Agriculture is just one of the human activities that puts pressure on Ireland’s water quality. However, it is the largest pressure on water quality in Ireland as it is the largest land use in the country. Water quality is impacted by diffuse nitrogen (N), phosphorous (P), sediment losses, point source losses, pesticides and toxicity and ammonium losses.
How does agriculture contribute?
The utilisation of soil and the addition of fertilisers and pesticides leads to the loss of nutrients, sediment and pesticides to waters. Inputs need to be carefully managed and farming practices that are appropriate to each individual farm and weather conditions should be used.
N and P are key nutrient inputs into grassland and tillage systems and can contribute to poor water quality if not managed correctly.
P loss typically occurs on soils that have low permeability. These are heavy, poor draining soils with a high clay content, which quickly get saturated from rainfall. This leads to water remaining on the surface of the soil, which then becomes overland flow of water.
This overland flow of water across fields brings P available to plants in a soluble form from fertiliser application. It also washes off soil particles that have P attached to them. P binds tightly to soil particles, then the soluble P and soil particles can be washed into the drainage network and streams on the farm.
N loss usually occurs on soils that have high permeability. These are lighter, free draining soils with a high sand content. Water permeates quickly through these soils.
When excess nitrogen fertiliser is applied, this N is not utilised and is left in the soil. N in the soil is also naturally mineralised, especially in autumn, and this needs careful management to minimise N losses. Unlike P, N does not bind to the soil tightly, and therefore, when rainfall is heavy, the water leaches N into groundwater and other watercourses.
However, it is important to note that water quality is improving in some areas as a result of different measures implemented by farmers, such as taking up-to-date soil samples to get a better idea of which areas need certain fertilisers, using LESS technology when slurry spreading, the use of buffer strips, riparian margins and reduced P applications.