ASSESSMENT OF METHODS FOR STUDYING THE DISSOLUTION OF PHOSPHATE FERTILIZERS OF DIFFERING SOLUBILITY IN SOIL .3. THE RELATIONSHIP BETWEEN RATE OF DISSOLUTION AND PLANT-RESPONSE

Relationships between plant response and rates of dissolution of groun d (< 150 mum) North Carolina phosphate rock (NCPR), NCPR 30% acidulate d with phosphoric acid (NCPAPR) and monocalcium phosphate (MCP) were a ssessed in pot experiments. The three fertilizers were incubated for 1 , 50 and 111 days, at the rates of 75, 150 and 750 mug P g-1 soil, usi ng two soils with different P-retention capacity. After each period of incubation, four pots were set up from each treatment, and perennial ryegrass (Lolium perenne) was grown in a growth chamber for about six weeks to asses the agronomic effectiveness of the fertilizers. Results in dry matter yield and P uptake showed that immediately following ap plication (1 day incubation), the MCP (solution) was supplying more P to plants than either the NCPR or the NCPAPR applied at the same rate. After 50 and 111 days of incubation, the NCPR and NCPAPR were just as effective in the lower P-retention Tekapo soil. The relative agronomi c effectiveness (RAE) of the NCPR and NCPAPR compared with MCP was gen erally poorer in the higher P-retention Craigieburn soil than in the T ekapo soil shortly after application, but improved with time of incuba tion. Ryegrass responses to the application of the three fertilzers co rresponded to the changing trends of exchangeable P in the soils, meas ured by the isotopic method. Regressions were made between plant P upt ake and indices describing the intensity factor (water extractable P), quantity factor (Bray I P, Olsen P, 0.5 M NaOH extractable P and isot opic exchangeable P) and the kinetic factor (F(in)) of soil P supply t o plants in the Tekapo soil. The percentage of variation in plant P up take explained by individual indices was generally less than 80%, no m atter which of the three single variable models, the Mitscherlich, the quadratic or the power function was fitted. However, more than 96% of the variation in plant P uptake in the Tekapo soil could be explained by the power function models involving two variable. The rate of P di ssolution (F(in)) determined by the isotopic dilution method was inclu ded in all the two variable models. The results suggest that assessmen t of soil P supply to plants should consider the kinetic factor in add ition to the intensity and quantity factors, particularly where P fert ilizers with differing solubility are applied.