Do I need to apply phosphorus fertiliser?

In Northern Ireland, soil phosphorus levels have increased steadily over the past 50 years to the extent that many soils have Phosphorus (P) indices greater than 2, which is the index required to maintain optimal production on intensive grassland.

For example a recent survey of intensive grassland farms showed that 60 percent of silage fields had soil P indices greater than 2. This contrasts with the situation in Northern Ireland 50 years ago when less then 5 percent of soils had P indices greater than the agronomic optimum.

This increase in soil phosphorus status reflects the continuous overuse of phosphorus on farms since the 1940s. Phosphorus inputs in fertilisers and animal feeds have been greater than phosphorus outputs in agricultural product by about 15 kg P/ha/year. This surplus phosphorus has accumulated in soil year-on-year and is now at risk of being lost into drainage water by leaching and surface runoff.

In the Nitrates Directive Consultation Paper the Proposed Action Programme states that Chemical fertilizers containing phosphorus shall not be applied unless it can be demonstrated that there is a crop requirement taking into consideration soil fertility status and the supply of phosphorus available from the application of organic manures on the farm.

For further information on the Nitrates Directive Consultation Paper contact local DARD Countryside Management staff.

Recommendations for phosphorus (P) fertilisers based on soil analysis Apply phosphorus according to this table

Index	Advice
0 Deficient	Low P levels likely to limit production.Increase P applications
1 Optimum for extensive grassland	Extensive grazing: Adequate. If current level of production is satisfactory continue with your usual fertiliser and slurry policy. Intensive grazing and silage: Low P levels are likely to limit production. Increase application with slurry.
2 Optimum for intensive grassland and silage	Extensive grazing: High P levels present. No yield response to added phosphate. Use zero P fertiliser. Avoid slurry application. Redistribute slurry to suitable fields on the farm. Intensive grazing and silage: Optimum for intensive grazing and silage fields. If current production is satisfactory, continue with your usual slurry and fertiliser policy.
3 High	High P present: No yield response to added P. Use zero P fertiliser. Avoid slurry applications on grazing fields. Redistribute slurry to suitable fields on the farm, concentrating on February/March application for first cut silage and dilute slurry applied at a low rate within three days of cutting between cuts.
4 Excessive	Excessive levels of soil P: No yield response to added P. Use zero P fertiliser. Avoid slurry applications on grazing fields. Redistribute slurry to suitable fields of lower P status on the farm, concentrating on February/March application for first cut silage and dilute slurry applied at a low rate within three days of cutting between cuts.