Accelerated transformation of the fumigant methyl isothiocyanate in soil after repeated application of metham-sodium

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Abstract
The transformation of metham‐sodium into methyl isothiocyanate and the further transformation of the latter active compound were measured in incubation studies with moist soils in gastight flasks at 15°C. Soils were collected from seven “problem” fields, from six frequently fumigated glasshouses and from nine trials with plots treated annually and plots left untreated. Metham‐sodium was transformed to methyl isothiocyanate to a high extent (more than 90 %) in all soils tested, and the maximum amounts of methyl isothiocyanate were measured within 0.1 to 1 day after addition of metham‐sodium. The transformation rate of methyl isothiocyanate varied considerably: the time for 50 % transformation ranged from 0.5 to 50 d. In general, methyl isothiocyanate was transformed appreciably faster in soils that had previously been treated frequently. However, in soils from two trial plots, that had been treated 13 times or more, no clearly enhanced rate of transformation was measured. Poor nematode control on arable fields could not always be explained by high transformation rates.

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