Phosphine gas is generated by agitating a reaction mixture of a metal phosphide and water with agitation air in a reaction pot of a phosphine gas generator. The resulting phosphine gas is then diluted with dilution air to produce a fumigant phosphine gas which is directly delivered to a commodity for fumigation. The reaction pot does not have any rotating means such as agitators, rotors, or stirrers. The generator provides on-site generation of phosphine gas in a rapid manner improving the fumigation efficiency for a commodity, such as grain, preferably contained within a storage structures, such as a grain silo. The generator has a built in deactivation system for the unused metal phosphide and phosphine gas.