Where to get your N: Legume Rotation vs. Fertiliser
Both fertilisers and legume rotations are known to supply nitrogen to cropping soils, but which is the better option?
A GRDC-funded project has researched this question by measuring inputs of fixed nitrogen (N) by different legumes, monitoring legume induced changes in available soil N, and comparing crop responses following legumes to applied fertiliser.
CSIRO Agriculture Flagship Research Director Mark Peoples says the project found that both fertilisers and legume rotations have their place in a cropping program.
Various trials examined N fixation by pulse crops and pasture legumes in southern and central NSW, the south-east region of South Australia, and in both high and low rainfall zones of Victoria over the past 4 years. These studies compared the net inputs of fixed N, which represents the fixed N remaining in legume residues at the end of the growing season.
“The trials demonstrated that, as expected, rotations that resulted in highest returns of fixed to the soil were brown manured crops, and grazed or hay-cut legume pastures , since large amounts of N are removed from pulse crops in the harvested grain,” Dr Peoples said.
An experiment undertaken near Junee in southern NSW between 2011 and 2013 found that soil mineral N measured just prior to sowing wheat in 2012 was 42 kg N/ha greater following lupins in 2011 compared to wheat when both were harvested for grain. This figure rose to 92 kg N/ha greater than wheat if the lupins were brown manured. Higher concentrations of available soil N persisted through to the second wheat crop in 2013 when the soil mineral N was 18 or 34 kg N/ha greater where lupins had been harvested for grain, or brown manured respectively, in 2011, compared to continuous wheat.
Not all the N in legume residues became available or was recovered by wheat the first year. In the Junee trials, approximately 28 per cent of N fixed in the lupin residues were taken up by wheat in 2012, compared to 47 to 59 per cent of top-dressed fertiliser when it was applied at the start of stem elongation (GS31), just prior to wheat’s period of peak N demand. A further 11% of the N in the 2011 lupin residues was subsequently utilised by the second wheat crop in 2013.
In addition, losses of N are usually lower from legume sources than from fertiliser (see ‘What are the types of nitrogen losses’), and more legume N remains in the soil, which will further balance the initial lower recovery.
Legumes also have other benefits, including increasing soil organic fertility, weed and disease management, and the development of beneficial soil biology, therefore growers should consider both options as both have a place within a farming system.
GRDC Project Code CSP000146
Region South, North