If you’ve grown peas, lentils, or any other pulse crop for a while, there may be background levels of rhizobia in your fields. However, it’s still important to use a “fresh” inoculant when you seed.
“Inoculants are the rhizobial products that provide the right species of rhizobia for the right crop,” says Sherrilyn Phelps, agronomy research manager at Saskatchewan Pulse Growers. “You need the right rhizobia in order to have decent nodulation in your crops.”
Using the right inoculant for the crop is “cheap insurance” to make sure you’re getting the right level of rhizobia near the seed to illicit the maximum nodulation response, says Phelps. It’s also an insurance that the rhizobia applied is effective. Over time, rhizobia can become naturalized, less competitive with commercial strains, and less effective. These strains might form a nodule but they might not actively fix N.
It’s particularly important to inoculate a crop that hasn’t been grown in the crop rotation, or have a specific rhizobia species that will biologically fix N with the crop, says Phelps.
Soybeans and their specific rhizobia species, Bradyrhizobium japonicum, is a good example of this — when grown for the first time, soybeans can’t rely on background levels of rhizobia used for other pulse crops. Chickpea is another example, which needs Rhizobium cicero instead of the Rhizobium leguminosarum used for faba beans, peas, and lentils.
Dry beans are an exception, says Phelps, as the inoculant used does not produce consistent and effective nodulation, in general.
Diving deeper, we know that biological N-fixation happens in a nodule, but why does the inoculant used need to be specific?
A pulse crop will signal to rhizobia and the bacteria will respond to this signal, by inducing the genes responsible for nodulation, and will attach itself to the root to start forming a nodule. The compounds secreted by both organisms as part of that signalling process are specific — they can recognize each other.
The wrong strain of rhizobia, or naturalized rhizobia might attach and form a nodule, but if it’s not the correct strain, or it’s not competitive, it will not effectively fix N. The easiest way to check on your nodules is to dig up a few plants around flowering, and crack open a nodule. Look for the pink to red colour — that means it’s fixing N.
The other issue with effective nodulation, is that there needs to be close contact between rhizobia and root hairs in order for attachment to occur. By inoculating, you’re guaranteeing rhizobia are around those root hairs, and don’t have to travel once the pulse crop initiates that signal.
If a pulse crop is not actively fixing its own N, in symbiosis with rhizobia, it will use reserve N from the soil, and if that N is low, the crop will run low by the time it gets to filling its seed, which could result in lower yields and lower protein.