The N Bank – A nitrogen management strategy

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Take home messages

  • High urea prices in 2022 will reduce profit and optimal N rates. Growers can offset this by planting a higher area of legumes (grain, hay, pasture, brown manure) and using organic wastes (manure, compost, biosolids) where available and cost effective.
  • Making N fertiliser decisions based on Yield Prophet® Lite or an environmentally appropriate N Bank target maximises profit, stops soil organic N decline and prevents accumulation of excessive mineral N.
  • N decisions based on 50% Yield Prophet® or 125kg/ha N bank strategy apply more N (60 – 80kg/ha) and are over $100/ha per year more profitable than the district average N rate (21 – 30kg/ha N).
  • The most profitable strategies all have neutral to positive N balances (more N applied in fertiliser than removed in grain) indicating soil organic N is not being mined
  • Nitrogen (N) deficiency is the single biggest factor contributing to the Australian wheat yield gap
  • You need to soil test in order to make any sort of rational decision about N fertiliser.
  • Yield Prophet®, back of the envelope calculations and ‘N bank’ targets are all valid ways of managing N fertiliser to achieve potential yields and avoid running down total N.
  • Don’t fear overapplication of N – on most soils in the Wimmera-Mallee unused N carries over and is available for use by subsequent crops and helps maintain soil organic matter.

Australian wheat yields are only half what they could be for the rainfall received (Hochman et al. 2017). Nitrogen (N) deficiency is the single biggest factor contributing to this yield gap. This is also likely to be true for other non-legume crops (barley, canola and oats) which reduces farm profitability and global food security. Alleviating N deficiency would increase national wheat yields by 40 per cent (Hochman and Horan 2018), and substantially improve farm profit.

On farms with no legume pastures, most of the crop N supply must come from fertiliser. Grain legumes do not provide enough N to support yield of subsequent crops at the intensity at which they are currently grown. N fertiliser is a costly input and use of it increases cost of production and value-at-risk for growers. Growers fear that over-fertilisation will result in ‘haying off’, which reduces both yield and quality. There is also the concern that overapplied fertiliser that is not used by crops is lost to the environment by leaching, volatilisation and denitrification. Consequently, efforts continue to be made to match N fertiliser inputs to seasonal yield potential. This is difficult in southern Australia due to the lack of accurate seasonal forecasts for rainfall.

The difficulty in matching N supply to crop demand and a tendency for growers to be conservative in their N inputs is responsible for a large proportion of the yield gap that can be explained by N deficiency. Chronic N deficiency has also caused soil organic matter to decline (Angus and Grace 2017) and has driven a rise in the proportion of low protein grain produced in Australia, which has eroded our standing as a producer of quality wheat in export markets.

‘N banks’ are a strategy for managing N in crop production areas with low environmental losses (leaching, denitrification). Most of the Wimmera-Mallee has soils which are free-draining and hold a reasonable amount of water, and therefore, environmental losses of N are low. N banks are therefore an effective strategy for managing N in most of the region.

Exceptions are areas prone to waterlogging or have very sandy soils. The advantages of N banks are that they are simple to calculate, ensure crops are rarely N deficient, and if set at an appropriate level for the environment, soil organic N is not mined. They also shift the cost of N fertiliser into years following a year of high production, rather than in the year of possible high production.

N banks require growers to set a locally relevant target for crop N supply (soil mineral N plus fertiliser N) that is enough to maximise yield in the majority of seasons. Soil mineral N is then measured early in the growing season, and if less than the target N bank, is topped up to the target value with fertiliser N.

A nutrition management tool such as Yield Prophet® uses a forecast of the future to provide a N app strategy rate for growers, using APSIM, and a probability factor to determine the application rate.

The N bank management strategy that is currently being developed does not even attempt to match crop N supply to seasonal demand, it simply makes sure that the crop has enough N supply (soil mineral N measured early in the season + fertiliser) to achieve water limited potential yield in most seasons. We do this by selecting an N bank yield target appropriate for the environment and apply N accordingly. A target of 125kg/ha N is proving most profitable in the southern Mallee (average wheat yield ~3.0 t/ha), but it is likely to be more like ~200 kg/ha N (average wheat yield >5 t/ha) at higher rainfall locations in south eastern NSW (Smith et al., 2019). We then use soil mineral N measurements from soil cores to work out how much mineral N the crop has available and top up the balance with fertiliser.

