Why use this calculator
Nitrogen is usually the largest variable input in a cropping budget, and the two biggest errors in setting the rate are both avoidable. The first is ignoring the legume credit entirely and applying a full rate after a pulse, which wastes money and pushes nitrate down the profile.
The second is assuming a legume has done more than it really did, and starving a wheat crop of the nitrogen it needs to make protein. This tool builds a proper nitrogen budget: demand driven by a realistic yield target, supply made up of soil test nitrogen plus a legume credit that scales with the previous crop's yield and its nodulation, and a rate that is the gap between them. It then prices both the saving and the remaining bill.
How to use it
- Select the previous crop, remembering that only legumes fix nitrogen from the atmosphere.
- Enter that crop's yield, using grain yield for a pulse and dry matter production for a pasture, lucerne stand or brown manured vetch, because the credit scales with how much the legume actually grew.
- Rate the nodulation you saw when you dug plants at flowering, since white or absent nodules mean very little nitrogen was fixed no matter how well the crop yielded.
- Choose the crop you are about to sow, which sets the nitrogen demand per tonne.
- Enter a realistic target yield for the season ahead based on stored moisture rather than your best ever year.
- Put in the mineral nitrogen from a deep soil test, then the paddock area and the delivered urea price so the tool can price the decision.
What each result means
- Urea to order for the paddock
- Tonnes of urea to buy so every hectare gets the rate still needed after the soil and legume credit, before rounding up to what your supplier sells.
- Nitrogen credit from the previous crop
- Net nitrogen the legume leaves behind after its own grain removal, scaled by yield and nodulation.
- Mineral nitrogen in the soil
- Nitrate and ammonium already available from your deep soil test, the first call on the crop's requirement.
- Nitrogen the next crop needs
- Total demand to reach your target yield, based on the nitrogen each tonne of that crop takes up.
- Nitrogen still to apply
- The gap between demand and supply. This is the rate that should actually go on as fertiliser.
- Urea to apply
- The nitrogen gap converted into kilograms of granular urea per hectare at 46 per cent nitrogen.
- Urea saved by the legume
- Product you do not have to buy because the legume fixed the nitrogen, capped at what the crop could actually use.
- Fertiliser saving per hectare
- Dollar value of the urea the legume replaced, one of the real returns from a break crop.
- Fertiliser saving across the paddock
- Whole paddock value of the nitrogen credit, which should be counted in the break crop's gross margin.
- Remaining urea bill for the paddock
- What the nitrogen still to apply will cost across the paddock at your delivered urea price.
The formula
Nitrogen credit equals the per tonne fixation factor multiplied by the previous crop's yield multiplied by a nodulation factor of 1.15 for excellent, 1.0 for good and 0.6 for poor, capped at 300 kilograms per hectare. Per tonne factors are field pea 20, faba bean and lupin 25, lentil 15, chickpea 12, brown manured vetch 40, lucerne 30 and subterranean clover pasture 25 kilograms of nitrogen per tonne. Nitrogen demand equals target yield multiplied by a crop factor of wheat 40, barley 35, oats 30, canola 80, sorghum 30 and maize 22 kilograms of nitrogen per tonne. Nitrogen to apply equals demand minus soil nitrogen minus credit, floored at zero. Urea rate equals that divided by 0.46.
How this number is used in practice
The nitrogen credit is often the largest part of a break crop's value, and it is routinely left out of the gross margin comparison that decides whether to grow one. A pulse that yields modestly can still beat a second cereal once you count the nitrogen it leaves, the disease break it provides, the chance to use a different herbicide group on resistant weeds and the yield lift in the following crop.
Run the comparison over two years rather than one, with the fertiliser saving credited to the legume year, and the ranking frequently changes.
Nodulation is the check that separates a real credit from a hopeful one. Legumes only fix nitrogen when the right rhizobia are present and active, and a paddock that has not carried that legume for many years, or that has become too acid, may nodulate poorly despite looking healthy above ground.
Dig a few plants at flowering and cut the nodules open. Large pink or red nodules on the crown mean active fixation. Small white or green nodules mean the crop is drawing on soil nitrogen rather than adding to it, and the credit should be discounted accordingly or the paddock limed and inoculated before the next attempt.
Set the yield target to a realistic decile, not to your best ever season. Nitrogen applied for a top decile crop that does not eventuate is money spent, a haying off risk in a dry finish, and nitrate left to leach below the root zone.
The usual approach is to apply a base rate at sowing that covers a conservative target, then top up in crop once the season has declared itself. The yield target table here makes that trade off explicit by showing the rate and the paddock bill at every yield level.
Worked examples
2.5 t/ha field pea with good nodulation, then wheat targeting 4 t/ha, soil test 40 kg N, 120 ha
A 50 kg N/ha credit, leaving 70 kg N/ha to apply as about 152 kg/ha of urea.
The same paddock after wheat instead of peas
No credit at all, so the full 120 kg N/ha gap has to be bought.
Brown manured vetch at 4 t/ha dry matter, then canola targeting 2.5 t/ha
A 160 kg N/ha credit, which covers the whole 200 kg N/ha demand with the soil test.
