Agriculture and Water Quality
Agriculture is just one of a number of human activities that can put pressure on water quality. Other activities impacting water quality include hydromorphology (physical changes to waters), urban and domestic waste water, forestry, peat extraction, industry and roads. Climate change is also impacts water quality.
Agriculture 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)
- Phosphorus (P)
- sediment losses
- point source losses
- pesticides and toxicity
- ammonium losses.
How does agriculture and farming contribute to these losses?
Modern farming practices and food production utilise the soil to grow grass for dairy, beef and sheep farming and crops, vegetables and fruit for the tillage and horticulture sectors. This utilisation of the soil and addition of fertilisers and pesticides that can lead to losses of nutrients, sediment and pesticides to waters. These inputs need to be carefully managed and farming practices implemented that are appropriate to each individual farm and weather conditions.
Nitrogen (N) and Phosphorus (P) are key nutrient inputs into grassland and tillage systems and can be applied to crops by organic fertilisers (slurry or farm yard manure, etc.) or by chemical fertilisers (Urea, CAN or 10-10-20, 18-6-12 etc.).
The rates and timing of N and P fertiliser applications are set out in the Good Agricultural Practices Regulations 2017. The regulations outline the maximum limits of N and P that can be applied. These are based on stocking rate and soil fertility levels on farms. The times of year that these can be applied is based on what county the farm is located.
Diffuse P and Sediment losses:
Phosphorus (P) loss typically occurs on soils that have low permeability. These are ‘heavy’, poorly draining soils with high clay content and get quickly saturated with rainfall. When there is heavy rainfall on these saturated soils this leads to the water staying on the surface of the soil. This in turns leads to overland flow of water, particularly on fields with slopes.
The overland flow of water across fields brings with it P available to plants in soluble form from fertiliser application. It also washes off soil particles that have P attached to them. P binds tightly to soil particles. The soluble P and soil particles can then be washed into the drainage network and streams located in the farm and end up impacting on the quality of water in the streams.
Diffuse N losses:
Nitrogen (N) loss typically occurs on soils that have high permeability. These are ‘light’ free draining soils with a high sand content and water can quickly permeate through these soils.
Where excess Nitrogen fertiliser is applied above crop requirement, this N is not utilised by the grass or tillage crop and is left in the soil. Nitrogen in the soil is also naturally mineralised, particularly in autumn, and this requires careful management to minimise N losses. N does not bind tightly to soil like P and therefore when there is heavy rainfall, the water leaches N away to groundwater, streams and rivers.