Calculating how much N is needed in pulse‐cereal rotations

Take home messages

• Nitrogen (N) - the element that is removed in the largest quantity with crop production.
• Calculate the contribution of pulse crops to the N supply and consider if you can change the N fertiliser rate for crops.
• Keep a watch on soil organic carbon (soil organic matter).
• When calculating target yield for crops, consider the plant available soil water.
• Consider the need for any other nutrients to be applied.
• How about a year-to-year nutrient budget?
• As a monitoring tool, place zero N and high N strips in the paddock.

Background

The question is often asked, “How much nitrogen (N) does a pulse crop contribute and how do I account for this in calculating the N fertiliser rate for a following cereal crop?” To give a definitive answer is tricky, as there are many factors that influence this decision:

• How the pulse crop performed which directly correlates to nutrient removal and N fixed;
• The fallow leading into the cereal crop;
• Estimating the yield potential and nutrient requirements for the cereal crop;
• Underlying fertility of the paddock; and
• Your goals for soil fertility management.

How much N will the pulse crop supply to the following crop?

Pulse crops have the ability to ‘fix’ atmospheric N (in association with bacteria) for their own requirements. This assumes that the root inoculation by the bacteria is sufficient and effective. Otherwise, soil N will be utilised the same as for cereals. Also, if available soil N is greater than 100kg N/ha, the N fixation by the bacteria is significantly reduced. The pulse stubble breaks down rapidly and hence the N will be available more quickly to the next crop.

The extra benefit from a chickpea crop compared to an equivalent wheat crop at the next planting opportunity will be in the range 20 to 70 kgN/ha. For example a chickpea crop yielding 2.4 t/ha should supply approximately 35 kgN/ha.

Calculating the extra N benefit form a winter pulse compared to a wheat crop

The extra contribution of a chickpea crop compared to a well-fertilised wheat crop (wheat yield approximately usually 50% more than a chickpea). This also applies to faba beans.

1. For a well-grown crop without any yield restrictions:
   Extra N contribution (kgN/ha) = grain yield (t/ha) * 20 (approximately)
2. For a semi-failed crop (e.g. frost, insects):
   Extra N contribution (kgN/ha) = crop biomass (t/ha) * 10 (approximately)

Should this be included in a subsequent N fertiliser rate calculation?

If the calculation indicates that the N benefit could be more than 40 kgN/ha a reduction in the N fertiliser rate using the calculation, may be appropriate. Perhaps a partial reduction in the N fertiliser rate could be made, with the potential extra N ‘banked’ for the future.

If the calculation indicates that the N benefit could be smaller (10 to 20 kgN/ha) it may be better not to reduce the fertiliser rate. Any extra N could be considered as a bonus that is available for supporting yield of future crops.

How much N does a cereal crop require?

Nutrient removal

Grain that is harvested from a paddock contains a range of elements which can only be derived from the soil or from fertiliser. Of all the elements contained in grain, N is present in the greatest quantity. Table 1 shows the typical quantity of the main elements that are contained in grain. The quantity removed by high yielding crops is significant. For example, the total nitrogen removed by a 4 t/ha wheat crop, with 12% protein, over 1ha is 85kg of elemental N and 14kg of elemental P. This is calculated using the typical grain concentrations in Table 1, but the nutrient concentrations can vary quite widely because of different seasons (yields), inherent soil nutrition, applied fertiliser rate and cultivar. Thus, annual testing the grain for nutrient concentration will give a clearer quantification of the actual nutrient removal.

Table 1. Typical nutrient removal levels (kg element/t grain)

Calculating an N fertiliser rate

This simple budgeting process involves calculating the nitrogen demand from a crop of certain yield and protein, subtracting the soil supply value and the difference is the N fertiliser requirement.

1. Total nitrogen demand can be calculated from the equation:
   N demand (wheat) = Grain yield* protein * 10/5.7 * 1.7
   N demand (sorghum) = Grain yield* protein * 10/6.25 * 1.7
   This takes into account the efficiency of uptake of N from soil, and in-crop mineralisation.
   The grain N removal is the equation without the 1.7 multiplier.
2. The soil supply is calculated from a soil test for N to a depth of 90cm, although approximately 80% of the nitrogen is accessed from the top 60cm (M Bell pers comm).
3. The N fertiliser rate is the difference of Soil N supply – Crop N demand

The Nitrogen Book, DAF Qld contains more detail on calculating N fertiliser rates.

Estimating the target yield

The above equation requires the use of a target yield. This can be an estimate from experience, or there are a range of tools available to assist in the understanding of the yield ranges that may occur. The CropARM tool can be used to create scenarios of yield ranges, effect of N application rates and the effect of SOI phases on yield outcomes.

