Bacterial inoculant improves spring peas

Judith Adam

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A bacterial inoculant helps boost nitrogen fixing in peas. (Photo by Brendan Zwelling)
A bacterial inoculant helps boost nitrogen fixing in peas. (Photo by Brendan Zwelling)
A bacterial inoculant helps boost nitrogen fixing in peas. (Photo by Brendan Zwelling)

I’m perfectly aware that the ground is frozen solid, and there are eight to 10 weeks of waiting before trowels and shovels can go into the soil. What’s a gardener to do, but live in hope and order seed for spring planting? My early plantings of peas were especially productive last year, most likely because I remembered to use a bacterial inoculant to boost their nitrogen-fixing ability.

Peas are legumes (just like dry beans, runner and bush beans, and soybeans), and have the ability to grab atmospheric nitrogen from air and fix it in solid form on their roots as ammonia nitrogen. The result of this process is that legumes can manufacture a considerable amount of the nitrogen they require. Legumes will attempt to do this without assistance, but using an inoculant at planting time greatly increases plant productivity.

Inoculant products contain living rhizobium bacteria, a benign organic material that should be kept refrigerated (but not frozen) for best viability, and the package should have an expiration date. Rhizobium bacteria form a symbiotic relationship with plant roots, producing results that are mutually beneficial to both partners. The bacteria are instrumental in encouraging legume roots to form the small lumpy root nodules that are mini-factories for ammonia nitrogen production. It’s interesting to examine the roots of legumes, looking for these nodules. When pea or bean plants are finished and lifted from the soil, carefully ease one clump out and spray a fine stream of water over the roots. Look carefully, and you might find small bumps along the roots. Those are the nitrogen-fixing nodules. (You can also do this with a clump of white clover, another legume, carefully eased out of the lawn in midsummer.)

Inoculant products come in several forms — solid, liquid and freeze-dried. The product prepared for home gardening is usually a packet of dust-like black powder that can be purchased at a garden centre (usually near the seed displays) or through seed catalogues. There are several species of rhizobia bacteria, and it’s important to get the right one for the plant you intend to grow. Soybeans will need a particular species of rhizobia, while another species is compatible for peas, lentils, snap and lima beans. This information is always printed on the package, and should also be available when purchasing inoculant through a catalogue.

Using inoculant is quick and easy. When ready to plant peas or beans, put the seeds into a plastic container and spray them lightly with water (no big drops). Then sprinkle the rhizobium over the seeds and swirl them around to coat in the black powder. Proceed with planting, tapping the seeds out of the container and into the furrow carefully so that not too much of the inoculant is brushed or knocked off. There’s no problem touching the inoculant with bare hands, but using fingers to plant the seeds will remove some of the powder.

All this encouragement about planting peas is making me think of what more I can grow with inoculants. For more information about inoculants, see

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