Cover crops can be grown for several purposes, but generally they fall into three main categories: a nitrogen trap crop, a source of cover and residue, or a source of nitrogen.

Dave Mengel, Kansas State University soil fertility specialist, says trap crops are used to absorb nitrogen from the soil. If the cover crop is not grazed or harvested, the nitrogen taken up by the crop remains onsite and is preserved within the residue for use by future crops.

"This helps reduce potential nitrogen losses from the soil through leaching or denitrification that may have occurred during the period between cash crops," Mengel says.

For trap crops, the use of fast-growing, nitrogen-demanding crops are ideal. Mengel says good choices would include millet or forage sorghum for planting in the summer after wheat; or cereal rye, wheat, triticale or canola for planting in the fall after summer crops.

"Keep in mind that the trap crops will use the soil nitrogen taken up to support growth," Mengel says. "In most cases, trap crops will have a high carbon-to-nitrogen ratio, so the release of the nitrogen to subsequent crops may be slow. In fact, it may be the second or third subsequent crop that actually benefits from the trapped nitrogen."

The main benefit, Mengel says, is that the trapped nitrogen is not available to move through the soil to contaminate groundwater. Most trap crops are also well-suited as residue cover sources.

"Remember, the rate of residue decomposition can be controlled to some extent by selecting a cover crop," he adds. "If the goal is to produce a residue cover, a crop with high-carbon, low-nitrogen residue, such as forage sorghum, millet or cereal rye would be a good choice.

"Letting it become fairly mature in a nitrogen-deficient environment will increase the carbon-to-nitrogen ratio, and slow decomposition."

If wanting to grow cover crops to provide supplemental nitrogen to future cereal or forages, Mengel says legumes are preferred. But cereals can also be useful, especially due to lower seed costs, if killed at an early growth stage when the residue has a low carbon-to-nitrogen ratio and decomposes more quickly.

Legumes fix atmospheric nitrogen, if they are well-nodulated, but the amount produced will vary widely. Mengel says the carbon-to-nitrogen ratio is important when determining how quickly the fixed nitrogen will be available to subsequent crops. Fine-textured, low carbon-to-nitrogen plants, such as alfalfa, clover, soybeans, or peas, will decompose much more quickly, and release nitrogen much more rapidly, than coarse-textured, high carbon-to-nitrogen plants, such as mature sunn hemp.

"Nitrogen fixation is a very energy intensive process," Mengel says. "That means conditions must be favorable to photosynthesis and high yield to achieve maximum nitrogen fixation. In many cases, short daylength, cool temperatures and dry soils limit nitrogen fixation by many legumes planted after wheat or summer crops.

"Also, keep in mind that high soil nitrates will feed the legume crop and reduce nitrogen fixation. So the actual nitrogen fixation from many legumes can be less than expected."

Research in Kansas has shown that the amount of nitrogen that can be produced, or trapped, can vary widely, Mengel says. For trap crops planted after wheat, 20 to 60 pounds of nitrogen can be trapped by crops such as millet, forage sorghum or sudangrass.

In fields with a history of manure applications, or when planted after a failed wheat crop, values could be higher. But Mengel says that the vast majority of the nitrogen has already been taken up by the time the wheat heads.

Winter cereals planted after corn will often trap 20 to 40 pounds nitrogen per acre. Less will normally be trapped by cereals after sorghum.

Mengel says the amount of nitrogen produced by legumes planted after wheat and terminated by frost can vary widely. Generally, long-season soybeans which stay vegetative and sunn hemp will produce the largest amounts, perhaps as high as 100 pounds nitrogen per acre.

But what portion will become available can be a question, especially with very aggressive crops such as sunn hemp. Cowpeas would be expected to produce significantly less nitrogen.

"The amount of nitrogen produced by winter legumes, such as vetch and winter peas planted after corn or other summer crops, is primarily determined by when they are terminated in the spring," Mengel says. "Generally, the equivalent of 30 to 50 pounds of fertilizer nitrogen can be produced if winter legumes are allowed to grow until mid- to late May. Killed earlier, nitrogen production is considerably less.

"Unfortunately, the cost of seed and planting the legume cover crop can exceed the value of nitrogen produced."