Abstract

The effect of four herbicides (atrazine, primeextra, paraquat and glyphosate) on soil microbial population, soil organic matter and dehydrogenase activity was assessed over a period of six weeks. Soil samples from cassava farms were treated with herbicides at company recommended rates. Soil dehydrogenase activity was measured at four-day sampling intervals up to the 20^th day. Bacterial, fungal and actinomycetes populations decreased upon treatment with herbicides when compared to the control. There was significant reduction in percentage organic matter after the herbicides were applied to soils. Soil organic matter then increased after continuous application from the second to the sixth week of treatment.Herbicide treatment resulted in a significant drop in dehydrogenase activity when compared to the control soil samples. Obtained results indicated that soils treated with primeextra had the lowest dehydrogenase activity of 16.09 µg (g^-1min^-1) after the sixth week of treatment, while soils treated with glyphosate had the highest dehydrogenase activity of 20.16 µg (g^-1min^-1) when compared to other herbicides used for treatment. Dehydrogenase activity increased from the second to the sixth week of treatment. This study indicated significant response of soil microbial activity to herbicide treatment and increased adaptation of the microbial community to the stress caused by increase in concentration of the herbicides over weeks of treatment.

Key words: Herbicides, soil organic matter, dehydrogenase activity, treatment.

Abbreviation

Abbreviations: NA, Nutrient agar; PDA, potato dextrose agar; INT,iodonitrotetrazolium chloride; DH, dehydrogenase; INTF, iodonitrotetrazolium formazan.