The active Mn in the soil includes the readily available manganous Mn and the easily reducible manganic Mn. The manganous-manganic Mn exists in an oxidation-reduction equilibrium reflecting the state of oxidation of the soil. The status of the manganous-manganic Mn equilibrium can be established by a measurement of the active Mn. A procedure is described for the measurement of the 3 forms of Mn constituting the active Mn; namely, that which is soluble in water; that which takes part in the cation-exchange phenomena; and that which exists as easily reducible manganese dioxide. The forms of active Mn are detd. in the leachates from successive extractions of the soil with distilled water, neutral normal ammonium acetate, and neutral normal ammonium acetate soln. containing 0.2% hydroquinone. The procedure described for determining the active Mn will identify: Neutral to alkaline soils that are Mn-deficient for plant growth. A soil of this type having < 25 ppm. of easily reducible manganic oxides is likely to be deficient. Strongly acid soils that will become Mn-deficient with liming. Strongly acid soils haying < 25 ppm of active Mn are likely to become Mn-deficient when limed. Soils with high and low capacity to oxidize added Mn salts. In general, Mn-deficient soils have a high capacity for the oxidation of added Mn.