Predicting the potential of soils to supply phosphorus by integrating soil chemical processes and standard soil tests

Samenvatting

In agriculture there is a growing need to use phosphorus (P) fertilizer more efficiently because of P related environmental problems and diminishing P reserves. A key factor to achieve this is an accurate prediction of the P supply potential of a soil. To improve the choice of soil tests and interpretation of the corresponding results a new methodology is proposed. The methodology is derived based on the continuous removal of P from soils using an artificial P sink (Fe oxide-impregnated paper) and linking the results to standard soil tests. To achieve this, the desorption results are modeled based on the adsorption characteristics of the P sink and a soil specific Langmuir desorption isotherm in which the parameters are calculated a priori from standard soil tests. To be able to make any prediction of the P supply potential a minimum of two parameters is needed: a measure for the reversibly adsorbed P (Q) and for the P concentration in solution (C). The best prediction is obtained when Q was approximated by P-Olsen, followed by PAL and Pox. The measure for C is an indication of the rate with which P can be removed from the soil and is approximated by P-CaCl2. The ratio Q over C is an indication of the capacity of the soil to buffer C, and thus the capacity to maintain the flux from the soil to a sink. The accuracy of this prediction increases for soils with high buffer power, i.e. with low P saturation of the reactive surface area associated with the readily desorbable P. In this case the Langmuir isotherm is linear. To increase the accuracy of this prediction for soils that do not have a high buffer power, and the Langmuir isotherm is thus nonlinear, a measure for the reactive surface area (e.g. Fe-ox and Al-ox) of the soil must also be taken into account. In addition, to be able to extend the prediction of the soil P supply potential to an amount of P exceeding the amount of readily desorbable P. a measure for the total desorbable P content must be included (e.g. P-ox). This total desorbable P content dictates the level at which C is highly buffered once the reversibly adsorbed P has been depleted. The methodology was verified in pot- and field experiments. Implementation of the methodology based on two parameters in a Dutch routine soil laboratory has resulted in a more accurate P fertilizer recommendation for grassland.