Phosphorus supplying capacity of rice soils in Northeast Thailand and the Philippines and amendments by rock phosphate and rice straw

The P supplying capacity of three Thai rice soils (Ubon, Roi-Et and Warin Series) and one Philippine soil (Luisiana soil) was evaluated and the effects of different phosphate sources and rice straw amendment on growth of rice on the P-deficient soil from the Philippines were studied. The three Thai soils have low fertility, acidic, have silt loam texture, low clay content, low organic matter, low CEC, low available N, K, and Zn and low active Fe and Mn. Available P is adequate in the top soil of the Ubon and Roi-Et soil, but low in the top soil of Warin. Luisiana soil has high clay content, low pH, moderate CEC, high OM, high active Fe and Mn, high available N and moderate Zn. P sorption isotherms revealed very low P adsorption in the Thai soils compared to the Luisiana soil. Especially the high active iron content of the Luisiana soil decreases the availability of P. Leaching rather than P fixation likely cause the P deficiency in the Thai soils. In the greenhouse experiments, the pH of Luisiana soil increased upon flooding from 4.3-6.4 while the Eh decreased from +350 to 20 mV. The reduction of Fe (3)-Fe (2) and Mn (4)-Mn (2) resulted in increased P released of adsorbed and occluded phosphates during the first 6 weeks of flooding. Thereafter P concentration declined. Fe 2+ concentration in the soil solution increased to toxic levels (300 mg/L) during the first 6 weeks and remained above 200 mg/L thereafter, which hampered particularly NH4+-N uptake which in turn result to additional N-immobilization and cause N-deficient rice, especially in rice straw treatment. Other elements i.e. Cu2+, Zn2+, Ca2+, Mg2+, HCO3-, Si, Na+, K+ were affected by the treatments but differences were generally not significant. Agronomic data revealed that TSP (tripe superphosphate) was superior to rock phosphate and the combination 1:1 was superior to 2:1 (RP:TSP). Rice straw treatment depressed growth and yield fo IR36 except that of IR26 in the TSP treatment.