The soil skeleton as a source for the short-term supply of "base cations" in forest soils of the Black Forest (Germany).

Abstract

For soil chemical analyses, the soil skeleton is normally rejected because this size fraction is considered to have no significant short-term nutritional potential. In order to revise this practice, the short-term potential for ion storage and mobilization of the isolated and cleaned soil skeleton was investigated by model experiments, using undisturbed and homogenized soil samples as references. The study considered two soil profiles in the Black Forest, Germany: one on granite and one on gneiss bedrock. The method allowed for the investigation of soil columns at a water status near field capacity. After the extraction of water soluble ions with deionized water, cation exchange properties were determined by percolation of the soil cores with ammonium chloride (NH4Cl). Moreover, the influence of different carbon dioxide (CO2) partial pressures in the soil air for ion mobilization was studied in a closed percolating system. The first results revealed site-specific ion mobilization potentials of the soil skeleton. Below the A-horizon, the skeleton fraction of the gneiss site plays the dominant role as a source for short-term base cation supply. The fine earth of the corresponding soil horizon had lost this function, since the base saturation was less than 5 %. The skeleton of the granite site had much lower ion mobilization rates, but nevertheless, its importance for ion mobilization must not be neglected. The results of the mineralogical study supported the findings of the model experiments.