Reducing Soil Compaction from Equipment to Enhance Agricultural Sustainability

Abstract

The compaction of agricultural soils cannot be solved, only managed. As a compressible media, soil travel without causing some collapse of the existing structure is impossible. If left uncorrected, farmers can see up to a 50% reduction in yield from long-term compaction. This chapter will describe the effects of soil compaction on the environment, crop quality, and economic sustainability. The base causes will be examined, along with the engineering designs for vehicles that minimize the problem. The tracks versus tires debate will be thoroughly discussed, and the advantages and disadvantages of each system will be detailed. It will be shown that although tires represent the likely current best economic option for vehicle support, the potential of tracks to reduce compaction has been fully exploited. The advantages of four-wheel drive vehicles in reducing soil compaction will be shown, along with the mitigation potential of independently driven wheels and active soil interaction feedback loops. The design of crop production tillage equipment and tillage tool working points will be explored, along with the concept of critical tillage depth. Equipment for compaction relief will also be discussed, as will the sustainable agricultural protocols of cover crops, crop rotation, and controlled traffic farming.

1. Introduction

Since the late 1960s, the agricultural industry has taken an increasing interest in the effects of soil compaction on soil health, agricultural practices, water runoff, and the sustainability of grain production. Compaction results from any practice that includes traveling over the soil. This can be caused by heavy-axle machinery, excessive ground working, livestock, or specific geotechnical practices, such as rolling, which is used to compact the soil in preparation for construction. Repeated soil compaction experiences have cumulative negative effects for agricultural soils, such as a decrease in pore space, reduced pore nutrient and water uptake, denitrification, and enhanced difficulties in seed germination. The effects of compaction also extend beyond agriculture and are of concern to environmental specialists all over the world. For instance, high compaction rates increase the likelihood of water retention issues, water runoff, and erosion. From the last 60 years of research, modern agricultural operations have progressed to incorporate a variety of soil compaction reducing approaches. These approaches include equipment solutions like tracked implements, happy-seeders, and complex multi-crop planters that reduce field traffic. Within the scope of production agriculture, many existing practices unrelated to vehicle design, like no-till seeding, have decreased the impact of soil compaction and help to repair damaged and heavily compacted soils. These design improvements and management practices will be explored in this chapter, and their effectiveness will be measured. This topic is particularly timely and relevant because present-day tractors have increased in size compared to traditional row crop tractors for better productivity and field efficiency. Although most smaller-scale agricultural equipment is used for multiple tasks, the presence of a variety of different off-road vehicles on the market indicates a broad need for various equipment types and provides an opportunity for exploring the existing and potential solutions to soil compaction problems in different off-road vehicle designs. The chapter will proceed with an analysis of how soil compaction is addressed in machine design, as well as new areas that deserve more specific research and improvement. Multiple factors are involved in soil compaction, and multiple designs exist to address these various factors. The present off-road vehicle offerings are clearly less than ideal for long-term soil health. There is a potential for improvement in existing designs to benefit all involved stakeholders, and this potential will be explored.

2. The relevance of soil compaction and its effects on sustainability

Interest in soil compaction dates back to the time when humans started to use draft animals as a main source of power in agriculture. Many authors have addressed the problem since the 19th century. One of the first recognitions of the problem in academic literature dates back to 1857, with a description of the Fowler steam engine-powered plowing system [1]. Draft animals are still being used on vast areas of land in developing countries, and the animal-induced compaction problem continues to this day. The growing use of steam-powered tractors added to soil compaction concerns in the second half of the 19th century. While the mass-power ratio allowed for the widespread use of powerful tractors, the vehicles were still very heavy, and the need to minimize wheel impact on the soil was quickly recognized.

