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Researching legume nitrogen fixation

Meagan Schipanski and Laurie Drinkwater

Even though many organic cropping systems rely heavily on the nitrogen (N) fixed by legumes, we know little about what controls N fixation within organic cropping systems. Our interest in researching legume N fixation evolved from efforts to calculate the N inputs in organic cropping systems. The N content of composts and other organic amendments are relatively easy to measure. In contrast, we realized that our estimates of legume N fixation were just thatórough estimates. At the same time, we were also getting questions from organic farmers who were wondering about how much N they were actually getting from their legume cover crops. Cover crop handbooks provide estimates of how much N different legume varieties will fix, but N fixation in the field can be extremely variable.

This module provides an overview of field experiments and methods used to study how soil fertility and plant species affect legume N fixation.

Two field studies were conducted to answer the following questions:

  1. How does soil fertility affect legume N fixation?
  2. How much N do different legumes fix when grown in pure stands and in mixtures with grasses?

Through these studies of the relationship between soil fertility and N fixation in several legume species we hope to identify simple soil measurements that can be used to provide better estimates of legume N fixation. This could allow farmers to determine the relative costs and benefits of different N sources such as legumes, composts, and manures at different stages of soil fertility development.

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