Fumigation with Phosphine and Carbon Dioxide in Metal Grain Tanks for On-farm Use in Swaziland

Abstract

Metal grain tanks, as constructed in Swaziland, were demonstrated to be extremely effective structures for fumigation on a small scale; phosphine gas concentrations were maintained well above the recommended minimum concentration of 150 p.p.m. throughout the minimum seven days exposure period. Current methods of sealing the tanks were shown to be inadequate but were improved by sealing the tank inlet and outlet using plastic sheeting and adhesive tape. Application rates of 3·33 g of phosphine per tonne of grain (as currently recommended in Swaziland) were more than adequate, provided the tanks were well sealed. “Tiny Bags” (produced by Degesch GmbH) were slightly slower in releasing phosphine when compared to conventional tablet formulations. However, the pattern of gas release and subsequent decay in concentrations were similar for “Tiny Bags” and tablets. As a result of the high degree of gas tightness which can be achieved using metal grain tanks, the recommended application rates for phosphine could be reduced.A system was devised for applying carbon dioxide to the base of the grain tanks using a probe inserted through the top of the tank. High initial gas concentrations were achieved but these fell quickly, necessitating frequent regassing of the tanks in order to maintain gas concentrations for the required 10 day exposure period. Given the proven gas tightness of the tanks, the most likely cause for the apparent loss in carbon dioxide gas would be adsorption by the maize.