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Breakthrough in Effluent Disposal

Spreading dairy shed effluent directly onto poorly drained soils may be the best way of preventing the leaching of nitrate into groundwater, according to results obtained by Landcare Research soil scientist Peter Singleton.

The Hamilton-based researcher has found that soils with a high water table can remove up to 99% of nitrogen from effluent, compared with minimal removal on well drained soils.

He says that farmers have traditionally used freely-draining soils for disposing of effluent. "The results come as a surprise. A proper drainage system and ripping (ploughing or breaking up the soil) can overcome the problems of a high water table often associated with irrigating poorly drained and clay soils."

Nitrogen in effluent breaks down to ammonium and nitrate. Under normal conditions in free-draining soils, the nitrate travels straight through the soil, down to where it may contaminate the groundwater.

A high water table on poorly drained soils causes anaerobic conditions where there is little oxygen. Under these conditions the nitrate breaks down to nitrogen gas and nitrous oxide, depending on the soil pH, instead of being leached from the soil and into the groundwater.

Singleton says that provided poorly-drained land is prepared and irrigated with the effluent at a low rate, there will be little difference in nitrate leaching compared with land not used for effluent disposal.

He will continue his research on soils this summer, looking at how much nitrogen poorly drained soils can remove. The work is part of a programme investigating sustainable land use.