Machinery Options for Reducing Soil Compaction in Crop Production
Crop Insights written by Mark Jeschke, Ph.D., Pioneer Agronomy Manager
Crop Insights written by Mark Jeschke, Ph.D., Pioneer Agronomy Manager
Soil compaction is a pervasive problem throughout modern agriculture. The need to move machines through the field to conduct planting, harvest, and other tasks makes some degree of soil compaction nearly unavoidable. Soil moisture is the most important factor influencing the risk of soil compaction (Hamza and Anderson, 2005; Soane and Van Ouwerkerk, 1994), so the best solution to compaction is simply to avoid operating when soil is too wet and compaction risk is high. However, the realities of getting crops planted and harvested often mean growers have essentially no choice but to sometimes operate when portions of a field are too wet. Given this situation, it is important to evaluate the full range of options available to minimize compaction, manage soil to be more resilient against compaction, and remediate or manage compaction that has already occurred. This Crop Insights will focus specifically on machinery options for managing compaction.
One of the most significant factors that has contributed to increasing soil compaction issues has been the dramatic increase in the size and weight of farm machinery over the past several decades (Figure 1). Even just within the last 20 years, the weight of some of the largest machines has gone up dramatically. The largest combine in the Case IH lineup in 1998 was the 2388, weighing in at 28,329 lbs. With an 8-row corn header and a full grain tank, the maximum weight tops out at 44,311 lbs. Compare that to a maximum weight of 77,020 lbs for the largest combine in 2018 (Table 1). Larger machines have facilitated much greater efficiency by allowing one operator to cover more acres, but the greater loads being applied to the soil have increased the potential for compaction that is both more severe and extends deeper into the soil profile.
Additionally, some growers may face greater pressure to conduct field operations when conditions in at least part of the field are too wet. Research has shown the benefits of planting corn (Jeschke and Paszkiewicz, 2013) and soybeans (Van Roekel, 2018) as early as practical to extend the growing season and maximize yields, but this means spring tillage operations may be pushed earlier when soils are more likely to be wet. The need for machines to cover more acres on larger and more geographically dispersed operations during the spring and during harvest can also increase the likelihood of being forced to operate in suboptimal conditions. Trends toward greater annual precipitation, particularly in the spring, and more intense precipitation events in the U.S. Corn Belt driven by climate change are likely to add to the problem (U.S. EPA, 2016).
The primary negative effect of soil compaction on crop production is a reduction in the ability of soil to supply water and nutrients to the crop. Compacted soils limit the ability of plant roots to grow into new soil to extract water and nutrients, effectively reducing the amount of the soil profile that is available to contribute to supplying water and nutrients for crop growth. The reduction in pore space in the soil also reduces the overall water holding capacity of the soil, meaning less water is available for plant uptake. Compacted soils can delay crop emergence, reduce stand establishment, inhibit crop growth, and ultimately reduce yield.
Table 1. Machine, header, and maximum grain weights for a top-end Case IH combine in 1998 and 2018.
Deep compaction caused by heavy loads is the most challenging form of soil compaction for crop production. Equipment with extremely high axle loads such as fully loaded grain carts can compact soil more than three feet down into the soil profile. Effects of deep compaction on crop growth and yield can persist for years and often go undetected, with resulting growth and yield issues often attributed to other factors. Compaction created by high axle loads can reduce crop yields by more than 15% in the first year, with yield reductions of 3-5% persisting as many as 10 years after the initial compaction event (Duiker, 2004). Compacted areas due to machinery traffic in a field often run parallel to the rows, making yield effects difficult to detect and measure from yield monitor data since all harvest passes tend to be affected. Additionally, deep compaction is difficult or impossible to fix once it occurs.
In order to effectively manage compaction, it is necessary to understand how the soil is affected at different depths by loads applied to it. Compaction in the topsoil is determined by contact pressure. Compaction in the upper portion of the subsoil is determined by both contact pressure and axle load. Compaction in the lower subsoil is determined primarily by axle load (Figure 2). The number of passes and load dwelling time (i.e., how fast the machine is moving) will also influence how the load affects the soil.
