RETENTION OF PHOSPHATES BY SOILS

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Abstract
A laboratory procedure of using soil-Ca(OH)2-H2O-air systems for obtaining equilibria at which the solubility of phosphates can be studied is described. Data concerning the effect of additions of ferric and aluminum chlorides upon the solubility of phosphates in this system are given. Reproducible equilibria in regard to treatment with lime and pH values are obtainable by the method. The reversion of added phosphates to dilute acid-insoluble forms continues to some extent after equilibration. The liming treatments decreased H20-soluble phosphates in all series, but increased the P soluble in CO2-saturated water or in 0.002 [image] H2SO4. Retention of phosphates by the soil against extraction with these acids was decreased by liming. The retention of phosphates insoluble in dilute H2SO4 at pH 3 was increased by additions of FeCl3. 6H2O but decreased by similar additions of AlCl3.6H2O. The results indicate that the effects of the added Fe and Al upon the fixation of phosphates are controlled by the soil colloid, i.e., they are largely absorbed as exchangeable bases and only a minor fraction forms iron and aluminum phosphates insoluble in 0.002 [image] H2SO4 at soil reactions above pH 4.5.

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