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Should you 'Inoculate' your Peas & Beans?

Question. I purchased some bush beans and the package recommends I treat the seeds with a "garden inoculant." What do they have in mind, and is this necessary?

    ---Russell in San Diego
Answer. 'Necessary'? No. Wonderful? Yes! These strains of naturally occurring soil bacteria enable plants in the legume family to suck nitrogen—the primary food plants need for growth—right out of the air.

Community Gardener Phil Williams in Augusta, Georgia recently sent me an excellent article from the University of Florida's Extension Service explaining that an astounding 35 tons (!) of "free nitrogen" is floating above every acre of land. That's a lot of plant food just waiting to be tapped, and the plants didn't wait for us to do the tapping! In a really cool act of Nature, certain plants have formed a symbiotic relationship with bacteria in the soil, causing gall-like growths to form on the roots and allowing the plants to suck in that air-borne food, an act known as "nitrogen fixing" or "nitrogen fixation."

The absorbed nitrogen feeds those plants directly, and some remains sequestered in the plant tissue, so that the plants themselves can be used to fertilize other crops. This is utilized in organic farming—and savvy home gardens—in the form of cover crops or green manures, in which nitrogen fixing plants are grown and then plowed into the soil to fertilize it. On a smaller scale, home gardeners who compost the remains of their peas and beans add some nitrogen to the pile.

There are many different species of such bacteria, each a specialist. Farmers typically buy individual varieties, but most inoculants sold for home garden use are a blend of organisms designed to work well on the most common home-grown leguminous crops: peas, beans and soybeans. If you want to grow things like alfalfa or clover, buy inoculants specific to those plants.

It's very easy to use in the home garden; you just dampen your seeds and roll them around in the powder, or dust some in the hole at planting time. Just don't inoculate seeds until you're ready to plant them; they should go into the ground right away afterwards.

The bacteria need fairly warm soil to get to work, so be patient with early pea plantings; those tell-tale nodules might take a while to form, but you'll get the benefits when they do. The bacteria also work best in soils that have a neutral pH, as acidity inhibits their ability to colonize. But that's cool, as peas and beans actually prefer a slightly sweet (alkaline) soil. That's why I dust a little bit of wood ash in their planting holes; it raises the pH and provides potassium and calcium (which also assist with inoculation).

Just don't use any nitrogen fertilizer if you inoculate; the plants will use up all of yours before they bother to make any of their own.

(Click here for a previous Question of the Week with lots more wood ash info.)

Question. Are pea and bean inoculants "natural"? I've heard that they protect the seed from mold and rot after planting, particularly in cooler climates where germination may be slow. (Like for me! I garden at 7, 000 feet; and we had a frost on June 18th last year!) Or is it the nitrogen from the air that does this?

    ---Tina; 45 minutes West of Denver
Answer. Sorry Tina—although they ARE natural, inoculants don't prevent seed from rotting in cold soil. You're confusing them with 'treated seed'; seeds that are sold coated with nasty fungicides like Thiram and Captan.


And these fungicides inhibit inoculation. So using treated seed would make your plants unable to suck that nitrogen out of the air, even if you did provide the necessary bacteria. If you have to plant in cold soil, as is often the case with Springtime runs of snow peas, snap peas and English shelling peas, just pre-sprout the seeds instead. Which brings us to….

Question. Mike: What are the details of this process of yours for pre-sprouting pea seeds before planting in the spring? We have heavy clay soil that is cold and waterlogged until the 2nd half of May. Even broccoli transplants are chancy in Mid April, technically a month after our last spring frost!

    ---Kay the Gardener in the Pacific NW
Answer. Place the seeds inside damp (but not soaking wet) paper towels, put the towels inside plastic bags, close them loosely and leave them out in a warm room. Check them every day, and plant them when they all—or mostly all—have little sprouts poking out. Although they won't sprout in cold soil, peas don't mind a chill once they're up. The plants actively grow anytime its 50° F or above, and can survive such cold temps that 'the book' says to plant peas about a month BEFORE your last average frost date.

But see what you can do about un-logging that soil with lots of compost first; only water chestnuts like wet feet…


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