Abstract—Soil quality is critical
for plant growth, and therefore important to agriculture and
rural ecosystems. Management practices that are appropriate for
local soil characteristics and climate can enhance soil quality.
These beneficial practices include crop rotations, crop residue
management (including cover crops and conservation tillage),
and various field/landscape structures and buffers. Crop residue
management is generally a cost-effective method of erosion control.
It usually maintains or increases crop yields, but requires fewer
resources than intensive structural measures and can be implemented
in a timely manner to meet conservation needs.
Introduction
Crop production and its environmental effects depend on the quality
of soil. Soil provides the physical, chemical, and biological
processes required to sustain most terrestrial plant and animal
life. Soil regulates water flow from rainfall, snowmelt, and
irrigation between infiltration, root-zone storage, deep percolation,
and runoff (National Research Council, 1993). Soil acts as a
buffer between production activities and the environment by facilitating
the cycling and decomposition of organic wastes and nutrients
(carbon, nitrogen, phosphorus, and others), as well as the degradation
of nitrates, pesticides, and other toxic substances that are
potential pollutants in water or air (Kemper et al., 1997). Soil
quality determines how well soil performs its functions.
Soil has both inherent and dynamic qualities. Inherent qualities
are those factors, such as texture, that affect a soil's
natural ability to function, but do not change easily. Dynamic
qualities depend on how a soil is managed. Soils respond differently
to management, depending on the inherent properties of the soil
and the surrounding landscape. Traditional measures of soil quality
include land capability and suitability, productivity, erodibility,
and vulnerability to leach pesticides and nitrates (Karlen et al.,
1997). A comprehensive soil quality measure would combine these
physical attributes with broader societal concerns, such as potential
surface-water pollution from field runoff, protecting long-term
soil productivity, and the health of agricultural/rural ecosystems.
Soil quality can be maintained or enhanced through the use of
appropriate crop production technologies and related resource management
systems that involve the composition, structure, and function of
entire ecosystems. Beneficial farm-level soil management practices
are designed to maintain the quality and long-term productivity
of the soil and to mitigate environmental damage from crop production.
These practices include crop rotations, crop residue management
(including cover crops) and conservation tillage, and field/landscape
scale engineering structures and buffers like grass waterways,
terraces, contour-farming, strip-cropping, underground drainage
outlets, and surface diversion/drainage channels. Also beneficial
to soil quality are certain nutrient (see AREI
Chapter 4.4), pest (see AREI Chapter 4.3),
and irrigation practices (see AREI Chapter 4.6).
The appropriateness of soil management technologies depends on
topographic and agro-climatic conditions; site-specific technical,
economic, and financial feasibility; farmer attitudes, perceptions,
and resources; and society's attitudes toward the range of
offsite effects associated with agricultural production (USDA,
1997). Soil management practices can enhance soil quality by:
Increasing ground cover and organic matter,
Tilling sparingly to reduce organic matter degradation and
compaction,
Managing fertilizer and pesticide use to minimize their impact
on nontarget organisms and water/air quality, and
Increasing the diversity of plants, wildlife, and other organisms to
help control pest populations.
Crop Rotation Systems
Crop rotation (see box, "Cropping Pattern Definitions") can help
conserve soil, maintain its fertility, and control pests, diseases,
harmful insects, and weeds. Rotating high-residue and/or closely
grown crops with row crops can reduce soil losses on erosive
soils. Closely grown field grain crops—such as wheat, barley,
and oats, as well as hay and forage crops—provide vegetative
cover to reduce soil erosion and water runoff while adding organic
matter. In addition, these crops help to control broadleaf weeds
and may help control weed infestation in subsequent crops. Crop
rotation also helps to break disease and insect cycles. Leguminous
crops can increase nitrogen levels in the soil, and cover crops
planted in the fall help reduce erosion from winter and spring
storms, hold nutrients that might otherwise be lost, enhance
the soil's biological processes, and lengthen periods of
active plant growth (to increase nutrient cycling, disease suppression,
soil aggregation, and carbon sequestration).