Different engineers have attempted to address the problem in multiple ways. These attempts did not lead to a unified design, but they moved the engineering thinking forward and were instrumental in creating the more successful designs of the 20th century. Between the last decade of the 19th century and 1904, internal combustion engines (ICE) replaced steam engines on tractors in America, and a new era of agriculture began. The better mass-power ratio of the ICE provided for lighter designs and less impact on the soil, but other problems emerged. Mass agriculture meant the more intensive use of fields and more impact on the soil per given period in time. One of the first experiments describing soil compaction problems was run in 1944. The topic has continued to be a strong focus for agricultural researchers. Raney, Edminster, and Allaway conducted the first review of literature on compaction in agricultural soils in America, which included 43 references [2]. The so-called “load index” started to grow at about the same time and exceeded that of the early 20th century by the 1970s. The soil compaction problem continues to be of a major focus of agricultural, industrial, and academic practitioners and researchers. One industrial example is Caterpillar’s efforts to use tracks in agriculture in agriculture to decrease soil compaction [3]. Other modern solutions have attempted to address the problem, and the current review will introduce them to the reader [4, 5, 6].

2.1 Environmental impact of soil compaction

Soil compaction has a measurable influence on the environment, specifically on atmospheric, water, and soil resources. Agricultural operations have a major impact on the atmosphere through the emission of greenhouse gases. Soil “compaction may change the fluxes of these gases from the soil to the atmosphere because of its influence on soil permeability, soil aeration and crop development” [4, p. 8]. Water resources include both surface and ground water volumes. Soil compaction negatively affects the infiltration of different substances into the ground. Ammonia injected into the soil can escape into the atmosphere faster in a compacted soil than in an uncompacted one. Soil compaction also perpetuates the accumulation of rainwater on the surface in low parts of the field and increases the likelihood of runoff events. The latter leads to excessive sediment and chemical transfer into surface ground water resources, such as local rivers, lakes, ponds, and bigger regional natural water reservoirs [5, 7].

Soil biota is responsible for the decomposition of organic matter, release of nutrients and formation of aggregates [8]. Such tasks are performed by microfauna (bacteria, fungi), which are fed upon by meso- and macrofauna (protozoa, nematodes, arthropods) within the soil food web. Figure 1 illustrates some of these various interconnections between the living things in the soil.

Figure 1.

Soil biota species and food web (duiker, 2005) – [8].

Soil compaction creates a rearrangement of soil particles that leads to a reduction of void space, a phenomenon that can be measured in several different ways. At first glance, there are visual and tactile methods that can provide a quick assessment, but to quantify the effects of soil compaction, physical parameters must be measured. Direct and indirect measures are used together to enable a deeper understanding of the characteristics of the total volume of the soil, such as bulk density (direct), soil strength (indirect), soil electrical resistivity (indirect) and water infiltration rate (indirect). Figure 2 shows two examples of soil profiles exhibiting compaction effects. The soil on the left has a better structure above and below the compacted layer located between 10 cm and 40 cm depth [9]. On the right, a compacted layer in wetland creates a toxic environment for roots and soil biota. Soil compaction effects vary by location based on multiple interconnected factors, making a comprehensive assessment of specific fields the key to securing the sustainability of any agricultural operation over time.

Figure 2.

A compacted layer under dryland canola (left), and a gray anaerobic layer in a clay loam soil (right) (Nawaz et al., 2013) – [9].

2.2 Effects on harvest quality and farmlands

Soil compaction has a directly visible effect on the crop that is being grown in the degraded area. As soil compacts, it reaches a point of root growth restriction that is highly detrimental to both the quality and health of plants, as well as the quantity of the cultivated crop yield [10]. The lack of loose soil aggregates prevents strong root formations. This leaves crops more susceptible to wind and water damage. There is reduced nutrient uptake, since the root mass of the plant is diminished in both absorption volume, as well as effectiveness. Individual plants are less healthy and produce significantly less grain and forage mass. Perennial crops, like many fruit plants, stop root growth when confronted with significant compaction. Beyond the lack of void space, nutrient uptake in compacted topsoil is greatly reduced as the biological health of the soil diminishes [11]. Crops growing in densified soils can be expected to be brittle, due to the reduced nutrient intake as soil compaction causes reduced aerobic microbial activity and denitrification [12]. Soil compaction has a progressively negative effect on the biosphere. As the soil is compacted and continuously depleted, natural vegetation, such as weeds and grasses, quickly gets restricted from lack of soil aeration. The crushing of the soil and diminishing amount of additional biomass that would otherwise be introduced into the soil can eventually lead to an elimination of plant life causing an open and exposed soil surface. Soil in this condition is more easily impacted by wind and water erosion. If preventative measures are not taken, the effects of soil compaction on crop quality and farmland are cumulative, can take place quickly, and have lasting damage [13].

