Phosphorus has an important role in grass growth

By Colm Kelly, Climate Action & Sustainability Adviser, Teagasc Galway/Clare

Phosphorus (P) has an important role in grass growth with deficiency of P leading to poorer root development and a drop in overall grass yields throughout the grazing season. It is also a pollutant when it enters watercourses that can cause water quality problems. Therefore, it is an important nutrient to apply to grassland but management around its application is crucial to avoid its loss to watercourses. A trend of lower chemical P sales may indicate under application on some farms. This reduces grassland performance having a negative effect on our ability to reduce slaughter age, use chemical N efficiently to meet climate targets and farm profitably.

Phosphorus Needed?

The way regulation works farmers need to demonstrate a need for chemical P. This requires a Nutrient Management Plan that uses stocking rate, meal purchased, soil sample results and on some farms soil organic matter maps to calculate allowances. Peaty soils can only have a maintenance allowance, as they do not hold P. The standard advice is to know how much P fertiliser you can purchase for your allowance. If I buy my chemical P in the form of 18-6-12 for example my advisor can calculate that I can buy x tonnes and if I stay below this figure I know I am compliant.

Scenario 1: Jim has a low Phosphorus allowance:

Farms with good or high P soil test results often have small to no allowances for chemical P. In that case, it is good news, P is there in sufficient levels and Jim won’t need to buy more that he won’t get a response from. There will be P offtakes from Jim’s farm each year from stock, crops or milk he sells so he should consider testing more regularly such as every second year to catch a decline. Typically, 60% of P taken up through grazing can be recycled onto pasture through dung or urine.  Once P soil test results start to drop to an index 3 Jim will begin to get maintenance allowances again and can go back in with P to keep the land optimal. What Jim could focus more on is the P distribution from slurry in particular.  Looking at soil test results a reading above 8 mg/l is an index 4. On some farms there can be samples reading much higher. These fields typically are easy to access fields that have ended up with surplus P built up over years. More economic responses could be achieved directing slurry to paddocks with lower indexes or land with high offtakes such as silage fields.

Scenario 2: Kate has low Phosphorus soil test results:

In this case, Kate is likely not optimising grass growth. If Kate was lowly stocked, perhaps she is growing enough grass for her system and grass utilisation is the ultimate goal. Kate however has a moderate stocking rate and she is feeling the pinch early/late in the year and in silage yields. This is leading to longer housing periods, lower animal performance from grass and more reliance on purchased feed to drive performance. Kates plan is to apply lime where required and then target slurry and chemical P to high offtake fields such as silage and the best grazing fields with low indexes avoiding saturated land when high rainfall likely. Part of Kate’s farm is in the ACRES scheme and she knows from her advisor if lower P rates apply to the actions chosen for these fields. On low intensity biodiverse pastures additional nutrients can have a negative outcome for plant diversity. A P build-up option provision applies to intensively stocked grassland farmers with a grassland stocking rate ≥130 kg N/ha prior to manure export. An advisor is required to apply for this additional allowance.  

Reduce Phosphorus loss?

Most P loss is associated with land that is slow to drain. In heavy rainfall conditions, overland flow occurs where P contained in slurries, fertilisers, soiled water or soil sediment is washed into surface water. Catchments.ie is a website where farmers can view predicted p pollution impact potential (PIP) maps, which predict which areas on your farm may be vulnerable to P loss. Complying with buffer zones and avoiding overland flow risk areas particularly when heavy rainfall is expected are practical measures to reduce losses.

Teagasc FBD Environmental Sustainability Awards 2025

These awards recognise farmers that are doing a good job to improve environmental sustainability while also farming efficiently and profitably. We want to share the stories of farmers that are embracing the actions that reduce greenhouse gases, enhance biodiversity, improve water quality and soil health.  We have introduced two new categories this year – organic farming and diversification.  Diversification options that improve profitability but also improve environmental sustainability.  If you would like to share your story and win up to €10,000, apply today at www.teagasc.ie