14 CIRCULAR 862, U. S. DEPARTMENT OF AGRICULTURE 



involving the several thousand observations recorded. For the 

 several repeated tests of various species, through the range of 

 seasons and on different soils, the trend of the results can be 

 shown dependably in summary tables. To indicate the confidence 

 that may be placed in any line of averages in the tables of com- 

 bined results the number of tests involved in each line is shown, 

 except in table 3, together with the number of single tests within 

 which significant differences were evident. In the tables of less 

 extensive accumulations of data, the least significant differences 

 are shown for each set of comparisons among treatments. Over- 

 all variance analyses of appropriate parts of several tables in 

 their present form were run to derive a generalized value for the 

 response — or lack of response — of a whole group of plants to 

 DDT or other chemical in the soil. 



Results With DDT 



RESPONSES OF SPECIES AND VARIETIES 



Marked differences in response to DDT in the soil were shown 

 among species and among varieties within species. This feature 

 of the results to date is one of the most striking that has emerged. 

 Because there is a large number of cultivated varieties of each 

 of the crops tested no exhaustive studies of varieties could be 

 made within species. Enough has been done, however, to show 

 without a doubt that some varieties of some species are far more 

 sensitive "than others. 



To emphasize the differences that exist among species the 

 reactions of the more sensitive and of the less sensitive species 

 are shown in separate parts of tables 1 and 2. First, it will be 

 seen in table 1 that DDT in the mineral soils affects the stand 

 of plants very little. A barely significant reduction is evident, 

 from an average of 54 percent for the sensitive species on the 

 controls to 50 percent on the 400-pound treatments. The further 

 reduction to 45 percent on the 1,000-pound treatment confirms 

 the generally small and relatively unimportant effect of DDT 

 upon the germination process. In later experiments where con- 

 ditions were more favorable for pre- and post-emergence damp- 

 ing-off, DDT did have a marked effect on stands of beans. 



In these experiments the germination and stand of the three 

 kinds of beans, of beets, and of spinach appeared to be depressed 

 more than the other crops shown that are rated as generally 

 sensitive. At the same time there seems to be no marked effect 

 of DDT on germination at concentrations below 400 pounds per 

 acre. Among the less sensitive crops observed there was no 

 significant effect of the DDT on germination and stand. 



The growth responses shown in table 1 are far more striking 

 than the effects on germination and stand. For the average of 

 the 11 crops rated as sensitive the application of 400 pounds of 

 DDT in the fall of 1945 reduced plant growth nearly one-half 

 during the succession of crops grown over a period of 2 years. 

 The average reduction on the 100-pound plots was significant, 

 and the reduction for several individual crops — snap beans, lima 

 beans, spinach, beet, and tomato — was especially severe. On the 



