Relevance of Organic Matter in Some Chemical and Physical Characteristics of Volcanic Ash-Derived Soils

Abstract

The accumulation of organic matter in volcanic soils may modify their properties and characteristics. Soil complexity makes difficult to assign the relative importance of organic matter in determining soil behavior. If changes in soil properties can be associated with the soil organic carbon (OC) content, a better understanding of the organic matter role may be achieved. A total of 20 samples of uncultivated volcanic soils, located in Central-Southern Chile, classified as Andisols and Ultisols were studied. Properties such as active/free iron oxide ratio (activity ratio), isoelectric point (IEP), cation exchange capacity (CEC), and potassium–calcium (K–Ca) cation exchange equilibrium were studied in relation to the soil OC content. The activity ratio increases when OC increases showing the inhibitory effect of organic matter in iron oxide crystallization. The IEP increases when OC decreases, this relationship being useful to follow the soil organic matter destruction in sample preparation. The ΔCEC (CECpH = 8.2 − CECsoil pH) is directly related with OC, showing that an important fraction of the negative surface charge is the result of ionization of organic matter active groups when pH increases over field value. Selectivity for K over Ca was observed, but decreases when OC content increases and a Ca selectivity may be expected in soils whose OC content is higher than 12.6%.