CONSERVATION TILLAGE IN THE UNITED STATES: AN OVERVIEW
Daniel Walters and Paul Jasa
Institute of Agriculture and Natural Resources
University of Nebraska – Lincoln U.S.A.
INTRODUCTION
The growth of agricultural production in the United States over the past 100 years has
followed the westward expansion of population across the North American continent. In
the 1930’s, the Dust Bowl, which resulted from a severe long-term drought and
excessive tillage, prompted the formation of the U.S. Soil Conservation Service (now
the Natural Resources Conservation Service NRCS) and a concerted effort to improve
soil conservation practices to preserve soil resources for future generations. There are
currently 295 million acres (120 Mha) of cropland in the U.S. subject to some form of
tillage. Significant advances in machinery and cropping systems as well socioeconomic
changes in U.S. agriculture and government programs have influenced the rate of
conservation tillage (CT) adaptation in the US. This paper will briefly outline the state of
CT in the US with a discussion of the principal determinants governing adaptation of CT
across the nation.
DEFINITIONS (5)
Conservation Tillage (CT): Any tillage and planting system that covers 30 percent or
more of the soil surface with crop residue, after planting, to reduce soil erosion by water.
Where wind erosion is the primary concern, any system that maintains at least 1,000
lb/acre ( 1.1 Mg/ha) of flat, small grain residue equivalent on the surface throughout
the critical wind erosion period. The following define three broad classes of conservation
tillage.
No-till or Strip-till (NT): A tillage/planting system where the soil is left
undisturbed from harvest to planting except for nutrient injection. Planting is
accomplished in a narrow seedbed or slot created by coulters, row cleaners, row
chisels or roto tillers. Weed control is accomplished primarily with herbicides.
Less than 25% row width disturbance is considered no-till.
Ridge-till (RT): A tillage/planting system where the soil is left undisturbed from
harvest to planting except for nutrient injection. Planting is completed in a
seedbed prepared on ridges with sweeps, disk openers, coulters, or row
cleaners. Residue is left on the surface between ridges. Weed control is
accomplished with herbicides and when ridges are rebuilt during cultivation.
Mulch-till (MT): The soil surface is disturbed prior to planting. Tillage tools such
as chisels, field cultivators, disks, sweeps or blades are used. Weed control is
generally accomplished with herbicides and/or cultivation.
Reduced Till (RDT): Any tillage system that leaves 15–30 percent residue cover after
planting, or less than 500 lb/acre (0.55 Mg/ha) of small grain residue equivalent
throughout the critical wind erosion period.
Conventional Till (CVT): Any tillage system that leaves less than 15 percent residue
cover after planting or less than 500 lbs/acre (<55 Mg/ha) small grain residue during the
critical wind erosion period. Generally involves plowing or intensive tillage.
DETERMINANTS GOVERNING ADAPTATION OF CT SYSTEMS IN THE U.S.
Socioeconomic forces:
Food Security Act of 1985: Conservation tillage was only practiced on 2.3% of
harvested cropland nationwide in 1965 (5). Although this percentage increased to 16%
in 1979 the rate of increase was stagnant. The passage of the Food Security Act by
Congress in 1985 tied soil conservation practices to farmer eligibility for government
sponsored crop deficiency payments, crop loans, storage payments, Federal Crop
Insurance and disaster payments (4). The overall purpose of the act is to remove
incentives for persons to produce crops on highly erodible land (HEL). Highly erodible
land is defined as soils that would erode at eight times that soil’s tolerance (T) value
based on the universal soil loss equation (USLE) or the wind erosion equation (WEQ).
This massive program affects more than 125 million acres (50 Mha) nationwide. The
current nationwide percentage of cropland under CT stands at 37.2 percent, RDT at
25.8 percent with CVT reduced to 36.2 percent of cultivated lands.(5). (Figs. 1 and 2).
Tillage by Region, 1998
Lake States
Northeast
Pacific
Northern
Plains
Corn Belt
Mountain
Appalachian
Southern
Plains
Delta
Southeast
Tillage practice by Type
Conventional
Ridge-Till
No-Till
Mulch-Till
Source: Conservation Technology Information Center
Pacific
Circle size represents tillage area in million acres (range in
ascending size): 7.7 million acres B 78 million acres
Figure 1. Proportion of agricultural land area under conservation and conventional
tillage practice in the continental USA. Source CTIC (5).
