What is a farm Phosphorus (P) balance?

Date published: 14 November 2025

A farm Phosphorus (P) balance is the difference between the amount of Phosphorus entering and leaving the farm expressed over the eligible land area controlled in a 12-month period.

Farm inputs such as feed and fertilizer, as well as manure imported and livestock purchased will all contain differing amounts of phosphorus as do farm outputs such as milk, grain, potatoes, livestock sold and manure exports.

It is important to differentiate between Phosphorus and Phosphate. Phosphorus (P) balance measures the movement of phosphorus on and off farm. Chemical fertilisers are identified by phosphate content and this figure needs adjusted to phosphorus before adding into the calculation (CAFRE online calculators do this automatically).

How to calculate a whole farm P balance?

A farm’s P balance is calculated retrospectively for the previous calendar year (1 Jan to 31 Dec), by calculating the phosphorus coming onto the farm and subtracting the phosphorus contained in produce and manure leaving the farm. This figure is then divided by the eligible land area controlled. Animals born on the farm and crops retained for home use during the year are not included in the calculation.

Currently derogated farms must complete a P balance annually and this must be below 10 kg P ha yr.

An example of how to calculate a P balance for a beef farm is shown below:

Land area 30 ha

*Percentage on bag or label is expressed as phosphate, P balance is calculated on phosphorus.

P balance = (588kg P brought on – 277kg P leaving) / 30 hectares = 10.4 Kg P/ha

If the farmer was able to purchase 25 0 5 fertiliser instead of 25 5 5 this would reduce the P balance by 4.9 kg P/ha making their P balance 5.6 kg P/ha. Use of soil analysis results would help to show if this was an option for the farm.

CAFRE has an online P balance calculator that will help you calculate your farm’s P balance. It is accessed via your Government Gateway at: https://www.daera-ni.gov.uk/services/daera-online-services. This programme is confidential, and data will not be shared or viewed by anyone but the farm business owner or their agent (if permission is granted).

Why is this useful?

Where the farm P balance is in surplus (more P coming on farm than leaving) P starts to build up in the soil. This can benefit soil fertility and farm production where soil indices are low, but where excessive amounts start to accumulate in the soil beyond that necessary for optimum farm production, this is when loss to waterways can occur.

A negative farm P balance shows more phosphorus is leaving the farm than coming in. This can be useful when trying to reduce soil indices but equally on some farms could impact production due to reduced available phosphorus.

In many cases purchased animal feed is the major source of phosphorus coming onto farm, which can often be overlooked. Improvements in feed efficiency in livestock enterprises will result in significant reductions in P balance. For example in a 100 cow dairy herd producing 9,000 litres of milk per cow, if the feed rate is reduced from 0.4kg/litre down to 0.35kg/litre (a figure closer to the benchmarked average) 248kg of P can be removed from the system.

In summary

Whilst P Balance is primarily a measure that can be used to identify excess Phosphorus or Phosphorus buildup on a farm, it can be reduced through the improvement in efficiencies which have been understood for many years.

P is a finite and expensive resource, and its managed use will not only contribute to reduced losses from the soil but also improved profitability.