A study of the distribution of the electric charges in the Ap and B2 horizons of two highly weathered Oxisols and one Alfisol from Brazil was made by means of potentiometric titration and by direct measurement of adsorption of ions in the presence of varying concentration of electrolyte. The titration curves at different ionic strengths crossed at the common point of intersection, the zero point of charge (ZPC), or the pH at which the net electric charge is zero. Thus, the electrochemical behavior of these soils was found to be similar to that exhibited by many metallic oxides in which the surface potential of a reversible double layer is determined solely by the activity of potential determining H+ and OH‐ ions in the bulk solution. Direct determination of the adsorption of ions from NaCl, CaCl2, and MgSO4 solutions showed that pH, electrolyte concentration, and valence of the counter ion influence the magnitude of the electric charges on the soil particles. From known surface areas and ZPC of these soils, the values for the net electric charge calculated by the application of the Stern model of the double layer theory were found to be in good agreement with the experimental results.

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A study was made of the effect of varying soil pH, by the addition of either HCl or Ca(OH)2, on the concentration of phosphate in 0.01M CaCl2 extract as well as the amount of labile phosphate, as determined by isotopic exchange, in both acid and limed soils to which liberal amounts of superphosphate had been applied over a long period of time but to which no phosphate had been applied for at least five years prior to sampling.For any given soil, the amount of labile phosphate in the soil and the concentration of phosphate in soil solution both reached a minimum value at about pH 5.5 and increased rapidly as the pH was increased or decreased from this value. When the value of pH + pH2PO4 was plotted against pH — ½pCa on the conventional calcium phosphate solubility diagram, the experimental points did not fall on the solubility isotherm of any known crystalline phosphate compound except when the pH was increased sufficiently to cause supersaturation with respect to octocalcium phosphate at which point octocalcium phosphate began to precipitate and the values of pH + pH2PO4 fell on the octocalcium phosphate solubility isotherm.The observed variation in the amount of soluble and labile phosphate with pH was found to be reversible. The addition of soluble phosphate to a soil also resulted in increases in the amounts of soluble and labile phosphate at any given pH. These findings would support the conclusion that the phosphate concentration in soil solution was determined by the amount of labile phosphate rather than by the solubility of some crystalline phosphate as the pH of the soil varied.

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