Research Confirms Link Between Extended Crop Rotations-Soil Quality

Farmers who want to maintain soil quality may want to get back to planting extended rotations of grain and forage crops. Data collected as part of a project funded by the Leopold Center show that crop rotations covering at least five years and which include at least three years of forage crops interspaced with corn and soybean, resulted in higher soil quality ratings than either continuous corn or a two-year corn-soybean sequence. The longer-term rotations had an additional benefit: they were more profitable than continuous corn production. The study team was headed by soil scientist Douglas Karlen of the ARS’ National Soil Tilth Laboratory in Ames.

The study confirms what many in the sustainable agriculture community have believed but have been unable to document: diversity is needed in crop production. “Extended crop rotations have value for maintaining our soil resources,” Karlen said. “We still are in the early phases of measuring soil quality, but we know that physical, chemical and biological characteristics must be considered.”

Karlen also said the study points to a need to create markets and uses for forage crops so that producers will have financial incentives to diversify their crop rotations. “Larger farm size, specialization and separation of agricultural crop and animal enterprises – along with pressure to maximize short-term profit throughout the nation’s corn and soybean belt – have decreased implementation of long-term crop rotations over the past 50 years,” Karlen said. “The result has been crop rotations that leave land bare for nearly six months each year, spurring organic-matter decomposition and erosion if the soils are tilled.”

The researchers collected almost 1,000 soil samples from three long-term crop rotation studies and one long-term organic study in Iowa and Wisconsin. They analyzed the samples for several physical, chemical and biological soil quality indicators, which were then used to develop an overall soil quality index (SQI). Soil samples from extended rotations that included at least three years of forage crops such as alfalfa and oats scored the highest SQI values. The lowest SQI values were associated with continuous corn.

Soil samples from continuous corn had low scores for compaction, percent water stable aggregates (an indicator of potential crusting, runoff or slow infiltration), acidity, soil organic matter and biological activity (measured by microbial biomass carbon). Total organic matter was the most sensitive indicator, showing significant differences at all locations.

Profit was calculated by subtracting costs of production from potential income based on actual crop yields and the 20-year average non-government supported commodity prices from the National Agricultural Statistics Service (NASS) database.

Karlen said researchers are using similar processes to study crop rotations in other parts of the country. The ARS study in Iowa and Wisconsin was significant because it confirms that extended rotations are important in the northern Corn Belt. Karlen said that when the information was first analyzed a decade ago, only some of the soil quality indicators showed significant differences between the rotations. Karlen continued this work after the Leopold Center grant ended, further refining the framework to measure soil quality. “The investment the Leopold Center made almost a decade ago is beginning to pay off,” he said. “It was a real jumpstart in our work on trying to figure out how to measure soil quality because it gave us a database to work with. And like most scientific endeavors, sometimes it takes time to develop.”

ARS is the U.S. Department of Agriculture’s (USDA’s) chief scientific research agency. Collaborators in the study included colleagues Cynthia Cambardella and David Meek from the National Soil Tilth Laboratory; scientists with the Soil Quality Team of USDA’s Natural Resources Conservation Service; and Iowa State University faculty Michael Duffy (economics) and Antonio Mallarino (agronomy). Results of the Agricultural Research Service (ARS) study are published in the May/June 2006 issue of Agronomy Journal.