When it comes to cover cropping, producers’ objectives should determine which cover crop or cover crop mix they plant. Some cover crop benefits, like preventing soil erosion, are undeniable.
One of the benefits often promoted by cover crop enthusiasts is nitrogen fixation by legume cover crops. But just how much nitrogen can you expect those peas, clover or hairy vetch to fix?
Daren Redfearn, Nebraska Extension forage crop residue specialist, notes not all legumes are created equal.
Many growers use soybeans as a guideline. Soybeans typically contribute less than 50 pounds of nitrogen per acre back to the system.
"The difference between soybeans and legume cover crops is with soybeans a lot of the nitrogen is mobilized up to the grain and removed from the system," Redfearn says. "We're looking at some residue, but that consists of the hulls and stems, so it's pretty low quality. The N you get is contributed by the root system."
Because legume cover crops aren't harvested for grain, they don't lose nitrogen this way. The amount of nitrogen fixed is based largely on the amount of biomass produced — about 500 to 1,000 pounds of biomass per acre. In legumes, most fixed nitrogen can be found in the leaves and stems, which is why biomass is important.
"The way legumes are managed in cover crop systems, oftentimes they're in a mix, and they don't compete very well with annual grasses they're mixed with. The additional N they contribute is pretty low."
Take hairy vetch. "It takes a little time to get established, and it's fall-planted, so it's going to require some spring growth to fix a lot of nitrogen," Redfearn says. "That biomass and nitrogen concentration are pretty critical to what drives the additional nitrogen in these cropping systems through legumes."
But what about perennial legumes, like alfalfa? Redfearn notes alfalfa will typically produce 100 to 150 pounds of nitrogen per acre. So why don't annual cover crops contribute as much nitrogen to the system? Perennial legumes like alfalfa have a root mass that's nearly equal to that of their aboveground biomass, while short-season annual legumes have a much smaller root system. This is another reason biomass for leaf and stem production of cover crops is important.
"We're trying to mimic some of those perennial rotations using annual cover crops," Redfearn says. "The difference is that the annuals don't have the length of time to contribute fixed nitrogen like the perennial legumes that are growing for three to five years."
Then there are summer annual legumes, like sunn hemp. Sunn hemp is different from most legumes in that it can put on a lot of biomass — by some estimates, as much as 5,000 pounds of biomass per acre in the summer. However, like many summer legumes, sunn hemp doesn't have as high of a nitrogen concentration in the plant.
"The higher biomass production compensates a little for the lower N concentration, so total nitrogen contributed is pretty similar," Redfearn explains. "Also, when soil mineralization is occurring, there's soil nitrogen available. It's an energy-demanding process for the plant to allow the rhizobia to fix their nitrogen. So when there's excess nitrogen available, the fixation process shuts down. When nitrogen is depleted, those nodules kick back in gear again."
Give it time
Meanwhile, before legumes start fixing any nitrogen, they must form nodules — a six- to eight-week process that's temperature-driven. And plants require some N early on in establishment, although they won't fix it themselves until they use up most of what's available in the soil, Redfearn adds.
"They've got to be well-established before they start fixing a substantial amount of N," he says. "And the amount of N legume cover crops fix is pretty minimal. That may only be 20, 30, 40 or 50 pounds, depending on how long they've got to accumulate biomass."
Some legumes, like winter peas, may produce as much as 50 to 100 pounds of nitrogen, depending on the amount of biomass they accumulate. That may be the best bet for growers who want to include legume cover crops in the mix.
However, Redfearn notes, nitrogen fixation is largely a side benefit of the main objective of cover cropping: preventing soil erosion.
"Cover crops are used primarily to cover the soil. The additional nitrogen contribution is an additional benefit from planting legume cover crops. Adding an additional benefit complicates the cover crop system," Redfearn says. "To get the most nitrogen from cover crops, planting only or mostly legumes is recommended. This will provide both soil cover and additional nitrogen. If you are just trying to get some soil coverage, you're not going to go to the legumes."