As shown by a review of the soil compaction literature [14], studies detailing the continuing long-term effect of compaction on a specific piece of ground are rare. However, as shown in Figure 3, it can take years for soil to naturally recover following a single compaction event [15]. Studies in cotton, as displayed in Figure 4, show a significant decline in crop yield within the initial season of the compaction event [16]. Since the effects of compaction are cumulative and continue from one season on into the next, it can be inferred that the decline from unmitigated soil compaction will continue to grow and magnify under the same management processes. Figure 5 presents the general effect over time on production costs and gross margin of the farming operation [14].

Figure 3.

Yield recovery following a significant compaction event (Voorhees, 1986) – [15].

Figure 4.

Difference in same year cotton yield between compacted and uncompacted ground (Jamail et al., 2021) – [16].

Figure 5.

The generalized trends for production cost and gross margin for avoided compaction, relieved compaction, and compacted soils in production (Chamen, 2015) – [17].

2.3 Social and economic impact

Soil compaction has a negative impact on the economy of agricultural operations in the long-term. Soil compaction decreases the quantity and quality of harvest. Continuous and unaddressed soil compaction does not allow soil to sustainably recover through natural means [17, 18]. This affects the local food security in the regions where the traditional economy relies on agriculture. The local quality of life and general economic health of an agricultural region is adversely affected when local soils become compacted. Multiple potential solutions can help minimize the impact. Some come from farmer experience and depend on the operator in the field. Others are industry-wide, general practices. Academic researchers model the problem by studying economic impact. These models provide for better forecasting, equipment selection, and targeted problem solving. One model suggests that in the short-term, the negative impacts of soil compaction can be compensated by “more timely field operations.” Profits and productivity generate energy costs, air pollution, capital costs, timeliness costs, and soil erosion, which are also evaluated [19]. Other studies address the problem through even more sophisticated modeling. Additional effort is needed to standardize the corporate and academic researchers’ efforts to improve these models for specific predictive tasks. Soil compaction models have the potential to help the businesses and governments to develop advanced solutions for real-world agricultural problems through an improved understanding of the social and economic impacts.

The extent of the economic effect of soil compaction is difficult to quantify as it is ultimately very situational. Under circumstances where soil requires additional operation to alleviate the effects of long-term compaction, the cost of crop production rapidly increases to unviability. Soil health does not always deteriorate to the point where intervention is required, but this does not mean that these farming operations are unaffected. The most common issue caused by soil compaction is the decrease in crop productivity. Figure 6 below summarizes the impact soil compaction has on soil and crop health [20]. Reduction in plant growth and development, such as biomass accumulation, stomatal photosynthesis, and poor proliferation, as well as poor nutrient and water uptake decrease yield and overall crop productivity. Figure 7 depicts the impact on potato yield resulting from varying irrigation levels [21]. This graph shows the availability of adequate water can increase yield by at least 100%. Because soil compaction so negatively impacts water availability and uptake, the conclusion can be drawn that compaction issues can decrease crop yield and productivity by up to 50%. In short, this also means that farmers risk losing 50% of expected profits, when the soil compaction problems are not properly addressed. Inattention to this vital issue in land management can destroy the resource’s ability to be productive both now and in the future. It is imperative that farm managers understand the connection between management of soil compaction today and the long-term sustainability of the agricultural ground into the future.

Figure 6.

Summary of the knowledge of the effects of soil compaction on soil plant morphological and physiological growth and soil properties (Shah et al., 2017) – [20].

Figure 7.

Potato yield at different irrigation levels for subsoil and control fields (Ghosh & Daigh, 2020) – [21].

3. The causes of soil compaction

Soil compaction is the phenomenon associated with the collapse of soil media to support the loads imposed upon it. All agricultural operations on the surface of the ground cause soil compaction. Heavy axle loads, wet soil operations, livestock grazing, and materials stored directly on the surface can all result in unwanted compaction. The details of these agricultural process root causes of soil compaction will be explored in this section.

