PLANT AVAILABILITY OF 2 PHOSPHATE ROCK MATERIALS IN ACID MALAYSIAN SOILS

The amount of phosphorus (P) dissolved in a closed-incubation system, in soils receiving Christmas Island grade-A phosphate rock (CIPR), Gaf sa phosphate rock (GPR), and triple superphosphate (TSP), as measured by extraction with 0.5M NaHCO3 (DELTAPb) or 0.5M NaOH (DELTAP) and exp ressed as DELTAP(b)/DELTAP100 (PDP) was compared to P uptake (DELTAPs ) by Setaria in a glasshouse experiment. There was no direct relations hip between DELTAPs and PDP for CIPR, GPR, and TSP added at 50 and 150 mgP/kg soil to three Malaysian soils (Bungor, Kundor, and Segamat) du ring a 10-month period. Averaged across soils, rate of addition, and P sources, DELTAPs was 17% higher than PDP. Over-estimation of PDP by D ELTAPs could be due to the ability of the roots of Setaria to use more of the P which is dissolved from the three P sources and then chemiso rbed, than can be extracted by the NaHCO3 reagent. The chemisorbed P p ool is extractable using 0.5M NaOH. There was a close relationship bet ween DELTAPs and DELTAP, with a correlation coefficient of 0.85*. Res idual P, determined by Pb method after each harvest, successfully pred icted by Ps by Setaria in the subsequent harvest with correlation coef ficients varying between 0.74 and 0.99** for CIPR, GPR, and TSP in fi ve soils (Bungor, Durian, Kundor, Segamat, and Tok Yong). The critical Pb values ranged from 5 for Durian to 10 mgP/kg soil for Kundor.