Catch crops, also known as cover crops or 'green manure', are a valuable tool for protecting soil health and water quality.

They are commonly used in Ireland as part of various environmental schemes, such as the Green Low-Carbon Agri Environmental Scheme (GLAS) and the Agri-Climate Rural Environment Scheme (ACRES).

Planted during fallow periods to protect the soil from heavy rainfall and erosion as well as to increase water infiltration, catch crops also utilise residual soil nutrients and prevent leaching of soluble nutrients, contributing to soil health and water quality.

Additionally, their strong root system condition improves soil structure, making cultivation easier, while the remaining plant matter enhances the soil's organic carbon content.

Nitrate loss to water

The EU Water Framework Directive has increased the focus of environmental policy-makers on nitrogen entering estuaries, with the Barrow and Slaney being noted as having particularly high amounts.

There are many things that influence nitrogen loss to water, and in almost all cases where it happens, you don’t see it. This is because nitrate loss does not occur by overland flow or run-off into surface streams.

Nitrate loss takes place below the soil surface, where it is carried in rainwater as it percolates downwards through the soil. Nitrate is easily soluble, does not bind to soil and is very mobile because of this.

As a result, any nitrogen that has not been taken up by a growing crop can be lost to ground water when weather conditions lead to water moving down through the soil profile.

Overwinter green cover

On free draining sites that are prone to nitrate leaching, research has shown that overwinter green cover can substantially reduce nitrate leaching to ground water from fallow crop land. The Agricultural Catchments Programme has been monitoring a tillage dominant river catchment in Castledockrell, Wexford since 2008, and has been focusing on winter green cover research.

Ireland’s Fifth Nitrates Action Programme (NAP) requires, in all circumstances, that 75-80% of harvested fields be shallow cultivated or sown with a crop, within 14 days of harvest. Clearly, the earlier the green cover is established, the more N it can capture.

Study

As a preliminary study in 2021, four catch crops with a range of sowing dates were selected for sampling. As was to be expected, the August-sown crops yielded more biomass than the September-sown crops through the sampling period.

Catch crops that were not grazed had a peak uptake of nitrogen of 48kg N/ha and 43kg N/ha for the respective sowing dates, 20 August and 4 September.

Additional surveys are required to quantify year-to-year variation, but it is clear that specific actions implemented at the appropriate time of year will result in much greater reductions of nitrate loss to water than cuts in the total annual nitrogen limits.

To learn more about the catch crop research at Teagasc, visit the Agricultural Catchments stand at the Crops and Cover Crop Cultivations event in Teagasc Oak Park on 21 June. The Irish Farmers Journal has teamed up with Teagasc to bring you a demonstration on cover crop establishment.