20 June 1998

PLUGGING THE P GAP

Its easy to neglect the phosphorus status of soils when there are no apparent difficulties with yield and the focus is on nitrogen supply.

FOR all the talk of using NPK fertilisers, the emphasis for the vast majority of growers is on getting nitrogen supply correct for the greatest possible effect on yield.

Allowance is usually made for phosphate offtake in the previous years cropping but nitrogen remains the number one nutrient. Results from a 30-year trial on the same continuous barley site may stimulate a rethink for growers.

It is clear, says Ian Richards, managing director of Levington Agriculture, which has monitored the trial since 1968, that the total amount of phosphate needed to give the economically optimum grain yield over the period of the trial was considerably greater than the amount of P necessary to replace offtake in grain.

Nearly twice as much, in fact. Over the 29 years for which there are results – only in 1994 did the barley fail and have to be re-drilled as spring rape – the phosphate requirement was 70kg P2O5/ha/year. Annual phosphate offtake in the grain and straw averaged out at 40kg P2O5/ha/year.

In the years following the Second World War, the P index in soils was built up by regular applications of basic slag so that once common index 0 soils largely disappeared. Now slag is no longer applied on a routine basis there is likely to be a gradual decline in P levels over 15-20 years unless the field supply is kept at more than the amount taken off annually by cropping.

Dr Richards says that for any meaningful results from monitoring phosphorus, the work has to be carried out over a long period of years. On the Cornish barley site, four rates of fertiliser P – 0, 22.5,45 and 90kgP2O5kg/ha – were applied over the period from 1968 to 1996 when Hydro Agri asked Levington to carry out soil analyses. Both phosphorus and cadmium were measured in seven soil layers down to 50cm (20in). No evidence was found of phosphate enrichment of the soil below the plough layer, even where 90kg P2O5/ha had been applied every year for 29 years. Nor was there any sign of build-up of cadmium in the soil or crop.

In the Cornish trial, the Powys series soil was at MAFF index 2 for P. As all straw was removed the current recommendation would be for applications of 22kg P2O5/ha annually to replace the estimated P removal in grain. However, the Levington results show that this amount of phosphate to keep the soil status at index 2 was insufficient to obtain optimum yield of barley.

The loss of grain by fertilising just to cover offtake and maintain the P index was small but nevertheless significant at almost 0.2t/ha (1.5cwt/acre) each year. Dr Richards says it may be necessary in future to reconcile the conflicting requirements of water protection and crop production if sufficient phosphate is to be applied to enable growers to attain the most economic crop yields.

Evidence from the trial indicates that cadmium levels in grain or straw were not increased significantly. Since all phosphate fertilisers inevitably contain cadmium present in the source rock, it seems possible that cadmium was leached below 50cm on the free-draining Cornish site with its relatively low organic matter content.

Extra phosphate may be needed to replenish reserves lost – and more.