Transformation in chemical forms of various phosphate fertilizers and their effects on the growth and yield of rice in acid sulfate soils

These are conclusions obtained from the 3 main parts of the 10 experiments: 1. Available soil P level for rice production in acid sulfate soil (Rangsit soil series) was very low, having average Bray II-P only 4 ppm PRRC P fertility status was slightly higher, however than that of the KRES because of higher T-P, O-P, I-P, Al-P, Fe-P and R-P though similar Ca-P; 2. Response to phosphate fertilizers of RD 23 rice could be indicated by: grain yield; such yield components as number of panicle/hill and member of good grain/panicle; such growth component as dry weight of straw, top and root and content of grain P and uptake of P in grain and top parts. The responses were very clear (highly significant) in dry season at KRES and highly significant positive correlation between the P rates and the crop behaviors mostly observed the positive responses (less clear than that of the KRES) in wet season at PRRC could also be obtained, if environmental factors were under good control: 3. Residual effect of RP for the rainfed acid sulfate soil could be persisted at least 2 years, while for the MAP, CP, DAP and TSP annual application should be made for the 6.4 kg P2O5/rai or less; 4. All phosphate fertilizers used could be transformed into I-P, Al-P, Fe-P, Ca-P, R-P and O-P in the first cropping season in acid sulfate soil and most of them positively and closely correlated with the P rates at KRES and similarly at PRRC but less clear; 5. Depletion trend of the residual P in the first 3 years could be predicted if proper forms of soil P for particular P fertilizer were selected; 6. Residual T-P and all other P forms including Bray II-P were positively correlated at significant and nearly significant levels but only few at highly significant level because of too few P rates were used at PRRC and 7. Residual I-P forms should positively correlated with Bray II-P at significant level and highly significant one if more P levels at PRRC were observed.