With this program, you are able to create a scenario showing the potential yield response to N rate (Figure 1) or add an effect of a climate outlook (Figure 2). Note that an SOI analysis for summer crop is shown because there is less opportunity to use the climate outlook for winter crop. The skill level is generally low for earlier winter-crop planting dates. The result indicates an increase in the median yield from approximately 5 t/ha to 7.5 t/ha and yields generally in a higher range.

Figure 1. Modelled effect of N fertiliser rate on potential wheat yield (at 11% grain moisture). The scenario relates to Pittsworth, soil N = 50 kgN/ha, soil 190mm PAWC, 90% full at planting.

Figure 2. Modelled effect of a negative SOI phase (Phase 1) vs a positive SOI phase (Phase 2) in July/August, prior to sorghum planting on 15 September. Soil N = 50 kgN/ha, N fertiliser rate = 150 kgN/ha, soil 190mm PAWC, 60% full at planting.

Considerations to maximise N application

How efficient is an N fertiliser application?

It is only in relatively infrequent circumstances that loss of urea occurs. The most significant losses can occur from denitrification under the combined conditions of; warm temperatures, high soil water content, a source of nitrate-N and a source of energy for micro-organisms. Surface applied N fertiliser can also be subject to losses under conditions of warm temperatures, high pH, moist soil (but not enough water to move the N into the soil), wind and high humidity. Ammonium sulphate is less subject to volatilisation, as are the treated products such as Green urea® and Entec®. These products are more expensive per unit of N than straight urea.

N fertiliser is transformed in the soil by micro-organisms to forms readily taken up by plants, principally nitrate-N (but also ammonium-N). Figure 3 shows generalised quantities of losses and uptake.

Figure 3. Generalised percentage distribution of applied N fertiliser in a winter crop system. Source: Adapted from D Herridge 2013.

Thus, 80% of applied N fertiliser is potentially available for plant uptake. However, in the northern cropping region, the figure is likely to be closer to 50% to 60%, with perhaps 20% of product left in the soil on many occasions. This N will be mostly available to the next crop unless an exceptional loss event (denitrification) occurs. Total losses are typically 15% to 20% in winter (Dr Wayne Strong’s trials) but more recent work by Dr Mike Bell has found losses of 20% to 40% in summer systems even with only an occasional wet event.

The percentage of applied N that ends up in the grain will be in the range of 30% to 45% in average to good seasons (average around 38%) (Table 2). In dry seasons, relatively less applied N (an average of 20%) will have made it into the grain, with the remainder distributed in stubble or soil.

The N in the stubble, or remaining in the soil, will be largely re-cycled in the soil N pools and available for future crops. In good seasons, with high yield and with well balanced N supply, a greater proportion of the applied N fertiliser will end up in the grain relative to the stubble (up to 85% in grain vs 15% stubble). With a dry finish, less of the applied N fertiliser will remain in the grain relative to the above-ground stubble (typically 70% in grain vs 30% stubble). More of the applied N fertiliser will remain in the soil (typically 20% to 40%).

Under waterlogged conditions, high losses can occur (at least 40%). High stubble loads will tie up more N (typically 7 kg/t of grain removed). This could be up to 35% of the applied N fertiliser.

Table 2. The effect of season type on the percentage partitioning of an N fertiliser application in wheat. Adapted from Herridge 2013.

¹ Denitrification, volatilisation, ² in micro-organisms, ³ percent of the N fertiliser application left in soil, ⁴ ratio of N fertiliser application in grain vs above ground residue, ⁵ percentage of the N fertiliser application that is in the grain.

How can I make the N fertiliser application as efficient as possible?

Seasonal conditions will largely determine if the applied amount of N was adequate, excessive or efficiently used. However, there are some things that can be done to make the best estimation.

Assuming that the fallow period has not been unusually wet or dry, the steps for efficient application are:

• knowing the soil N prior to the crop (soil test)
• calculating the N demand for a realistic target yield
• allowing for the contribution of a previous, failed or semi-failed pulse crop
• do test strips with nil N, intermediate and high N rate. The nil N rate is needed to calculate Agronomic Efficiency. The high N rate will show what might have been (in a good season) and be a point of reference for future applications.

After the crop is harvested

• evaluate the grain yield and protein of the crop (how did the removal match the application rate?)
• calculate the Agronomic Efficiency (AE) (yield increase with amount of N applied);
  • the formula is; AE = (Yield with N rate – Yield with nil N)/N rate    (all rates in kg/ha)
  • a value of > 25 is good in a well-managed system

Remember- there are usually greater economic losses from under-fertilising than over-fertilising (in the longer term).

