Upper Midwest Tillage Guide
Reducing tillage intensity
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- Benefits of reduced tillage
Reducing soil tillage intensity presents many benefits, challenges, and some required changes to your field operations. Benefits range from reduced soil erosion, fuel use, time, and labor, as well as building soil organic matter, improved soil structure, and maintaining options for soil warm up and dry down in the spring months. Conversely, the challenges include learning a new tillage system, changing equipment costs, managing residue build-up over time, patience, and perhaps going against local traditions.
Benefits of reduced tillage
Seedbed preparation is all about setting a hospitable environment for seeds to germinate and thrive. Much like your own home, you optimize your house's heating, cooling, and insulation, while also stocking the pantry to provide comfort throughout the changing seasons. The seedbed is no different. By optimizing your seedbed to handle whatever Mother Nature decides to throw at you, you are providing your crop the best chance to quickly develop a stronger stand and produce to its full potential.
Spring soil temperature: Why we care and how it works
The higher the soil temperatures in the spring months, the faster your crop will emerge and establish a strong stand. Early crop emergence and stand establishment promotes an earlier crop canopy closure, reducing the germination of weeds during the mid- and late-season, as well as providing your crop a better chance to withstand disease and insect pressure.
Crop residues on the soil surface have a low density, which means they are light and have a large amount of air in and around them. This air gives crop residues an insulating effect on soils, similar to fiberglass insulation in attics. The thickness of the crop residue layer covering the soil surface governs how quickly heat from solar radiation will move into the soil during the spring months until the crop canopy closes. A soil with a thick crop residue layer covering 70 percent of the surface will heat up more slowly than a soil with a few fragments of residue covering 10 percent (Photo 1). Therefore, it is important to make sure residue is spread evenly behind the combine to avoid areas of excessively thick residue. In addition, crop residue is typically light in color, which reflects more solar radiation back up into the atmosphere than a darker color would. As a result, less heat reaches the soil surface.
Photo 1. A visual guide to differences in corn and soybean residue cover.
Source: USDA-NRCS https://www.nrcs.usda.gov/
Spring soil temperature: Linking tillage practices
Soil temperature changes due to the depth and aggressiveness of the tillage implement used. Deep tillage implements, such as the moldboard plow and disk ripper, leave less than 15 and 45 percent of crop residues covering the soil surface, respectively.
Medium depth tillage implements, such as the chisel plow, strip till, ridge till, and disk also leave 30 to 60 percent of crop residues covering the soil surface. If a field cultivator is used as a secondary tillage option, the amount of crop residue covering the soil surface will decrease to 20 to 30 percent.
Soil temperatures in the plant row may differ somewhat among all these tillage options during the hottest portion of a few select days in the spring. However, the soil temperatures will most often tend to be quite similar, particularly during the evening, nights, and morning, as well as after rainfall.
Strip-till implements move crop residues to the side of where the plant row will be placed. Because the residue is all but completely removed from the plant row, soil temperatures in the strip will be similar to soils tilled with a moldboard plow, disk ripper, chisel plow, and disk. North Dakota State University research in the Red River Valley (Prosper, ND, and Moorhead, MN) during 2007 indicated comparable soil temperatures between a single fall strip till pass and a fall primary chisel plow pass followed by a spring secondary field cultivation pass (Overstreet et al., 2007).
Legend: DMI_CP=Disk ripper; ST_btwn=Strip till, between row; and ST_in row=Strip till, in-row
Figures 1 and 2. Soil temperature measured daily at a 2-inch depth across three tillage systems in two fields near Clarkfield, Minnesota in June, 2011 (top) and May, 2012 (bottom)
Legend: ST=Strip-till; DR=Disk ripper, and MBP=Moldboard plow
The University of Minnesota performed similar research during 2011 and 2012 in southern Minnesota with an aggressive strip till implement, a disk ripper, and a moldboard plow in a corn-soybean rotation (Figure 1 and 2). They also observed similar soil temperatures among these tillage options. However, soil temperatures were lower and more soil moisture was conserved in the areas where the strip tiller moved crop residues between the plant rows.
Figure 3. Average soil temperature taken at a 4-inch depth from thaw to crop canopy closure for four tillage systems near Wahpeton, North Dakota and Fergus Falls, Minnesota, 2015.
