Carbon Connections - 1 05/20 07:33
Tillage Practices, Fertilizer Use at Heart of Reducing Greenhouse Gases
Most of the research on farming's ability to sequester carbon and reduce
greenhouse gases has been driven by the hope of a carbon payment scheme.
Meanwhile, that research also reveals the more esoteric, but yield-increasing,
benefits to better soil and fertility management.
By Greg D. Horstmeier
DTN Production Editor
COLUMBIA, Mo. (DTN) -- To most growers, organic matter was one number among
many that showed up on their soil test report. While they recognized it
indicated soil quality and tilth, they also considered it in much the same way
as their ability to drain 30-foot jump shots in basketball. Some folks were
naturally blessed, others weren't. If they farmed soils with strong organic
matter levels, good for them. If not, there was little sense pining about it.
Along came climate change and concerns about too much carbon dioxide and
other greenhouse gases in the atmosphere. By weight, soil organic matter is 57
percent carbon. Increase organic matter in fields and you pull carbon from the
air and safely sequester it in the soil. Suddenly, building organic matter
wasn't just for the soil tilth fanatics. Carbon credit markets, and the debate
over "cap-and-trade" legislation, offered the promise of a paycheck for
sequestering carbon in the soil.
Once again, the world quickly divided into "haves" and "have-nots." The
basic distinction was no-tillers sequester carbon, tillers release it. Growers
in regions where no-till hasn't worked saw the idea of carbon credits as just
an extra support payment for those lucky no-tillers.
Most participants in carbon credit contracts traded by the Chicago Climate
Exchange are using no-till practices. At the time, no-tilling was the most-sure
bet that a farmer was reliably sequestering carbon.
A new report, on greenhouse gas and carbon sequestration, created under the
auspices of the Soil Science Society of America, may help bring more farmers
into the sequestration fold. Due in June, the report looks at the science of
assessing farming's ability to lower atmospheric greenhouse gases and offers
more definitive ways to measure that ability.
BASED ON AVERAGES
"For growers now involved in carbon trading contracts, those value indexes
are essentially based on averages. If you farm in a certain area, and use
certain practices, this is the sequestration rate you get," said Charles Rice,
Kansas State University soil microbiologist and a member of the panel that
reviewed the upcoming SSSA report. Regional averages are low-cost and reliable,
but conservative, Rice said. They can substantially underestimate the carbon
sequestered, eliminating some practices from consideration.
Rice, who also is director of the Consortium for Agricultural Soils
Mitigation of Greenhouse Gases, said the other extreme is to take actual carbon
measurements in every field. "Here again, it's standard soil science to take
measurements of bulk density, which can be converted to accurate carbon
estimates." The problem with such accuracy, Rice said, is it's doubtful a
carbon credit market, or a government payment, would be enough to warrant the
cost of those field-by-field measurements.
Rice said the report points to a hybrid of those extremes. It discusses
establishing a series of "benchmark sites" throughout a region, GPS-located,
that would be resampled every three to five years to re-evaluate carbon
sequestration. Growers would use values from those sites, then add their
production inputs, tillage, yields, climate and other variables, which would
feed greenhouse gas models to give a more accurate picture of their farms'
influence on climate. On-farm numbers could be generated from a farmer's yield
maps, or from remote sensing to gather vegetative data, Rice said.
More accurate greenhouse gas estimations would tell a truer story of how
much effect various practices have on carbon and other gases. It could broaden
the kinds of farm practices that would benefit from greenhouse mitigation
payments, should they become reality.
"You may be tilling, but if you're using cover crops, or have an intensive
crop rotation that sequesters carbon, you would get credit for that."
In the meantime, soil scientists like Jerry Hatfield use the buzz around
sequestering carbon to remind growers about their organic matter levels: higher
"The benefits to higher organic matter, which is what we're talking about in
sequestering carbon, go way beyond some potential payment," said Hatfield,
supervisory plant physiologist at the National Laboratory for Agriculture and
the Environment in Ames, Iowa. Improving soil structure and tilth by increasing
organic matter improves water-holding capacity and reduces potential for soil
erosion, he said. Higher organic matter soils are better at releasing nutrients
throughout the growing season.
Hatfield, like Rice, stressed that creating better soils is not just for
no-tillers. He notes the up-tick in tillage the past few seasons, due to higher
crop residues from better yields, the need to fix field ruts from wet harvests,
and the need to control some Roundup-resistant weeds. "You can do those things
and still minimize the carbon lost," said Hatfield, who also is familiar with
the upcoming SSSA report.
Vertical tillage, being careful not to disturb soil below the two-inch
depth, lessens the impact. Tilling in cooler temperatures oxides less soil
carbon than tilling on a summer day. "It's really more about, if you need
tillage, that you have a game plan on when and where to do it, realizing the
impact you're having on the soil," Hatfield said. Tilling heavy weed
populations into the soil adds organic matter, just as plowing in a clover crop
once did, he said.
FERTILIZER PART OF EQUATION
Fertilizer plans also enter into the greenhouse gas equation, he said.
Nitrogen fertilizer in the soil, under the right conditions, can produce
nitrous oxide (N20), which is 296 times more potent than carbon dioxide as a
greenhouse gas. According to a 2007 study compiled by the International Plant
Nutrition Institute, agriculture contributes 78 percent of the N20 emissions in
Those emissions can be reduced, Hatfield said, by applying the right amount
of nitrogen for the crop, and using nitrogen tests and sidedressing when the
crop needs more N. Using nitrogen inhibitors also can help, and the benefits of
new nitrogen management schemes are not yet included in typical greenhouse gas
Recent Colorado research studied various tillage and fertility programs for
N20 emissions. In no-till continuous corn plots, researchers found untreated
urea produced the most N20 emissions. Using urea coated with ESN reduced
emissions an average of 33 percent over two seasons. UAN nitrogen treated with
AgroTainPlus likewise reduced N20 levels by 35 percent, versus untreated UAN.
Greg D. Horstmeier can be reached at firstname.lastname@example.org
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