Diffusion of potassium, calcium, iron and acidity in reduced rice soils

To understand the diffusion of cations and acidity in anaerobic soil as influenced by the nature of the co- and counter-ions, profiles of cations and pH were measured in cylinders of anaerobic soils that had been exposed to a planar layer of cation exchange resin at one end. Four soils representing the main lowland rice growing areas of the Philippines were used. The resin was saturated with either K+ or H+. The amounts of K+, Ca 2+ and Fe 2+ adsorbed by the resin were determined after 1, 3, 5 and 7 days (d) of resin-soil contact. The concentration and pH profiles in the soil were determined after 3 and 7d by slicing the soil into 0.2 mm thick sections parallel to the resin layer. The soil diffusion impedance factor, cation ion exchange characteristics and pH buffer power were also determined. In the runs with K+ saturated resin, differences among soils in rates of Ca 2+ and Fe 2+ diffusion to the resin sink followed differences in the concentrations of exchangeable Ca 2+ and Fe 2+ in the soil. The concentration was least in the soil with the lowest CEC and clay content. In all the soils, the ratio of Ca 2+ to Fe 2+ diffusing to the resin sink far exceeded the ratio of exchangeable Ca 2+ to Fe 2+ in the soil. In the diffusion runs with H+ saturated resin, the soil pH in the acidification zone dropped from approximately 6.7 to 2.6 in all the soils. The pH profiles were steepest in the pH range 3.5 to 5.5, which is the range in which the soil acidity diffusion coefficient, D sub HS, is expected to be minimal. Values of D sub HS averaged over the zone of pH change were estimated from the measured soil moisture content diffusion impedance factor, soil pH buffer power, and the difference in concentrations of H3O+ and HCO3-over the zone of pH change. The value of D sub HS ranged from 2.7 to 4.8 x 10 raised to negative nine mol/sq.dm/s in the four soils and the spreads of the pH change in the soil roughly matched the values of D sub HS. Equations are given for developing a mathematical model of multiple cation and acidity diffusion in anaerobic soil. The data presented in the thesis are sufficient to test such a model