In considering the interaction between microorganisms and herbicides applied to the soil, two effects may become of great practical significance. One involves the potential inhibition or even germicidal action of the chemical to the microscopic inhabitants of the soil. These organisms, chiefly the bacteria, fungi and actinomycetes, occupy a unique position in biological cycles in terrestrial habitats and are essential for plant growth and soil fertility. The microorganisms decompose root and crop residues, mobilize the reserve of organic nitrogen and phosphorus, carry out transformations involving a change of oxidation state and bring about many other reactions necessary for crop production. Any deleterious influence, even of a temporary nature, could have a profound effect upon crop yields.