EFFECTS OF INCUBATION AND PH ON SOIL SOLUTION AND EXCHANGEABLE CATIONRATIOS

Bioavailability of nutrients is dependent on their concentration in th e soil solution and on exchange equilibria between solution and adsorb ed nutrient species. Elucidation of these nutrient supply relationship s has been hampered by the fact that soil solution is not only difficu lt to extract, but its composition may change significantly during sto rage or incubation of held-moist soil prior to solution extraction. Th e objectives of this study were to evaluate the effects of liming on t he exchange relationships of the major cations (Ca, Mg, K, and Na) and to determine the causes of incubation-induced changes in their soluti on concentrations. For periods of up to 1 yr, soil solution was extrac ted from soils treated in the laboratory with CaCO3 and incubated at a pproximately held capacity. Solution samples were also obtained from f ield-limed soils (lime applied 17 yr previously) after 1 and 10 wk of incubation in held-moist condition. The concentrations of cations in s olution increased substantially during incubation. These increases wer e balanced by NO3-, indicating that the changes in cation solubility w ere microbially mediated. In contrast to cation concentrations, cation ratios (i.e., Mg/Ca, Na adsorption ratio, It adsorption ratio) were l ittle affected by incubation. These ratios were directly related to th e composition of the exchange phase and were strongly influenced by li ming. Protons generated by mineralization of organic N during incubati on 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 ratio s can be obtained from exchangeable cation data.