Routes to improving the efficiency of phosphorus use in arable crop production

Phosphorus (P) fertilisers are essential to sustain and improve crop productivity but they contribute to P surpluses in soil and to P exports from crops to livestock and from land to water. Consequences of P use are economic cost, eutrophication, and depletion of finite rock phosphate reserves. This review examines evidence underlying current P nutrition strategies for cropping and assesses scope for innovations to achieve more sustainable approaches.

Current P recommendations largely rely on soil P storage rather than fresh P to optimise crop P supplies. This reliance results from poor capture of freshly-applied P by plant root systems and rapid immobilisation of plant-available P into less available forms in the soil matrix. Soil structure, moisture, temperature, pH and redox conditions can also constrain P supply from soil to root. Some inappropriate P use may also arise through growers justifiably lacking confidence in current soil P tests.

The balance method of assessing efficiency of fertiliser P use appears misleading because it discounts P contributions from non-labile soil sources. Long term studies indicate that soils release at least 5-9 kg/ha/year P without any fertiliser use, and net recoveries of conventional fertiliser P are only 10-15%. The philosophy of feeding the soil rather than the crop must therefore be questioned.

Three key strategies are proposed to improve sustainable P use: (i) minimising crop P requirements, (ii) maximising root recovery of soil P, and (iii) developing targeted fertiliser technologies with as complete P recovery as possible. Large genetic variations in leaf P and crop P (% DM) and in rooting and soil P acquisition offer much potential to improve crops and varieties by breeding; low phytate cereal mutants offer particular promise. Sites should be prepared and maintained to enable testing for and breeding of P-efficient varieties.

Integrated P management strategies including P fertiliser placement, seed P coatings, foliar P applications, industrially re-cycled products, and products that modify soil P availability provide many potential opportunities to improve fertiliser P recovery. These should be developed, and particularly targeted to meet crop P demand at the most susceptible stages (i.e. establishment to stem extension), so as to reduce reliance on soil P fertility. However, current high levels of soil P fertility inhibit P fertiliser improvement; networked experimental sites should be developed and sustained to facilitate development and validation of P fertilisers. New high-precision fertiliser testing methods are also required. Proven reliability of any P targeting technologies will be crucial to their general adoption. Successful innovations in crop P nutrition could offer significant progress in reducing growing costs, preserving finite global P reserves and reducing export of P to livestock and in runoff from land, whilst also facilitating renewed enhancement of crop productivity.