PLANT RESPONSES TO INSECTICIDES IN THE SOIL 27 



With successive crops grown on these plots for nearly iy 2 years 

 from the time of applying parathion to the soil, it may well be 

 that some of the test crops failed to encounter toxicity, although 

 parathion was temporarily toxic. Be that as it may, table 8 

 shows no important persistent toxic effects on growth. 



Table 9 suggests a temporary toxic effect of parathion on ger- 

 mination and stand. In the tomatoes sown immediately after 

 soil treatment germination was depressed significantly by 3 

 pounds or more of parathion per acre. Seven weeks later in a 

 succeeding planting the 100-pound treatment did not depress 

 stand significantly. In beans planted immediately after treatment 

 the heaviest treatments depressed stand, 100 pounds being sig- 

 nificantly lower. This effect on beans did not persist significantly 

 in sowings made more than 2 months after starting the tests. 

 Sowings of rye 2 and 4 months after treatment showed some 

 trend toward poor stand following the larger amounts of para- 

 thion, but they were not significant. None of the sowings made 

 a year or more after treatment showed definite trends or signifi- 

 cant differences. 



The high variability in the parathion tests is believed to be due, 

 in part, to the difficulty of mixing very low concentrations of 

 parathion with large volumes of soil. 



Hazard to personnel also interfered somewhat in getting the 

 best mixtures with the soil. 



DDT-TREATED SOILS EXPOSED TO WEATHERING 

 Materials and Methods 



Late in 1947 it appeared desirable to replace the series of soil 

 treatments in the greenhouse, discussed in the preceding section, 

 and to set up additional and different studies. The treated soils 

 and their respective check plots were transferred quantitatively 

 from the greenhouse benches to an open coldframe, 6 by 100 feet, 

 where they have since been planted with various crops and con- 

 tinuously exposed to natural weathering. The several soils and 

 treatments of each were placed in the frames in duplicate at ran- 

 dom. Two plots from duplicate blocks were combined to make 

 larger plots in the frames than in the greenhouse benches. 



In 1948 the plots were planted with Abruzzi rye, but were so 

 surprisingly damaged by rodents and dogs as to render the harvest 

 weights of little or no value. Observations of the plots before the 

 crop was damaged indicated that the growth of the rye was af- 

 fected by the treatments very much like the growth of the sensi- 

 tive species in the greenhouse. 



After taking measures to protect later crops from depredations 

 of animals, the plots were planted with Henderson Bush lima 

 bean and Stringless Black Valentine snap bean in 1949 for ob- 

 taining yield and quality data (table 10). Rows of lima beans 

 were planted half as far apart as they were intended to grow to 

 maturity, with the purpose of removing the plants of alternate 

 rows for recording plant weights at an early stage of growth. 



When the beans reached a stage at which they are normally 

 harvested for canning or freezing, all pods were removed from 



