A management framework for phosphorus use on agricultural soils using sorption criteria and soil test P

The variation in sorption and desorption of phosphorus (P) among soil types is not captured in current agronomic advice for agri-environmentally sustainable use of P. Phosphorus use is typically based on soil test P (STP) and soils are assumed to have the same rate of response to added P, regardless of sorption properties. The development of P sorption categories, coupled with STP information could improve fertiliser decision making, by making it more site specific and soil type specific. A framework for P sorption specific advice is proposed here integrating soil P sorption dynamics with STP for agronomic and environmental management, at farm and catchment scale. Using a national population of agricultural soils, laboratory measurement of Langmuir sorption maximum (Smax50, mg kg(-1)) and binding energy (k50, l mg(-1)), were coupled with STP (Morgan P) to derive novel categories for P management advice, specifically addressing the build-up and draw-down phases of P in soils. In addition to measured values, Smax50 and k50 were predicted from MIR spectroscopy and pedotransfer functions and used to allocate soils into these new sorption categories. The allocation of soils into a P management category using predicted values indicated that pedotransfer functions offered greater reliability (90% allocation accuracy using an independent test set), however MIR spectroscopy is faster and less resource intensive (67% allocation accuracy using an independent test set). Phosphorus sorption dynamics should be interpreted alongside soil test P and P Index information so that water quality policy can consider the difference between high and very high STP soils based on sorption information. In the absence of laboratory data on P sorption, soils can be classified into P management classes using predicted values from spectroscopy (rapid and cheap) or pedotransfer functions (greater reliability). Further development of the MIR methodology is recommended along with field validation.

Automated summary

The variability in phosphorus sorption and desorption among different soil types is not reflected in current agronomic guidance for sustainable phosphorus use. A framework integrating soil phosphorus sorption dynamics with soil test P is proposed to improve fertilizer decision-making. Predicted values from MIR spectroscopy or pedotransfer functions can be used for classifying soils into phosphorus management categories, with pedotransfer functions offering greater reliability but MIR spectroscopy being faster and less resource intensive.