The influence of some pesticides on soil microbial flora in relation to changes in nutrient level, rock phosphate solubilization and P release under laboratory conditions

Two soil types, loamy sand and sandy soils, were treated with atrazine, pyrethrin and a mixture of metobromuron and metolachor for eight weeks in the laboratory to determine the effect of the chemicals on soil microbial populations and their mineralization activities. The experiment was also aimed at evaluating the changes that occurred in soil nutrient levels as a result of the pesticide treatments. Significant reductions in microbial populations after an initial rise, as well as in percentage carbon, nitrogen, potassium and pH were recorded in the two treated soil types. Eight bacterial isolates, two actinomycetes and five fungal species were identified in the control loamy sand soil. Marked reductions in species numbers were recorded in the treated soils as compared to the control. Compared to similar species of organisms isolated from control soils, Pseudomonas, Achromobacter, Bacillus spp., Aspergillus niger and Streptomyces sp. isolated from the treated soils released less phosphorus when inoculated into a broth containing 'Sokoto' rock phosphate complex as the only source of P nutrition. (C) 1997 Elsevier Science B.V.