EFFECTS OF INCUBATION AND PH ON SOIL SOLUTION AND EXCHANGEABLE CATION RATIOS

Bioavailability of nutrients is dependent on their concentration in the soil solution and on exchange equilibria between solution and adsorbed nutrient species. Elucidation of these nutrient supply relationships has been hampered by the fact that soil solution is not only difficult to extract, but its composition may change significantly during storage or incubation of held-moist soil prior to solution extraction. The objectives of this study were to evaluate the effects of liming on the exchange relationships of the major cations (Ca, Mg, K, and Na) and to determine the causes of incubation-induced changes in their solution concentrations. For periods of up to 1 yr, soil solution was extracted from soils treated in the laboratory with CaCO3 and incubated at approximately held capacity. Solution samples were also obtained from field-limed soils (lime applied 17 yr previously) after 1 and 10 wk of incubation in held-moist condition. The concentrations of cations in solution increased substantially during incubation. These increases were balanced by NO3-, indicating that the changes in cation solubility were microbially mediated. In contrast to cation concentrations, cation ratios (i.e., Mg/Ca, Na adsorption ratio, It adsorption ratio) were little affected by incubation. These ratios were directly related to the composition of the exchange phase and were strongly influenced by liming. Protons generated by mineralization of organic N during incubation displaced exchangeable Ca, Mg, IC, and Na in ratios that maintained the equilibrium between solution and exchangeable phases. The results suggest that a reasonable approximation of soil solution cation ratios can be obtained from exchangeable cation data.