In 2015, a joint project by the University of Minnesota and North Dakota State University at four field sites with varying soil textures (sandy loams to silty clays) demonstrated similar patterns among these tillage practices. However, in these fields, shallow vertical tillage and no-till were included and evaluated for soil temperatures. As expected, the shallow, less aggressive vertical-till implement caused soils to be several degrees cooler and no-till temperatures to be the coolest due to the higher crop residue cover during the spring months as compared to strip till and chisel plow implements (Figure 3).
Research on strip till has consistently shown over time that soil temperatures in the spring depend more on where crop residue is placed rather than the percent soil covered of the whole field. The advantage of higher soil temperatures in strip till berms as compared with no-till is demonstrated by faster plant emergence and crop development. This advantage is greatest when soil temperatures approach the lower threshold for crop seed germination. For example, early-planted strip till corn or soybeans will likely emerge sooner than in a no-till system. Early plant establishment frequently translates into higher crop yields and quality.
Soil moisture: Why we care and how it works
Crop residues left on the soil surface help snow and rainwater enter the soil and then conserve that water as soil moisture for later use by the crop. Crop residues covering the soil surface over winter help to trap snow (Photo 2). The trapped snow can help prevent the soil from freezing as deeply as a bare soil without crop residue cover. When the snow melts, it can then move through the residue layer and easily into the soil.
When spring and summer rains come, the crop residue protects the soil from the falling water, preventing it from forming a crust and sealing. As the crop residue intercepts rainwater, the water can gently move through the layer of residue and into the soil. However, the crop residue's job is not done yet. The layer of residue then reduces the amount of soil water lost to the atmosphere via evaporation by acting as a barrier. By reducing evaporative losses and conserving the soil moisture, that water will be available for plants, particularly later in the growing season during the plant's reproductive stages.
However, too much moisture can be as bad as too little moisture. During planting in the spring months, excessively high soil moisture can create issues with trafficability on weak, poorly structured soils. Nearly all soils are most vulnerable to compaction when 80 to 85 percent of the soil pores are full with water. Additionally, if compaction layers prevent the soil from adequately draining excess soil moisture, the vulnerability of the seedling to seed rot and damping-off diseases, such as Pythium, increases as the soil warms up.
Soil moisture: Linking tillage practices
Figure 4. Soil water loss (mm) 80 hours after four tillage system operations in Morris, Minnesota. (MBP=Moldboard plow, CP=Chisel plow, DH=Disk harrow, NT=No-till)
Source: Reicoski, et al. USDA-ARS
Aggressive tillage implements, such as the moldboard plow, disk ripper, chisel plow and disk, cause the soil to quickly lose some of its moisture to the atmosphere. In a USDA-ARS study, researchers measured the loss of soil moisture during the first 80 hours after using a variety of soil tillage implements. Areas tilled with a moldboard plow (MBP) lost 30 percent more moisture than areas tilled with a chisel plow (CP) and twice as much moisture than areas with no tillage (NT) (Figure 4).
Figure 5. Average soil moisture levels at a depth of 2-inches taken from thaw to canopy closure for four tillage systems near Wahpeton, North Dakota and Fergus Falls, Minnesota.
In 2015, during the same joint project by the University of Minnesota and North Dakota State University, four field sites of varying soil textures (sandy loams to silty clays) showed soil moisture differences among tillage practices during the spring months. In these fields, shallow vertical tillage resulted in the soils being 7 percentage points drier on average (Figure 5) than no-till. In contrast, the more aggressive chisel plow and strip till implements caused soils to be 13 to 14 percentage points drier than no-till, due to little crop residue covering the soil surface. However, soil moisture between the strip till berms was between the moisture levels measured in the no-till and vertical till plots. Soil in the strip tilled berm dried down and warmed up similar to the chisel plowed plots. However, while the areas between the strip till berms, which were just a few inches away, were not disturbed, they still provided a modest amount of drying and warming (3 percentage points more than the no-till plots). Strip till tended to present the best of both chisel plow and no-till management. The promotion and preservation of soil structure between the strip till berms will provide better drainage of excessive moisture during high rainfall springs, as well as provide more strength for spring traffic as compared to tillage practices that demote soil structure.
Aeration: Why we care and how it works
Plant roots require a minimum amount of oxygen at all times to maintain metabolic functions. Plant roots and soil microbes release carbon dioxide, which can affect oxygen availability if aeration is limited. Oxygen moves through water 10,000 times slower than through air. Therefore, if soil moisture persistently stays near saturation, the crop has a high likelihood of drowning (Photo 3). This emphasizes the need for soils to be able to quickly drain the excess soil moisture during years with heavy or persistent rainfall.
