upon those parts of the respective crops 

 normally harvested at maturity. The results do show, however, 

 that some crops are sensitive to DDT mixed with certain mineral 

 soils in amounts no greater than 100 pounds per acre and are 

 seriously retarded in growth by as much as 400 pounds. The 

 data further indicate the relative susceptibility of several crops 

 to DDT in the soil. 



A later section (pp. 27 to 30) contains data on a few crops 

 grown to maturity on different soils containing different amounts 

 of DDT. 



The short list of less sensitive crops in table 1 shows no signifi- 

 cant differences in growth among the several DDT treatments. 

 The figures for cabbage at the 1,000-pound level suggest sensi- 

 tivity, but 1,000 pounds is a very heavy treatment. In only one 

 of the nine tests with cabbage were significant differences found, 

 and in that instance the effect of the 400-pound treatment was 

 not especially marked. 



In a single test Irish potato appeared rather highly tolerant, 

 showing no significant reduction in tuber vield at 400 pounds or 

 less of DDT per acre. The 1,000-pound treatment, however, 

 reduced the yield about 40 percent. 



INFLUENCE OF SOIL CONDITIONS 



Table 2 contains the data for plants grown on muck soil treated 

 with DDT at the same rates as the mineral soils shown in table 1. 

 Much greater amounts of DDT can be mixed with muck soil 

 without affecting sensitive crops than in the instance of the 

 mineral soils observed. On muck there was no effect of the DDT 

 treatments on germination and stand of either the less sensitive 

 or more sensitive crops. At amounts of 25 to 400 pounds of 



