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HomePasture HealthRhizobia = Free Nitrogen!

Rhizobia = Free Nitrogen!

Fritz Haber and Carl Bosch were German scientists.
Fritz Haber and Carl Bosch were German scientists.

Nitrogen is one of the most important macronutrients for plant growth and ever since Fritz Haber and Carl Bosch invented a process that can produce nitrogen out of “thin air” we’ve been making, bagging and then applying more nitrogen to our fields and pasture. The process earned its inventors two Noble Prizes and was arguably one of the highest contributions to humanity in the last 200 years because, it enabled the production of nitrogen fertilizer which helped increased food production, allowing population to rise to a current 7 billion. But making fertilizer via the Haber-Bosch process requires vast amounts of energy (up to 3,600 psi of pressure and up to 1,022 ̊F). So is there some cheaper, more natural way to get the same results?

If you’re thinking ‘germs or microbes’ you’re right! A good example is a soil bacterium genus Rhizobia, that associates with legume roots in a a mutually beneficial partnership. Rhizobia “infects” legume roots looking for the energy it needs to live (stored in plant sugars or carbohydrates). In return, rhizobia is sequesters nitrogen from the atmosphere, making it available to plants for free. This is equivalent to having a nitrogen factory right in your soil!

To obtain this benefit, if specific soil bacteria is not present in the soil, we inoculate legume seeds. Each legume seed has its own preferred rhizobia

How to Inoculate Your Legume Seeds

Here's the Rhizobia for your legume.
Here’s the Rhizobia for your legume.

First, each legume seed has a preferred rhizobia choose the right rhizobia for your legume (e.g.: Clovers like “Rhizobium trifolii”).  Check the table at the right to pick out the one you need.  If the package you’re looking at is out of date don’t buy it. Once you have your inoculant, keep it in a cool, dark place as heat and sunlight can kill these tiny living beings.

To inoculate, place legume seeds in a bucket in the case of small quantities, oroin a flat surface (it could be a swiped barn floor or on a tarp). In a clean container, mix warm water with inoculant and sugar to activate the rhizobium. Sugar will provide the stickiness to glue inoculant and seed together. Next form a mound with the legume seeds. Open a “hole” in the middle of the seed mound and slowly start pouring the warm sweet gooey mix of rhizobia inoculant. Gently mix it with a shovel. Lastly, sprinkle lime over the mound until seeds get a whitish dry coat. This will also prevent the gooey stickiness from being transferred into the seeder where it could form clumps. When the mix is dry and loose, seed it.

Symbiosis between rhizobia and legumes roots cause pinkish nodules. Rhizobia benefits from sugars exudated by roots. Plants absorb nitrogen captured and processed by rhizobia.
Symbiosis between rhizobia and legumes roots cause pinkish nodules. Rhizobia benefits from sugars exudated by roots. Plants absorb nitrogen captured and processed by rhizobia.

In general, establishing forage species can be as simple as broadcasting them in a pasture strip (by hand or with a spreader) just before animals are to be moved to a new strip or pasture. Use animals to briefly walk around the strip or paddock to promote seed-to-soil soil contact to stimulate better germination rates. This is the most inexpensive way to get legumes and nitrogen in your pastures however, be aware that rates of germination could be lower than when compared to no-till or tilling methods.

Want more? Check this factsheet.

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Juan Alvez
Juan Alvez
Juan comes from a two-generation pasture-based family farm in Uruguay. He obtained his BS in Agronomy in Brazil, his MS in Plant and Soil Science with Bill Murphy and his Ph.D. in Natural Resources at the University of Vermont. He has experienced interdisciplinary research in grazing management, agroecology, ecosystems goods and services, land use change, conservation policy, green markets, and ecological economics. His work addresses environmental, social and productive aspects of grazing farms, with emphasis on dairy management, ecosystems conservation and sustainable livelihoods in Vermont and New England. In his study, grasslands play a key role because they are complex ecosystems that sustain a vast array of functions and processes delivering benefits for supporting healthy environments and communities.

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