Figure 6. Oxygen diffusion rates in soil for ideal plant growth functions for three agronomic crops.
Source: Glinksi and Stepniewski, 1985
Different crops have different minimum soil oxygen diffusion rate requirements before nearly every metabolic process in the plant deteriorates (Figure 6). This means that some crops in your rotation may handle wet springs fairly well, whereas other crops may be affected severely in a matter of a few days of enduring high soil moisture.
Aeration: linking tillage practices
Aggressive tillage implements "fluff" up the soil, increasing the amount of pore space and the amount of air in the soil. The fluffed-up soil can readily dry by evaporation after most rainfall events allowing oxygen to meet crop needs. However, drying by evaporation can only provide so much benefit. If rainfall becomes heavy or frequent, high soil moisture levels may persist for long periods causing risk to the crop. During these times, soils with good soil structure (soils with several years of reduced or no tillage) are much more efficient at draining this excess moisture from the seedbed and allowing air to enter through the largest pores. However, if rains are so heavy and frequent to bring the groundwater level up within the topsoil, then evaporation or drainage may not be able to provide much relief to the crop unless fields are tile drained.
Soil structure is formed by the aggregation of individual soil particles (clay, silt, sand, pieces of organic matter) into peds. Soil aggregation is the movement and then sticking of soil particles together. There are many large pore spaces between the aggregates, which allow roots to penetrate the soil easier and air and water to pass readily through. Microscopic bacteria and fungi in the soil, as well as plant roots, play a vital role for soil particles to stick and stay together as peds. Their sticky exudates and hyphae physically hold the soil together, helping soil structure to form and persist over time (Photo 4). The more diverse and abundant the microbial population, the faster the soil aggregation. Benefits of improved soil structure include the following:
Photo 4. Well-aggregated soil with fungal hyphae (branching filaments) taken from a long-term strip tilled field in southern Minnesota.
- Reduced bulk density
- Increased aggregate stability
- Resistance to soil compaction
- Improved water infiltration and drainage
- Enhanced retention of plant available water
- Reduced nutrient leaching
- Less soil erosion
- Enhanced biological activity
- Increased soil organic matter
All of these benefits are based on building or preserving soil structure. Tillage breaks apart soil aggregates, damaging the existing soil structure. Over time, tillage also reduces soil biological life and diversity. The deeper and more aggressive the tillage, the less structure the soil will have. This leads to an abundance of individual soil particles, which can clog pores and crust the soil surface, slowing water infiltration and increasing runoff (Photo 5). Smaller soil particles are also highly susceptible to being swept away with wind and water. Valuable topsoil moves into the ditch, or the neighbor's field, or the next state, and is lost forever.
Additionally, as the soil loses structure, it becomes denser and more susceptible to compaction, which is the loss of larger pore spaces. Compaction slows root growth and limits water holding capacity. Repeated tillage operations at the same depth may cause serious compacted layers, or tillage pans, just below the depth of tillage. As a result, higher horsepower equipment is needed to get through compacted soil and it suffers more wear-and-tear.
Reducing tillage helps preserve the soil's natural structure, making the soil more resistant to erosion and the negative effects of heavy field equipment.
Soil is a non-renewable resource and cannot be built within our lifetime. When it is gone, it is gone. While erosion is a natural process, cultivation of the prairie and the dominance of annual crops have significantly sped up soil erosion. The loss of topsoil severely diminishes a field's productivity.
The soil that is moving downslope or completely off the field is the most productive soil. It contains carbon, nitrogen, phosphorus, sulfur, is lower in salts and, in the upper Midwest, has a more neutral pH than the remaining soil left behind. The USDA-NRCS estimates that cultivated land in Minnesota and North Dakota has 4 to 7 times more water erosion (Photo 6) than non-cultivated land and 30 to 50 times more wind erosion.
Crop residues absorb the energy from high-speed raindrops, reducing the likelihood of aggregate dispersion and soil crusting. Residue also slows down the overland flow of runoff, allowing more time for the water to infiltrate and move through soil pores.
Photo 7. Wind erosion in a region of west central Minnesota with 80 percent of the fields aggressively tilled.
