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Long-term conservation-tillage study shows benefits, concerns

The technical capability and popularity of conservation-tillage in North Carolina has greatly increased since the early trials of the practice nearly a half-century ago. In a recent presentation George Naderman, former North Carolina State University Extension soil specialist (now retired) said that for various reasons “we have come to count on it.”

Many farmers appreciate the efficiency conservation-tillage provides them, and with diesel costs at two and three times higher than three years ago, the fuel savings alone are now especially important. This may nudge producer interests even more towards no-till, strip-tillage or minimum-tillage practices.

Besides this usefulness to farmers, adoption of conservation-tillage helps meet the goals of various government programs (local, state and federal) in the protection of our natural resources, including soil, water and wildlife habitat. Current debate over farm program supports likely will likely include the environmental benefits of long-term conservation-tillage.

Even an issue seemingly unrelated to farming, like global warming, can be helped by long-term conservation-tillage. These days the non-farming sector of the country more easily understands the merit of supporting farmers in conserving these public resources. These considerations are often also critical to legislators.

Naderman contends that farmers closely watch the general year-to-year benefits of using no-tillage culture, but they may not fully understand the variability of yields within fields, nor the long-term effects — both good and bad — associated with continuous use of conservation-tillage systems.

Before retirement, Naderman and colleagues conducted a six-year comparison of conservation-tillage and conventional-tillage at the Center for Environmental Farming Systems, located on the Cherry Farm near Goldsboro, N.C. That study included rotations of corn/soybeans, corn/wheat/double-crop soybeans and corn/peanuts/cotton.

In 2003, he returned to do some detailed sampling of the soils within that experimental area. The results revealed something much more clearly than he had expected. The research team found strong and consistent correlations between total soil carbon content and the soil’s “bulk density.”

Of course, soil carbon content is also readily expressed as soil organic matter, by simply multiplying the carbon percentage by 1.724. Soil bulk density is an agronomic term closely related to soil compaction.

Naderman’s research team found that organic matter and soil compaction was an “inverse” correlation, meaning that when soil carbon content was lower, soil density was higher and vice versa. These results indicated that when soil carbon is adequately high, this protects the soil from an unfavorable density, hence from being readily compacted.

Even when topsoil was sufficiently sandy in texture, the carbon content below 2 inches depth was still too low to provide compaction protection, despite careful use of conservation-tillage, including the use of wheat or oats as winter cover crops.

In those soils where the surface texture included less sand but more silt, the soil carbon content was higher, and the density more suitable. This would be helpful to production, especially for root system development during early crop establishment.

From his results with practical research and statistical tools, Naderman said, “this inverse relationship was undeniable.” So, with cost reimbursement from North Carolina grower associations for corn and soybeans, and the research program of Cotton, Inc., the team then took those research findings to the field — literally.

Seventy-four areas within farm fields were sampled in the same manner, all of them selected because of a history of continuous no-till culture. These came from every corner of North Carolina. There were nearly 1,200 samples, spread over 31 counties in the state, varying greatly in texture and other soil properties, and from differing cropping histories.

Results of these tests showed the same basic relationship between soil carbon content and soil density (compaction), and were clearly related to surface soil textures. These results convinced the team, including North Carolina-based U.S. Natural Resources Conservation Service agronomist Bobby Brock, to take the information to North Carolina farmers and others interested.

Farmers in North Carolina and surrounding states will soon have opportunities to see these research results and the recommendations that come from them. There will be a series of meetings across North Carolina this winter. These meetings will focus on keeping conservation-tillage profitable by understanding this “carbon connection” and the importance of surface soil textures.

Plans are to hold regional meetings in the Cooperative Extension facilities in the following North Carolina cities: Halifax, Greenville, Kenansville and Lumberton in the Coastal Plain. Two meetings will be held in western North Carolina. Look for announcements of these regional meetings to be held in late January and early February, 2006.

“Looking at soil compaction from a plant root’s perspective, all it “wants” is a clear, easy path to grow readily and take advantage of available nutrients and water. But, when soil compaction increases from a density of 1.4 up to 1.65 grams per cubic centimeter, seemingly a very small difference, the total soil pore space drops by 20 percent,” Naderman points out. This will strongly influence how fast, and where, the growing roots extend in the soil beneath the crop.

He explains that this is a double whammy. When soils are dense, air and water movement is slow during wet periods. However, if the weather turns abruptly drier, the compact soils get much harder too quickly, making further root growth nearly impossible when the plant needs more moisture.

The six-year study also demonstrated that corn-cotton-peanut rotations, so common on North Carolina farms, are less “carbon friendly” than where wheat or wheat/soybeans are in the rotation. Peanut production, in particular, and cotton to a lesser degree, return less “durable biomass” carbon to the soil, Naderman says. “For the more sandy soils it looks like we will need special considerations in keeping long-term conservation-tillage culture highly productive, and this may depend on the choice of crops,” he said.

Although the previous production finding is helpful to farmers, the Goldsboro study showed more information that may be of potential interest to those concerned with future farm policy. In several of the comparisons Naderman made, the conservation-tillage system placed at least 2,000 pounds more carbon per acre to a 5 inch depth, compared to the same rotation with the traditional clean-tillage (chisel plow/disk) system.

“While we don’t know how much of this carbon happened in the first year or sixth year, or how much it would continue to add over further crop years, we do know there was over this much more carbon (and it can also be expressed as organic matter) per acre-5 inches from the conservation-tillage” he stresses. This is called “carbon sequestration.”

Putting more carbon back into the soil helps reduce the extra carbon dioxide placed in the air from all of our combustion of fossil fuels; and this helps to reduce the “greenhouse effect” and the concerns about global warming that everyone is hearing more and more about these days.

The same conservation-tillage system that “sequestered” more carbon also captured (in the topsoil) more than 200 pounds more nitrogen per acre. This is directly related to the use of cover crops (and lack of tillage) from the conservation-tillage system used. And, Naderman points out, this nitrogen is in a biological, or non-leaching, form.

Clearly, any reduction in loss of unused soil nitrogen by deep leaching into the ground water has a positive impact for society. And, if over time, farmers can use less fertilizer nitrogen because of conservation-tillage, it is an important benefit to them, as well. In the Goldsboro study the recommended rates of fertilizers were always used on the crops grown; and no fertilizer was used on the cover crops.

Naderman stresses that earlier guidelines for use of subsoiling and deep tillage still are applicable. “What we are talking about from these new results is the possibility of even more shallow soil compaction,” he says. “It may be equally hard soil, but maybe even more serious than the deeper pan layers, especially in the early growing season, the North Carolina soil scientist says.” This might be called a “low carbon/high density concern.” If compaction is found, the use of some type of non-inversion soil loosening tools may be needed. Strip-tillage to appropriate depth is certainly one of those alternatives.

Statewide only about 20 percent of the course-textured soils in the long-term no till fields sampled had carbon levels high enough to sustain favorable soil density for good root activity. However, in the medium and finer textured soils over 95 percent had these conditions at the depth of 2-5 inches.

From the statewide sampling of fields under long-term, no-till production, Naderman also checked soil fertility status at the two sample depths. This showed that even where animal waste had long been applied, phosphorus was being held in the soil, as was potassium, especially in the top two inches. This is important for efficient and environmentally careful use of these nutrients.

“In general, he says, farmers should be aware of the relationship of soil compaction tendency to sandy soil textures, along with the importance of cover crops, and residues in building soil organic matter.”


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