METHOD FOR DETERMINING RELATIVE PHOSPHATEFIXING CAPACITY OF ACID SOILS

Abstract

A method was developed for determining the relative phosphate-fixing capacity of acid soils. The method depends on the extraction of Fe and Al from the soil by 0.5 [image] citric acid under fixed conditions of temp. and time. The amt. of Fe and Al extracted was found to be a measure of the relative phosphate-fixing capacity of the soil. A method was developed for determining Fe and AT together in soln. by precipitation with phosphate under fixed conditions. The resulting precipitate is then dissolved and analyzed for phosphate to give the total Fe and Al equivalent of the precipitate. The advantage of this method is its simplicity and the elimination of necessity for separation and individual detn. of the 2 elements. This method can be employed for determining the total sesquioxides, Fe and Al, in soil extracts, clays, cement, limestone, and other materials. The relative phosphate-fixing capacity of 39 representative soils from Alabama, Maine, Massachusetts, New Jersey, North Carolina, and Vermont was detd. by the developed method and by a direct phosphation method. The values obtained by both methods were practically the same for each soil. The soils used showed a range of values from 4.7 to 81.4 m[image] phosphate-fixing capacity per 100 g. Both methods used gave only relative values for phosphate-fixing capacity of soils. Changes in the conditions of the detn. would change the values obtained. The choice of standard conditions for the method developed here was based on preliminary investigations which showed that 1-hr. digestion and extraction of a soil with citric acid gave values for phosphate-fixing capacity which were of similar magnitude to those obtained by 4-hr. digestion in the direct phosphation method. All the results obtained here indicate that there is no absolute value for phosphate-fixing capacity of a soil; that the agents responsible for phosphate fixation in acid soils are Fe and Al; that the most active Fe and Al can be removed from a soil by extraction with citric acid; and that the citric acid extracted soil has very little phosphate-fixing capacity except that which results from the degrading action of the phosphate reagent on the more resistant minerals of the soil.