Modification of phosphate uptake model considering changing buffering capacity of soil and release threshold limit of phosphorus in soil due to rhizosphere effect

To study the effect of root-secreted organic acids on phosphorus (P) uptake by wheat and rice and for better understanding and predictability of phosphorus uptake model, a series of green house and laboratory experiments was conducted in three different types of soils: Alfisol, Vertisol, and Inceptisol. Root length density and phosphorus uptake were estimated from pot culture experiments. Phosphorus desorption isotherms were determined by equilibrating soil with organic acids mostly found in rice and wheat rhizosphere. Considering the change in buffering capacity of soils in the presence of organic acids, introducing the concept of phosphorus release threshold level a phosphorus uptake model was modified. Buffering capacities of soils are greatly influenced by organic acids. The desorption isotherm was used instead of adsorption isotherm of P for estimating buffering capacity and P release threshold level. Release of nonlabile P was more in the presence of organic acids. Maximum root influx rate (I-max) decreased with plant age, but Michaelis-Menten constant (K-m) was independent of plant age. The model was run by taking into account interroot competition for prediction of phosphorus uptake by crops. The model outputs were used for comparison of P uptake in pot culture experiments. The observed and predicted values agreed well as tested by chi(2) analysis.