For example:

IF the soil mineral N measured in soil cores (0-1 m) = 75kg/ha N AND the N bank target for you 125kg/ha of N THEN the fertiliser required to meet N bank target = 125 – 75 = 50kg/ha N (or 100kg/ha urea).

Assessment of the N bank is achieved by soil testing for nitrate and ammonium. The ‘N bank’ management system relies on growers knowing how much mineral N (nitrate and ammonium) they have available to a crop early in the growing season. Consequently, for any rational decision to be made on N management, it is critical that paddocks are soil tested to measure mineral N. This can be done any time from March through to June, but if done following sowing it is essential that samples are taken from the inter-row to avoid sampling any fertiliser N applied at sowing. Soil cores should be taken to at least 0.6m (ideally >1.0m) and segmented into different depths (e.g. 0-0.1 m, 0.1-0.3 m, 0.3-0.6 m). At least six cores need to be taken per paddock or production zone within a paddock, and bulked samples carefully mixed. Samples should be kept cool and ideally air dried before being sent to an accredited laboratory for analysis. A good soil sampling contractor will do all of these things for you!

The yield gap due to N deficiency is always bigger in high production years. Growers either need to make a realistic assessment of current economic yield and associated crop nitrogen demand or choose an N bank target (see calculations in following section). They then need to calculate how much mineral N was available to crops early in the season from soil samples (refer preceding section), and therefore how much fertiliser N needs to be applied to achieve economic yield or the N bank target.

The bulk of this fertiliser should be applied in July-early August when cereal crops are at Z30-31 and canola hasn’t started elongating. Applying at this time minimises volatilisation losses (cold often wet soil with rain fronts coming through) and maximises chances of crop uptake. If you are applying N during July you do not need to do it right in front of a substantial rain, just do it by the calendar. Crops can be topped up later in the season (e.g. Z37), but risk of volatilisation losses are higher and substantial rain is needed to make sure the N becomes available to crops. If top-dressing later than mid-August you should do it in front of a substantial (>15 mm) forecast rain.

Growers in the Wimmera and Mallee shouldn’t fear over-application (supplying more N than the crop needs this year) on paddocks that aren’t sandy or don’t reliably get waterlogged. Leaching and denitrification losses are very low and N that is not used by the crop this year will be available for next year’s crop.

Crop simulations and the BCG-La Trobe long-term N field experiment at Curyo is showing that occasional over-application is more profitable in the long-term than under-application. This agrees with findings from a similar long term experiment in semi-arid China. The only other reason not to overapply is if your cash flow is tight and you can’t afford not to get a return on investment in fertiliser N this year.

It can be beneficial to soil sample high and low yielding zones of paddocks independently to get a better picture of what is happening across the paddock. Some growers are making variable rate N applications based on protein maps from previous crops and are reporting a high level of success at achieving higher yield and protein in high performing areas of paddocks and avoiding chronic over-fertilisation in low yielding areas. Whilst protein maps are an effective way of informing how N is best allocated across a paddock, they can’t help with estimating the base rate, with soil testing and the management systems described above necessary to achieve this.

N management performance should be reviewed over the longer term (>3 years), and there are two effective ways to do this and make sure growers are achieving your production goals. The first is by reviewing wheat grain protein. If growers are producing wheat with less than 10.5% protein (ASW), then yields are highly likely to be N limited, and profits will almost certainly be increased by increasing N application rates. If protein is between 10.5% and 11.5% (APW), yields are likely to be N limited and profits improved by increasing N application. If protein is between 11.5% and 13% (H2) yield is likely maximised and there will be no benefit from further N application. If protein is over 13% (APH) it is extremely unlikely that yields are N limited, and growers may be over applying N at the expense of some yield and perhaps profit, depending on whether they are attracting price premiums for high protein grain.

The N Bank project has only one current research site, located in the Southern Mallee at Curyo, and although modelling has been undertaken through CSIRO to determine the theoretical N bank targets for other grain growing regions, applied in-paddock research is required to validate and demonstrate the potential of this strategy.

 

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