| Abstract: | Influence of potassium supply on the availability of potassium in the rhizosphere of rape (Brassica napus) Potassium depletion of the soil in the proximity of roots was studied in order to obtain quantitative information on the availability of potassium. For this purpose rape seedlings were grown in pots which separate roots from soil by a fine meshed screen; root hairs penetrated the screen. The soil adjacent to the screen was sliced by microtome into layers about 0.1 mm thick which were separately analysed for k. Plant roots strongly depleted the soil in their proximity; further distant ranges remained unchanged. A loess derived loam, brought to different levels of exchangeable K by precropping or K application, was equally depleted to 150 μmoles/100g soil at the root surface. Therefore, the quantity of K released from this source increased with initial K level. In addition, the distance of the depletion zone extended with K level from 4,6 to 6.3 mm from the surface of the root cylinder. Hence, the volume of soil contributing K to the root increased from 0.7 to 1.2 cm3 per cm root length. The combination of these two parameters, i.e. the quantity of exchangeable soil K released per unit of root length increased by a factor of 20 whereas exchangeable soil K was raised by a factor of 4.5 only. K uptake of the plants after 4 days was in agreement with the depletion of exchangeable soil K in the high K treatment only. The other treatments had obviously taken up considerable percentages of nonexchangeable K. This fraction was released from the soil ranging less than 1.5 mm from the root. The distance of the K depletion zone was also extended by application of NaCl and MgCl2. Because of cation exchange, K concentration of the soil solution was increased, K buffer power decreased and, therefore, K diffusion was enhanced. It is concluded that plants in the field do not uniformly deplete the total rooted soil volume. Whereas roots in their proximity strongly deplete the soil including parts of nonexchangeable K they do not even use exchangeable K in a slightly greater distance. The quantity of K available per unit of root length is, therefore, determined by both - the degree of soil K depletion at the root surface and -the distance of the depletion zone, i.e. the volume of soil that contributes K to the root. Either factor was markedly affected by the level of soil K and thus by K application. |
