The decay of plant and animal residues in soils produces brown organic compounds commonly designated as humus. Some of it is soluble in dilute alkaline solutions, and agricultural chemists and investigators have often assumed that this part plays an important function in the nutrition of plants by reason of its solubility. As a result numerous methods have been devised for determining this readily soluble material, and its amount has been regarded as a measure of the fertility of the soil. Since however the amount of nitrogen in the soluble humus material is commonly 40 per cent, to 50 per cent, of the total nitrogen, the results for ordinary soils gave no better indication of potential fertility than did the total nitrogen determination itself.