Peg phosphate levels to avoid vulnerable zoning

30 January 1998

Peg phosphate levels to avoid vulnerable zoning

ACT now to prevent the introduction of phosphate vulnerable zones. That is the advice of phosphate specialist Paul Withers of ADAS Bridgets, Hampshire.

Up to half the phosphate pollution entering water comes from agriculture, according to the Department of Environment, Transport and Regions recent consultation document on preventing water pollution.

To get a better understanding of phosphate issues MAFF is researching the risk of eutrophication, algal blooms, oxygen depletion and animal and plant death in affected waters.

Although the output of phosphate from farming is generally expected to be small compared to that from sewage outfalls, there is considerable variation across catchment areas, notes Mr Withers.

"Farms that use large amounts of manure, especially pig and poultry, risk a build up of phosphate in the soil to a level when it will leak out," points out Mr Withers.

Farmers should aim for P indices of 2-3 and no higher, with extra provision for crops with a high P requirement, like potatoes, only.

Furthermore, work at IACR Rothamsted shows different soil types have different critical levels beyond which P is released. That makes it impossible to set a universally safe upper limit for phosphate.

"Crops have a relatively low P off-take, so it takes a long time for them to pull reserves down if the index has climbed to a leaking level," comments Mr Withers.

Much of the direct release of phosphate to water can be controlled by good management. He urges farmers to:

&#8226 Avoid soil erosion – it washes phosphate reserves straight into watercourses.

&#8226 Control slurry and manure storage to avoid run off.

&#8226 Apply no organic manure or inorganic fertilisers on fields that are frosted, flooded or at field capacity.

&#8226 Use manure and slurry as a resource matched to P index and crop need, not a waste to dump on one field.

"The EU Urban Waste Water Directive already sets standards for phosphate discharges in nutrient sensitive areas. So it will be prudent for farming to keep phosphate leakage to water as low as possible," notes Mr Withers. &#42

PHOSPHATE RISKS

&#8226 P can cause water problems.

&#8226 MAFF investigating dangers.

&#8226 Vulnerable zones possible.

&#8226 Act now to reduce risks.

&#8226 Avoid erosion, run-off and excess applications.