Effect of water soluble phosphate on the utilization of rock phosphate by cabbage and rye plants

The effect of water soluble phosphate, added together with rock phosphate, on the availability of rock phosphate to plants, was investigated in pot experiments utilizing 3 2P labelled KH2P04 and superphosphate. In soil/rock phosphate systems, all the ? that can be taken up by the plant does not become uniformly labelled because rock phosphate ? exchanges only to a limited extent. In such systems, calculated values do not indicate amounts of labile ? but are indices of the ratios in which rock ? and soil ? are absorbed by the plants. A low L value will, therefore, indicate that relative to soil P, less rock phosphate was absorbed than when a high value is obtained.