Cation exchange processes in structured soils at various hydraulic properties

Abstract

The determination of the cation exchange capacity is one of the most important analysis of chemical properties to predict ion ad- and desorption as well as to device the filter and buffer capacity of soils. In general, soils are homogenized prior to the analysis, thus only the maximum amount of the exchange capacity will be determined. Although soils are never homogenized and rarely water saturated, important influences like soil structure and actual soil water content for plant nutrition or retention of nutrients in soils are not considered. To find out the ecological significant cation exchange processes it is necessary to use natural structured soil samples (cylinder samples), because they represent the field conditions more precisely. The results of the experiments show, that the amount of exchangeable cations of a water saturated structured soil sample is smaller compared to a homogenized sample of the same material. This difference amounts to 15% e.g. for the Bt-horizon of a Luvisol. A drainage of the samples reduces the available pore volume and thereby the amount of accessible exchange places. Subsequently the amount of exchangeable cations decreases. Provided that always the same percolation volume is used per soil volume, obviously there is a correlation between the cation exchange capacity and the accessibility of exchange surfaces which again depends on the water content and water suction. Furthermore, under unsaturated conditions it is necessary to consider the gradients of water suction and the hydraulic conductivity, because they influence the time the exchange solution remains in the soil. This results in a clear dependence between hydraulic conductivity and the amount of exchangeable cations: with increasing percolation time, the time for exchange- and transport processes increases, whereby more cations in the soil are exchanged. Consequently it can be concluded, that it is necessary for the estimation of soils to consider the dependencies between soil structure, hydraulic properties and the cation exchange as well as transport processes.