Effect of soil fumigation with vapam on the dynamics of soil microflora and their related biochemical activity

Summary Fungal propagules, in general, were drastically reduced in vapam amended soils. The toxicant from 250 to 1000 ppm was lethal toFusarium spp. Only in the early part of the experiment, higher concentrations of vapam appreciably reduced the population of bacteria and actinomycetes, though later on, their population gradually increased. The numbers of Azotobacter in soil amended with 125, 250 and 500 ppm did not alter appreciably, but their population in treated soils increased over the check on the 45th day. All concentrations of vapam lowered the population of Rhizobia.Vapam produced inhibitory effect on soil nitrification, inhibition period varying with the amount of chemical applied initially. Vapam from 125 to 500 ppm stimulated the ammonification process, while its higher concentration (1000 ppm) produced detrimental effect for 15 days. The toxicant increased CO₂ production for first 32 days.Results suggest that extraordinary success of vapam in controlling soil-borne infections of Pythium and Rhizoctonia was not entirely due to its fungicidal action. Partly it was also due to an increase in the population of known antagonists of these pathogens in fumigated soils.