Match nitrogen to soil type to lift crop profits

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Two light blue water drops on a darker blue backgroundIncreasing nitrogen inputs on deep sands can increase WUE by up to 91 per cent relative to district practice.

Wheat yield and farm profit can be increased significantly by lifting nitrogen rates on sandy soils and reducing those on heavier soils in the Mallee region of South Australia

Three years of CSIRO research trails as part of the GRDC-funded Water Use Efficiency Initiative show there is money to be made by even a modest increase in nitrogen inputs on the deep sands of the Mallee dune cropping systems.

The same trials indicate growers using current district practice were wasting nitrogen fertiliser and money on heavier soils, which failed to respond with extra yield above nil fertiliser input.

CSIRO scientist Dr Therese McBeath says the results were consistent across a range of different seasons from the very wet years of 2010 and 2011 to the average-rainfall season of 2012.

The yield improvement on the sandy soils was associated with a significant increase in the water use efficiency (WUE) of the wheat crop – a critical success factor for cropping systems in this region.

Variable soil types

The Mallee region is characterised by dune-swale systems. The crest of the dune is typically deep sand with low organic matter and nitrogen fertility. Moving down the slope, the mid-slope soils were characterised by low-fertility sands overlying a clay layer, enabling these soils to hold more water and nutrients than the deep sands. At the base of the slope, swale soils tend to have the highest levels of organic matter and fertility but often suffer poor production in low-rainfall seasons due to subsoil constraints (Figure 1).

These highly variable soils coupled with the area’s variable seasonal rainfall have resulted in an understandable risk-averse approach to cropping. Growers in the region typically minimise the risk of not getting a return on nitrogen fertiliser in poor seasons by applying fixed, low rates of nitrogen to their cereal crops across all soil types.

However, nitrogen deficiency on the sandier dune soils has been identified as a major cause of the gap between actual and potential yields in wetter seasons in these environments. Poor crop performance on the swales in drier seasons led to accumulation of nitrogen beyond levels required to produce average crops.

Dr McBeath says the main message from the research was to tailor fertiliser inputs to soil type and shift inputs from heavier soils to deeper sands to reap yield and economic benefits.

Using a bio-economic model based on three years of field data, she found that nitrogen applied at the district practice rate of 15 kilograms per hectare resulted in an average loss of $30/ha on the dune soils, a low net return of $7/ha on the mid-slope soils and a relatively higher net return of $66/ha on the swale.

The best net return on the dune was achieved by applying 60kg nitrogen/ha.

Compared with the district practice of 15kg nitrogen/ha, this higher rate increased average net return by $136/ha, break-even probabilities by 71 per cent, and net return on fertiliser investment by $0.8 per dollar of invested nitrogen fertiliser.

The best performing nitrogen rates applied to the more fertile mid-slope soils were lower than those applied to the dune soils, of about 30kg nitrogen/ha. For the swale, the district practice of 15kg nitrogen/ha gave the best net returns.

Catering to risk attitude

Dr McBeath then ranked the nitrogen management strategies based on the range of risk attitudes among Mallee growers from very risk averse through to risk neutral. At the level of risk aversion some growers chose to apply the district practice nitrogen rate of 15kg/ha on the mid-slope zone instead of a more profitable higher rate. These same growers should have been applying a higher rate of nitrogen to their dune soil and a lower rate on their swale soil.

This indicates that at a level of risk aversion likely to be common in the grower population, crop yield and profits can be increased by applying a variable rate of nitrogen across the dune system rather than a blanket 15kg/ha.

FIGURE 1 Typical Mallee dune system - with heavier more fertile soil in the swale at the bottom of the dune and sandy, low fertility soil at the crest.

Figure representing a typical Malleee dune system

Numbers indicate the wheat yield response to applying nitrogen at either the district practice of 15kg/ha or 40kg/ha at sowing or nil fertiliser. Growers in the region tend to apply a uniform, low amount of nitrogen across the soil types (15kg/ha). In 2012 wheat yields were doubled on the sandy soils at the top of the dune system by adding 40kg/ha nitrogen at sowing. No yield advantage was gained from adding nitrogen to the swale, which had accumulated significant nitrogen stores over time.

More information:

Dr Therese McBeath,
08 8303 8455,
therese.mcbeath@csiro.au

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GRDC Project Code CSA00025

Region South