Effect of higher concentrations of herbicides on bacterial populations in tropical soil

Abstract

It is a common practice for local farmers in the tropics to apply high concentrations of herbicides. Studies were carried out on the effect of high concentrations (× 1.5 and 2.0 recommended rates) of two commonly used herbicides (glyphosphate and paraquat) - on soil pH, bacterial counts, and species in a tropical soil. Treatment herbicides resulted in a general reduction of soil pH from 6.22 to as low as 5.28 when compared to the control soil treatment day (day 0). Glyphosphate (at × 1.5) resulted in initial lowering of bacterial counts from 3.2 × 105 cfu/ml to2.5 × 105 cfu/ml at week one and 3.0 × 105 cfu/ml at × 2.0 recommended rates. While paraquat (at x 1.5) resulted in an initial bacterial increase from 2.0 × 105 cfu/ml to 2.4 × 105 cfu/ml at week one; paraquat (× 2.0) resulted in an initial increase from 2.2 ×105 cfu/ml to 2.5 × 105 cfu/ml. High herbicides concentrations generally resulted in bacterial population reductions when compared to the control soil. Bacillus sp (glyphosphate - 39 .7%; paraquat - 20.8%) and Pseudomonas sp (glyphosphate - 23.5%; paraquat - 31.7%) were the most frequently isolated bacteria from herbicides treated soils.