The Effect of pH, Salt Concentration, and Nature of Electrolytes on the Charge Characteristics of Brazilian Tropical Soils

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Abstract
A study of the distribution of electric charges in the A and B horizons of some Oxisols, Ultisols, and Alfisols from Brazil was made by direct measurement of retention of ions in the presence of varying electrolyte concentrations. Soil samples were collected from the Amazonian Jungle and from the principal cocoa‐producing zone of the country, the southern part of Bahia State.The electrochemical behavior of these soils was found to be similar to that of the constant potential systems in which the surface potential is determined by H+ and OH‐ ions in the equilibrium solution, hence charge distribution varied substantially with pH and electrolyte concentration. Direct measurement of adsorption of ions from solutions of MgSO4, K2SO4, KCl, and MgCl2 showed that the nature and valence of the counter‐ions also influenced the magnitude of the electric charges on the soil particles. The pH at the zero point of charge (ZPC) of the soils varied from 1.2 to 3.4 in the A horizons and from 4.0 to 6.1 in the B horizons. At or near their field pH the surface horizons were, in general, net negatively charged, while the subsurface horizons were either isoelectric or positively charged. This fact was found to be related to differences in organic matter contents of the horizons.