Legumes and Soil Quality

Legume Confusion

Leaves and flowers of the 'Lead Tree'

It is common to find reference to the great benefits of growing a crop of legumes such as Beans or Peas in terms of soil health. Indeed the Mayans were know to cultivate the Leauceana or 'Lead Tree' for the specific purpose of improving soil quality. Six bags of dried leaves from this tree are comparable to an equivalent bag of store bought Ammonium Sulfate fertilizer. AMAZING!!

This discussion arises from the fact that legume species from the Fabaceae plant family form mutually beneficial (Symbiotic) relationships with bacteria in the soil know as Rhizobia. These Rhizobia in-turn process Nitrogen from air pockets in the soil and make it available to plants as Ammonium. Nitrogen in the air is after all not useable by plants before it is 'fixed' in such a process.

Why are we talking about Nitrogen in the first place? Because it is possibly the single key nutrient to plants and thus life on earth. Low Nitrogen levels are the most common cause of crop failure or low yield and Nitrogen is readily lost from agricultural and garden soils alike during extended wet periods and to the natural Nitrogen cycle (Diagram below).

Rhizobia 'Nodules' on a bean plant

This relationship costs the legume in question quite a substantial sum as it must transfer a large portion of the energy it makes through photosynthesis to the Rhizobia. As such, contrary to popular belief, there is actually little to no 'leakage' of Nitrogen from the the Rhizobia into the surrounding soils. Its like asking your next door neighbor if you could borrow the carpenter they have hired after he finishes fixing the front door. 

Rhizobia on a Pea

That is to say you will most likely not get a bigger tomato crop simply by planting beans near them during the growing season. The true benefits of the legumes come only if you turn the whole growing portion of the plant back into the soil after it is done, let it rest and then plant where the legumes have decayed. Note also that the presence of the nodules i have shown does not necessarily mean you are getting nitrogen fixation taking place. Nodules that are larger than 2mm and a pink/red color are working efficiently and fixing nitrogen.

This is not to say that planting legumes is not worthwhile, simply that the right procedures need to be followed in order to get the maximum benefits.