Why use this calculator
Legume crops fix atmospheric nitrogen into the soil through their root nodules, providing a valuable nitrogen credit for the following crop. Accurately accounting for these credits reduces fertiliser costs and prevents over-application of nitrogen, which saves money and reduces environmental impact.
How to use it
Select the previous crop grown on the field, choose your planned next crop, and enter the total field area. The calculator determines the nitrogen credit available, remaining nitrogen needed, and the dollar value of fertiliser savings based on urea pricing.
The formula
N Credit varies by previous crop: soybeans 40, clover 80, alfalfa 120, peas 30, lentils 25 kg N/ha. N Still Needed = max(0, Next Crop Requirement - N Credit). Savings based on urea at 46% N and $600/tonne.
Worked examples
Soybeans followed by corn on 50 hectares
40 kg N/ha credit, 110 kg N/ha still needed, $26.09/ha savings, $1,304.35 total savings
Alfalfa followed by wheat on 30 hectares
120 kg N/ha credit, 0 kg N/ha still needed (credit exceeds requirement), $156.52/ha savings, $4,695.65 total
When people use it
- Planning crop sequences to maximise nitrogen fixation benefits
- Reducing fertiliser budgets through strategic rotation planning
- Meeting sustainability goals by minimising synthetic nitrogen use
- Comparing the economic value of different legume break crops
Tips
- Nitrogen credits vary with the vigour and biomass of the previous legume crop
- A healthy legume crop with good nodulation provides the highest nitrogen credits
- Credits are most available in the first season after the legume and decline in subsequent years
- Always validate with soil testing as actual credits depend on seasonal conditions
Questions people ask
- How reliable are nitrogen credit estimates?
- Credits are general guidelines based on average conditions. Actual nitrogen fixation depends on legume crop health, nodulation quality, biomass produced, soil conditions, and climate. Soil testing before planting is recommended to verify available nitrogen.
- Do non-legume crops provide any nitrogen credit?
- Non-legume crops like wheat, corn, and canola do not fix atmospheric nitrogen and provide zero nitrogen credit. However, crop residues do return some organic matter to the soil which can slowly release small amounts of nitrogen through decomposition.
- What is the environmental benefit of using nitrogen credits?
- Reducing synthetic nitrogen application decreases the risk of nitrous oxide emissions (a potent greenhouse gas), groundwater contamination through nitrate leaching, and eutrophication of waterways. Strategic use of legumes in rotation is a key sustainable farming practice.