3.1 Operation of equipment with heavy axle loads

An axle load is the total load supported by a single axle, usually across two points of contact on either side of the vehicle. Although most agricultural equipment uses two axles for load distribution, each point of contact carries harmful loads into the soil. A large agricultural vehicle weighing 20 ton, creates 10 ton of force on each axle and causes the soil beneath each point to compact, until it can support the imposed load. The biggest factor to consider in reducing soil compaction is large axle loads. For two vehicles with the same weight distribution, the bigger the vehicle’s contact area with soil, the lesser the pressure is applied to the soil surface. Figure 28 illustrates an advantage of tracks over tires by the contact area parameter [22]. Research has shown that having an axle load of 10 ton can cause deep (more than 45 cm) subsoil compaction under moist conditions [8]. Grain carts and other heavy trailing implements behind the power units add to the problem of soil compaction, since axle load is determined by the total weight of the vehicle divided by the number of axles. Reducing single axle loads below five ton or less will diminish subsoil compaction, and only cause topsoil compaction [8]. Using heavy machinery under wet or moist conditions always increases soil compaction dramatically over use under dry conditions for most soil types [23]. The relationship among pressure applied, water content and bulk density varies across different soil types as particles rearrange with changing water contents [24].

3.2 Operation during non-optimal soil conditions

Under non-optimal soil conditions, field farm operations should be considered with great reluctance, due to the potential for severe damage to the soil matrix. As farm equipment crosses through a wet field, ruts are formed from soil compaction around the tire path. Tillage is a common practice to relieve soil compaction due to poor soil management. However, tilling breaks-apart the soil structure and causes further traffic, in addition to deeper compaction in the field. A tilled soil is more easily compacted, since the subsoil beneath the tillage line is now in a more vulnerable state for soil compaction [25]. Under good soil conditions, the integrity of the soil is reasonably strong and minimizes the loss of pore space from heavy equipment travel. When soil conditions are non-optimal, the structural integrity of the soil is significantly reduced, and this results in the elimination of pore space with vehicle traffic. As shown in Figure 29, when the same pressure is applied in a loam soil, the bulk density significantly increases with increasing soil water content, thus, leaving the soil susceptible to compaction [24]. Additionally, water within the soil matrix reduces the coefficient of friction between neighboring soil particles and promotes the ease of displacement and flowability of the soil.

3.3 Livestock grazing

Livestock grazing can affect soil stability and functionality if not managed properly. The severity of soil damage due to livestock grazing is related to the soil type, texture, and moisture content. Pugging, the formation of soil around the hoof of the livestock, can result in increased soil compaction and a reduction in soil surface water infiltration rates [26]. When water does not infiltrate through the soil surface during rainfall or irrigation, puddling occurs in fields. The trampling and pugging from livestock onto soil surfaces damages the subsurface soil integrity. The density of the livestock per unit of area in a pasture impacts the level of soil compaction due to pugging. This effect also negates the value of winter grazing on crop land to glean harvest losses. The long-term damage from soil compaction to the crop ground greatly outweighs the value of the “free” feed gained.

3.4 Other

Aside from intensive farming and grazing practices common in modern agriculture, there are other factors, some environmental and some man-made, that can have a noticeable effect on soil compaction. Depending on the region of agricultural production, the type of soils, as well as natural and artificial drainage, some fields can be subject to prolonged ponding of water in localized areas. Over time, the weight of the water ponded on the soil surface causes the soil pores to collapse further, slowing the movement of water through the soil and increasing the weight of water on top of the soil surface during future precipitation events. Water ponded on the soil surface adds 10 kPa of pressure per m of depth. Additionally, slowed water movement through the soil increases the risk of farming operations occurring during non-optimal soil conditions. Another non-conventional contribution to soil compaction is the relatively new practice of storing grain in large plastic bags that are laid-out on the soil surface. Producers using this method of temporary grain storage have noted significant soil compaction on the surface due to the weight of the grain.

4. Off-road vehicle designs for soil compaction management

Agricultural tractive power units are the largest source of unwanted soil compaction today. Significant research and financial investment have been made in methodologies to reduce the compaction from these vehicles. Tracked systems and advanced tire systems are both designed to spread the loads imposed upon the soil below detrimental levels. This section will review these common undercarriage systems, along with advanced compaction reduction technologies for off-road vehicles.

