Legumes (peas, vetches, clovers, beans and others) grow in a symbiotic relationship with soil-dwelling bacteria. The bacteria take gaseous nitrogen from the air in the soil and feed this nitrogen to the legumes; in exchange the plant provides carbohydrates to the bacteria. Do beans contain nitrogen?
An important food crop for centuries, beans are soil improvers, adding nitrogen to build soil fertility . Beans improve the soil with bacteria, which forms nodules on their roots. The nodules absorb nitrogen from the air in the soil, fertilizing not only the bean plants, but others as well.
Some insights into this can be found in Designing and Maintaining Your Edible Landscape Naturally, Robert Kourick, which shows a bean plant and its relative nitrogen levels. In a green plant before flowering, 60% of the fixed nitrogen is found above ground in leaves and stem and 40% below ground.
A inquiry we ran across in our research was “Are beans good for soil?”.
Beans are one of the only plants — I used to say the only plant, but I just read about a tree in Africa — that give back to the soil. In other words, when you raise them, they don’t deplete the soil of nutrients during their growing phase. They actually give back; they enrich the soil by adding nitrogen.
What plants add nitrogen to soil?
Legumes such as peas, peanuts, beans, clover, and alfalfa are the best plants for adding nitrogen to soil. According to Wikipedia, a legume is a plant that has “symbiotic nitrogen-fixing bacteria in structures called root nodules.” (The specific type of bacteria is called Rhizobia ).
Grow corn one year and follow that up with beans or peas the following year to restore the level of nitrogen in the soil. There is no doubt that legumes are able to capture atmospheric nitrogen (using bacteria) and convert it to plant usable nitrogen, but how much of this actually benefits other plants?
Do broad beans absorb nitrogen from garlic?
In one study broad beans were injected with radioactive urea to see where it goes. The garlic growing nearby absorbed some of the nitrogen from the bean, clearly showing it moved while both plants were alive. Other research has shown no movement of nitrogen between plants .
How do plants get nitrogen from the air?
From their homes in the plant roots they take nitrogen from the air and convert it into ammonia, a form of nitrogen that plants can use. For this valuable service the legumes feed the bacteria a steady diet of plant sugars. Your soil may already have several varieties of rhizobia present that live on from year to year.