Axle load is the total weight carried by one axle, typically expressed in lbs, kg, or tons. For machines or implements with more than one axle, the average axle load can be calculated by dividing the total weight by the number of axles. The maximum axle load will be some fraction of the total weight and varies depending on how the machine is balanced. For example, combines typically carry most of their weight on the front axle, whereas 4WD tractors have a more even weight distribution (Table 2). So for a combine and a 4WD tractor of equal weight, the average axle load would be the same but the maximum axle load would be greater for the combine.
Table 2. Approximate weight balance of modern combines and tractors (Hoeft et al., 2000).
Research has shown that axle loads greater than approximately 10 tons can cause compaction that penetrates into the subsoil (Voorhees et al., 1986). Compaction caused by axle loads less than 5 tons is generally limited to the topsoil and does not extend into the subsoil. Modern tractors, combines, and grain carts often greatly exceed the 10-ton threshold (Table 3) and therefore run the risk of causing subsoil compaction in susceptible soils.
Table 3. Approximate axle loads for field equipment (DeJong-Hughes, 2018).
Contact pressure is the axle load divided by the surface area of contact between the load and the soil and is measured in pounds per square inch (psi) or kPa and is the primary factor determining topsoil compaction. Reducing contact pressure will reduce compaction in the topsoil. This can be achieved by lowering tire pressure or by increasing the contact area between the load and the ground, such as by using wider tires (Figure 3).
Contact pressure for radial agricultural tires is generally 1-2 psi above inflation pressure. The use of low-pressure radial tires can help reduce topsoil compaction. Grain trucks and other vehicles with high axle loads and high-pressure road tires can cause much more severe topsoil compaction.
Larger tires, duals, lower tire pressure, and rubber track systems are all effective options to reduce contact pressure and minimize topsoil compaction; however, axle loads greater than 10 tons still can cause subsoil compaction. Research has shown that lower contact pressure can reduce compaction in the upper soil profile (Table 4) but that it has little to no effect on subsoil compaction, which is primarily determined by axle load (Table 5).
Table 4. Maximum pressure at a range of soil depths associated with different tire inflation pressures (Arvidsson and Keller, 2007).
Number of Passes
Conventional wisdom for managing soil compaction holds that the majority of compaction occurs on the first pass over the soil, so growers are better off concentrating repeated traffic into the same travel lane rather than spreading traffic out over a greater portion of the field. This is true; research shows that 70 to 80% of compaction effects happen on the first pass (Wolkowski and Lowery, 2008). However, this does not mean that effects of repeated passes are inconsequential. The compaction caused by repeated passes may cause as much damage to crop growth because the incremental increases in soil density are being applied to a soil that is already above optimum bulk density (Duiker, 2004). The compactive effects of lower axle loads applied repeatedly can eventually exceed the effects of fewer passes with a heavy axle load as well as extend into the subsoil Balbuena et al., 2000).
This can be important when considering the value of tandem or triple axles vs. single axles on heavy equipment such as slurry tankers and grain carts. Adding an additional axle cuts the axle load in half and doubles the surface area of contact, both of which can help reduce compaction. However, it also effectively adds a pass since the same track is being trafficked twice instead of once, which is likely to offset some of the aforementioned benefits (Raper and Kirby, 2006) (Figure 4).
Load Dwelling Time
Travel speed of machines operating in a field can influence the amount of compaction they cause. Longer dwelling times of loads applied to the soil increase the amount of compaction they cause. Increasing travel speed will decrease the load dwelling time and, consequently, the severity of compaction.
Tire Pressure and Configuration
Soil compaction in the upper part of the soil profile is greatly influenced by the contact pressure, so lower tire inflation pressure can help reduce compaction. From a practical standpoint, conducting field operations at the lowest recommended tire pressure can be challenging, as proper tire pressure for road speeds can be 2-3x higher than optimal pressure for field conditions. New lower-pressure agricultural tires have been introduced by multiple tire manufacturers in recent years, expanding options available to growers for reducing compaction. On-board compressor systems have also been developed that allow growers to reduce tire pressure when entering fields and then re-inflate tires before travelling on roads.