Cropping Pattern
Definitions
The following definitions were applied to 3-year crop sequence
data reported in the Agricultural Resource Management Survey
to identify a cropping pattern for each sample field. The
data were limited to the current year's crop plus the
crops planted the previous 2 years on the sample field, with
the exception of winter wheat in 1996. For this crop, only
2 years were used to determine the rotation due to data
limitations.
Monoculture or continuous same crop: crop
sequence where the same crop is planted for 3 consecutive years.
Small grains (wheat, oats, barley, flax, rye, etc.) or other
close-grown crops may be planted in the fall as a cover crop.
Continuous row crops: crop sequence, excluding
continuous same crop, where only row crops (corn, sorghum,
soybeans, cotton, peanuts, vegetables, etc.) are planted for
3 consecutive years. Small grains or close-grown crops may
be planted in the fall as a cover crop.
Continuous small grain crops: crop sequence,
excluding continuous same crop, where only small grain crops
(wheat, barley, oats, rye, etc.) are planted for 3 consecutive
years
Row crop/small grain rotation: crop sequence
where some combination of row crops and small grains are planted
over the 3-year period.
Rotation with meadow crops: crop sequence
that includes hay, pasture, or other use in 1 or more previous
years. The rotation excludes any of the above rotations and
any area that was idle or fallow in one of the previous years.
Idle or fallow in rotation: crop sequence
that includes idle, diverted, or fallowed land in 1 or more
of the previous years.
Crop rotation
system use for major crops
With the exception of cotton, rotational cropping in some form
dominates major crop production in the United States. The most
common rotation system for both corn and soybeans is a corn-soybean
rotation. This combination reduces erosion (compared with continuous-corn
or continuous-soybeans) helps control disease, insects, and weeds
and enables soybeans to fix nitrogen for use by the subsequent
corn crop. Approximately 75 percent of corn acres and 80 percent
of soybean acres in the 10 major producing States used this rotation
system in the most recent surveyed year (2001 for corn and 2002
for soybeans) (figures 4.2.1 and 4.2.2).
Economic factors affecting farmers' choices
A farmer chooses a cropping pattern based mostly on the
relative rate of return resulting from differences in yields, costs
and returns, and government policy. Crop rotations usually result
in yields higher than those achieved with continuous cropping under
similar conditions. Rotations that add organic matter can improve
soil tilth and water-holding capacity, and thus increase crop yields.
Grain yields following legumes are often 10 to 20 percent higher
than continuous grain, regardless of the amount of fertilizer applied
(Heichel, 1987; Power, 1987). Corn following wheat produces a greater
yield than continuous-corn with the same amount of fertilizer,
even though wheat is not a legume and cannot fix atmospheric nitrogen
(Power, 1987). Rotations with legumes can increase available soil
nitrogen and reduce the need for commercial nitrogen fertilizers.
Legumes in a rotation are most effective in humid and sub-humid
climates where they do not decrease subsoil moisture for subsequent
crops.
Crop rotations—by alternating a susceptible crop with a
nonhost crop—can help to control a variety of pests by disrupting
their life cycles. Soil microbiology and beneficial insects thrive
under crop rotations, and this helps control disease and other
pests, particularly those that attack plant roots. For example,
rotating corn with soybeans can reduce the need for insecticide
treatment when the field is in corn by reducing the number of corn
rootworm larvae in the soil (although the effectiveness of this
practice may be decreasing in some areas).
The diversification inherent in rotations can be an economic
buffer against fluctuating prices of crops or production inputs
and against the vagaries of weather, disease, and pest infestations.