Crop residues protect the soil from winds near the soil surface. These residues shift the column of wind upwards away from the soil. In Minnesota, the average wind erosion rate is 5.2 tons of soil loss per acre per year. North Dakota is slightly lower at 4.7 tons and South Dakota is at 2.4 tons per acre per year. While these levels have decreased in the past three decades, wind erosion is still occurring at detrimental rates (Photo 7). The most severe areas of erosion are well above the general estimates of 5 tons per acre per year.
Tillage can erode more soil than wind and water erosion combined. A team of USDA-ARS researchers in Western MN studied highly erodible fields, observing that moldboard plowing loosened and moved 27 tons of soil per acre per year. Once loosened, the soil was also vulnerable to further movement from runoff flowing downslope. The researchers calculated that on this field, water moved almost 9 tons of soil per acre per year (Photo 8). As expected, crop yields were also severely affected. In 2003, areas of the field where the soil accumulated, wheat yields were between 80-95 bu/ac. In the area of the worst erosion (hill tops and shoulder slopes), yields were as low as 45 bu/ac, a yield loss of near 50 percent or loss of $180 per acre if wheat is $4.00 per bushel.
Soil loss via wind and water erosion cuts your profits and reduces productivity by removing a non-renewable resource. Erosion is very costly in terms of nutrient removal, lower water holding capacity, and loss of productive organic matter. Management to reduce the potential for erosion include the following:
- Maintain at least 30 percent residue cover to protect the soil surface.
- Reduce tillage to improve soil aggregation and structure.
- Keep the soil covered for longer periods using cover crops and perennials.
- Increase the height of standing stubble and replace shelter belts to reduce wind speeds across your fields.
Soil organic matter
A producer cannot change the soil texture in a field. However, the level of soil organic matter can be increased or decreased due to the chosen set of management practices. This makes a producer’s management choices very important, since soil organic matter is related directly to soil fertility, soil structure, and agricultural productivity potential. Other advantages to increasing or maintaining a high level of soil organic matter include the following:
- Reduced bulk density
- Increased aggregate stability
- Resistance to soil compaction
- Enhanced fertility
- Reduced nutrient leaching
- Resistance to soil erosion
- Improved water infiltration and drainage
- Enhanced retention of plant available water
- Enhanced biological activity and diversity
You may have read articles where the term soil organic matter (SOM) is used interchangeably with soil organic carbon (SOC). This is because soil organic matter is 58 percent organic carbon. Carbon is invisible to the eye, making it difficult to understand how soil management affects it. Luckily, researchers can measure carbon in the soil and in the atmosphere.
Soils continuously store carbon and then release some of it in natural processes. Soil tillage speeds up this process by warming the soil and incorporating oxygen and crop residues into the soil. The soil microbe population, particularly bacteria, increases in response to the tillage and these additional food sources. Microbes consume the carbon from the crop residue and the soil organic matter, accelerating the conversion of organic carbon into carbon dioxide gas (CO2), which is then released into the atmosphere.
By accelerating this process, the carbon in the crop residue quickly turns into a gas and leaves the soil instead of slowly breaking down and forming soil organic matter. Over time, soil organic matter levels and their related advantages decrease with soil tillage.
Figure 7. Pounds of carbon dioxide per acre per day lost to the atmosphere using three tillage operations near Jeffers, Minnesota (MBP=moldboard plow; DR=disk ripper, and ST=strip-till)
Source: Faaborg et al., 2005
Identifying tillage methods that reduce the amount of carbon released into the atmosphere is important. A Minnesota study conducted in 2005 compared soil CO2 emissions following fall moldboard plowing (MBP), disk ripping (DR) and strip tilling (ST) and determined that strip tillage maintained more soil carbon than moldboard plowing and disk ripping. Disk ripping and strip tillage released 53 and 83 percent less CO2 from the soil than moldboard plowing (Figure 7). Moldboard plowing disturbed and exposed the greatest amount of soil, allowing carbon as CO2 or previously stored as organic matter to escape into the atmosphere. The deeper and more aggressive the tillage, the more soil carbon was lost.
Figure 8. Pounds of carbon dioxide lost to the atmosphere under four tillage operations in western Minnesota. (MP=moldboard plow; DH=disk harrow; CP=chisel plow, and NT=no-till)
Source: Reicoski et al.
