Initial data reports from research projects in central South Dakota are showing that cover crops are an excellent tool for storing nitrogen in the soil for next year's crop, says Jason Miller, Conservation Agronomist with the Natural Resources Conservation Service (NRCS), Pierre.
The use of cover crops has benefits for both the soil and the subsequent crop, explains Miller. A number of cover crop species will utilize nutrients, especially nitrogen, which would otherwise be lost to leaching or denitrification before the next cash crop could capture them.
"Typically, we see the legume component of a cover crop mixture lag a little initially until the brassicas use the nitrogen leftover in the soil after the wheat crop," explains Miller, "Then that is when the legumes do their thing - fix nitrogen. Inoculation is important; the correct strain is important," says Miller.
Producers with fields in a winter wheat rotation may see big benefits by planting a cover crop after harvesting wheat. Brassica cover crops such as radishes, turnips, canola, and rape, help increase microbial activity in the soil which will help cycle small grain residue. "Depending upon what part of the state you're in, there are positives and negatives to an increased decomposition process in regard to moisture conservation," Miller explains. The residue cycling will result in warmer and drier seedbeds for planting the following spring; and it also helps fix nitrogen for the following year's crop.
The U.S. Department of Agriculture Agricultural Research Service (USDA ARS), the South Dakota State University (SDSU) Plant Science Department, and the Dakota Lakes Research Farm are conducting research on South Dakota sites. The information being gathered from the research sites is showing a significant amount of nitrogen was available last year after harvest, explains Miller. For example, one field site was sampled on two dates - September 25, 2008, and October 15, 2008. The cover crop was a mixture of radishes, lentil, and chickling vetch. The yield on September 25 was 1.2 tons per acre (dry weight) which sequestered 88 pounds per acre of nitrogen.
The same research site cover crop plot was sampled again October 15. Samples showed the biomass doubled to 2.4 tons per acre (dry matter); and the corresponding nitrogen yield was 134 pounds per acre. "So, last year a large amount of nitrogen was sequestered in that cover crop. However, the million-dollar question is...," says Miller, "How much of that nitrogen will become available for the 2009 corn crop? It could be anywhere from 25- to 75 percent of the total nitrogen that was sequestered and/or being fixed on that site."
The implications of this research won't be known until later this year. "The full value of cover crops for sequestering and fixing nitrogen could be tremendous," says Miller. "Plant tissue tests are being taken throughout the 2009 growing season; and this fall's yield data will be analyzed."
The research will give producers valuable data for making planning decisions for their cropping systems. "Keep in mind," Miller cautions, "that varying conditions such as soil type, temperature, and moisture; tillage systems; and in a no-till system-the length it has been in no-till, will dictate the amount of nitrogen release during the growing season."
"The bottom line," says Miller, "is that there's a tremendous amount of value in keeping the nitrogen from being lost to leaching or denitrification. Cover crops are a great conservation tool for improving soil quality."
In addition to using cover crops for improving the soil quality, Miller suggests that producers who have cattle with their farming operation may be interested in using cover crops as an additional source of forage. Most, if not all, cover crop species can be utilized for livestock grazing. "However," Miller says, "No single cover crop can accomplish all field objectives." He recommends that producers know their soil types and check with specialists to ensure any particular single crop or mixture will continue to meet their goals for that parcel of land. Miller recommends selecting a cover crop species or a majority of a mix that is an opposite crop type to the next year's planned cash crop.
Interested producers should visit with an agronomist or their local NRCS staff for science-based information on cover crops and for networking contacts to help producers at any point in their soil conservation efforts.