Soil-root interactions in the rhizosphere affecting plant availability of phosphorus

Abstract

Phosphate depletion of the soil around plant roots was studied by quantitative autoradiography and thin slicing rhizosphere soil. In addition, desorption of soil phosphate, pH and phosphatase activity in the rhizosphere as well as root hair length of several plant species were measured in order to quantify factors affecting soil P depletion by plant roots. Steep gradients of soil P developed around plant roots within a few days. P concentration at the root surface was decreased in the soil solution to one μmol/l and the isotopically exchangeable and HC1‐soluble soil P fractions by 30 ‐ 50% of the initial value. The radial distance of the depletion extended to about 2 mm from the surface of the root axis. The shape of the depletion profile depended on soil texture, pH of rhizosphere soil and root hair length. Relative P influx of seven plant species in a low P soil was significantly related to root hair length. Both acid and alkaline phosphatase activity in the soil in a zone of 2 mm from root surfaces were strongly increased. Fractionation of soil P indicates that the organic soil P fraction was depleted in the same zone. It is concluded that the quantity of soil P available to a crop is markedly affected by interactions between soils and plants.