Can grain P concentration be used as an indicator of fertilizer requirements in winter wheat?

Available phosphorus (P) management is a continuous task in wheat-based systems of the UK, primarily to balance applying enough P to support high yields while avoiding unnecessary costs and damaging losses to the environment by applying too much. Grain P concentration with a corresponding threshold value of 0.32 % has been proposed as a new method for P management, supporting or replacing soil test-based evaluations. The objective of this study was to investigate if this approach was a reliable option. We used data from the long-term “Exhaustion Land” experiment on the Rothamsted Farm in southeast England, to investigate the relations between winter wheat grain yield, grain P concentration, and Olsen P values in winter wheat over the last 32 years. Our results show that maximum grain P concentrations in high yielding years are much lower than in low yielding years, indicating a dilution effect through high assimilate transfer to grains. We could not confirm a lower threshold of 0.32 % grain P as an indicator of crop P deficiency at high yields, in our trial the value was closer to 0.24 % grain P. The Olsen P test at our site was a good indicator of P response, and the Olsen P threshold value of 20 mg P kg –1 was sufficient to support the highest yields of winter wheat. We conclude that P recommendations for cereals should continue to be based on soil Olsen P values, possibly supported by better estimations of P exports using grain analysis. Evaluation of the suitability of grain P concentration as a tool for P fertilizer management in cereal based systems would require more research. In the future, the existing Olsen P Index classes in the current UK Nutrient Management Guide (‘RB209’) should be reviewed to possibly increase P fertilizer use efficiency and reduce P losses to the environment whilst maintaining current production levels. • We assessed the suitability of grain P concentration in winter wheat for P management. • We studied long term trial data on yield, grain P, and Olsen P from 32 years in the UK. • Maximum grain P in high yielding years was much lower than in low yielding years. • We could not confirm a threshold of 0.32 % grain P as indicator of crop P deficiency. • We propose that winter wheat P recommendations remain to be based on Olsen P values.