Reinventing Agriculture for Environmental Enhancement
AgMRC Renewable Energy Newsletter
Robert C. Brown
Bergles Professor of Thermal Science
Department of Mechanical Engineering
Iowa State University
Director of the Center for Sustainable Environmental Technologies and
the Office of Biorenewables Programs.
The prairies of North America are among the world’s greatest natural resources, their soils providing unparalleled fertility for the production of food, feed, and fiber. Before agriculture, they were also a gigantic sink for carbon dioxide, sucked from the atmosphere by forbs and grasses and stored underground as carbon in decaying plant material—nature’s process for sequestering greenhouse gases from the atmosphere.
The sodbusters changed all that. Their steel plows ripped open the tangle of roots beneath the prairie, making its fertility available to agriculture. But the exposed soil was subject to erosion, washing and blowing its fertility elsewhere—often to unwanted locations—at the rate of 5 tons per acre per year or higher. Moreover, agriculture had the further effect of exposing soil carbon to oxidation, releasing carbon dioxide to the atmosphere where it contributes to global climate change.
Although natural soil building can replace some of these losses, Iowa is thought to have lost half of its soil carbon since cultivation began 150 years ago. That number is open for debate, but it is generally recognized that, as currently practiced, agriculture is slowly depleting soil fertility.
Good land stewardship, as currently defined, minimizes the negative impacts of agriculture. However, this alone is not enough to pass the test of sustainability, which requires that natural resources be employed in a manner that assures their availability for future generations. Modern agriculture’s reliance on fossil fuels to power tractors and produce nitrogen fertilizer, as well as its role in the depletion of topsoil and soil carbon, add up to a less than stellar record of sustainability.
We can reinvent American agriculture in a way that doesn’t just minimize its impacts on soil, air, and water, but actually enhances the environment, a “new” kind of agriculture drawing inspiration from the pre-Columbian peoples of South America.
Recent archaeological studies in the Amazon basin have revealed what local inhabitants have known for years: the highly oxidized, infertile soils common in this region are interspersed with well-defined areas of highly productive soils. Known as “Terra Preta” (“dark earth”) soils, they are substantially darker than the surrounding soils and contain large amounts of charcoal and pottery shards. These anthropogenic soils were created by the gradual addition of manure and charcoal (formed by incomplete burning of plant material) by indigenous people.
Thought to arise from the increased biological activity of bacteria and fungi colonizing the porous char, these soils have remarkably enhanced fertility compared to untreated soils in the same locations. Furthermore, this carbon appears to have been stably sequestered as soil organic matter for hundreds if not thousands of years.
Could we duplicate these results, yielding a system that not only produces food crops but also rebuilds soils, meets the energy demands of modern agriculture, and sequesters greenhouse gases from the atmosphere?
Iowa State is launching a project to investigate the feasibility of just such a system for corn production. In this system, about half the stover (the residue of stalks, leaves, husks, and cobs) is harvested along with the grain. In a process known as fast pyrolysis, the stover is partially burned to form charcoal and an energy-rich liquid known as bio-oil, which is transported to a fertilizer plant to be reacted with steam to form hydrogen in lieu of the natural gas usually employed in manufacturing anhydrous ammonia fertilizer. The ammonia and charcoal are then injected into the soil to serve as a nitrogen fertilizer, a biologically active soil amendment, and a carbon sequestration agent.
In effect, the farmer provides all of the energy to manufacture the fertilizer required for his own farm, with substantial benefits to both farmer and the environment. A 640-acre farm in corn production would save one million cubic feet of natural gas annually by using stover to manufacture ammonia and would avoid releasing 65 tons of CO2 into the atmosphere. For supplying stover to the fertilizer manufacturer, the farmer would also receive a fuel credit equal to about 50% of the cost of anhydrous ammonia.
While switching from conventional tillage to no-till would sequester only about 310 tons of carbon dioxide per year, the charcoal produced by this farm would effectively sequester 1,800 tons of carbon dioxide—the annual tailpipe emissions from 340 automobiles. Although their value in the U.S. is only speculative at this time, the value of carbon credits in international markets averages about $3.50/ton, or about $6,900 for a 640-acre farm.
Not least among these benefits is the anticipated improvement in soil quality as a result of the application of char. And though these results have not been fully demonstrated, the experience of the pre-Columbian peoples of the Amazon basin—not to mention the futures of our children and the land—encourages this attempt to reinvent agriculture.
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*Reprinted with permission from Innovate, Iowa State University College of Engineering, Spring 2006