In another Minnesota study, three tillage systems ranging in soil disturbance were compared to no-till. In this study, wheat residue produced from the previous season's crop added 2,840 pounds of organic matter per acre. When the soil was moldboard plowed (MP), the soil lost over 3,800 pounds of organic matter per acre within 19 days after the primary tillage pass, which is 1,000 pounds more than what was added by the previous crop's residue (Figure 8). This system will continue to lose more carbon in the spring when the field is prepared for planting. If this MP system is used continuously, the organic matter content will decrease over time. These values are substantially greater than the no-till (NT) treatments, which lost only 770 pounds of organic matter per acre due to natural carbon cycling processes. The disk harrow (DH) and chisel plow (CP) were in the middle range of organic matter lost.
Evaluating the economics of tillage systems is very complex. Consideration must be given to the initial cost of the implement, financing charges, maintenance costs, the size of tractor needed to pull the implement, equipment depreciation, labor costs, conservation program incentives, and increased management costs related to fertilizer and pest management.
Reducing tillage means fewer trips across the field, conserving energy, fuel, and labor, and reducing machinery maintenance. Refer to Chapter 4, Economics of Tillage for more information.
Changing tillage means a change to the system
When considering a change in tillage practice, producers will need to make changes to many other parts of their system. For instance, weed control and fertilizer applications that worked with chisel plowing will likely not work well for no-till, and vice versa. Changes to the system may not always line up well with family or neighbor traditions and perceptions. A keen eye and willingness to increase a farm's efficiency will likely require a combination of traditional and innovative adjustments to field operations in order to achieve the greatest economic gains.
Shift to perennials
Reduced tillage and additional surface residue may increase perennial weed pressure; therefore, it is important to adjust your weed management program to fit your tillage system. When reducing tillage, start with relatively weed-free fields. Effective weed management in a reduced tillage system includes the use of soil-residual herbicides, well-timed post-emergence herbicide applications, crop rotation, and use of multiple herbicide modes of action.
Tillage effect on weed seed survival
The persistence of the weed seed bank varies by tillage system and weed species. Less-intensive tillage will likely favor small-seeded weeds that emerge from shallow depths. More-intensive tillage buries weed seeds, protecting them from seed predators and increasing their persistence.
Weed seed bank degradation is greatest near the soil surface. Seeds near the soil surface are more accessible to seed predators like rodents and insects and are more likely to be degraded by soil microorganisms. No-till systems do not have tillage to incorporate weed seeds, which concentrates the weed seeds near the soil surface where they are more likely to be degraded. More intensive tillage buries weed seeds deeper in the soil and results in a more uniform distribution of weed seeds in the soil.
Weed seeds become more dormant when buried, which prevents them from germinating until they are brought back to the soil surface. No-till and minimum-till systems that concentrate weed seeds near the soil surface generally result in an increase in small seeded weeds that germinate at shallow soil depths. Large-seeded weeds generally decrease in prevalence with less tillage, as seed predators can easily find weed seeds on the soil surface and large-seeded weeds generally do not germinate unless incorporated. Although tillage system influences seed bank degradation and seed persistence, there is a large amount of variation among weed species (Table 1).
Table 1. Weed seed persistence in soil. The approximate number of years it takes to reduce weed seed population by 50 and 99 percent. Adapted from Michigan State University, 2005.
|Seed population reduction|
|Weed type||Species||50 percent||99 percent|
Nutrient management practices will be affected by tillage system choices. For example, surface-applied nitrogen (N) fertilizer, such as granular urea or UAN solution, should be incorporated mechanically or by rainfall within three days of application to eliminate N loss from volatilization. Other N application alternatives in a reduced tillage system include applying urea or UAN to the side of the row with a coulter at planting time. Additionally, because anhydrous ammonia is injected into the soil, it is considered a minimum tillage pass (Photo 10).
Stratification of immobile nutrients in the soil does occur with no-till and shallow tillage, but in the Northern Corn Belt, this is rarely a problem for plant growth. Research shows that crops respond similarly to broadcast, planter-banded or deep-banded phosphorus (P) and potassium (K), regardless of tillage system.
It is more important to apply the P and K at rates recommended by the soil test. No-till and strip till greatly reduce P losses to surface waters. Injecting or subsurface-banding P also reduces dissolved P loss, compared to broadcast application (source: Antonio Mallarino, Iowa State University).
