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What is Rhizobia? Why Your Crops, Cover, and Turf Need It

May 8, 2023

Successful farmers are quick to know the root of any production issue. When it comes to nitrogen fixing in between rotations, however, farmers may not know what’s going on at the actual roots of their cover crops: a natural process that involves rhizobia inoculants in addition to cover crop seed.

What is rhizobia— or, more accurately: what are rhizobia? Rhizobia (also called rhizobacteria or rhizobium, singular) are soil bacteria that have formed a relationship with plant members of the legume family. These bacteria beneficially touch the roots of these plants to draw out atmospheric nitrogen down into the soil, making it available to plants down and around their roots.

Growers have utilized legumes paired with rhizobia inoculant to naturally restore nitrogen to farmland for over a century. This tiny bacteria can be pivotal to successful farming— read on to learn more about rhizobia, and here’s why your crops, cover, and turf need it.

Legume cover crops wouldn’t work without it.

For major legume cover crops like alfalfa, peas, and clover to accomplish nitrogen fixation at their roots for rotating crops to come, rhizobia bacteria must be inoculated into the soil alongside these plantings.

These plants may even form the telltale nodules at their roots to signal nitrogen fixation could happen; but, without a present relationship with rhizobia bacteria, nitrogen pull from the atmosphere into soil won’t take place. To signal that nitrogen fixation is taking place at cover crop roots, nodules will turn a pink color: this indicates rhizobia have beneficially infected the nodules and fixation is happening.

Legume cover crops can still be very useful and offer benefits like winter cover, green manure, and other micronutrient accumulation without the help of rhizobia. Lacking rhizobia inoculation alongside legume plantings, however, means no added nitrogen, which may defeat the purpose of some growers’ planting intentions.

It’s essential for naturally occurring nitrogen in soil.

No other soil bacteria or microbe creates nitrogen in the soil with plant roots like rhizobia does. The relationship rhizobia and legume plants have is unique and singular: there is no other soil relationship like it, meaning that a legume cover crop and rhizobia bacteria together are both specifically required.

That said, legumes can be interplanted with other plants, like turf— or before the next rotated crop, of course— for nitrogen fixation to happen and to benefit other plants, along with rhizobia inoculants. Proper termination of legume cover crops is also crucial to ensure nitrogen stays within the soil for other plants and crops to come.

Many legumes utilize all the nitrogen they fix for the flowering or seeding process. When this happens, the fixed nitrogen during its growing phases is not available for the next crop. As such, terminating legume cover at the right time to get the benefits of rhizobia nitrogen— before this happens— is critical.

It may be vital for the bioavailability of other nutrients.

Increasing nitrogen levels in soil is shown to boost the uptake of other vital micronutrients for plants as well, including sodium, phosphorus, copper, and zinc. Besides amending with nitrogen fertilizers, pairing legume cover crops and rhizobia can be a strategic way to naturally increase the overall absorption (though not actual present mineral levels) of these essential elements for healthy plant growth and successful yields.

One study shows that increasing nitrogen in soil has an effect on increasing the uptake of other minerals by plants— especially phosphorus, the next major important nutrient for crops after nitrogen. It can also have some impact on potassium uptake; however, some research shows that the more nitrogen builds in soils the more likely potassium uptake may eventually be inhibited.

If phosphorus, potassium, or other micronutrient uptake is at issue with your crops, implementing rhizobia with cover crops may help boost uptake.

It can play a helpful role in adequate soil structure.

As living soil microbes, rhizobia can play an important role for soil health as one of many other living organisms in the soil microbiome. Rhizobia also make nitrogen available for other living things in the soil around your plants’ roots. The result is greater diversity of soil life and better (and healthier) soil structure, which means healthier crops overall.

Soil assessments again and again show that better soil structure— including both more soil bacteria and organic matter— improves yield outcomes and even drought resistance among crops of all types. One study on rhizobia bacteria emphasizes its important role in enhancing naturally healthier soil structure; another study showed that inoculating poor or even compacted soil with rhizobia could help restore not only microorganism diversity but soil structure as well.

Bacterial inoculants of rhizobia are incredibly vital to successful and ecologically friendly farming, helping growers curb nitrogen fertilizers. As a cost-effective, labor-saving alternative to typical field inoculants, many seed treatments or seed coating technologies are beginning to incorporate rhizobia bacteria directly into their products (and especially cover crop seed): meaning that as soon as your cover crop plantings touch the soil, they are accompanied by the rhizobia bacteria they need for a successful season of nitrogen fixation ahead.

seeds with seed coatings being planted by hand in soil

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