Conservation Expert Guest Editorial
Conserving Water with
Crop Residue and Cover Crops
A
s we talked about in the last issue, residue and
growing vegetation protects the soil surface
from erosion by absorbing the impact energy
of raindrops, thus reducing soil particle detachment.
Reducing the impact, also reduces surface crusting and
sealing, enhancing infiltration and decreasing runoff.
By creating small dams and obstructions along the
flow path, residue slows the water velocity, reduces the
soil transported and reduces the additional soil particles
detached by flowing water. Also, when flow velocity is
reduced, more time is allowed for infiltration and some
soil particles in the runoff water are redeposited.
the straw mulch. This savings of almost 4" of water in
evaporation during the growing season because of the
crop residue is important, but represents only part of the
story. Additional benefits of crop residue extend beyond
the growing season and include runoff reduction,
reduced evaporation, and snow capture. These other
benefits could easily add 2" or more of soil moisture.
More recent KSU irrigation research by Norm Klocke
at Garden City, Kansas, showed that corn and wheat
residues reduced evaporation by about 50%, or about
0.03" to 0.05" per day (see below). This quantity is impressive on a daily basis because, extrapolated over 100
days of a growing season, the water savings total
3" to 5". Reducing soil water evaporation with the
Wheat straw cover can reduce bare
of crop residue management can lead to
soil evaporation by half under a fully adoption
reduced pumping and energy costs for irrigators
with adequate water and increased crop producirrigated corn crop.
tion for irrigators with limited water supplies.
Residue Reduces Evaporation
In addition to reducing evaporation, the residue
Throughout the year, residue protects the soil from the
keeps the soil surface cooler by protecting it from
sun and wind, reducing water losses through evaporathe sun. This cooler, moist surface allows better root
tion. During the growing season, a good crop canopy
development in the heat of summer, improving the
also keeps the sun and wind off the soil surface. In the
standability of the crop. The better roots near the soil
off season, a cover crop can do the same and provide
even more residue. During the winter, standing residue
Soil Water Evaporation
increases soil moisture by trapping winter snowfall
Evap. Rate
Evap. Rate
rather than letting it blow away. Standing cover crops,
(in./day)
Year and
(in./day)
especially those seeded after soybean harvest, can also
Reduced
Crop Cover
Full ET Irrigation
Irrigation
aid in keeping the snow where it falls.
University of Nebraska–Lincoln (UNL) research
2003 Soybean
at North Platte demonstrated that the equivalent of
Bare soil
0.06
6,000 lb./acre of wheat stubble lying flat could reduce
Corn residue
0.03
bare soil evaporation by half under a fully irrigated corn
Wheat stubble
0.03
crop. The projected full-season evaporation (120 days) is
2004 Soybean
shown in the table below.
Bare soil
0.08
0.06
Evaporation in the fully irrigated trial was reduced by
Corn residue
0.03
0.04
6.8" due to the crop canopy and another 3.8" due to
Wheat stubble
0.04
0.03
Evaporation Protection with
and without Wheat Straw
Year
1986
1987
With Crop Canopy
Without Crop Canopy
Bare soil Straw Cover Bare soil Straw Cover
Evaporation (in./season)
7.6
3.8
15.1
8.5
8.5
5.7
14.6
9.4
Bare soil
Corn residue
Wheat stubble
Bare soil
Corn residue
Wheat stubble
2004 Corn
0.06
0.03
0.03
2005 Corn
0.07
0.04
0.04
0.05
0.03
0.02
0.07
0.04
0.05
Information provided by a grant from the Howard G. Buffett Foundation
surface are also more able to use the light rain showers
that don’t soak into the soil profile very far.
Cover Crops Build the Soil
While most producers are concerned about the soil
moisture used by a cover crop, many would benefit
from the use of cover crops. Cover crops can keep the
sun and wind off the soil surface to reduce evaporation,
both as growing vegetation and by providing additional
residue after terminating the cover crop. Many producers report that properly managed cover crops use less
water than is lost through evaporation from unprotected soil surfaces, particularly following low residue crops.
In addition, the growing cover crop provides biological activity to help “feed” soil organisms during the
non-crop season. The biological diversity of properly
selected cover crops can reduce pest problems in the
production crops and improve the soil environment for
beneficial organisms. The cover crop biomass added to
the system from harvesting sunlight, water and carbon
dioxide during the off season can increase soil organic
matter, improve soil tilth and recycle or scavenge
unused nutrients. The additional roots from the cover
crop adds structural stability. Some cover crops even
offer nitrogen fixation and grazing potential as well.
When it comes to water use, if there is more water
available than can be stored in the soil profile at the
time, a cover crop can use some of that water to
reduce losses, making room for future rainfall events.
For example, a sandy soil may only be able to store
about an inch of water per foot, or 4 in. of water in a
4-ft. rooting zone. If the precipitation is much more
than that after the harvest of one crop and before
planting of the next, some water is lost to deep percolation and could have been used by a cover crop. Likewise, if the soil is bare and unprotected, the evaporation loss from the soil surface is water that could have
Paul Jasa, extension engineer with the University
of Nebraska Lincoln, develops and conducts
educational programs related to crop production that
improve profitability, build soil health and reduce
risks to the environment. He has been working with
planting equipment and tillage system evaluation at
the university since 1978. Jasa has become one of
the best sources of information in the Midwest on notill planting equipment and system management. If
there is a mistake to be made with no-till, Jasa admits
he’s either made it himself or has seen it done. More
importantly, he has learned from those mistakes and
shares information on a systems approach and the
long-term benefits of continuous no-till.
been used by a cover crop to protect the soil.
Even on heavier soils that can store more water,
producers may have “extra” water that can be used to
grow a cover crop. If they over-irrigate and leave the soil
moisture profile too full as their crop matures, a cover
crop could use some of that water and scavenge unused
nutrients to reduce off-season losses. This is especially
true for wet falls and on soils that typically have excess
water in the spring. Too often, producers are willing to
till the soil to dry it out when they could instead use a
cover crop to grow the water out and build the soil.
About the Howard G. Buffett Foundation
Established in 1999, the Howard G. Buffett Foundation’s primary mission is to catalyze transformational change to improve
the world and the lives of the most impoverished and marginalized populations. The Foundation’s focus is on international
programs that operate in challenging environments, including conflict and post-conflict countries. The Foundation has to
date invested nearly $250 million to support sustainable agricultural development and improved nutrition initiatives in 50
countries around the world, including in the United States. The Foundation believes global food security efforts must include
all countries – both rich and poor – therefore, the United States must consider its domestic agricultural practices and policies
if we are to successfully address hunger, malnutrition and achieve global food security.
To follow this series visit www.HarvestingThePotential.org