Legumes Should Be Supplemented with Nitrogen for Optimum Results



One of the bird of the desert bird of paradise, a legume.
Most legumes have characteristic flowers and leaves.

Q. Do nitrogen fixers like trumpet vines and locust trees provide significant nitrogen to nearby plants?  For example the Bermuda lawn surrounding the locust or the iris and daffodils in a flower bed anchored by a trumpet vine.

 

A. No they don’t produce enough for our high expectations in landscapes and gardens. Nitrogen fixers (such as legumes, there are others) supply enough to help make sure they can reproduce and make seed. The objectives (if I can put it in human terms) of plants and humans are different. Plants want to survive, reproduce and out-compete with other plants for their niche. In nitrogen poor soils nitrogen fixers, like legumes, take nitrogen from the air and supplement what they can’t get from the ground. In nitrogen poor soils, legumes are fantastic competitors. In nitrogen rich soils they are not.

Typical legume flower leaf and pod

            The expectations of humans for the plants they care for are far greater than plant objectives. We want beauty and lushness from landscape plants and we want a good production of food from our legume crops.

            The nitrogen needed to meet human expectations is far greater than the nitrogen needed to meet plant objectives. So for this reason, we need to fertilize nitrogen fixing plants with nitrogen to meet our objectives. The basic rule of thumb I use is the question, “So I want my plants to meet what they consider to be adequate (reproduction and beat out the competition) or do I want them to do more than that?”

Snow pea flowers and leaves are good examples of what
many legumes resemble

            Most people want these plants to do far more than successfully reproduce. Some people are purists and they want that “native look” or for philosophical reasons they prefer the plant produce what it can they are happy living in the "nitrogen cycle".  Nothing wrong with that and it meets their expectations. If you want lushness or greater production, then add extra nitrogen.

            The general rule of thumb you can follow is that many nitrogen fixing plants receive only about 25%, at best, of the nitrogen they need to meet our expectations. However you can treat legumes just like any other plant and feed them extra nitrogen. Plants can be lazy. If you give them all this nitrogen, they may produce little to no nodules on the roots (the nodules contain the nitrogen fixing bacteria). Hey, its alot easier to take available nitrogen than it is to build these homes on their roots for these symbiotic bacteria that take nitrogen from the air.

root nodules of legume can resemble root knot nematode infestation

            We are lucky in that nitrogen fixation by legumes is far more efficient in our alkaline soils of the desert than in acid soils of high rainfall areas. So to answer your question with a short winded response, no, they will not produce enough nitrogen for surrounding plants if your landscape expectations are high. If you are a eco-purist, then maybe they will.