Phosphorus sorption isotherms of selected acid sulfate soils of the Philippines and Vietnam and improving the productivity of acid sulfate soils for wetland rice

More than 99% of added P was sorbed. The amount of P sorbed under reduced condition was higher than that under oxidized condition. Organic matter and exchangeable Al highly correlated with P sorptivity of these soils. The desorption of freshly adsorbed P by Bray 2 extractant was higher in reduced than in oxidized soils. P retained by the soil system after desorption correlated highly with Fe oxides and active Fe content. P fertilizer required to achieve 0.12 microgram P/ml, a critical level of maximum rice growth is very high for most of the acid sulfate soils studied. Especially, Hoa An, Hau Giang and Ben Ke, Long An, Viet Nam soils required more than 500 microgram P/g soil under reduced conditions. Electrical conductivity [EC] rose up to 12 mS/cm upon submergence of the strongly acid salic acid sulfate soils from Lanjagan, Iloilo, Philippines. Puddling and surface flushing before transplanting lowered water soluble cations. Alternating drying and wetting and leaching the soil alleviated the Fe, toxicity and lowered EC but leached out also essential nutrients. Amelioration of acid sulfate soils by leaching requires large amounts of water, and is time consuming. Leached nutrients have also to be replaced. Mode of level of P application did not improve rice yield on an acid sulfate soil from Rodan, Aparri, Philippines. Nitrogen and P content of rice shoot, straw and grain increased with higher amounts of P applied (up to 100 mg/kg soil) were higher but P supply remained below the critical level and toxicities adversely affected rice growth. Split application of P fertilizer is not effective because most of the P broadcast in splits are sorbed before taken in by the plants. Root dipping in P enriched soil slurry to delay P sorption and improve P uptake merits further research. The P concentration (25 and 100 mg/kg) for root dipping adversely affected initial seedling growth but improved growth after recovery.