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Release of Herbicides from Clay Minerals as a Function of Water Content: I. Kaolinite¹

ABSTRACT

The influence of water content on the release of six herbicides from kaolinite was studied. Kaolinite suspensions were treated with ¹⁴C-labeled herbicides at rates of 1 and 4 µg/g (based on dry weight of clay). Extracts of the soil solution were obtained at successively increased increments of pressure from 0.3 to 15 bars, using the pressure membrane extractor. The following determinations were made: (i) volume of extract for each pressure-increment; (ii) concentration of herbicide in the extract, as measured by liquid scintillation; (iii) amount of herbicide remaining, as calculated by difference; and (iv) pH of the extract.

The results show that for all herbicides used, except simazine, the concentration in the extracts decreased with increasing pressure increments. The decreases were greatest with diuron and chlorpropham. Simazine concentration was from two to three times greater in the high than in the low-pressure extracts. High concentrations in low-pressure extracts suggest that plant availability is high and that movement with water could be a problem with high rainfall under field conditions. Fluometuron and 2,4-D were negatively adsorbed; the others were positively adsorbed. The order of release in terms of total recovery of herbicide applied at the 1-ppm rate was: fluometuron > 2,4-D > atrazine > chlorpropham > simazine > diuron. The order was the same for the 4-ppm rate except that 2,4-D was greater than fluometuron. The total recovery of the herbicides in the extracts ranged from 12.0% for diuron to 92.1 for fluometuron.

¹ Paper no. 3253 of the Journal Series of North Carolina State Univ. Agr. Exp. Sta., Raleigh, N.C. Part of a thesis presented by the senior author in partial fulfillment of the requirements for the degree of Master of Science. This study was supported in part by Water Resources Research Institute Annual Allotment Grant A-008-N.C.

² Research Assistant and Professor of Soil Science, respectively. Senior author now Research Assistant at the University of Kentucky, Lexington.

Received for publication July 31, 1970. Accepted for publication January 18, 1971.