Fixing Your Leaky Phosphorus Cycle!

14 Apr 2020

UK dairy farmers could reduce their feed costs, minimise phosphorus lost to the environment and reduce their vulnerability to trade prices by ‘closing’ leaks in their farm’s phosphorus cycle.

A phosphorus cycle follows the movement of phosphorus throughout a farm. Excessive import of phosphorus can lead to leaks which can linger in soils for decades. This “legacy phosphorus” may be taken up by future crops or lost as runoff to waterbodies. Phosphorus that reaches waterbodies accelerates algal growth, degrading water quality and reducing aquatic biodiversity. Farmers can improve the efficiency of phosphorus use on their farms by ‘closing’ these leaks, making them more environmentally and economically sustainable.

A common ‘leak’ on UK dairy farms is the overfeeding of phosphorus, which is on average fed 20% in excess of National Research Council’s (2001) recommendations. Phosphorus fed in excess of the dairy cows’ requirement is excreted primarily in faeces, which creates phosphorus-rich manure. This is a challenge for farmers to handle without over-applying phosphorus above the crops requirement. As manure application in the UK is limited by nitrogen, it’s likely that phosphorus is over-applied above the crop’s requirement. Reducing the feeding of phosphorus to the recommended level could reduce feed costs and minimise phosphorus loss without compromising health, productivity or fertility.

Identifying phosphorus leaks

Calculating a phosphorus balance can identify leaks in the cycle to close and improve efficiency. A ‘farm-gate balance’ subtracts phosphorus that is exported off the farm, typically milk, from the phosphorus imported into the farm, typically feed. A ‘soil phosphorus balance’ is calculated by subtracting phosphorus exported from soil (typically grazed grass or harvests) from phosphorus imports into the soil (typically manure and fertilisers). A balance in surplus indicates a risk of phosphorus loss but also the opportunity to improve efficiency.

In the UK, soils are often highly saturated with legacy phosphorus due to previous heavy fertiliser use. So balances, even with little imports, can result in phosphorus being lost to waterbodies. These stores can be drawn down by reducing phosphorus imports or increasing phosphorus exports out of soil. Currently, UK specific practices that dairy farmers can adopt to reduce phosphorus loss have not been identified.

Best practices to reduce phosphorus loss

A PhD project funded by AHDB-Dairy at the University of Reading, in collaboration with Harper Adams University, is recruiting fifty dairy farms across the UK to identify best practices to reduce phosphorus loss. Participants are asked to complete a simple form to allow the calculation of farm-gate and soil balances. If data on the farm’s soil, feed or manure is missing, a farm visit (approx. 1 hour) between Sept 2019 and March 2020 will be arranged for samples to be collected.

Participants receive phosphorus balance and sample phosphorus data in a personal performance report, which contains anonymized data of all participants to benchmark. 

For more information or to get involved please contact PhD student Brad Harrison (Email: or supervisor Dr Partha Ray (e-mail: , phone: +44 (0) 118 378 6783).