Nitrogen cycling via cover crops protects groundwater  - 2009-10-27

One of the range of reasons for growing cover crops is to “trap” nitrogen in the soil and preserve it for future use.

A trap crop actually absorbs nitrogen from the soil, said Dave Mengel, soil fertility specialist with Kansas State University Research and Extension. Unless the crop is then grazed or harvested, however, the nitrogen it absorbs will remain on-site, preserved within the residue for use by future crops.

This helps reduce the soil nitrogen losses that can occur between cash crops, he said, due to either leaching or denitrification (natural process by which bacteria convert nitrates into nitrogen gas).

Fast-growing, nitrogen-demanding crops are ideal as trap crops, Mengel said. Good choices include millet or forage sorghum, planted in summer after winter wheat. Others choices are cereal rye, wheat, triticale or canola, planted in fall after summer crops.

“Keep in mind that trap crops use the soil nitrogen they take up to support their own growth. And, in most cases, they end up with a high carbon-to-nitrogen ratio,” he added. “So, the release of that nitrogen to following crops may be slow. In fact, the second or third subsequent crop may be the one that finally benefits from the trapped nitrogen.”

Nonetheless, trapped nitrogen simply isn’t available to move through the soil to contaminate groundwater, Mengel pointed out.