Review of AHDB-funded research on phosphorus management in arable crops

Abstract

AHDB has recently published three Research Reviews and three Project Reports on strategies for phosphorus (P) nutrition of cereals. Current strategy involves maintenance of a level of soil available P that does not restrict full economic yield of the crop. P is applied annually to ensure this level does not decrease due to offtake by crops. The level of soil P is checked at intervals by soil testing. The P recommendation system in the Nutrient Management Guide (RB209) derives from this strategy with index 2 as the level of soil available P to be maintained (the target index) and 3–5 years as the interval for soil testing. One of the Project Reports (No. 529) and one of the Research Reviews (No. 74) reviewed here assessed the application of this strategy in RB209. The general conclusion was that index 2 remained appropriate as the target for most soils to achieve 95% or 98% of potential yield. For well-structured soils, index 1 might be the appropriate target. Application of fresh fertiliser P could raise crop yield at index 1 to that achievable at index 2. However, application of P could not raise yield at index 0 to that at index 2. This Project Report and Research Review provided support for the current recommendation system.

An alternative strategy (‘feed the crop’) was described in Project Report No. 569 and Research Review No. 83. This was based on the premise that the current strategy for P fertilisation is inefficient, wasteful of resources and should be replaced by one that allows a lower concentration of available P in the soil and increased recovery of applied P. Various ways to improve the efficiency of fertiliser P were described: placement, foliar application, seed treatment, a coating for TSP and use of water-insoluble P sources (struvite). None of these showed reliable and convincing benefits. This is not to say that these techniques or products are not worth pursuing and most of them already are used on farms. However, they do not appear to provide a sound basis for a general ‘feed the crop’ strategy. ‘Feed the crop’ as described is not a recommendation system. Some guidance on the effectiveness of P use might be provided by grain or other tissue analysis but this is retrospective whereas soil analysis for P index is predictive.

Available data for P offtake by wheat, barley, winter oilseed rape and forage maize were collated and assessed in Research Review No. 92. It was concluded that the typical offtake of 7.8 kg P₂O₅/kg grain assumed for wheat should be reduced to 6 kg P₂O₅/kg grain. It also was concluded that a single value would be appropriate for both wheat and barley. However, there were fewer data points for barley (400) than there were for wheat (700) so a change to the typical value for barley might wait for further data.

Themes developed in these Project Reports and Research Reviews were drawn together in Project Report 570. Use of grain P concentration was proposed as a method for assessing crop P status, with 0.32% P in grain dry-matter suggested as the critical value. Grain P analysis could be a useful way to assess P fertilisation strategy. P offtake in wheat grain of 6.5 kg P₂O₅/t was proposed, slightly greater than that indicated in Research Review No. 92.

Based on these reports, some amendments to RB209 have been proposed: reduction in typical offtake in wheat grain to 6.5 kg P₂O₅/t, periodical use of grain P analysis for assessing farm P strategy, calculation of P balances to supplement regular soil analysis and indication of circumstances where soil index 1 might be an appropriate target.