Minimising Nitrate losses to Waters

In Ireland, all water policy and management is led by the Water Framework Directive. Under this directive Ireland has been set a target of achieving ‘good status’ for all waters in Ireland by 2027. However, despite a lot of good work over the last 20-30 years we are falling short in achieving this target and water quality has declined in recent years.

One of the areas of concern highlighted by the EPA is the elevated levels of nitrate in waters with estuaries, coastal waters and groundwater drinking supplies. The south and east of the country is particularly at risk. Agriculture provides 85% of the nitrate load in rural catchments.

Estuarine waters are in the poorest condition with just 38% of these meeting their WFD water quality targets and are especially sensitive to elevated nitrogen concentrations.

Factors influencing nitrate loss to waters

There are a number of factors that influence the quantity of nitrate lost to waters and include:

Type of land - free draining/poorly draining soil,
The management of the land - intensive/extensive farming and enterprise type
Weather - soil temperature, rainfall and drought.

Typically in Ireland the catchments where elevated levels of nitrate occurs is in the freer draining and more intensively farmed catchments in the south and east of the country. It is in these catchments that the EPA have indicated that reductions in overall tonnes of nitrogen lost to waters is required.

Minimising diffuse nitrate losses to waters

There are a number of practices and technologies that farmers can utilise to minimise diffuse nitrate losses to waters. The Agricultural Sustainability Support and Advisory Programme (ASSAP) offers farmers in 190 Priority Areas for Action (PAA’s) a free and confidential advisory service. ASSAP provides advice on how to reduce the levels of nutrients, (including nitrate), lost to the environment. Some of the key components on how to achieve reductions in nitrate losses are outlined here:

Improving your farms nitrogen use efficiency (NUE) by implementing the advice provided below. Grassland based systems of production are “leaky” by nature in terms of N loss. Current National Farm Survey Sustainability reports put Irish dairy farms at 25% NUE in terms of their ability to recover N from their N imports (feed, fertiliser etc.) via milk sales, cattle sales and grass/crop yield.
Taking soil samples and implementation of a nutrient management plan. This will address deficiencies in phosphorus, potassium and lime that can impact on utilisation of nitrogen.
For early season nitrogen applications, only spread if fields are suitable for tractor work, when water is drained sufficiently and where heavy rainfall is not forecast. Apply fertiliser N when soil temperature is greater than 6°C and rising. Typically this occurs around the end of February however, this will vary across the country and from year to year.
Target fields for early N that are most likely to respond to an early N application: fields at optimum soil fertility (pH, P and K), perennial ryegrass swards, recently reseeded or with a grass cover of greater than 400 kg DM/ha or 5 cm grass.
Match chemical N applied to grass growth rates as this varies across the country. Apply up to 30kg N/ha (24 units N/ha) maximum in 1st split and avoid fields that have received an application of cattle slurry.
LESS (Low Emission Slurry Spreading) increases the amount of N recovered for slurry and allows for a reduction in applied fertiliser N.
Applying slurry in spring - 25 m³/ha (2,500 gals/ac) by low emission application will supply ~25 kg/ha (~20 units/ac) of available N. It is important to reduce your chemical N application rates accordingly.
It is important to reduce or cease nitrogen applications during the year depending on the severity of soil moisture deficits or drought conditions. Nitrogen fertiliser will not be taken up by the plant when applied to soils where growth rates are well below normal due to drought and will lead to fertiliser being potentially leached when rainfall returns.
To ensure efficient and accurate application of fertiliser, calibrate fertiliser spreaders and use GPS equipment where available.
Ensure heavy or prolonged rain is not forecast when spreading fertiliser/slurry.
Use of grass-clover systems. Research in Moorepark has shown that white clover in the sward can replace 40% of chemical nitrogen(100 kg N/ha) and not affect herbage production, compared to a grass-only sward receiving 250 kg N/ha. This saving of 80 units N/acre is worth about €4,000 for a 100 acre farm.
Use protected urea for early chemical N applications as this will help reduce the risk of nitrate leaching as well as reducing ammonia emissions.
In tillage areas, grow cover or catch crops in autumn as these will capture nitrate in the soil that is available to be leached. Establish these crops as early as is possible in the autumn to maximise the nitrate taken up by the crop.