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Legumes & Nitrogen Fixation

A row of plants with a stake reading, "2/1 Provider Snap Beans".

Garden crops, such as peas and beans, are unique plants that can establish a nitrogen fertilizer factory in their roots. Members of the legume family develop a symbiotic relationship with Rhizobia bacteria that operate the nitrogen factory.  

When these bacteria are living in the soil near the planted seed or are attached to the seed coat at planting, they enter the plant’s roots and multiply. The bacteria fix or capture atmospheric nitrogen gas, convert it to ammonia and make it available to the plant. The plant reciprocates by providing organic compounds to sustain the bacterial colony in the plant’s root nodules.  

To ensure your pea or bean plants develop nitrogen-fixing capabilities, live Rhizobia bacteria specific to the crop being grown must be planted with the seed.  

Once the plants have fully developed and you have harvested the beans or peas, inspect the roots for nodulation. Carefully dig up the plants and wash the roots. Where you see nodules, squeeze them and they should turn a reddish color. This indicates a fully functioning symbiosis – healthy bacteria generating plant-available nitrogen for the legume crop.   

Gardeners should consider inoculating one other crop. In the fall after all vegetables have been harvested, lightly till the soil and plant a winter cover crop mix, including legumes like hairy vetch, Austrian winter pea and crimson clover. Include cereal rye or annual ryegrass to help hold up these legumes as they grow. Treat the seed prior to planting with the correct inoculant.  

A successful winter cover crop can provide much of the nitrogen needs for next year’s garden crops.

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By Tom Basden, WVU Extension Service Specialist – Nutrient Management