Basic nutrient cation chemistry of two Ecuadorian andisols

Abstract

Andisols are noted as soils with constant surface-potential colloids dominated by amorphous aluminosilicates. A combination of routinely performed analyses was used to evaluate nutrient status, clay mineralogy, and the charge and cation sorption properties of two acidic Ecuadorian Andisols from experimental fields under crop production. Additionally, a highly weathered soil of south-central Kentucky was studied to contrast the constant surface-potential behavior coming from a mineralogical source lacking in amorphous aluminosilicates. Mineralogical analyses indicated that the clay fraction in those Andisols was dominated by amorphous material but enough crystalline minerals were present to be quantified. Surface charge characterization confirmed that those soils were dominated by constant surface-potential colloids and required an unbuffered approach to assess cation exchange properties. Also, the magnitude of the effects of pH on the negative charge was different among the three studied soils, which confirms the presence of different kinds of major ion-exchange materials, and the predominant role of soil organic matter as a constant surface-potential colloid. Additionally, the marked increase in cation exchange capacity and consequent enhancement in the retention of exchangeable “bases” with increasing pH observed for the Haplustand, Melanudand, and surface Typic Paleudalf was more marked for Ca than for K. In contrast, K was more preferred than Ca in the subsurface Typic Paleudalf, whose clay fraction was dominated by kaolinite. Finally, K/Ca+Mg quantity-intensity relationships for both Andisols showed that the release of K to the soil solution was influenced by the presence of competing cations such as Ca and Mg, and their close relationship with changes in soil pH that were directly affected by soil fertilizer and/or amendment applications.