Vertical Distribution and Persistence of Insecticidal Residues in Soils as Influenced by Mode of Application and a Cover Crop1

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Abstract
To determine, the effect of mode of application and a cover Crop on the persistence and vertical distribution of insecticides in soils, field tests were conducted over a 3-year period. Aldrin and heptachlor were applied either to the soil surface only or incorporated into the soil by rototilling to approximately 5 inches. Recoveries of insecticidal residues ranged from 2.7% to 5.3% of the applied dosages, 4 months after a soil-surface application. However, incorporation of insecticidal deposits into the upper soil layers increased the persistence of the residues by a factor of 10. One year after treatment of the upper soil layers, 90% of the recovered residues were located within the upper 3 inches of the soil. The highest concentration of the insecticides was found within the second in the of the soil layer. A deeper penetration and a more equal distribution of the insecticidal residues was noticed 3 year after soil treatment. Two to 3 times more insecticidal residues were recovered from alfalfa-covered plots than from fallowed ones. Volatilization appears to be the major factor in the loss of insecticidal residues. Their longer persistence after having been incorporated into the soil, may be extended by reducing the volatilization of the insecticides at the soil-air interface with a dense cover crop.

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