THE STUDY OF SOIL CHEMISTRY THROUGH QUASI STEADY-STATE MODELS .2. ACIDITY OF SOIL SOLUTION

The acidity of three hypothetical soil solutions was studied with a quasi-steady-state model. Processes were assigned to one of three time scales: fast (processes at equilibrium), slow (processes for which the kinetics are specified explicitly), or very slow (processes that have a negligible effect on the steady-state solution). Reactions among species in solution and precipitation/dissolution reactions of solid Al(OH)3(s) were considered to be at equilibrium. The slow processes were uptake of nutrients by plants, root respiration, release of nutrients, production of CO2 and organic acids during decomposition of organic matter, decomposition of organic acids, nitrification, atmospheric deposition, exchange of CO2 with the atmosphere, and weathering. The relative importance of biological, chemical, and physical processes controlling the acidity and speciation of soil solution was examined; three cases illustrate high vs. low rates of weathering and absence vs. presence of organic acids. The sensitivity of the calculated solution pH to values of parameters in the model was examined through calculated sensitivity coefficients and an analysis of equations governing the system. Fluxes of acidity due to biological processes are larger than fluxes due to other processes, but because of internal feedback their effect on pH may not be as large as expected. © 1990.