The influence of rhizosphere in rice crop on resin-extractable phosphate in flooded soils at various levels of phosphate applications

Summary Poor or lack of response of lowland rice to P fertilization is a well-known fact. Several studies were conducted in this direction however, our understanding regarding the underlying mechanism has been far from clear. A remarkable influence of rice plants on P transformation in submerged soil is identified in this experiment which may shed light on this problem. Accordingly, in presence of rice plants P was mobilized during the initial growing period followed by immobilization. The increased microbial activity in presence of physiologically active roots was responsible for P mobilization, while capacity of rice plants to reoxidize the rhizosphere, by secretion of oxygen from roots, during later growing period was responsible for P immobilization.The extent of P mobilization decreased while that of immobilization increased with increasing P levels in different soils. At a given P level the ratio of P mobilization to immobilization was higher in a soil where crop growth was better and P uptake was higher as compared to another soil where crop growth was poor with lower P uptake.Thus, lowland rice plants appear to possess an unique physiological mechanism, to regulate the contrasting changes in P availability in the rhizosphere depending on P requirement by the plants or P availability in soil, which in turn is responsible for the poor or lack of response to P fertilization.The experiment was conducted in a growth chamber. Two soils with widely varying properties were used.