Fixing N lacks promise in cooler climates

Nitrogen fixation is a very biologically expensive process. “The word on the street still is that legumes are just like any other plant,” said Peter Bottemley. “If there’s soil nitrogen, they’re going to take it.”

So nitrogen-fixation of the legume isn’t all a free silver bullet for N, explained the Oregon State University soil microbiologist.

Speaking next to a red clover planting at OSU’s Hyslop Farm near Corvallis, Ore., Bottemley said that crop is taking whatever nitrogen is left over from the previous crop to use itself.

Nitrogen fixation “is a very warm-blooded process,” he added. “It evolved much farther south” than Oregon, he noted. “With the kinds of springs we get [in the Willamette Valley], there’s next to no nitrogen being fixed.”

Key Points

Nitrogen fixation is not as promising if your fields are wet and cool.

Jump-starting legumes with some nitrogen may help a bit.

Don’t worry about rhizobia: It sticks around.


Bottemley said he hates to say that, because he was one of the big advocates for nitrogen fixation.

But there is a positive, he adds, in the fact these crops do retain nitrogen residuals from the previous year’s crop, and “putting the N into a pretty good residue that will turn it over pretty quickly.”

But again, there is a flip side even to that, he added. “As soon as the fall rains come, the nitrogen gets converted into ammonia and nitrate, and then it is gone,” Bottemley said.

As a result, “there is a balancing act with nitrogen,” he said. “But to be honest, we researchers do not know a lot details about that process.”

Crops that fix nitrogen won’t shift into doing so until they take up the available N in the soil, Bottemley said.

Getting its fix

Jump-starting legumes with a little fertilizer “works in a low-nitrogen system,” he noted. “But you have to get everything just right to get good nitrogen fixation.”

Here’s his general rule of thumb on fixation: “If you have somewhere in the region of 10 to 30 parts per million of available nitrogen, I suspect that a legume growing in those kinds of soils is going to get only about 50% of its nitrogen from fixation at best,” he said.

“Once that [existing] nitrogen moves up into the 100 ppm, the percentage of fixation will drop down to about 20%.” Put on 150% of N, and fixation is zero for the legume, he said.

While legumes in the Willamette Valley may be retaining nitrogen instead of returning it to the soil, the plants do provide organic matter, “and that ain’t all bad,” Bottemley said.

If you’re worried about the rhizobia in your soil, he has some words of assurance. These will stick around for years, he said of the bacilli that fix the nitrogen in legume nodules it subsequently forms by invading the roots.

What happens is really an “infection” of the plants by the rhizobia in which the legume, as well as the bacteria, plays a symbiotic role.

Multiplying greatly in numbers once the nodule cells are infected by the rhizobia and then taking on a bacteroid role, nitrogen fixation is triggered.

In effect, these plants than create their own nitrogen, helping the grower save money in the expensive commercial fertilizer marketplace. But not all plants that produce nodules are good N-fixers. Beans, for example, have been bred to the point where their fixation process is all but gone. Alfalfa, however, remains the hero plant in the N fixation scenario.

This article published in the September, 2011 edition of WESTERN FARMER-STOCKMAN.

All rights reserved. Copyright Farm Progress Cos. 2011.