In the first half of the 20th century, southwestern Minnesota's farms looked different than they do today. They were diversified landscapes that mixed corn, small grains, livestock, and pasture on their land. Today, over 90% of the region is devoted to growing corn and soybeans. According to Deborah Allan at the University of Minnesota, many farmers feel "locked in" to a two-year corn and soybean rotation, forced by the markets to alternate between the two crops year after year after year. Allan is among the growing number of people who believes alternatives to this practice are necessary. She is part of a long-term research project studying the effects of using a system that rotates crops over four years instead of the conventional system. In her system, the first year's crop is corn, followed by soybeans, then oats and alfalfa, and then an all-alfalfa stand.

From years of data to important conclusions

Trials of both conventional two-year and the above four-year rotation have been underway at the Southwest Research and Outreach Center in Lamberton, MN, for thirteen years. "It's unique to have a site that we've studied for such a long time," says Allan. "We know it's gone through the transition period." Long-term data, explains Allan, helps ensure more accurate results.

In Lamberton, each cropping system is being grown on two plots, one with high and one with low-fertility soil. On each of the two plots, each crop is subdivided into four sections of different management systems, with each section receiving either high inputs (fertilizers and herbicides), low inputs, no input, or organic inputs only (manure). Since 1989, each management system in each soil type has been studied under both rotation practices for their yield potential and economic viability.

After so much time, what's the result? "Economic analysis of the data has shown higher long-term net profitability in the four year rotation… across all management systems," says Allan. This, she says, is partially due to the buffering effect a diverse set of crops provides against commodity price fluctuations, but it is also a result of increased yields in the four year system.

Allan found that in the conventional high-input management system, yields showed no difference between two-year and four-year rotation practices. In the low-input, no input, and organic input systems, yields were higher in the four year rotation.

However, says Allan, using the high-input system to acquire high yields cannot be continued for long. "Farm failures, resistant crop pests, and environmental degradation resulting from current cropping practices suggest that the present system is neither economically nor environmentally sustainable," says Allan. If, as the research indicates, the same or higher yields and profits can be achieved with alternative practices, then it is clear that they need to be pursued as vigorously as possible.

The link between crop yields and the soil

A soil scientist, Allan joined the cropping study in 1999 to assess soil quality in the varying rotation and management systems. Prior to her work, there was no comprehensive information regarding the effects of the four year rotation on the soil itself, which Allan thinks is an integral part of understanding the benefits of the alternative system. "Characterization of soil quality in the four management systems will give producers another yardstick by which to evaluate the system's long-term sustainability," she explains.

So what makes a good quality soil? Allan has been analyzing a wide array of physical, chemical, and biological components of the soil in her research; including organic matter content, nutrient availability, earthworm presence, microbial biomass, and soil stability. However, she considers one of particular characteristic of the soil's physical structure pehaps the most important indicator of its health: tiny soil aggregates.

"Aggregates integrate all the elements of soil health," she says. They are small, microscopic clumps of soil particles, bound together by chemical and bacterial interactions. A soil with many aggregates would be the opposite of a compacted soil, she explains, and would be characterized by better water infiltration and holding capacity, reduced runoff, and more biological activity from microbes. In her research, Allan found significantly more soil aggregates in the four-year rotation than the conventional corn-soybean system.

Why would this be? In the microscopic world of soil biology, says Allan, the four-year rotation increases the amount of organic matter in the soil and helps keep it there. The oats' fibrous root system provides nutritious organic matter and binds aggregates together, and alfalfa's two-year presence in the rotation system limits the amount of annual tillage that needs to occur.

Though tillage does not directly break up these crucial aggregates - they're only several millimeters in diameter -- it does allow huge amounts of oxygen to enter the soil. For microorganisms, this oxygen is the crucial ingredient that helps them decompose the carbon in the soil. This process decreases the organic matter content in the soil, dissolves aggregates once bound by carbon, and eventually lowers the overall quality of the soil.

"With physical aggregation, the organic matter can't be easily decomposed by microorganisms; it's sequestered carbon," explains Allan. "Then those nutrients can be released over several years - in a way, it's kind of like a slow-release fertilizer. But if you till, you release those nutrients all at once. Once broken up, there's more oxygen, and the organisms go to work. You want decomposition to happen, but you want it to happen at a rate synchronous with what the plants need."

As a result, a soil with a high number of aggregates, says Allan, is simultaneously rich in organic matter and supportive of a steady amount of nutrient availability. Such a system provides plants with the necessary resources to produce continuous high yields without the need for high agrochemical inputs on the side.

It has been pointed out that low-input and organic systems sometimes actually require more tillage because of weed control. But Allan's data suggest that within the four-year crop rotation, the oat roots' additional contribution of organic matter and alfalfa's two-year rotation offsets this effect, effectively retaining aggregates in the soil, maintaining quality and crop yield.

Allan's study of four-year rotations is using the soil quality of a nearby prairie as a benchmark - "something to aim toward," as she says. She believes in increasing the amount of plant, animal and landscape diversity in farmland ecosystems, and her work is centered around the specific soil conditions and soil types of the individal regions where she works. Doing so means finding solutions relevant to each region's climate, soil type, and native vegetation and wildlife. "Agroecoregions should be how we're thinking about cropping system design," she says.

Remembering the economics of sustainability

Of course, Allan also realizes that ultimately, it is the bottom line that will make or break an alternative agricultural practice. "The idea is that hopefully farmers will see that it doesn't hurt them economically and get involved," she says, referring to thirteen years' worth of data. "Alfalfa stays on the soil through the winter and then grows for a whole second season. You take the hay off it, but leave the crowns for regrowth." In other words, a single planting yields two harvests for the farmer.

And Allan is quick to point out that her alternative system is really nothing new. "This is what everyone did here for years and years. Without having the animals on the farm these days, [growing oats and alfalfa] is hard to do for the market. My hope is that we can figure out how the system works so that we can imagine crops that will fit into the markets farmers have today."

Deborah Allan: allan001@umn.edu