The Agricultural Sustainability Support and Advisory Service (ASSAP) interim report shows that the main water quality pressures identified from catchment scientists in the Local Authority Waters Programme was the loss of phosphorous (P) and sediment to water bodies.
Since 2018, ASSAP is providing advice for farmers on suitable mitigation measures to improve water quality from farming. Achieving ‘good status’ for all waters in Ireland is a key national goal.
In grassland soils, applied phosphorous accumulates close to the surface. During heavy rainfall on heavy soils or when the soil is saturated, the majority of the water will run off the surface.
As a result, the water moves in or on the soil where P is most concentrated. The run-off carries both dissolved P and P attached to clay particles.
According to research by Teagasc, eight to 10 heavy rain storms will carry more than half the P lost to rivers and lakes over a given year.
When sediment enters a stream, it settles on the river bed which results in the loss of macroinvertebrate habitat and spawning ground.
P binds to this sediment and this leads to excess nutrient load and the formation of algal blooms or eutrophication. This reduces the oxygen levels in the stream.
What can farmers do?
Fertiliser and slurry application should be targeted at optimum times throughout the main growing season, especially on low-permeability soils.
When soils are wet, cold (less than 6C) and heavy rain (more than 10mm) is due within two to three days, there is a greater risk of run-off from slurry and fertiliser.
To reduce this, farmers should lower the application rate accordingly, target the driest fields and avoid steep slopes near water bodies.
P does not bind to peat soil particles, therefore peat soils do not have the ability to build up a store of P.
A riparian buffer zone and the planting of trees or dense vegetation in these areas are other measures which can be put in place to protect watercourses from nutrient and sediment losses.
Other mitigation options to reduce sediment loss include preventing livestock access into drains and streams and providing alternative drinking water sources and employing correct drain maintenance.