Policies and programs affecting cropping patterns
Federal policies influence farmers' choices of crops and management
practices. Past commodity programs that restricted base acreage
to program crops encouraged monoculture or continuous planting
of the same crop. Starting with the 1990 Food, Agriculture, Conservation
and Trade Act, farmers were given the option to diversify (without
incurring a penalty) their program crop base acres. Farmers began
to grow other crops and/or use rotations in response to changes
in prices and loan deficiency payments.
Under the 1985 Food Security Act and subsequent farm legislation,
highly erodible land (HEL) used for crops required implementation
of a conservation plan in order to be eligible for USDA farm program
benefits (see AREI Chapter 5.3,
"Compliance Provisions for Soil and Wetland Conservation"). Rotating
row crops with less erosive crops such as small grains and hay/pasture
is a key part of some conservation plans for HEL, usually in combination
with cover crops, crop residue management, and conservation tillage.
Crop Residue Management
Crop residue management (CRM) maintains additional crop residue
on the soil surface through fewer and/or less intensive tillage
operations. CRM is generally cost effective in protecting soil
and water resources and can lead to higher returns by reducing
fuel, machinery, and labor costs while maintaining or increasing
crop yields, but requires fewer resources than intensive structural
measures and can be implemented in a timely manner to meet conservation
needs (USDA, 1997). CRM systems include reduced tillage, conservation
tillage (no-till, ridge-till, and mulch-till), and the use of
cover crops and other conservation practices that leave sufficient
residue to protect the soil surface from the erosive effects
of wind and water (see box, "Crop Residue Management and
Tillage System Definitions").
Crop
Residue Management and Tillage System Definitions
Unmanaged
Crop
Residue Management (CRM)
Intensive-
or conventional-till
Reduced-till
Conservation
tillage
Mulch-till
Ridge-till
No-till
Moldboard plow or other intensive
tillage used
No use of
moldboard
plow and
intensity
of tillage reduced
Full-width tillage, but further
decrease in tillage intensity
Only the tops of ridges are
tilled
No tillage
performed since harvest of previous crop
<15% residue
cover remaining
15-30% residue
cover remaining
30% or greater
residue cover remaining on soil surface after planting
Crop Residue Management (CRM)—A
year-round system that usually involves a reduction in the
number of passes over the field with tillage implements and/or
in the intensity of tillage operations, including the elimination
of plowing (inversion of the surface layer of soil). CRM begins
with the selection of crops that produce sufficient quantities
of residue to reduce wind and water erosion and may include
the use of cover crops after low-residue-producing crops. CRM
is an umbrella term encompassing several tillage systems including
conservation tillage (no-till, ridge-till, and mulch-till),
and reduced-till. (Note: reduced-till is not considered a part
of conservation tillage.)
Conservation tillage—Any tillage
and planting system that maintains at least 30 percent of
the soil surface covered by residue after planting to reduce
soil erosion. Two key factors influencing crop residue are:
(1) the type of crop, which establishes the initial residue
amount and its fragility, and (2) the type of tillage operations
prior to and including planting. No-till, ridge-till and
mulch-till are thee common types of conservation tillage
systems.
No-till—Residue from the previous
crop is undisturbed from harvest to planting except for nutrient
injection or narrow strips. Weed control is primarily accomplished
with crop protection products.
Ridge-till—Residue from the previous
crop is undisturbed from harvest to planting except for nutrient
injection. Planting is completed in a seedbed prepared on
4 to 6 inch high ridges that are formed and rebuilt
during row cultivation for weed control. Residue is left
on the surface between ridges.
Mulch-till—A full-width tillage
system usually involving one to three tillage passes over
the field performed prior to and/or during planting, that
leaves, after planting, at least 30 percent of the soil surface
covered with residue.
Reduced-till(15-30% residue)—Full-width
tillage usually involving one or more tillage passes over
the field performed prior to and/or during planting, that
leaves 15-30 percent residue cover after planting.
