Water Quality - Phosphorus & ACRES

By Ivan Kelly, ASSAP Adviser, Teagasc Galway/Clare

The protection of water quality from nutrient, sediment and pesticide losses and achieving ‘Good Status’ for all waters in Ireland is a key national goal. Since 2018, The Agricultural Sustainability Support & Advisory Service (ASSAP) is providing advice to farmers on suitable mitigation measures to improve or reduce the risk to water quality from farming.

The ASSAP Interim Report shows the main water quality pressures identified and referred to ASSAP from catchment scientists in the Local Authority Waters Programme was the loss of phosphorus & sediment to waterbodies. In grassland, applied phosphorus accumulates in soil close to the surface. When you get high rainfall on heavy soils, or when the soil is saturated, most of the water will “run-off” the surface.  Consequently, the water is moving in or on the soil where P is most concentrated.  This “run-off” carries both dissolved P and Phosphorous attached to clay particles.  The Teagasc Agricultural Catchments Programme found that in a typical year, 8 or 10 heavy rain storms will carry over half the Phosphorous lost to our rivers and lakes. 

If sediment finds its way to the stream, it can settle on the river-bed in slow flow areas, resulting in the loss of macroinvertebrate habitat and spawning ground. Phosphorus often binds to this sediment and can cause excess nutrient load and promotes algal blooms (eutrophication) which reduce oxygen levels in the stream. In fresh water, a very small amount of phosphorous (under 1 unit per acre) can cause eutrophication.

Targeted fertiliser and slurry application at optimum times throughout the main growing season, particularly on low permeability soils, along with suitable land management can help mitigate against the risk of phosphorus & sediment making their way to waterbodies.

This risk of run-off from slurry & fertiliser is greater where soils are wet, the soil temperature is low (< 6°C) and heavy rain (>10mm) is due within 2 to 3 days. Farmers can reduce the risk by lowering the application rate where necessary, target the driest fields and avoid steep slopes adjacent to waterbodies or areas susceptible to run off.

Phosphorus does not bind to peat soil particles, so unlike mineral soils, Peat soil do not have the capacity to build up a store of phosphorous. Phosphorus should only be applied at rates the plant needs and can use for growth immediately.

A riparian buffer zone is an area adjacent to a water body where no chemical and organic fertiliser, cultivation or spraying can be carried out. These zones vary in width and are required to protect waters from diffuse losses of nutrients, sediment and chemicals. The introduction of trees or rough dense vegetation in these areas can act as a barrier, shade streams and stabilise river banks while the roots can absorb soil nutrients. To be effective, riparian buffer zones must be located at the points most likely to allow nutrient, sediment or pesticides enter a waterbody.

Agricultural practices such as land drainage, cattle access drinking points to streams and poor management of farm roadways can also lead to loss of sediment & phosphorus.

Mitigation options to reduce sediment loss include:

• Prevent access by livestock into drains and streams and providing alternative drinking water sources.
• Divert all surface runoff from farm roadways to a field or soak pit
• Establish targeted riparian buffer zones
• Employ proper drain maintenance

The new Agri Climate Rural Environment Scheme (ACRES) that will open shortly, will include financial support to encourage farmers to take on suitable water quality protection measures for their farm. If a farmer has land within the “High-Status Water objective” mapped area in 2021, they may be eligible for Tier 1 priority entry to the general scheme, provided they take on a suitable action such as Riparian buffer zones, planting trees or hedgerow in an appropriate area or the management of grassland next to a watercourse. Similarly, farmers within the Vulnerable Water mapped area in 2021, may be eligible for Tier 2 priority entry to the general scheme.

Agricultural Advisors preparing ACRES plans for farmers must consider risks to water quality and identify Critical Source Areas (CSAs). Critical sources areas are areas on the farm that deliver a disproportionally high amount of pollutants and represent the areas with the highest risk of impacting a water body. Pollution Impact Potential (PIP) maps are a tool to help highlight these risk areas. Maps identifying PIP risk rankings for two pollutants – phosphate and nitrate are available to assist ACRES advisors in targeting actions to the areas where they will be most effective. Where phosphorus is a risk, the EPA mapping also shows Focussed Flow Paths and Delivery Points on farms i.e. where this phosphorus is most likely to be lost to waterbodies through overland flow.

This Information is also freely available to all farmers on the EPA interactive map. The map can be accessed on the website www.catchments.ie