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Figure 2. Trends in CT adaptation in the United Staes, 1990-1998. Source (5)
Farm Size: The number of farms in the U.S. has declined from a high of 6.8
million in 1935 to 1.9 million today (11). At the same time, farm population has declined
from 33 million people to under 4.8 million or 1.9 percent of the national population (Fig.
3). Average farm size in the U.S. today is 508 acres (206 ha). The reduction in farm
labor and increase in farm size requires more timely tillage operation and less time with
machinery management. Where plowing required an average of 1.22 hrs/acre (3
hrs/ha), NT requires only 0.5 hrs/acre (1.2 hrs/ha) because of the reduction in primary
and secondary tillage operations.
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Figure 3. Trends in U.S. farm size, population and farm number. Source (11)
Environmental Awareness: Public awareness of environmental issues has
increased throughout the U.S. as information transfer has become more sophisticated.
Water quality is a principal public concern and there is increasing public financial
support for conservation programs to reduce non-point pollution of surface and ground
water. Nearly every major agricultural state has some tax support for cost sharing of
conservation structures on private land. Protection of watersheds with CT and forested
buffer strips along the nation’s watersheds is a part of national policy. Significant
progress has been made toward protecting the Mississippi River watersheds in the
Central U.S. (Fig. 4) however sedimentation of the nations waterways is still a major
offsite effect of soil erosion which costs the nation more than $7 billion dollars annually
(7 ).
Figure 4. Proportion of major US continental watersheds under conservation
tillage. Source CTIC (5).
Climate and Soil Factors:
Agriculture in the U.S. is practiced under a wide range of climatic regimes with
agricultural eco-regions that span a mix of rainfall patterns, solar energy load, day
lengths, altitudes, crop requirements, soil properties, irrigation water availability/quality
and pest problems. The national pattern of CT adaptation tends to roughly follow
latitudinal trends with growing season length, energy flux and crop heat requirements
dominating regional acceptance of high surface residue CT systems (Fig. 5). Several of
the more dominant state variables in the Central U.S. that inherently influence tillage
adaptation are:
Soil temperature: Seed germination and growth (especially maize) is reduced
under cool soil temperatures. Radical elongation and phenology is delayed under cool
soil conditions (1, 2 ) which reduces yield potential and increases the risk of yield loss at
latitudes where heat units are marginally adequate to achieve maize maturity (Fig.6).
The inherent characteristics of conservation tillage likely to influence soil temperature
are surface residue reflectance (albedo), thermal conductivity and soil drainage class.
The relative popularity of ridge till and strip till systems in northern latitudes is
attributable to increased aspect (in RT) (13 ) and reduced albedo.
Conservation Tillage by Region, 1998
Lake States
Northeast
Pacific
Mountain
Northern
Plains
Corn Belt
Appalachian
Southern
Plains
Delta
Southeast
Tillage practice by Type
Ridge-Till
No-Till
Mulch-Till
Source: Conservation Technology Information Center
Circle size represents conservation tillage area in million acres
(range in ascending size): 1.6 million acres B 35.9 million acres
Pacific
Figure 5. Proportion of agricultural land area under conservation tillage practice in the
continental USA. Source CTIC (5).
Figure 6. Daily total undepleted solar radiation received on a horizontal surface as a
function of latitude and time of year (8). (US latitudes between 30 & 50)
Soil Drainage: Since the principal component of soil heat capacity is water
content, CT systems that improve energy absorption in poorly drained soils through
reduced albedo or increased aspect will improve conditions for seed germination and
timing of spring planting operations. Conversely, soils that have low water storage
capacity have improved water retention and are aided by high surface residue CT
systems. Poorly drained soils susceptible to compaction are especially vulnerable to
the high axel loads associated with heavier CT equipment. Even under NT, 30 to 50%
of the soil surface may be subjected to wheel traffic (14 ).
Water Conservation in Fallow Tillage Systems: Dryland fallow systems are
practiced in the High Plains of the western USA where two wheat crops are grown in
three cropping season. During the lengthy fallow period (14 mo.) soil is managed for
weed control, water harvest, snow capture and wind erosion protection. Tillage systems
that result in erect stubble and high surface residue with a greater reliance on
herbicides for weed control have dramatically increased fallow efficiencies (Table 1).
However, economic return to land, labor, capital, and management in fallow systems
are reduced under no-till as the cost of conversion to no-till from stubble mulch is
greater than the yield advantage from increase fallow efficiency. Costs for NT are $8 $11.00 more per acre than stubble mulch (6,10). Intensification of cropping toward
shorter fallow periods, however, can improve profitability under NT (Table 2) (12 ).
