Nitrogen Contribution from Pulse Crops
Large pulse crops with high biomass and relatively low grain yields will contribute significant amounts of residual Nitrogen to the soil. Shorter crops with less biomass that yield relatively well, export much of the fixed Nitrogen as grain and will return little residual Nitrogen to the soil.
Pulse crops fix Nitrogen at roughly 20-25kg N/tonne of Dry Matter (DM), although this can vary between crops.
A 2.0 t/ha Lupin crop with a harvest index of 40% will produce 120kg Nitrogen per hectare.
The same crop will export 90kg Nitrogen in the grain, leaving 30kg Nitrogen in the soil.
Harvest Index (HI) refers to the amount of crop biomass that is converted into grain. Small, high yielding crops have a higher HI, while large, bulky crops which do not yield as well have a low HI. Pulse crops with higher HI will generate more grain income, but return less Nitrogen to the soil.
Some pulses such as Field Peas will use existing soil reserves before beginning to fix Nitrogen. By contrast Lupins and Faba Beans will fix Nitrogen regardless of starting soil levels.
Pulse Choice
While Nitrogen production is an important consideration, matching the crop with soil type is far more important when choosing a pulse crop.
Lupins are suited to well-drained, slightly acidic soils and do not grow well under waterlogged conditions.
Faba Beans prefer neutral to alkaline heavy clay soils.
Field Peas are fairly versatile but do not grow well in acidic soils with high Aluminium.
Chickpeas prefer neutral to alkaline, well drained soils, hence do not perform well in southern NSW.
As long as pulse grain prices remain high, pulse crops will be an attractive option as a break crop. Growing pulses requires more attention to detail and additional inputs, compared to cereals or canola. While pulses do not require Nitrogen fertiliser, a high yielding pulse crop will export most of the fixed Nitrogen in grain, so should not be seen as a long term answer to declining soil Nitrogen.
The table below provides indicative soil Nitrogen levels after a range of pulse crops with different harvest indexes assuming 25 kg N/tonne DM.
Source: Modified from NSW DPI publication “Pulses: Putting life into the farming system”