Duals and triples can also help reduce compaction. Additional tires on a machine increases the total surface contact area and also reduces inflation pressure necessary to carry the axle load. A study comparing compactive effects of single and dual wheels found that duals reduced compaction in the upper part of the soil profile but that the advantage of duals over singles narrowed at greater depths (Figure 5). Duals and triples have the disadvantage of increasing the width of the trafficked area.
The availability of factory and aftermarket rubber track systems for farm machines has greatly expanded in recent years, making them one of the most readily-available equipment options for managing soil compaction. A variety of track options are now available for tractors, combines, grain carts, sprayers, and planters (Figure 6). There are a number of factors to consider in assessing the value of tracks vs. tires, and research has not necessarily shown a clear across-the-board advantage for tracks in mitigating soil compaction under wet conditions.
Tracks generally increase the surface contact area of a load relative to a comparable wheeled configuration, which can help reduce topsoil compaction and formation of ruts. Also, since tracks expand the surface contact area longitudinally within the path of travel, they do so without increasing the area of the field that is trafficked, in contrast to other options such as duals that increase the width of the compacted path. However, it is important to realize that surface contact pressure is not uniform across the entire track area. Rather, a zone of higher pressure is created under each wheel. As a track moves it will create multiple pressure spikes corresponding with each wheel passing over the soil. In that sense, tracks can be thought of as a form of multi-axle configuration – there are more axles carrying the load and the surface contact pressure is reduced, but the soil in the machine path is subjected to repeated pressure applications and greater total load dwelling time (Duiker, 2004).
Table 6. Soil compaction of a four-wheel drive and tracked tractors at different soil depths (Abu-Hamdeh et al., 1995a).
Table 7. Soil compaction (reduction in soil porosity) from a John Deere 9600 combine with various tire and track configurations (Abu-Hamdeh et al., 1995b).
The question of whether tracks provide an advantage over tires in reducing soil compaction is not one that necessarily has a straightforward answer. Research has generally indicated that it depends on the specific tire and track configurations being compared. An Ohio State study comparing compaction down to a depth of 20-inches caused by tracked and wheeled tractors found that the best result for minimizing soil compaction was achieved with duals running at low inflation pressure (Table 6). Another Ohio State study compared half-tracks and four different tire configurations on a combine. This study also showed that tires at a low inflation pressure provided the best results (Table 7). For machines such as sprayers or planters where the tires typically have a higher inflation pressure, tracks would likely provide a greater advantage relative to tires. For the heaviest machines such as combines and grain carts, tracks may provide an advantage in reducing surface compaction and rut formation, but will not eliminate the risk of subsoil compaction associated with heavy axle loads.
Controlling wheel traffic in a field is a tactic available to all growers to help reduce soil compaction. Research shows that 80% of wheel traffic compaction occurs on the first pass, so growers should try to limit the number of trips across field and use the same traffic pattern whenever possible. During harvest, try to follow combine wheel paths as much as possible when running the grain cart rather than cutting diagonally across the field between the combine and the grain trucks. Try to keep grain trucks confined to the edges of fields or out of the fields altogether if possible as the heavy axle loads combined with high inflation pressure road tires can cause significant compaction.
Multiple farm machinery manufacturers are currently developing autonomous farm machinery technology, and it is likely that the first autonomous farm machines will become commercially available in the near future. Initial experimental prototypes and concept vehicles have often resembled current tractors without the need for an operator, and in some cases without a cab or operator controls on the machine at all (Figure 7).
As autonomous technology progresses, however; it may move away from resembling current operator-based vehicles to take advantage of the inherent advantages of the technology – specifically the ability to run more machines simultaneously without additional operators and the ability for a machine to run 24 hours a day. Replacing a single large operator-controlled machine with multiple smaller autonomous machines could provide significant advantages in reducing soil compaction. Additionally, the ability to precisely manage traffic patterns across a field throughout the season could reduce the proportion of the field subject to compaction by concentrating traffic into regular paths.
Harvest photo and CaseIH MX 285 and Magnum 380 photos in Figure 1 are all courtesy of CNH.
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The foregoing is provided for informational use only. Please contact your Pioneer sales professional for information and suggestions specific to your operation. Product performance is variable and depends on many factors such as moisture and heat stress, soil type, management practices and environmental stress as well as disease and pest pressures. Individual results may vary.