Howard Cox, DAF QLD
Growing pulses can increase soil nitrogen (N) stocks. Researchers in QLD have developed some rules of thumb to estimate pulse-N contribution to the next cereal crop. The rules apply to chickpeas and faba beans.
Calculating N from pulses
In a good chickpea or faba bean crop:
Extra N (kgN/ha) = grain yield (t/ha) * 20
For example, a good chickpea crop yielding 3 t/ha will supply an extra 60 kgN/ha.
Another rule of thumb from southern NSW suggests using grain yield (t/ha) * 18 to calculate additional N contribution.
In a semi failed crop:
Extra N (kgN/ha) = crop biomass (t/ha) x 10
For example, a frost affected crop with a biomass of 1 t/ha will supply 10 kgN/ha.
Should you change your N rate?
Consider reducing N fertiliser if the pulse crop has supplied more than 40 kgN/ha. Consider N contributions less than 40 kg/ha a boost to soil reserves. The next crop probably still needs normal N rates.
Use soil tests as well
Always check soil N stocks with soil tests. The amount of N a pulse fixes varies and is affected by:
- crop species
- soil conditions (moisture, organic matter, N levels)
- seasonal conditions
- crop performance
If available soil N is > 100 kg/ha, pulses are unlikely to have fixed much N. In high N soils, pulses take advantage of the existing N rather than fixing their own.