Livestock manure should be incorporated for maximum agronomic benefits and minimum environmental risk. This can be a dilemma in a minimum tillage system; nitrogen use is greatest when manure is applied before corn, but incorporation largely destroys fragile soybean residue, leaving the soil unprotected. In continuous corn systems, incorporating manure with a chisel plow or by injection allows for good residue management. New application tools, such as vertical manure injection, are being developed to incorporate manure with minimal soil disturbance.
Soil pH management can be an issue in long-term no-till fields. Surface applications of ammonium-containing fertilizer, such as urea and UAN, acidify the surface. Lime must be incorporated to be effective, as the neutralizing benefit migrates very slowly in soils that are not tilled. A grower may consider collecting soil samples separately from the 0-2 and 2-8 inch depth. Where acidification is present only at the surface, apply one-third the recommended lime rate in no-till.
Tradition, perception and patience
Farming is ever-changing. Practices that were at one time the norm have been replaced by more efficient, profitable, and environmentally sound systems. This is very true for tillage, where within one's lifetime, moldboard plowing has been replaced by chisel plowing, and chisel plowing has been replaced by low disturbance systems such as strip till and no-till. Change is difficult for some, but those that are willing to break with tradition to try something different are often rewarded.
The public's perception of agriculture is also changing. Less than 2 percent of the U.S. population actively farms (USDA-NASS, year?). As the general population becomes further removed from food production, there is less understanding about modern agricultural practices and its balancing act between production and environmental risk. Tillage can play a major role in mitigating some of these risks. While selecting the proper tillage system may not solve all issues, it will help keep the soil and nutrients on the land.
As farmers deal with change, they must display patience. Growers seeking to produce crops at the lowest cost per unit of yield possible should factor in tillage selection and its implementation as a part of the calculation. Adapting to a tillage system may take time, as it is fine-tuned to the soil, crop rotation, and the farmer's ability. Patience will be an important key to reaching success.
DeJong-Hughes, J., D. Franzen, A. Wick. 2014. Reduce Wind Erosion for Long Term Productivity. University of Minnesota Extension. Available on-line at https://www.extension.umn.edu/Agriculture/soils/tillage/docs/reduce-wind-erosion-for-productivity-2014.pdf
Faaborg, R., C. Wente, J. DeJong-Hughes and D.C. Reicosky. 2005. A comparison of soil CO2 emissions following moldboard plowing, disk ripping and strip tilling. USDA-ARS research update.
Glinski, J., W. Stepniewski. 1985. Soil Aeration and Its Role for Plants. CRC Press. Technology & Engineering.
Gunsolus, J., D. Wyse, K. Moncada, C. Fernholz. 2010. Risk Management Guide for Organic Producers, Chapter 6. Weed Management. University of Minnesota. Available on-line at https://conservancy.umn.edu/bitstream/handle/11299/123677/Ch6.Weed_management.pdf?sequence=9&isAllowed=y
Mallarino, A. P., R. Borges. 2005. Phosphorus and Potassium Distribution in Soil Following Long-Term Deep-Band Fertilization in Different Tillage Systems. SSSAJ 70 (2): 702-707.
Overstreet, L.F., N.R. Cattanach, S. Gegner, and D. Franzen. 2007. Crop sequence effect in sugarbeet, soybean, corn, and wheat rotations. North Dakota State University. Sugarbeet research and extension reports, 2007.
Papiernik, S.K., M.J. Lindstrom, T.E. Schumacher, D.A. Lobb, J.A. Schumacher, and A. Farenhorst. 2004. Variation in soil properties and crop yield across an eroded prairie landscape. American Society of Agronomy Abstract. 4797 [CD-ROM computer file] ASA, Madison, WI.
Reicosky, D., and M. Lindstrom. 1993. Fall Tillage Method: Effect on Short-Term Carbon Dioxide Flux from Soil. Agron. J. 85(6): 1237-1243.
United States Department of Agriculture-National Agricultural Statistics Service (USDA-NASS). Year. Year Ag Census. Available on-line at www.nass.usda.gov.
USDA-NRCS. 2010. National Resources Inventory Summary Report. Available on-line at www.nrcs.usda.gov
Upper Midwest Tillage Guide is a collaboration between University of Minnesota and North Dakota State University
Peer review provided by Richard Wolkowski, Extension Soil Scientist, Emeritus, University of Wisconsin-Madison
Thanks to Jared Goplen for providing assistance with editing.
Photos are provided by Jodi DeJong-Hughes unless otherwise noted