Table 1. Evolution of Fallow Tillage systems, Akron, Colorado. (9)
Years
Management system
Fallow Efficiency
% ppt. Stored as
soil water
1916 – 1930
Maximum Tillage (Dust Mulch)
16-22
1931 – 1945
Conventional Tillage : shallow disk and
rod-weeder
Improved Conventional Tillage
Stubble Mulch in 1957
Stubble Mulch, began minimum tillage
with herbicides
1946 – 1960
1961 – 1975
1975-present
No-Till with contact herbicides
24-27
27-30
33-38
40-55
Table 2. Relative Return to Capital Investment on a 1200 acre farm as
affected by cropping system and fallow tillage system in NE Colorado (12 )
Cropping System
Tillage Preceding
Wheat Planting
WF
WCF
WCFM
Conventional
100%
140%
127%
Reduced Till
92%
136%
120%
No-Till
72%
125%
113%
WF=Wheat Fallow; WCF = Wheat-corn-fallow; WCMF=Wheat-corn-millet-fallow
Water and Wind Erosion Hazard: Obviously conditions that reduce runoff and
improve water infiltration will reduce water erosion potential. Conservation tillage
practices tend to increase the tortuosity of surface runoff path and increase infiltration
time. Conservation tillage systems dominate regions of the country where farming is
practiced on sloping lands, soil texture is conducive to crusting and erosive rains are
frequent during soil preparation for planting. Wind erosion is the dominant hazard in the
wheat-fallow regions of the Central Plains and CT systems that preserve erect residue
will reduce wind velocity at the soil surface. At times, tillage operations are performed
to increase soil roughness and aggregate size to reduce saltation under extreme
conditions.
Crop Management Factors:
Weed Control: Conservation tillage systems most often result in increased
reliance on chemical weed control. Weed control can be a significant problem in CT
systems where pH stratification affects herbicide activity or where residue cover may
intercept herbicides rendering them inactive. At times, emergency tillage is required to
offset herbicide failures resulting in less than 30% residue cover. In specific situations,
the most effective herbicide compounds will require soil mixing limiting the choice of CT
systems. Conversely, CT systems can also afford reduction in herbicide use when
banded in strip- or ridge-tillage operations. Cooler soils in residue-laden zones may
suppress the emergence and competitive ability of weeds relative to the crop. Weed
control in continuous cropping systems are usually the most problematic for CT systems
because weeds with similar biological characteristics and growth periods to the crop
tend to predominate over time. In these situations, rotation of crop species may be the
most effective weed control measure under CT. The introduction of herbicide resistant
crops has revolutionized CT weed control strategies in the maize belt but the safety and
ecological impact of these crops is being questioned.
Crop Rotation: Crop rotations allow selection of different herbicides, planting
dates and tillage systems resulting in greater opportunities to control weeds, disease
and insect pests. Certain CT systems are not amenable to crop rotations. For example,
RT systems that require ridge construction during a row crop season are unsuited to
rotation with solid seeded or sod crops. This fact tends to limit the acreage devoted to
RT. Often CT systems alternate with crop rotations in response to residue management
needs or fertilization strategies. Crop rotations of all sorts are becoming the dominant
cropping strategy as they provide market diversity as well as management flexibility.
Soil Fertility: Stratification of soil chemical properties and nutrient distribution is a
commonly observed phenomenon of CT systems. Non-mobile nutrients such as P and
K can accumulate in the upper portions of the soil profile from surface placement of
fertilizer materials and/or deposition from decomposed residues (3). The extent of soil
acidification produced by nitrification of ammonium N will depend on soil buffering
capacity and inherent N mineralization potential. Redistribution and mixing of nutrients
varies with the degree of soil mixing so the choice of CT system and duration of CT
practice will depend on native soil fertility and the resistance of soil to chemical change.
SUMMARY
There are 294 million acres of cropland subject to tillage each year in the United States.
Conservation tillage systems (> 30% residue cover) are practiced on 37% of these
acres with 26% maintained with > 15% residue cover. Government incentives tied to
the Food Security Act of 1985 have been the primary reason for growth in CT in the
past decade. A growing threat from agriculture to surface water quality continues to
drive public initiatives and support for soil conservation programs. The diversity of
cropping systems, climate and soil resources across the agricultural regions of the U.S.
has resulted in a broad array of CT strategies that include innovative machinery design,
biological pest control, alternative crops and crop rotations, herbicide formulations and
genetically modified crops.
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