Crop Rotation and Soil Residue
Management Just as Important as Conservation Tillage, Expert Argues
As a student, Charles Mitchell was taught it was impossible to build
organic matter on cropland in the wet, humid South.
Now, after working
two decades as an Alabama Cooperative Extension System agronomist, he
doesn’t buy this argument. He’s seen firsthand the dramatic successes
producers have had building organic matter in their fields through
practices such as conservation tillage.
(Above: The Old Rotation at Auburn University. Recent findings
from the century-old research and similar studies reveal that crop
rotation and high residue management are just as crucial as
conservation tillage in building organic matter, according to Charles
Mitchell, Alabama Cooperative Extension System agronomist and Auburn
University professor of agronomy and soils.)
Even so, he is
convinced conservation tillage alone is no guarantee of adequate
organic matter. Crop rotation and high residue management are just as
important. Indeed, without them, you may end up farming dirt rather
always appreciated the merits of crop rotations --- an appreciation
gained from years of experience working with Auburn University’s Old
Rotation, the oldest continuous cotton rotation study in the United
“We’ve known about
crop rotation for more than 100 years now,” Mitchell said. “There are
benefits, and we’ve seen them. But they have not been dramatic. And
when you throw in the economics, it hasn’t been profitable up to now.”
However, in 1997,
when the Old Rotation was converted to conservation tillage, Mitchell
began noticing something different --- different enough that it
eventually turned his lukewarm enthusiasm for rotation into a red-hot
passion, one he now shares freely with producers. Rotation coupled
with conservation tillage, he discovered, resulted in marked yield
increases as more organic matter was added to the soil.
But that was only
one lesson. Earlier in the decade, another important lesson was
driven home to Mitchell while he was working with Extension agents in
central Alabama surveying cotton fields for the presence of nematodes,
soil fertility and hard pans. The survey revealed 63 percent of those
fields had significant hard pans.
In 2001, a similar
survey turned up the same problems, even though 55 percent of these
fields had been switched to conservation tillage.
“Most of these
fields had some type of in-row subsoil – a vast improvement from 10
years earlier when 100 percent of the fields were in conventional
tillage with no subsoiling. So we were pleased with that,” Mitchell
Even so, the hard
pan remained, despite the widespread use of in-row subsoiling.
“What’s going on?”
Mitchell recalled asking himself.
The problem, he
soon discovered, was that only 15 percent of these fields were planted
in winter cover crops. And in cases where fields were planted in
cover crops, usually rye, the crops were being killed before they
could provide any benefit from organic matter.
“Of the roughly 80
fields surveyed, we found the average amount of soil organic matter in
the upper two inches was only six-tenths of a percent,” Mitchell
said. “Our best yielding crops typically have between 2 and 2.5
percent organic matter.”
Simply put, it
seemed the vast majority of producers “were farming dirt, not soil,”
“The definition of
soil is something that has organic matter in it. But the survey showed
we didn’t have organic matter in these fields, and, equally bad, the
hard pans were returning.”
is not falling on deaf ears. In fact, several producers in central
Alabama already are practicing what Mitchell is preaching, including
Macon County farmer Shep Morris, who is rotating corn and cotton on
his farm near Shorter.
“Corn really makes
a difference,” Morris observed. “Just starting out with rotation, you
see slight differences, but as organic matter rises after three or
four years, you see a lot of benefits.
“I’ve seen less
crust on my soil and less nematode pressure. We’ve also seen less
seedling disease, and we’re not putting quite as many fungicides at
planting as we once were.”
matter, which started out at around a half percent, has now increased
to around 2 percent in some cases. Morris is also seeing reductions
in soil erosion --- a recurrent problem associated with the prairie
soil in which his crops are grown.
The buildup of
organic matter also is reflected in his yields. One Extension agent,
Lee County Extension Agent Jeff Clary, is predicating Morris could end
up with corn yields as high as 180 bushes an acre, and cotton yields
as much as three bales an acre.
“It all gets back
to soil organic matter,” Mitchell stressed. “’That’s the one key to
soil quality above all others.
tillage is a key, but so are high residue management and crop
rotation. All three will make a difference.”
Cooperative Extension System Agronomist, 334-844-5489.)
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