Conventional-till or intensive-till (less than 15%
residue)—Full-width tillage that is performed
prior to and/or during planting, that generally involves
plowing with a moldboard plow and/or other intensive tillage
equipment. Less than 15 percent residue cover remains on
the soil surface after planting.
Historically, crop residues were removed from farm fields for livestock
bedding, feed, or sale. Residues that remained on the field were
burned off to control pests, plowed under, or tilled into the
soil. Culturally, some farmers would take pride in having their
fields
"clean" of residue and intensively tilled to obtain a
smooth surface in preparation for planting. More recently, farmers
have adopted CRM practices—with government encouragement—because
of new knowledge about residue's benefits and improved planters,
crop protection technologies, and the like (USDA, 1997).
CRM can benefit society through enhanced environmental quality
and farmers through higher overall economic returns. However, adoption
of CRM may not lead to clear environmental benefits in all regions
and may not be profitable on all farms. Public and private interests
support cooperative efforts to address the barriers to realizing
greater benefits from CRM practices. For example, recent advances
in planting equipment permit seeding new crops through heavier
surface residue into untilled soil and even directly into killed
sod (USDA, 1997).
Trends in crop residue management use
According to the Conservation Technology Information Center's
National Crop Residue Management Survey, U.S. farmers practiced
CRM on about 172 million acres in 2004, or 62 percent of planted
acreage, up from 144 million acres in 1990. Conservation tillage
accounted for 41 percent of U.S. planted crop acreage in 2004,
compared with 26 percent in 1990. Most of the growth in conservation
tillage since 1990 has come from expanded adoption of no-till (fig.
4.2.5), which can leave 70 percent or more of the soil surface
covered with crop residue. U.S. crop area planted with no-till
more than tripled from 17 million acres (6 percent) to 62 million
acres (22 percent) between 1990 and 2004 (CTIC, 2005).
Yield response with soil-conserving tillage systems varies with
location, soil characteristics, climate, cropping patterns, and
level of management skills (Sandretto, 2001). In general, long-term
field trials on well-drained to moderately well-drained soils
or on sloping land show slightly higher no-till yields, particularly
with crop rotations, compared with intensive tillage (CTIC, 1996).
Benefits from improved moisture retention in the root zone usually
increase crop yields, especially under dry conditions. In some
areas, these benefits permit a change in the cropping pattern
to reduce the frequency of moisture-conserving fallow periods
(USDA, 1997). Other benefits derive from more timely preparation
for double cropping, with better yields as one result.
Crop yields can be significantly reduced by pest populations,
which frequently change under different tillage systems and are
also affected by cropping pattern. Maintaining or increasing yields
when changing tillage systems requires skillful use of the various
means of pest control, including crop variety selection, proper
application of crop protection products, row cultivation, cover
crops, crop rotation, scouting, and other integrated pest management
practices (see AREI Chapter 4.3). Use of
crop protection products on major crops differs among tillage systems,
but the effects related to tillage systems are difficult to distinguish
from differences in pest populations due to other factors, including
use of other pest control practices (USDA, 1997).
Choice of tillage system affects machinery, chemical, fuel, and
labor costs. Decreasing the intensity of tillage and/or reducing
the number of tillage operations (fewer trips over the field) reduces
labor requirements per acre, extends equipment life, increases
the area covered, and reduces fuel and maintenance costs. These
cost savings may be offset by increased crop protection costs and
the fertilizers required to attain optimal yields (Sandretto, 2001).
Conservation tillage may increase net returns on the entire farming
operation even if returns for a particular crop do not increase.
For example, a tillage system that requires substantially less
labor per acre and reduces returns per acre only slightly may free
up labor to serve more acres or generate more income elsewhere
(Sandretto and Bull, 1996).
Potential environmental benefits of crop residue management
Soil quality can benefit from minimum tillage and maximum residue,
and this combination contributes to improved ecosystem health
in several ways.
