Legumes: Self Fertilizing Plants

The green stem of the legume palo verde tree in the Sonoran Desert.

Determining how to fertilize a plant can be quite the difficulty.  You may have heard that most people over water their plants, but it is also true that most people over fertilize their plants.  Certain plants however never need to be fertilized simply because they have "figured" out a way to fertilize themselves.  Bean plants, also called legumes, form a symbiotic relationship with a bacterium known as rhizobium.  The air we breath is about 70 percent nitrogen, an essential nutrient for plant fertilization.  Atmospheric nitrogen however cannot be used by plants, it must be converted into a different form called ammonium.  Rhizobium bacteria has the ability to take nitrogen gas and convert it into ammonium.  This changing of nitrogen gas into ammonium is known as nitrogen fixation. 

Using legumes in the garden can be quite a useful way to fertilize your plants naturally.  Farmers in the Midwest will often alternate between corn and soybeans, taking advantage of the fact that soybeans naturally replenish the soil with nitrogen fertilizer that the corn can use.  In deserts, which have soils that are naturally deficient in nitrogen, plants must either be adapted to living in soils with low nutrients or have the ability to fix their own nitrogen.  For this reason, legumes are extremely common in desert ecosystems.  Legumes are not just your typical bean plant, they also grow into bushes and trees.  In the Sonoran Desert palo verdes, mesquites, ironwoods, and acacias are all small legume trees that form bean-like pods.  Typically, these legume trees will form islands of soil under their canopy that are rich in nutrients compared to soils beyond the canopy.  Because of the slightly richer soil many smaller plants will often be growing in this micro-environment. 

The darker growths on these plant roots are tumors infected with rhizobium bacteria.

Legumes and Rhizobium bacteria form their association with each other in the soil-root environment.  Rhizobium are naturally occurring soil bacteria but don't really do much if they haven't infected a legume.  When bean seeds germinate and begin to grow, rhizobium already present in the soil infects the new plants roots.  Points of rhizobium infection in the roots form into tumorous-like growths which are like little nitrogen fixing factories.  Typically we think of tumors as unhealthy but be assured, this type of plant tumor is very healthy and beneficial to not only the infected plant but also to the entire environment.  Legume plants that for some reason are not infected become extremely anemic with stunted growth and yellow coloration.