Phosphate sorption isotherms for evaluating phosphorus requirement of wetland rice soils
De Datta S.K.
Plant and Soil 86(2): 185-196
ISSN/ISBN: 0032-079X DOI: 10.1007/bf02182893
Phosphate sorption isotherms were developed for 5 Philippine wetland rice soils using the conventional technique and a modified one. In the conventional method, P requiremnts of soils varied between 280 and 810 .mu.g P/g soil. In the modified method, they varied from 160-540 .mu.g P/g soil at 0.2 ppm P in solution. Soils with high P-sorption capacities had vermiculite and halloysite as the dominant clay minerals. Soil reduction by flooding decreased P-sorption by 28-70% at 0.2 ppm P in solution. The decrease in P-sorption due to soil reduction was greatest in a crystalline soil with vermiculite and halloysite as the dominant clay minerals and least in a soil with dominant X-ray amorphous silicates in the clay fraction. Desorption of freshly adsorbed P under reduction was greater in HCO3- solution than in CaCl2 and it increased with level of applied P. Desorption patterns of freshly adsorbed P were similar to adsorption patterns but values of P in solution were lower at desorption. Soils varied with respect to desorption of freshly sorbed P. Desorption studies indicate that soils vary in intensity factor with respect to P and thus influence P availability to plants. Use of P-sorption and P-desorption data obtained under reduced soil condition was proposed for detecting P needs of submerged rice soils. Results of a pot study with IR36 at different levels of solution P (reduced) in one soil indicated a high degree of correlation between adjusted P levels and the measured growth parameters. About 0.12 ppm P in the soil solution or 0.46 ppm P desorbed in HCO3- solution (equivalent to 100 mg P/kg soil) was adequate for near-maximum plant height, tiller production, total dry matter yield, plant P content, and total P uptake.