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The determination of the fertilizer requirements of individual crops has furnished a fertile field of investigation, and although many contributions have been made, much still remains to be accomplished before rational methods of fertilizer applications may be relied upon to produce normal crops. In the past the physiological chemist has attacked this problem chiefly from two angles. He has investigated the content and availability of the nutrient elements in the soil, and he has determined the total content of nitrogen, phosphorus and potassium occurring within the plant. Both of these methods have failed to furnish a satisfactory basis for determining the mineral nutrient needs of plants. We cannot satisfactorily determine the quantity and kind of amendments needed on any given plot of land by making a soil analysis ; neither can we tell by an analysis of the plants grown on the soil the amount and kind of fertilizer that should be added. This is no doubt due to the very large number of factors involved in crop production, especially with reference to the relationship between the plant and the soil in which it grows. In this paper, work carried on during 1925-1926 is summarized, and a different mode of attack is suggested, which may prove helpful. Quite aside from the practical applications of this work, it is believed that other workers may find the methods here developed useful in studies of mineral nutrient metabolism.

THE ADAPTATION OF CERTAIN COLORIMETRIC METHODS TO THE ESTIMATION OF NITRATES, PHOSPHATES AND POTASSIUM IN PLANT SOLUTIONS