Einfluß einer Düngung auf den Diffusionsfluß nicht gedüngter Kationen

Effect of Mineral Fertilizer Application on the Diffusion of Cations that have not been Added with the Fertilizer . Mineral fertilizer application increases the concentration of all cations in the soil solution, because the cations added displace partly those cations occupying exchange positions into the soil solution. In pot experiments, therefore, the K concentration could be raised by Mg-fertilization and vice versa. In the first experiment the response of alfalfa to Mg fertilization was to be studied. 1.2 g Mg/pot (12 kg soil/pot) was applied before sowing and 0.5 g/pot after each cut. 1.0 g P/pot as CaHPO₄ was added, but no K was added in the first year (except soil No. 6 K). In the 2^nd year 1 g K/pot was added after each cut. At 3 sampling dates soil samples were taken for diffusion measurements. In a second experiment K availability was tested at different K and soil water contents. 4 K treatments were included (0, 9, 29 and 49 mg K/100 g). K and Mg diffusion were measured. In the first experiment an increase of K concentration in the soil solution was recorded (Tab. 1) and in the second one an increase of Mg concentration (Fig. 1). This increase of concentration took place without altering the amounts present in the soil, but it led to an increase of the diffusive flux of the cations in question (Tab. 1 Fig. 2). This is taken as further evidence that cation diffusion takes place in the soil solution and depends mainly on the cation concentration in the soil solution. In those cases in which the cation availability depends mainly on cation diffusion to the roots mineral fertilizer application may affect the supply of cations that have not been added to the soil. This effect will be important in pot experiments to which large amounts of fertilizer are applied and which are not subject to leaching. It cannot be detected by most of the conventional soil testing methods because the amounts of cations - except, of course, those that have been added - have not changed. Only an analysis of the soil solution will reveal whether this ?salt effect”? is significant or not in a specific case.