Maximising the nitrogen legacy from legume crops depends on maximising biomass and ensuring high nitrogen fixation rates.

Key points

• The value of legume nitrogen benefits can be significant at more than $200 per hectare when urea prices are $1200 per tonne
• To maximise the nitrogen legacy, maximise crop biomass and nitrogen fixation by selecting the best legume crop, variety and sowing time for your soil and get the agronomy right

Declining pre-sowing soil mineral nitrogen and grain protein levels, together with recent increases in nitrogen fertiliser prices, have refocused attention on the value of legume crops as a nitrogen source.

Soil organic nitrogen mineralises by an average of 2.5 per cent per year in continuous cropping systems. Failure to restore it with pasture phases or grain legumes means that the relative spend on nitrogen fertiliser could double from nine to 10 per cent of gross margins in 2017 to 18 per cent in 2067.

To maintain crop yields, growers either need to increase the rate of nitrogen fertiliser or incorporate more legume crops and pastures into the system.

Legume legacy

To evaluate the benefit of legumes, CSIRO Agriculture and Food and the NSW Department of Primary Industries established field experiments at Wagga Wagga, Condobolin, Urana and Greenethorpe in NSW with GRDC support.

The aim was to measure the impact of legumes on soil mineral nitrogen available to subsequent crops and the yield and dollar value of this benefit to the system.

All sites were sown to wheat in 2017 before commencing three-year phased rotations of barley/canola/wheat or legume/canola/wheat in 2018, 2019 or 2020 under a range of nitrogen fertiliser strategies. The legume crops included lentils, chickpeas, lupins, faba beans and vetch hay.

Soil nitrogen testing, undertaken when sowing canola in the subsequent season, showed that legume crops provided an average increase of at least 50 kilograms per hectare of additional mineral nitrogen compared to those following cereal crops across all legume crop types, seasons and sites (Table 1).

Table 1: The additional soil mineral nitrogen available at sowing following each legume crop at four field sites (averaged across 2019, 2020 and 2021 sowing dates) and final average of 50kg of nitrogen/ha across all sites x crops x years. The resulting urea saving (kg/ha) and additional canola yield (kg/ha) as an average for all legumes (crops x years) and an overall average (sites x crops x years).

Source: CSIRO

Much of this nitrogen was not available directly after the legume harvest but became available over the summer fallow period and as the nitrogen mineralised throughout the season.

At the relatively high urea prices of $1200 per tonne in early 2022, the value of legume nitrogen benefits can be significant at more than $200/ha.

These figures do not include the potential value of further in-crop mineralisation during the following growing season or beyond, even when the loss of nitrogen from the various end-uses, such as grain or hay removal, are accounted for.

Maximising the benefits

Importantly, the choice of legume crop was less important than ensuring that the best legume species and variety was selected to suit local soil and climatic conditions. Getting the agronomy right to ensure effective nodulation and high biomass is key.

While there was a significant amount of variability in the extra soil mineral nitrogen available to subsequent crops, this was related to the site, season, choice of crop and end-use option.

Maximising biomass production maximises the potential to fix nitrogen. A legume that is well-suited to its environment will likely derive more than half of its nitrogen requirements for growth from atmospheric nitrogen via fixation, provided there are no constraints to nitrogen fixation.

Poor nodulation can result from low rhizobia numbers, poor inoculation procedures, residual herbicides and subsoil constraints such as acid subsoils, but these can all be overcome with good management.

Legume crops with a high grain yield or those cut for hay production do not always provide a net input of mineral nitrogen; however, there are other benefits to a double break. These include weed and disease benefits as well as assisting in the breakdown of cereal stubble.

More information: Tony Swan, 0428 145 085, tony.swan@csiro.au; Mathew Dunn, 0447 164 776, mathew.dunn@dpi.nsw.gov.au, GRDC Update paper: What is the N legacy following pulses for subsequent crops and what management options are important to optimise N fixation?