THE ABSORPTION OF PHOSPHATE FROM SOIL AND SOLUTION CULTURES

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Abstract
Wheat plants were grown in soil cultures and in flowing solution cultures in an attempt to study the absorption of phosphate. The composition of the soil solution, especially with regard to phosphate, was determined by analysis of the displaced solution. In the several soils studied the concentration varied from 0.15 to about 10 p.p.m. PO,. The behavior of the phosphate under the influence of a crop of wheat was studied by comparison with uncropped controls. Flowing solution cultures were used to maintain the conc. (0.05-50 p.p.m. PO4) in a series of 3 exps. While the absorption of phosphate was affected by the conc. of the soil or culture solution with respect to phosphate, there was no direct proportionality. The data indicate that there were 2 categories under the conditions of the exps. (a) vigorous growth with tillering and high absorption of phosphate at 1.0-50 p.p.m. PO4 in the culture solution; (b) small growth with greatly reduced tillering and low absorption of phosphate at 0.05-0.1 p.p.m. PO4 in the culture solution. The amount of growth, of tillering, and quantity of phosphate absorbed by the plants in each group were remarkably similar. If a region of direct proportionality exist, it is suggested that it will be in the range 0.1-1.0 p.p.m. PO4[long dash]the range most commonly met with in soil solutions. The composition and buffer qualities of the expressed sap were reflected by the total absorption of phosphate. Phosphate was effective in buffering against alkali, but the conc. of phosphate had no observable effect against acid. An excess of base in the expressed sap suggested the presence of an organic anion. The non-sap phosphate of the plant showed no proportionality to the conc. of phosphate in the culture solution or in the expressed sap. The displaced solution of the soils studied, neutral and highly buffered in character, represented the physiological soil solution with respect to wheat. In these soils the conc. of phosphate in the soil solution was dependent on the nature of the solid phase rather than on the composition of the soil solution.