INTERACTIONS BETWEEN ACID-SOLUTIONS AND VENEZUELAN TROPICAL SOILS

This study addresses the impact of exogenous acid on tropical topsoil. The main objectives are to ascertain the physico-chemical parameters and processes that determine the buffer capacity (beta) and to evaluat e the effects of anions and pH on the leaching of Na, K, Ca, Mg, Fe, S i and Al. The topsoils were classified into two groups. First the carb onate group (n = 6), comprising samples collected in a tectonically-ac tive area with beta values of between 75 and 3600 meq/kg and CaCO3 val ues from 0.8 to 19 wt.%. Second the non-carbonate group (n = 15), comp rising mainly highly weathered soils from the plains and the Guiana Sh ield, which yield the lowest beta values display a linear correlation with % CaCO3 and CEC for the former group, and with organic carbon, CE C and BS for the latter. Fifty milliliters of water (initial pH = 5.6) added to 1 g dry-soil produced a release of cations ranging between 4 .3 and 68 meq/soil kg. This is explained by a desorption-dissolution p rocess. Strong acid solutions, e.g. HNO3, H2SO4 and HCl (initial pH = 3.3), in relation to water, induces increased cation release. This pro cess produces consumption = 60-70% of H+ added. Protons consumed were predominantly compensated by Ca2+ + Mg2+ released into solution. The a mounts and relative proportions of cations desorbed are independent of the initial anionic composition of the inorganic acid solution. Howev er, a weak acid solution, e.g. CH3COOH solution (initial pH = 3.3) pro duced remarkable increases of the relative proportion of both Na+ and K+. It may be concluded that ionic exchange, controlled by organic mat ter and carbonate dissolution, is the main proton consuming mechanism.