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The truth about legumes and nitrogen fixation

We are commonly told to plant legumes in pasture or in the garden to increase the nitrogen in the soil.  It is true that given the right minerals and microbes in the soil, legumes will develop a symbiotic relationship with rhizobial bacteria, which can “fix” gaseous nitrogen from the air and make it available to the plant (more here).  The important point is that this nitrogen is used by the growing legume plant and only minimal amounts are transferred to the soil orother plants.
“The amount of nitrogen returned to the soil during or after a legume crop can be misleading.  Almost all of the nitrogen fixed goes directly into the plant. Little leaks into the soil for a neighboring nonlegume plant.  However,  nitrogen eventually returns to the soil for a neighboring plant when vegetation (roots, leaves, fruits) of the legume dies and decomposes.”
 Therefore, the only way to harness the nitrogen produced by the legume/rhizobial relationship is to use the legume as a cover crop and mulch it onto the soil (or where you need the nitrogen) at the end of the season.  It will not provide nitrogen to other plants as it is growing.  With the exception of perennial leguminous trees and shrubs (e.g. Tagasaste, wattles (Acacia) and Pigeon Peas), which can contribute nitrogen to the soil by periodically losing their leaves and branches.

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