THE HETEROGENEOUS DISTRIBUTION OF MOBILE IONS IN THE RHIZOSPHERE OF ACID FOREST SOILS - FACTS, CAUSES AND CONSEQUENCES

The structure of forest soils contains chemical information relevant to the growth of the forest. Therefore, operational analysis procedures have been developed to preserve this chemical information by not homogenizing the soil sample. Under slight suction a quasi-equilibrium is established between a circulating soil solution and the surfaces of naturally structured and homogenized soil samples. A soil structure dependent, heterogeneous distribution of water soluble basic cations is normally found in the rhizosphere of acid forest soils. Maintaining the soil structure causes a decrease in the equilibrium concentration of basic cations, especially potassium. In both naturally structured and homogenized soil samples, the exchangeable ions were determined fraction by fraction in an open percolation system. When the exchange solution is percolated under slight suction, the spontaneous exchange on both natural and artificial aggregate surfaces controls exchange with the interior of the aggregates. Aggregate surfaces in the rhizosphere of acid forest soils also have low concentrations of exchangeable nutrients. Wi th potassium, large selectivity gradients within aggregates limit diffusion. Therefore, the soil structure causes a heavy delay of nutrient release in the soil macropores, which for many forest trees represents the effective rhizosphere.