When people use it
- Setting the nitrogen rate for a cereal following a pulse instead of applying a blanket figure
- Including the nitrogen credit in a two year gross margin comparison of a break crop against a second cereal
- Deciding whether brown manuring a vetch crop pays once the fertiliser saving is counted
- Budgeting the urea order for a whole cropping program before prices move
- Testing how much nitrogen a higher or lower yield target really commits you to
- Explaining a fertiliser plan to a lender or an agronomist with a transparent budget
Tips
- Dig plants at flowering and cut nodules open. Pink inside means active fixation, white means the credit is largely imaginary.
- Scale the credit to the legume's actual yield. A failed pulse crop leaves far less nitrogen than a good one.
- Take a deep soil nitrogen test to at least 60 centimetres. Topsoil only tests miss most of the mineral nitrogen available.
- Split the rate between sowing and in crop so you can respond to the season rather than commit everything in autumn.
- Credit the fertiliser saving to the legume year when comparing rotations, or break crops will always look worse than they are.
- Correct soil acidity before blaming the legume. Rhizobia struggle below about pH 5 in calcium chloride terms.
- Remember canola needs roughly twice the nitrogen per tonne that wheat does, so a canola year is not the place to skimp.
What it assumes, and where it stops being accurate
- Fixation factors are indicative district averages. Actual fixation varies with rainfall, soil nitrogen, rhizobia and crop health.
- The credit is expressed net of the nitrogen removed in the previous crop's grain, which is why a grain pulse credits less than a brown manured one.
- Most of the credit is available to the first crop after the legume. A smaller residual carries into later years and is not modelled.
- Nitrogen demand per tonne assumes typical protein levels. High protein wheat contracts require more nitrogen than the figures used here.
- Losses to leaching, volatilisation and denitrification are not deducted, so the applied rate is a minimum rather than an allowance.
- Soil test nitrogen is assumed to be plant available within the rooting depth and present at sowing rather than mineralised later.
Terms used on this page
- Nitrogen credit
- Plant available nitrogen left in the soil by a previous legume crop or pasture.
- Nodulation
- The formation of root nodules where rhizobia bacteria fix atmospheric nitrogen for the plant.
- Brown manure
- Terminating a legume crop with herbicide before grain set so all the nitrogen and biomass stays in the paddock.
- Break crop
- A non cereal crop grown to interrupt disease and weed cycles and to change the nitrogen and herbicide options.
- Deep N test
- A soil test to 60 or 90 centimetres measuring mineral nitrogen available to a crop's full rooting depth.
Questions people ask
- How much nitrogen does a pulse crop leave behind?
- As a rule of thumb, roughly 20 to 25 kilograms of nitrogen per hectare for each tonne of pulse grain produced, net of the nitrogen removed in that grain. A 2.5 tonne field pea crop therefore leaves about 50 kilograms. Brown manured legumes leave far more because nothing is removed, but they forgo the grain income.
- Does a cereal or canola crop leave any nitrogen credit?
- No. Only legumes host the rhizobia that fix atmospheric nitrogen, so wheat, barley and canola leave a credit of zero in this budget. Their residues do return organic matter that mineralises slowly over later seasons, but that is a soil building effect rather than a nitrogen credit you can bank on for next year's crop.
- How much nitrogen does wheat need per tonne of yield?
- About 40 kilograms of nitrogen per tonne of grain at typical protein levels, covering both the nitrogen in the grain and what stays in the stubble and roots. Barley is nearer 35 and canola roughly 80 because oilseeds are far more nitrogen hungry per tonne. High protein wheat contracts push the wheat figure higher still.
- Why does nodulation matter so much?
- Because legumes only fix nitrogen when compatible rhizobia colonise the roots and stay active. Dig plants at flowering and cut the nodules open: large pink or red nodules mean fixation is happening, while small white or green ones mean the plant is drawing on soil nitrogen instead. Poor nodulation is common in acid soils and in paddocks new to that legume.
- How do I convert kilograms of nitrogen into kilograms of urea?
- Divide by 0.46, because granular urea is 46 per cent nitrogen by weight. Seventy kilograms of nitrogen per hectare needs about 152 kilograms of urea per hectare. Other products differ: ammonium sulphate is 21 per cent nitrogen and monoammonium phosphate around 10, so always check the analysis before converting.
- Should I apply all the nitrogen at sowing?
- Usually not. Splitting between a base rate at sowing and one or two in crop applications lets you match the rate to how the season actually turns out, reduces leaching risk in a wet start and avoids committing to a top decile yield in autumn. The trade off is extra passes and the need for rain to wash surface applied urea in.
Use this with
Crop Rotation Nitrogen Credit Calculator answers one part of the question. These pick up where it stops: Fertiliser Application Calculator, Soil pH and Lime Rate Calculator, Harvest Yield Estimator, Seed Rate Calculator and Cost Per Hectare Calculator.
Where these figures come from
Figures and rules on this page follow Australian law and ATO guidance.
- Grains Research and Development Corporation. Australian grains agronomy, nitrogen budgeting and yield estimation research
- NSW Department of Primary Industries. Primefacts on stocking rates, soil acidity, livestock nutrition and breeding
- Agriculture Victoria. Extension notes on pastures, soils, liming and livestock management
- Agriculture and Horticulture Development Board, United Kingdom. UK crop and livestock benchmarking, nutrient management and growth data
Crop Rotation Nitrogen Credit Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.