3 August 2000 Nitrate leaching: how does dairying compare?

Although nitrate-leaching losses from dairy farms are of concern, it appears that they may be no greater than losses from other intensive farming systems in NZ.

Comparison of published research results shows that the highest nitrate leaching losses occur from vegetable crops and the lowest from extensive sheep grazing. Leaching losses from dairying are similar to, and sometimes less than, losses from arable mixed cropping systems.

Nitrate leaching from soil is of concern because of its impact on drinking water quality and the effects that nitrate can have on rivers and lakes. High nitrate concentrations in drinking water is a recognised health hazard and high concentrations in rivers and lakes can cause excessive growth of algae and weeds which reduce the fish population. Nitrate leaching also represents a financial cost to the farmer and a loss in soil fertility. At current prices a loss of 100 kg N/ha/y represents a cost of about $80/ha if this nitrogen has to be replaced by fertiliser.

As shown in the figure, large nitrate losses of over 300 kg N per hectare per year can occur under intensive vegetable production systems, whilst lower losses of between 18 to 110 kg N/ha/y have been measured under dairy pastures. Nitrate losses from dairying are generally greater than extensive sheep farming (6 - 41 kg N/ha/y).

Leaching losses from mixed cropping systems can be large (over 100 kg N/ha/y) when pasture is ploughed in the autumn (a seemingly benign process). This occurs because ploughing releases nitrate from the soil organic matter and this is then leached over the winter fallow period.

Comparing nitrate leaching losses between farming systems is not a simple exercise because the amount of nitrate lost by leaching depends on a wide range of factors, including the amount of drainage, plant uptake, soil immobilisation, stocking rate, fertiliser and effluent application, as well as soil type.

Recent dairying research at Lincoln University has found that fertiliser applications up to 200 kg N/ha/y did not result in the nitrate concentration in the drainage water exceeding the NZ Dept. of Health maximum accepted value.

However, there is no room for complacency in dairying, because as stocking rates increase and rates of nitrogen fertiliser use also increase there will be a greater risk of nitrate leaching. It is also timely to note that the effects of the current expansion of dairying onto irrigated shallow, free draining soils in the South Island are largely unknown at this stage. Research is being conducted at Lincoln University to make accurate measurements of nitrate leaching under these new dairying systems and thus help to ensure that environmental sustainability of the dairy industry is achieved.

Professor Keith Cameron, Soil Scientist & Director of the Soil & Environmental Quality Research Centre, Lincoln University, Canterbury, New Zealand