State-space estimation of the intrinsic soil phosphorus pools from soil phosphorus tests

The widely-used measurements of soil P by chemical soil tests cannot explicitly give the intrinsic status of soil P for quantitative management of soil P. Estimating the intrinsic sizes of soil P remains an unresolved problem. The objectives of this study were (1) to develop a new method to estimate the intrinsic sizes of soil P pools, and (2) to quantify the plant-available P in these intrinsic soil P pools. An adsorption experiment of soil P was carried out to determine the dynamics of chemically extractable soil P by the Olsen method (Olsen-P) after fertilizer phosphate was applied to soils. State-space models were developed to describe the intrinsic sizes of two soil P pools including a weakly adsorbed P pool (a reversible part of sorption) and a tightly adsorbed P pool, their transformations, and their relationship with Olsen-P. A cropping experiment was carried out to test the mass balance of plant P uptake and the intrinsic sizes of these two soil P pools. Results showed that state-space models of the two soil P pools could describe the dynamics of soil Olsen-P in the adsorption experiment. Olsen-P was a fraction of the weakly adsorbed P, and this fraction showed a linear relationship with soil P-sorption site density. The measured amount of P removal by plants in the cropping experiment could be estimated by the intrinsic sizes of the two soil P pools, and thus, the two pools both contributed to plant P uptake.