The effects of neem extract and azadirachtin on soil microorganisms

Both neem extract and azadirachtin are widely used in agriculture as organic pesticides because they are nontoxic to humans, animals, and the environment. However, their effects on soil microorganisms and plant growth-promoting rhizobacteria (PGPR), which directly affect soil quality, remain largely unexplored. In this study, the effects of neem extract and azadirachtin on the activity of soil microbes and rhizosphere microorganisms was evaluated. We found that 0.1 and 0.4 gmL-1 of the extract and 1.25 and 2.5 µgmL-1 of azadirachtin inhibited the activity of soil microorganisms in vitro. Treating soil with azadirachtin for two months reduced the number of microorganisms present, while two months of treatment with neem extract increased the number of microorganisms in both the soil and the rhizosphere. The phytopathogenic bacterium Pectobacterium carotovorum was more resistant to azadirachtin than Rhizobium sp. Moreover, treatment of mung beans with neem extract or azadirachtin reduced the number of root nodules and Trichoderma asperellum in the rhizosphere, when compared to the control.