Applying N fertiliser

Especially for the northern region, applying the most appropriate N rate is considered more important than the timing of the N application. However, with respect to timing, there are a few factors to consider;

1. Early application for winter crop (January to March) – biggest risk is denitrification losses
2. Close to winter planting (soil disturbance, N can be stranded in surface). Consider mid-row application which is showing good result in southern Australia.
3. At or close to sorghum planting (can be denitrification losses). Consider applying earlier in the fallow when the soil is dry

Surface application vs incorporated?

Recent research is indicating fairly small losses from surface-applied urea. Research by Graeme Schwenke et al. (2014) reported the following losses;

Average loss in fallow 11% - (range 5% to 19%), in wheat crop 3% to 8%, in pasture 27%.

However, note that the maximum loss was 19%. Thus under conditions of high temperature, high pH, high wind, and high humidity, losses can be moderately high. Incorporation of urea into soil will reduce losses from volatilisation but of course will be subject to denitrification losses if conditions become conducive for it to occur.

Fertiliser products

Urea is a cheap and convenient N fertiliser product. Modified urea products are available that can reduce losses by volatilisation or denitrification. Entec® has a component that delays the conversion of ammonium to nitrate nitrogen and thus can reduce denitrification losses. Volatilisation losses can still occur and the least losses will occur if the fertiliser is incorporated into the soil. Green Urea® has a component that slows the conversion of urea to ammonium. Thus when surface-applied, this product may allow more time for it to be moved into the soil by rainfall, irrigation or cultivation. Denitrification loss can still occur once the conversion to nitrate occurs. Losses have been shown to decrease and there are farming systems where these products are very suitable. Because these products are more expensive than standard urea, the cost:benefit ratio and overall risk of losses needs to be taken into account when making a decision to use the product.

Contribution of organic matter (using organic carbon results)

Organic carbon is a laboratory measure from which the amount of organic matter (OM) can be calculated. The organic carbon (OC) level is a good indicator of the ability of the soil to supply nutrients. OC levels have declined as nutrients have been removed in grain (Figure 4). Nutrients such as phosphorus (P) and potassium (K) will be declining along with N. Organic matter has biological benefits (energy for micro-organisms and provision of nutrients) but also has benefits to the physical (structure) and chemical (pH and CEC) functions of the soil. Declining OM levels under cropping systems has resulted in reduced soil nutrient reserves, creating a greater reliance on fertilisers. Allowing the soil OM level to decline too far may result in the soil being permanently unproductive and unable to grow crops or pastures.

The best way to prolong the productivity of a cropping soil is to supply adequate fertiliser to grow the maximum biomass, use minimum or zero-tillage, don’t burn residues. Including pulse crops in the farming system will almost certainly be valuable financially, but will not increase soil organic matter. The best way to restore OM levels is to return the paddock to a grass/legume pasture. Research has shown that approximately 0.65 t C/ha/year can be added (Dalal et al. 1995). This will equate to almost 0.1% increase in OC for each year that the grass/legume pasture was growing compared to continuous conventionally-tilled wheat. (Figure 5).

Figure 4. Soil N OC decline over years of cultivation. Soil 1 = Waco clay (vertosol), soil 2 = Langlands Logie, grey clay), soil 3 = Red Earth, Kandosol. Source: Dalal and Chan 2001.

Figure 5. Soil OC concentration in May 1989 after continuous conventional till wheat, and after grass+legume pasture commencing 1988, 1987 or 1986 (0-2.5cm). Source: Dalal and Chan 2001.

References

ARMonline

H Cox and W Strong (2015). The Nitrogen Book.

Dalal R. C. Chan K. Y. (2001) Soil organic matter in rainfed cropping systems of the Australian cereal belt. Soil Research 39, 435-464.

R. C. Dalal, W M. Strong, E. J. Weston, J. E. Cooper, K. J. Lehane, A. J. King and C. J. Chicken (1995). Sustaining productivity of a Vertisol at Warra, Queensland, with fertilisers, no-tillage, or legumes 1. Organic matter status. Aust. Journal of Experimental Agriculture, 35, 903-913.

D Herridge (2013). Managing legume and fertiliser N for northern grains cropping.

GD Schwenke, W Manning, BM Haigh (2014) Ammonia volatilisation from nitrogen fertilisers surface-applied to bare fallows, wheat crops and perennial-grass-based pastures on Vertosols.  Soil Research, 52(8), 805-821.

Contact details

Howard Cox
Department of Agriculture and Fisheries
Tor St, Toowoomba
(07) 4529 4181
howard.cox@daf.qld.gov.au

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