Dissipation of s-triazines and thiocarbamates from soil as related to soil moisture content

Abstract

Half-lives ( t 1 2 ) of two soil incorporated s-triazine (atrazine and prometon) and two thiocarbamate (EPTC and triallate) herbicides were determined in relation to soil moisture content in two California soils. Treated soils were incubated at three moisture levels in aerated glass vials at 25 ± 1°C and were analyzed at 0, 7, 16, 28, 56 and 112 day intervals. Loss of herbicides in all treatments followed first-order kinetics. The t 1 2 - values of all herbicides decreased with increasing soil moisture and followed an empirical equation, t 1 2 = aM −b (where t 1 2 is half-life; M the moisture content; and a and b are constants). Soil moisture had a greater effect on carbamates than on s-triazines. Prometon exhibited the longest half-life in both soils, whereas EPTC was least persistent in one soil and atrazine in another. The t 1 2 - values for atrazine, prometon, EPTC, and triallate with medium moisture levels and 10 μg/g concentration were 34·6, 43·2, 25·4 and 38·1 days in sandy loam and 26·5, 44·4, 44·1 and 25·9 days in loamy sand, respectively. Disappearance of 50% of the applied concentrations of most of the herbicide-soil combinations (except EPTC and triallate in one soil) took longer for lower initial concentrations (1 μg/g) than for higher concentrations (10 μg/g).