EFFECTS OF ORGANIC MATTER AND IRON OXIDES ON CATION EXCHANGE EQUILIBRIA AND POTASSIUM SELECTIVITY IN A VOLCANIC ASH SOIL OF CHILE

Abstract

The volcanic ash soils of Chile are naturally acidic, of variable charge, relatively rich in organic matter (OM) and contain little reserves of potassium (K). Ion exchange controls the availability of nutrient cations such as K and is strongly influenced by the OM and iron oxide content of the soils. Agricultural use has depleted OM, destabilized structure and made the soils more acidic. To obtain basic information to guide soil management for nutrient availability, we measured calcium–potassium (Ca–K), calcium–sodium (Ca–Na) and calcium–magnesium (Ca–Mg) exchange equilibria at 5°C and 25°C in three <2 μm fractions of a Vilcun Series volcanic ash soil of Chile: the whole fraction—Clay, the fraction with OM removed—Clay–OM, and the fraction with OM and iron oxides removed—Clay–OM–Fe. The cation exchange capacity of the fractions decreased in the order Clay–OM–Fe>Clay–OM>Clay because the OM, especially, and to some extent Fe blocks exchange sites, and because removing Fe increases the pH and the cation exchange sites. The selectivity coefficients for exchange were calculated by the Gaines and Thomas method. Graphs of the selectivity coefficient against fractional saturation suggested three types of exchange sites, a first group with some limited selectivity for Na and K over Ca at small Na and K saturations (15–20% of the exchange complex), and two different types with selectivity decreasing with increasing Na and K saturation. Calcium was preferred to Mg for the whole exchange.