I get very irritated at gardening books and experts which tell Aussie gardeners that peas and beans will fix nitrogen in their garden soil. The theory being that the nodules on legume roots hold nitrogen, and when the roots die off, the nitrogen is returned to the soil for other crops. Some crop rotation patterns and recommendations for fertilising legumes are based on this myth.
Legumes roots require the presence of suitable bacteria in soil to efficiently fix nitrogen in the nodules on their roots. The legume vegetables and flowers we grow originated in the Northern Hemisphere where nitrogen-fixing Rhizobia bacteria occur in soils, but they are not naturally found in Australian soils; they must be introduced by sowing seed that has been inoculated with a suitable rhizobium. Some groups of rhizobia are effective for several legumes; others are specific to a particular crop. Some crops, such as French beans, require a specific rhizobium that is very difficult to find here, and these will not be able to fix nitrogen effectively.
Our native legumes can fix nitrogen because the rhizobia they require is in our soils. You can verify nitrogen fixation yourself by gently digging down beside a legume plant to expose some of the roots with nodules. The nitrogen-fixing nodules can be clearly seen on the White Clover roots in the photo below.
Split one of the nodules in half with a thumbnail. If the interior colour is deep pink to dark red, your legumes are efficiently fixing nitrogen and a suitable rhizobium is present in that garden bed. If the interior colour is pale pink, your plants are trying hard, but there is little of the appropriate rhizobium available. If the interior of the nodule is white, your legumes are not fixing nitrogen. (See the close up of the split nodule, below.)
When sowing un-inoculated legume seed, you will have to include enough nitrogen in the fertiliser for good growth, and avoid lower leaves of legumes becoming yellow. This occurs when the plant contains insufficient nitrogen or magnesium and the plant draws these elements from the lower leaves to the young tips in an effort to keep growing.