Managing N fixation at different stages of soil organic matter development

Nutrient management strategies can vary depending on the starting point of soil fertility. For example, a field with low organic matter that is undergoing the transition to organic management requires different strategies than a field that has high organic matter. Because soil N status affects N fixation, legumes can serve as internal regulators of soil N—they help keep what N is already in the soil and add more N when there is not enough. This internal regulation can be a benefit or a detriment, depending on nutrient management goals.

Legumes compared with composts. Legumes are one of the few fertilizers that organic farmers can use that only add N to the soil. Composts, in comparison, also add phosphorus, potassium and the other nutrients needed for plant growth. In low fertility soils, using a mixture of legumes and composts is often the best option for providing enough of all of the critical nutrients at the levels that a crop uses. Using just compost to meet crop N demands tends to result in the addition of too much phosphorus and using only legumes will result in the depletion of soil phosphorus and potassium over time. The optimal balance of legumes and other nutrient amendments depends on the starting fertility status of a soil, the nutrient demands of crops being grown, and the relative cost and availability of nutrient amendments.

Transitioning to organic management. A field undergoing transition to organic management may have low organic matter levels. Composts, manures, and cover crops can be used to increase soil organic matter levels. Legume N fixation will probably be high under these conditions due to low N availability, as long as phosphorus and other nutrients are not too low. However timing is important if composts and manures are used in addition to legumes. Adding compost or manure directly to a legume cover crop can reduce the N fixation benefit.

Established organic field with high soil organic matter levels. The sensitivity of N fixation to soil N can be seen as a positive if you are trying to maintain fertile soils while still providing some quickly available N when a legume cover crop is incorporated. A reduction in N fixation with high soil N levels means that the legumes are efficiently recycling soil N and the following crop will still receive the benefits of having a proceeding N-rich legume grown. However, legume seed can be expensive and legumes can be “leakier” than grasses. Under high soil N conditions, growing legumes in intercrops with grasses or interseeded with growing crops, may provide the most cost-effective option for retaining N while still adding some new N to the system.

What our research suggests. From our limited research with soybeans on loam and clay loam soils of New York, it looks like it might take 10 to 20 years of organic management before organic matter levels reach a level that reduce soybean N fixation rates. The rate at which management practices result in increased soil organic matter levels will vary depending on many factors, such as how much compost, manure or other organic amendments are being added each year, soil texture, tillage practices, and climate.

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