Atmospheric Nitrogen Fixation

Cover Crops

PURPOSES OF COVER CROPS

Atmospheric Nitrogen Fixation

Leguminous cover crops fix atmospheric nitrogen. To ensure that nitrogen fixation takes place, it is important to use seed inoculated with the appropriate Rhizobium strain (Table 1.10-3). The amount of nitrogen fixed is directly related to the amount of biomass produced by the cover crop.

For cover crops established in the fall, the most popular leguminous cover crop currently available is crimson clover, followed by hairy vetch. Recent research has shown consistent winter survival of crimson clover in the southern half of Pennsylvania, even when planted after corn silage harvest. In a study conducted in Maryland, hairy vetch fixed approximately 2 lbs/A of nitrogen per day from early March onward, totaling 150 lbs/A of nitrogen when it was killed in early May. While crimson clover fixes less nitrogen, it does so earlier than hairy vetch.

If the cover crop can be established in the spring or summer, growers have more choices for leguminous cover crops, such as red clover, white clover, field peas, and sweet clover. Research has show that in Pennsylvania the leguminous cover crops do not need to be incorporated to obtain their maximum nitrogen benefit for the following crop. If incorporated, the leguminous cover crop will decompose more rapidly, and release greater amounts of nitrogen early in the season and lower amounts later in the season. In fact, incorporating large quantities of a leguminous cover crop such as hairy vetch in the soil may lead to nitrate losses in water draining from the soil profile in a wet spring. Research with hairy vetch and rye/vetch mix has shown that much of the nitrogen requirement of the following corn crop can be provided by hairy vetch without incorporation. If the vetch is mixed with rye, the nitrogen will be released more slowly. Perennial leguminous cover crops that are used for conservation plantings or living mulches are crownvetch, birdsfoot trefoil, and flatpea.