1. Sodium arsenite applied to a soil greatly increases the number of organisms in that soil which will develop on synthetic agar. 2. In only three dilute concentrations were the ammonifiers stimulated by the addition of sodium arsenite to a soil. The soil in which stimulation was noted was a calcareous loam high in organic content. If the number of ammonifiers in a soil are increased by the addition of sodium arsenite to a soil, their physiological efficiency is reduced to such an extent that there is a decrease in the accumulation of ammonia over that found in an untreated soil. 3. Sodium arsenite is toxic to nitrifiers when added to a soil in only one part per million, whereas even 85 parts per million of arsenic in the form of sodium arsenate stimulates. 4. Sodium arsenite is less toxic to ammonifiers and nitrifiers in a loam soil than in a sand, and still less toxic in an organic loam than in a silt loam. The organic colloid probably forms a loose chemical combination with the arsenic, thus protecting...