Surface heterogeneity in Osorno soil

ltilier Salazar G.1, Mauricio Escudey S.2 y Keith W.T. Goulding3

The surface heterogeneity of Ca/K exchange in volcanic ash subsoil of Osorno, Chile, was studied by microcalorimetry and exchange isotherms. Three types of exchange sites were indicated by differential enthalpy, selectivity coefficients, differential free energy and differential entropy of Ca/K exchange. This shows the high surface heterogeneity of the Osorno soil. The soil exhibited Ca-selectivity as noted by the positive integral (overall) free energy (∆Go) values. Only at small K saturations in the < 2 μm fraction ∆Go was negative showing thus any K-preference. However, a negative differential enthalpy of exchange indicated very strong K-binding on some sites of both the < 2 μm and > 2 μm fraction of the soil. This will be elucidated by mineralogical analysis. The differential entropy of exchange was negative, indicating that the water/K/soil complex is better arranged than the water/Ca/soil system, which does not favour the exchange of Ca/K. It seems that the organic matter in the soil causes thís phenomenon, thus blocking exchange sites.

Keywords: soil, cation exchange, sejectivity, thermodynamic differential parameters adsorptíon sites.
1 Departamento de Ciencias Químicas. Universidad de La Frontera. Casilla 54-D, Temuco, Chile.
2 Departamento de Química. Universidad de Santiago de Chile. Casilla 5659. 2, Santiago, Chile.
3 Soil Science Department, AFRC Institute of Arable Crops Researches, Rothamsted Experimental Station, Harpenden, Herts AL52JQ. U.K