Phosphorus - Where Does It Go?

Phosphorus (P) is an important nutrient for the productivity of grass, crops and farm animals. It is brought onto the farm either through chemical fertiliser or feedstuffs.

On a typical 100-dairy cow farm the amount brought on to the farm would equate to approximately 2 tonnes of P per year. However, only 40 percent of this P leaves the farm in produce such as milk, calves and cull cows. This difference between the output and input can be treated as a surplus. The highest surpluses are found on intensive and highly stocked farms with high P fertiliser usage and high levels of concentrates fed. So what happens to this surplus P?

P is accumulating in soils. The build up of P in soils is evident from soil analysis results over the last 50 years. Soils have changed from being predominantly deficient to predominantly excessive in P.

P is held tightly by the soil. However, as soil P levels increase an increasing amount of P is lost in drainage water. The level of leaching is small at approximately 1 KG P/ha/yr, but nevertheless highly significant in terms of nutrient enrichment of water.

A significant amount of P can also be lost directly to watercourses from farms with poor waste management practices. The most common sources of problems are leaking and overflowing tanks, dairy and parlour washings, and run-off from dirty yards. Run-off from fields following applications of slurry can also lead to significant loss of nutrients including P. Research shows that the risk of contamination of drainage water occurs not just in the days immediately after slurry is applied but continues for a number of weeks afterwards, if water is flowing in the drains.

In Northern Ireland there is a surplus of approximately 10,000 tonnes of P brought on to farms every year. This surplus will be reduced by the voluntary step taken by the feed industry to reduce the level of P in ruminant concentrates, and the recently introduced Phosphorus Regulations. These Regulations require that farmers must not apply any chemical fertiliser containing P unless there is a requirement. This can only be established through a soil test. Applying fertilisers at the correct amount and at a time when conditions allow uptake of nutrients will lead to greater nutrient efficiency and greater financial benefit. If you find low P levels in some fields you need to ask yourself – where has this P gone?

Details of the Phosphorus Regulations are found in the Guidance Booklet which has recently been issued to all farm businesses.

A series of information meetings to assist you to meet the requirements of these Regulations is being delivered by the College of Agriculture, Food & Rural Enterprise (CAFRE). If you wish to attend a meeting in your locality or have a query, please telephone 028 9052 2770.

All media enquiries to DARD Press Office, Tel: 028 9052 4619.