Tillage systems that leave substantial amounts of crop residue
evenly distributed over the soil surface reduce soil erosion,
from reduced wind erosion and reduced kinetic impact of rainfall,
surface sediment transport and water runoff; with increased water
infiltration and moisture retention (Edwards, 1995). Several field
studies conducted on small watersheds under natural rainfall on
highly erodible land have shown that erosion rates with the moldboard
plow can be reduced by 70 percent or more with conservation tillage
(USDA, 1997).
Surface residues help intercept nutrients and chemicals and hold
them in place until they are used by the crop or degrade into harmless
components, which provide cleaner surface runoff (USDA,
1997). Increased organic matter in the top layer of soil results
in cleaner runoff, and thus benefits water quality by reducing
the flow of contaminants such as sediment and adsorbed/dissolved
chemicals into lakes and streams (USDA, 1997; CTIC, 1996). Studies
under field conditions indicate that while the quantity of water
runoff from no-till fields was variable depending on the frequency
and intensity of rainfall, clean-tilled soil surfaces produce substantially
more runoff (Edwards, 1995). Average herbicide runoff losses from
treated fields under no-till and mulch-till systems for all products
and all years were about 30 percent of the runoff levels from moldboard-plowed
fields (Fawcett et al., 1994).
Crop residues on the soil surface, by creating tiny dams, enhance
infiltration, reduce surface-crust formation, and slow water runoff,
which increases water infiltration and soil moisture (Edwards,
1995). The channels (macropores) created by earthworms and old
plant roots, when left intact with no-till, improve infiltration
to help reduce or eliminate field runoff and provide water quality
benefits. Combined with reduced water evaporation from the top
few inches of soil and with improved soil characteristics, the
higher level of soil moisture can contribute to higher crop yields
in many cropping and climatic situations (CTIC, 1996).
Less intensive tillage reduces breakdown of crop residue and loss
of soil organic matter improving long-term soil quality.
Carbon sequestration may increase to build soil organic matter,
enhance biological (including earthworm) activity, and maintain
long-term productivity. Conservation tillage, particularly continuous
no-till, improves soil structure by increasing soil particle aggregation
(small soil clumps), aiding water movement through the soil so
plants expend less energy to establish roots. No-till also reduces
soil compaction through fewer trips over the field and reduced
equipment weight and horsepower requirements (CTIC, 1996).
These potential environmental benefits suggest a public
role in encouraging adoption of crop residue management
practices. Conservation compliance provisions of the 1985 Food
Security Act and subsequent farm legislation have given farmers
additional incentives to adopt CRM to control erosion (and thereby
improve water quality), particularly on highly erodible cropland
(HEL) (see AREI Chapter 5.3 "Compliance Provisions for Soil and Wetland Conservation" ).
Expanded use of CRM practices on non-HEL indicates that producers
are motivated by the potential to reduce costs, improve efficiency,
and/or increase soil productivity.
Conservation Buffers and Structures
Soil and water conservation structures and buffer zones can significantly
reduce erosion and sediment transport caused by rainfall and
water runoff. These structures allow for surface water to be
captured onsite or slowed and diverted from the field via erosion-resistant
waterways, channels, or outlets. While management practices,
such as crop rotation, crop residue management (including cover
crops), and conservation tillage practices help to control erosion,
they may not sufficiently control runoff water after heavy rainfall
events. Soil-and water- conserving structures, therefore, are
important in farm soil management systems. Engineering structures
and buffer zones for soil and water conservation vary significantly
across crop production regions to reflect the wide variation
in soil, climate, and cropping patterns.
A variety of USDA programs since the 1930s have provided cost-sharing
and technical assistance for conservation buffers, structures,
and practices (see AREI Chapter 5.4,
"Working-Land Payment Programs"). While recent program efforts have
been directed toward management practices, including vegetative
cover establishment and crop residue management, some cropland
continues to be served by installation of terraces and other structural
measures to better control sediment and water runoff.
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