Need Nitrogen? Plant Legumes!

As landowners and farmers start to plan for the upcoming planting season, they may consider the benefits of using legume cover crops to provide nitrogen. Some of the most common types of legumes include clover, alfalfa, and soybeans.

Nitrogen Fixation: the process used to convert atmospheric nitrogen into a form usable to the plant.

However, nitrogen fixation does not work unless a specific bacteria (rhizobium) is present to initiate the process. Once the legumes have the correct bacteria to fix nitrogen, they need to protect these nitrogen producing root nodules from exposure to oxygen. Exposure will destroy the enzyme in the plants that assists in fixing nitrogen. It would be like putting wax on your new car to protect it from the harmful winter salts that rust our cars from oxidation. Instead of wax, legumes make a protective substance called leghemoglobin to protect themselves from oxygen exposure.

Want to see the leghemoglobin? Try this:

  • Dig up a couple legume plants from your garden, lawn, or field.
  • Gently wash the roots in a bucket to remove all of the soil.
  • Identify the ball like nodules on the roots. (Nodules should be about the size of a Nerd candy)
  • Open/break the nodule to observe the leghemoglobin inside.
  • What color is it?

The relationship between the bacteria and the plant is called symbiosis, meaning both plant and bacteria benefit from the relationship. The bacteria use carbohydrates from the plant for energy to stay alive and the plant can grow and be productive without human-added nitrogen. How does the farmer/gardener benefit? Legumes provide at least 50 lbs. per acre of nitrogen for crops planted after the legumes are terminated. To release the nitrogen fixed by legumes for use by other forages, the stems and leaves must be recycled: the top growth is eaten by animals or the plants are tilled into the soil & decompose. Nitrogen fixation through the use of legumes can also reduce your carbon footprint as commercial nitrogen is produced using natural gas (a non-renewable resource).

Jeff Fewless is the Conservation Technical Assistance Initiative (CTAI) Technician for Missaukee Conservation District. Call 231.839.7193 to contact Jeff with questions about this article or related issues.

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