We know from past research that there’s usually a nitrogen (N) benefit from growing legume crops and that productive legume-based pastures are worth their weight in gold to crops on the following paddock rotation.

But what is the overall value to the system? How much N is supplied? How much do the following crops benefit? And what are the longer-term effects on soil fertility? With newer varieties and better agronomic packages, these legumes and legume-based pastures can be very productive — we might be underestimating their value to following crops.

Last year, CSBP set up a long-term rotation trial near Moora with blocks of lupins, canola and serradella. The site is a deep yellow loamy sand that has been ameliorated with lime, deep ripping and Plozza ploughing.

Grain analysis showed that harvesting the big lupin crop removed about 170 kg N/ha in the grain, and the canola removed 65 kg N/ha. Serradella cuts and analysis at the end of September showed about 100 kg N/ha in the above-ground biomass.

Since March, the site has been soil tested three times to look at the effects of the different rotations on mineral N levels (Figure 1).

The first lot of samples from 17 March showed higher levels of soil nitrates, as expected after the legumes. But, by 2 May, the levels had fallen. This was likely due to plant uptake by volunteers which germinated following late March rains and were not subsequently controlled until a knockdown was applied on 8 May.

The trial was sown to wheat on 16 May. Soil sampling at the end of May showed the soil nitrates after legumes returned to elevated levels.

Figure 1.

Soil nitrate N measured at 0–10 cm on 17 March, 2 May and 26 May 2022 after lupins, serradella and canola were grown in 2021. This graph indicates that soil nitrates after lupins and serradella may follow a similar pattern that is distinctly different from canola plots.

Deeper soil sampling in May also showed that the nitrate N levels were higher down the profile after legumes, with higher levels measured after lupins (Figure 2).

Figure 2.

Soil nitrate N was measured down to 60–90 cm on 26 May 2022 after lupins, serradella and canola were grown in 2021. Lupins were enriching the subsoil with nitrates more than the canola and serradella plots.

This season we will continue to monitor soil N levels and crop recovery of N from the different rotations. Three rates of N have been applied to a wheat crop that followed each rotation this year to compare responses in yield, protein and gross margins.

The intent is to continue the trial for many years and to evaluate the effect of rotations on N contributions, fertiliser N efficiencies and gross margins.

Image: Lupin plots at the start of August 2021.