The timing of phosphorus (P) application through chemical fertiliser or slurry is key to minimising the risk of losing the nutrient to waterways, Teagasc’s agricultural catchments specialist Eddie Burgess told Lakeland Dairies’ water quality farm walk in Longford on Friday.

Burgess stated that phosphorus should only be spread when land is not waterlogged and has sufficient soaking capacity to avoid the nutrient flowing into rivers or streams.

The researcher cited Teagasc Agricultural Catchments Programme data which found that across an entire catchment of around 1,000ha, an inch of rain on soils with a moisture deficit released only one-quarter of the phosphorus as the same rain does on a saturated soil.

“Rather than focusing on the total amount of P that has been applied to the soil, the timing of application is key to minimising nutrient loss.

“The timing of applying organic manures is much more likely to be at a time when soil is saturated because many factors influence when we are going to spread organic fertilisers,” Burgess said.

Minimising the amount of phosphorus that enters waterways will be important to improving water quality, as although nitrogen is the key nutrient monitored under drinking water quality testing, phosphorus impacts water’s biological quality.

The scientist explained that in rivers and lakes phosphorous is generally the nutrient which causes eutrophication, but this switches to nitrogen in the case of seas and estuaries.


“Nitrogen or phosphorus cause algae to grow which sucks oxygen out of the water.

“If that oxygen is sucked out of the water, the natural life that would normally be there finds it difficult to survive and that has an impact on the species that are living there,” he said.

He commented that Ireland’s current water quality results are “not all bad” as the number of waterways assigned to the lowest biological status has decreased in recent years and the country’s drinking water standards are “very good”.

The significance of waterway biology to farmers is that all water bodies monitored under the Water Framework Directive must reach at least good status by 2027.