A comparison of values for initial and residual effectiveness of rock phosphates measured in pot and field experiments

Abstract

The effectiveness of Calciphos (500¦C calcined calcium-aluminium C-grade rock phosphate from Christmas Island), Queensland rock phosphate (low carbonate substituted apatite, from Duchess deposit, Northern Queensland [QRP]), and granular North Carolina rock phosphate (highly carbonate substituted apatite from U.S.A. [NCRP]) were compared with the effectiveness of monocalcium phosphate (MCP) fertilisers in pot and field experiments. Three different lateritic soils from south-western Australia and different plant species were used. In the pot experiment, the effectiveness of the fertilisers was measured relative to freshly applied MCP for 5 successive crops. Subterranean clover, oats, triticale and barley were grown for about 1 month before harvesting. In the field experiment, the same plants were grown, and the effectiveness of the fertilisers was measured relative to freshly applied triple superphosphate for 3 successive years. On a dry matter yield basis, the effects of freshly applied rock phosphate dressings measured in the pot experiment were in a similar range to those measured in the field experiment for the same soil types, and the same plant species, indicating that pot trials can substitute for field trials for the evaluation of the initial effectiveness of fertilisers. However, the residual effectiveness values for both MCP and the RPs were generally much lower for the pot experiment, indicating that pot experiments cannot replace long-term field experiments for the reliable measurement of residual effectiveness of fertilisers. For the pot experiment, the relative effectiveness values of freshly applied NCRP and Calciphos for all soils were 0.2-0.3 and 0.1-0.2 respectively. The corresponding values for the field experiment were 0.1-0.3 for NCRP and 0.1 for Calciphos. For the pot experiment, the relative effectiveness of freshly applied QRP on all soils ranged from 0.03 to 0.1, compared with from 0.1 to 0.3 for the field experiment. The relative effectiveness of all fertilisers declined markedly with successive crops (i.e. crops 2-5) in the pot experiment, whereas in the field experiment the relative effectiveness of QRP and NCRP showed no systematic change for the 3-years of the experiment. For the pot experiment the relative effectiveness of residual MCP decreased to about 0.3, 0.2 and 0.1, compared with 0.5, 0.2 and 0.5 for the field experiment for West Dale, South Bodallin and Gibson soils, respectively. For the pot experiment and for all soils the residual relative effectiveness of NCRP and Calciphos was about 0.01- 0.04, compared with 0.003-0.02 for QRP. For the field experiment the corresponding values were 0.05-0.2 for NCRP and Calciphos and 0.01-0.1 for QRP. For both the pot and the field experiments the relationship between yield and phosphorus content was sometimes dependent on fertiliser type.