4 Reasons Sulphur Is Necessary for Nitrogen Fixation in Legumes.
In many regions across North America, legumes are valued as a rotational crop because of their ability to increase nitrogen levels in the soil for the following crop. In areas such as Ontario and the Upper Midwest, growers are planting legumes such as red clover and hairy vetch as a cover crop for winter wheat and sweet corn.
The reasoning is solid: a healthy legume crop can produce 25-75 lb of nitrogen per acre every year. These practices can benefit the current and following crops. Equally important, it reduces reliance on nitrogen fertilizer.
But did you know another nutrient – Sulphur – plays a key role in nitrogen fixation?
How does nitrogen fixation work?
Legumes have a unique relationship with rhizobia bacteria, a common microorganism in the soil. Rhizobia inhabit the root nodules of a legume plant. The bacteria produce enzymes called nitrogenases, which convert nitrogen from the atmosphere (N2) into plant-available ammonia (NH3). Ammonia is converted to ammonium and nitrate nitrogen or forms of nitrogen products that are then used by the plant (it can vary, depending on the type of legume).
After harvest, nodules, roots and dry matter that is left in (or returned to) the field becomes a valuable source of nitrogen for future crops.
4 reasons sulphur is required for nitrogen fixation:
1. Nodule Formation
When plants have ample amounts of sulphur, they produce larger nodules in greater numbers. Sulphur aids in the production of leghemoglobin, glucose and adenosine triphosphate (ATP), which provides fuel to the plant.
2. Nodule Metabolism
Sulphur aids in the uptake/transportation of nutrients and enzymes. It helps form chlorophyll and key proteins, which allow the legume to efficiently process plant-available nitrogen.
3. Production of Nitrogenase.
If a plant is sulphur-deficient, the production of nitrogenase will be inhibited. This results in a lower conversion of atmospheric nitrogen to ammonia. Maintaining recommended levels of sulphur in the soil throughout the growing season will pay dividends.
4. Nitrogen Assimilation by Rhizobium
Sulphur is necessary for the production of an iron-sulphur compound that is required for both nitrogen fixation and photosynthesis: ferredoxin. It helps the bacteria convert N2 to NH3.
Nitrogen fixation requires sulphur throughout the growing season.
Ensuring a legume crop has plenty of access to nutritional sulphur throughout the growing season is essential for maximizing nitrogen production.
Tissue testing will provide the most accurate measure of in-season sulfur availability, as soil testing for S can be challenging.
Just how much do you need? If you wanted to grow a 60-bushel soybean crop, you would require approximately 30 lb of sulphur.
AMS and other fertilizers containing sulfate can meet the early crop demand – and are important in the initial formation of roots and nodules. Degradable elemental sulphur fertilizers provide an economic season-long sulphur source, which will help increase nitrogen-rich organic matter.
When trying to boost nitrogen fixation, don’t forget sulphur!
Sulphur is an often overlooked nutrient that impacts plant functions in many ways. If you’re growing legumes as a rotational or cover crop, know your sulfur requirements and don’t get caught short-handed!
4 Reasons Sulphur is Important to Nitrogen Fixation in Legumes
- Promotes Healthy Nodule Formation (size and number of nodules)
- Enhances Nodule Metabolism
- Required for Nitrogenase Production
- Supports N2 Assimilation by Rhizobia