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Legume nitrogen fixation learning module

Meagan Schipanski and Laurie Drinkwater

Topics covered:

  1. Basic overview of nitrogen (N) fixation
  2. Why is legume N fixation so variable?
  3. How can legume N fixation be intentionally managed?

Legume N fixation in organic cropping systems

Legumes are grown for many purposes. In organic cropping systems legumes that serve as edible grain crops, forages and cover crops are considered to be key crops in many rotations. Both legume and non-legume cover crops serve many different purposes, including:

  • Improving nutrient retention.
  • Reducing soil erosion.
  • Increasing soil organic matter.
  • Reducing weed pressure.
  • Increasing rotational diversity.

Legume cover crops provide the additional benefit of adding new N to the soil through N fixation.

Ultimately, organic farming is dependent on biological N fixation as the primary source of N to replenish soil fertility. Organic farms vary in their reliance on legume N fixation as an N fertility source. In a study of N inputs on 12 organic grain and vegetable farms in the northeast, organic grain farms relied on legume N fixation for an average of 80% of all N inputs. In contrast, organic vegetable farms relied on legume N fixation for only an average of 18% of N inputs and the majority of N inputs were from compost additions.

Even though some organic cropping systems rely heavily on the N fixed by legumes to supply sufficient N for subsequent crops, we know little about how much N is actually being fixed within organic cropping systems. Cover crop handbooks provide estimates of how much N different legume varieties will fix based on data from published research papers. However, it is common knowledge that N fixation rates are extremely variable because the process is sensitive to many environmental and biological factors. For example, one very strong influence that controls N fixation is background soil fertility.

This learning module provides an overview of the basic biology and ecology of biological N fixation. We hope this information will be useful to farmers who want to improve their management of this important process.

What is legume N fixation? Go to the first unit:
The basics of legume N fixation

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