All plants are able to take up nitrogen from the soil in the form of ammonium (NH4+) or nitrate (NO3-); together these are known as available N. In addition to taking up available N from the soil, legumes (clovers, medics, peas and beans) are also able to acquire N from the abundant supply in the atmosphere via special soil bacteria (rhizobia) which are housed in nodules on their roots. With fully functioning nodules, legumes can grow in soils that are deficient in available N. These rhizobial ‘factories’ are subject to variation in establishment and performance and so a supportive environment must be provided to maximise N2-fixation.
Rhizobia tend to be widespread in soils, however they are not all equally effective. While most are happy to reside in nodules, not all are able to efficiently fix N2. To ensure large numbers of efficient rhizobia are present in the legume nodules (figure 1), it is advisable to inoculate legume seed at sowing with the recommended strain of rhizobia if the paddock has not been inoculated with that rhizobia, or has not grown a crop of a suitable host legume for that rhizobia in the last 4 years.
Figure 1: Good nodulation on faba bean).
Survival of rhizobia and legume nodulation will be reduced in acid soil (pH <5), except for narrow-leaf lupin. To maximise N2-fixation in low pH soils, more regular inoculation and/or liming is required.
Where available soil N is low, the amount of N2 fixed is directly proportional to legume dry matter production (figure 2). If legume crops in any one location have about the same total dry matter, then you would expect them to fix about the same amount of N (table 1), while equally productive pastures tend to fix more N.
Figure 2: Relationship between legume shoot dry matter and fixed-N in shoots for crops and pastures in Australia.
Moderate available soil N (>35 kg/ha) will reduce crop legume N2-fixation by a similar amount. In pastures with companion grasses this mineral N will be taken up by the non-legume component and N2-fixation will not be suppressed. Approximate amounts of N2 fixed by crops and pastures can be gauged from figure 2, although this does not include root N, which might contribute another 30 %.
Table 1: Average increase in wheat grain yield after deep ripping. (Source: Steve Davies DAFWA, Crabtree 1989, Davies et al., 2006, Jarvis, 2000.)
LEGUME SHOOT DRY MATTER (t/ha)
FIELD PEA, FABA BEAN
At maturity 30–40 % of the N in legume crops is in the seeds, which are typically 25–30 % protein. When this grain is harvested, much of the N that has been fixed will be exported off of the property. However, the N remaining in the shoot and root residues means that legumes usually make a positive contribution to soil N reserves in southern Australia.
A healthy and productive field pea crop could fix up to 200 kg N/ha
Drew E et al. (2012) Inoculating legumes: a practical guide, GRDC, Canberra.
Unkovich M, Baldock J, Peoples M (2010) Prospects and problems of simple linear models for estimating symbiotic N2
fixation by crop and pasture legumes. Plant & Soil 329: 75-89.
Author: Murray Unkovich (University of Adelaide).
The National Soil Quality Monitoring Program is being funded by the Grains Research and Development Corporation, as part of the second Soil Biology Initiative.
The participating organisations accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.