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Behavior of Vapam and Methylisothiocyanate in Soils¹

ABSTRACT

An investigation was undertaken to study the behavior of the fungicide Vapam (sodium-N-methyldithiocarbamate) and its toxic degradation product MIT (methylisothiocyanate) in soils to obtain the necessary parameters for predicting the distribution of these compounds when applied in irrigation water. These parameters include absorption by the soil components, the degradation of Vapam to MIT and the subsequent breakdown of MIT to nontoxic substances, and the distribution of MIT between the liquid, gaseous, and adsorbed phases in the soil. Adsorption was determined by both batch and flow methods. The results indicated that degradation of both Vapam and MIT followed first-order kinetics. The breakdown of Vapam to MIT was rapid, half-life generally < 30 min at moisture contents below saturation, while MIT was found to persist for 2 weeks or more. The rapid breakdown of Vapam will result in a soil solution containing mainly MIT. MIT adsorption was relatively low (Kads = 0.41 and 0.57 ml/g for the soils higher in clay and organic matter content). The soils with the lower adsorption coefficients (Kads < 0.05) will give a practically uniform MIT distribution with depth when Vapam is applied at a constant concentration in the irrigation water. The behavior of Vapam in heavy textured soils needs further investigation.

¹ Contribution from the Agricultural Research Organization (ARO), The Volcani Center, Bet Dagan, Israel. 1975 Series, no. 137-E. This research was partially supported by the Israeli Ministry of Agriculture and the National Council of Research and Development.

² Graduate student, Senior Scientist, and Head, respectively, Division of Soil Residues Chemistry, Institute of Soils and Water, ARO, The Volcani Center, POB. 6, Bet Dagan, Israel.

Received for publication May 27, 1975. Accepted for publication December 30, 1976.

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