Western growers are being warned they may need to manage their cropping systems more carefully after new research confirmed that inefficient nitrogen and water use by lupin-wheat rotations is speeding up soil acidification. The research is part of a joint effort supported by the Grains Research and Development Corporation (GRDC) and the Land and Water Resources Research and Development Corporation to assess the impact of legume and cereal crops on nitrogen in the soil and soil acidification.
CSIRO Plant Industries scientist Ian Fillery said while lupins and other legumes benefit crop rotations by adding nitrogen to the soil, they can acidify the soil if the nitrogen is leached in its nitrate form.
"Soils become acid when nitrate is not taken up by plants and allowed to leach through the soil profile," Dr Fillery said. "How much acidification occurs depends on the crop's ability to take up nitrate and soil water and the amount of nitrate leached as surplus water drains through the soil profile.
"Trials on a sandy soil at Moora last year showed high levels of leaching can occur if nitrate is not taken up by plants. Although the rainfall was higher than usual during the trial, these results show large quantities of water drain from sandy soils, leaching useful nitrate from the plant root zone. "Lupin crops add nitrate to the soil but the following wheat crops cannot put down deep roots early enough to use the nitrate before it is leached by opening rains.
"We calculated that 70 kg N/ha of nitrate was leached under the wheat crop following a lupin phase. Under a lupin crop 50 kg N/ha was leached while only 20 kg N/ha was leached from subclover-based pastures.
"The real cost in terms of replacement nitrogen is about $70/ha where wheat follows lupins."
Pastures may soak up nitrate
The lower rate of nitrate leaching under pastures was enexpected but we believe the capeweed, which made up 40 per cent of the pasture, acted like a sponge soaking up nitrate in the soil."
Dr Fillery said altough the trials were on a deep sandy soil and the rainfall was greater than average, the same principles would apply in other soils and seasons.
"It shows that growers may need to manage their lupin-wheat rotations more carefully to prevent the spread of soil acidification which already affects 4.7 million hectares of Western Australia's wheatbelt."
More variety, less tillage
"This does not mean growers need to throw out lupins from their crop rotations — it does mean that they may need to modify their crop rotations by looking at additional crop options to minimise the spread of acidity.
"The use of minimum tillage should reduce the quantities of organic nitrogen converted to mineral nitrogen at the start of the season."
Dr Fillery said additional strategies could include early sown wheat varieties which establish roots earlier and start using nitrates lower down the soil profile before leaching occurs during winter.
Canola uses more nitrogen than wheat and intercropping canola with lupins may also be an option.