The nature of the phosphates dissolved by various soil extractants
1945
Fraps, G.S. | Fudge, J.F.
The solubility of several phosphates and of 34 soils of low basicity in 0.2 N nitric acid, 0.75 N hydrochloric acid, 0.002 N sulfuric acid, and 0.52 N acetic acid in 10% sodium acetate was determined. Pure calcium phosphates were completely soluble in all extractants. Rock phosphates were almost completely soluble in 0.75 N hydrochloric acid and 0.2 N nitric acid, less soluble in 0.002 N sulfuric acid, and practically insoluble in 0.52 N acetic acid. Apatite was not completely soluble in the different extractants; the solubility was influenced by the strength of the acid and the period of extraction. The differences between acetic acid and the other solvents were probably due to the presence of calcium fluophosphates. The quantity of phosphoric acid dissolved from the soils by 0.52 N acetic acid was from 20% to 40% of the dissolved by the mineral acid extractants. Fixation of dissolved phosphoric acid from the acetic acid was about the same as from water, but was less from the other solvents. The amount of phosphoric acid removed by corn from 34 soils was related to the phosphoric acid dissolved by the solvent. The correlation coefficients for the relation between the phosphoric acid removed by the crops and the phosphoric acid dissolved by the solvents were much larger for the mineral acids. Chemical methods determine the easily soluble phosphates but do not determine the availability of the soil phosphates. Any interpretation of the chemical data secured in terms of availability should be aided by agronomic data.
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