6 October 1995

Phosphates are now firmly on the agri-agenda

Phosphate pollution of lakes and rivers has sparked a flurry of activity among scientists. Andrew Blake reports from last weeks International Workshop on "Phosphorus Loss to Water from Agriculture", in Ireland

PHOSPHATE fertilising of crops and the use of animal manures in particular seem set for considerable overhaul after the meeting at Johnstown Castle Research Centre, Wexford, Eire. Organised by TEAGASC, the countrys agriculture and food development authority, it attracted specialists from Europe and the US.

Phosphorus added to the soil has done much to boost food output in the past 150 years. But as reserves have built up so has the environmental run-off. Once pristine trout-fishing lakes are now threatened by P-induced algae.

So far phosphorus losses from farming in England and Wales have "not been high on the agenda", according to John Archer of ADAS. But a "substantial programme" soaking up 12% of the annual £5m spend on nutrient R&D."

The ministrys Code of Good Agricultural Practice for the Protection of Water, currently under review, could include a new section on phosphate, he said.

Nobody openly disagreed with TEAGASCs Hubert Tunney when he said: "You can have too much of a good thing." But the three-day event showed solutions will be hard to come by.

Swedish experience suggests the benefits from cuts in inputs could be a long time coming. The country has "drastically" reduced its use of P fertiliser since 1970. "But weve still not seen any reduction in phosphorus losses," said Barbro Ulén of the Swedish University of Agricultural Sciences.

The long-held view that phosphate does not leach and that any excess applied is effectively "money in the bank" for following crops took a hammering.

But unlike the nitrate issue, where leaching is the main concern, the meeting showed problems with phosphorus are more complex.

Until recently it was thought most P escaped to watercourses through surface erosion. Now it is clear that on some soils, especially lighter sands, relatively small but environmentally significant amounts can leak out.

Even where surface losses are the main route, the picture is far from straightforward. "Were finding that the area where run-off occurs is quite variable," said Andrew Sharpley of the Pasture Systems and Watershed Research Laboratory in Pennsylvania.

The word "diffuse" echoed through the proceedings. Assuming that so-called point sources, such as leaky slurry pits, have been eliminated, a key target, delegates agreed, is to identify specific areas in fields and larger catchments likely to cause trouble. This would allow economic remedies.

&#8226 EU soil P balance rising by 1m tonnes a year.

&#8226 Only small leakage needed to affect watercourses.

&#8226 Loss routes still far from clear.

&#8226 Big differences in soil testing methods.

&#8226 Wide variation in EU fertiliser recommendations.

&#8226 Livestock feed P and manure disposal adding to problems.

&#8226 Optimum soil P hard to define.

Slurry spread near water is an important source of phosphate pollution, but it is far from the only cause. Although phosphate use may stabilise soil reserves could keep rising, says Dr Hubert Tunney.