Soybean School: How many nodules are enough?

As many soybean growers are well aware, soybeans have a unique ability to biologically fix their own nitrogen (N) in a symbiotic relationship with the bacteria, Bradyrhizobium japonicum.

Inoculant containing the bacteria can either be applied on the seed or to the soil at planing, and then as the crop is putting on vegetation and getting into flowering, the bacteria can get to work forming nodules, which is where the fixation actually happens. You might be wondering, though, how many nodules is enough?

“First you have to remember that a 50 bushel crop of soybeans needs at least 200 pounds of actual N,” says Horst Bohner, soybean specialist with the Ontario Ministry of Agriculture, Food, and Rural Affairs. There’s a fair amount of N available in the soil, but most of the N provided to the crop is through N fixation.

The direct answer to the question of how many is enough is about seven nodules per plant says Bohner. But it depends on the size of the nodules and where they are on the root structure, either lateral or crown nodules, or both.

Don’t forget too, that a nodule needs to be actively fixing N for it to be effective and the best way to check on that is to dig up the plant and crush open a nodule — if it’s pink to red in colour, it’s active, but if it’s whitish or green, it’s not active.

“If nitrogen is so important [for yield], and we know that often in August, as the plant starts to senesce a little bit and all that energy is put into the seed, N fixation really slows down, and that’s just the time when nitrogen demand really goes up,” says Bohner.

Bohner is a firm believer in feeding soybeans throughout the growing season to get those high yields. In a trial, Bohner applied 300 pounds of N fertilizer product in total on one side of a field, and the other side didn’t receive any additional N fertilizer. It’s commonly believed that when fertilizer N is supplied, N fixation doesn’t happen, but interestingly enough, in the side that fertilizer N was applied, Bohner still finds nodules.

While he wouldn’t recommend 300 pounds of fertilizer N product, Bohner would recommend 50 pounds, without any concern for hindering effective nodulation.

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