4.1 Tracks

Commercially successful track-type vehicles, which were recognized under the trademark name Caterpillar, began production in the early 1900s [27]. These early agricultural tractors, similar to the one shown in Figure 8, paved the way for future tracked vehicles and the continued use of the more complex metal grouser style tracks on construction equipment. Tracks did not remain popular in the agricultural sector, once pneumatic tires became available. They faded from use for many decades due to some specific issues that later rubber-belted machines were finally able to address.

While construction equipment is traditionally shipped to a worksite on a large trailer, tractors are generally driven from field to field on the road. Track-type machines with metal grousers are slower than pneumatic-tired machines during road transport under their own power. This slower transport speed, combined with a poorer ride for the operator and higher costs, eliminated most traditional track-type tractors from the agricultural market during the 1920s and 1930s. Two revolutionary designs, which are still produced by major manufacturers today, reintroduced the use of tracks on tractors. In 1986, Caterpillar launched the revolutionary rubber track Challenger 65® tractor, shown in Figure 9 [3]. The Challenger used a two-track running gear system, similar to most construction equipment designs [28]. Shortly thereafter, Case IH introduced an articulated tractor with tracks at each corner of the machine. The Quadtrac®, shown in Figure 10, was configured like a traditional four-wheel drive tractor, with each of the contact points supported using a triangular track drive and bogie mechanism [29].

Today, rubber-belted tracks have become so successful that a common argument in the agricultural world is the debate of tracks vs. tires. However, opting for a tracked configuration creates a sizable increase in both purchasing and operating costs for the tractor. The price jump from tires to tracks can often be in the neighborhood of 10–25% of the cost of the machine. The operating costs jump too. As can be seen in Figure 11, the specific operational cost difference between tracks and tires for a 358 kW tractor is approximately $0.085/kWh [29]. Currently, available data and existing published studies seem to support both sides of the tracks versus tires debate.

A 2018 European study involving a comparison of tracks and tires on two identical sugar beet harvesters revealed that the use of tracks does have a positive impact on reducing soil compaction [31]. Stress transducers were placed under the soil to analyze the compaction effects of the tractive devices. The mean ground pressure for the tire undercarriage system was measured to be 107 kPa, while the rubber tracks had a mean ground pressure of 84 kPa. As shown in Figure 12, ground pressure for the tires was also more concentrated, and it was more distributed under the tracks [31].

Because of their larger footprint, rubber tracks are often assumed to have a uniform weight distribution, but this is not true. Multiple design elements in a rubber track system, along with the integration of the track system onto the vehicle’s frame, are critical to its effectiveness at reducing soil compaction. Common track systems, as shown in Figure 13, are traditionally composed of large driver and end wheels and smaller bogie wheels [29]. Bogie wheels, in theory, help to distribute the half the axle weight across the track’s contact surface with the ground. However, in reality, the bogie wheels create ground pressure spikes beneath their relative positions. As can be seen in the right graph of Figure 12, the individual soil pressure peaks can be attributed to the bogies and wheels of the tracks evaluated in the study. The ideal performance of a track can be identified by finding the theoretical applied ground pressure stress. This calculation is performed by dividing the load on the track by its contact area. In the sugar beet harvester study comparing tracks versus tires, the researchers discovered that the peak stress applied to the ground by the tracks was 5.7 times greater than the ideal ground pressure calculated value [31].

An analysis of the soil types across Europe was conducted to evaluate the maximum load capacity of different tractive devices, without causing permanent soil deformation [31]. Figures 14–16 convey this analysis, showing soil types and the respective loads that can be handled by tires, tracks, and ideal tracks having a uniform pressure distribution. It is worth noting that a substantial load bearing increase could be achieved through improved track design.

Regardless of which side of the track versus tires argument is seen as the correct economic option, tracks do serve utility for farmers beyond that of tires. Although farmers prefer to be in the field when conditions are good, the weather does pose challenges. Depending on the geographic location of a farm and its soil type, it is common to deal with wet field conditions. As a result of the need to beat seasonal weather patterns, farmers often push acceptable limits to finish critical tasks in the field. Saturated soils are easier to tackle with tracks, because of their improved tractive performance over tires. Tracks are also less prone to rutting the soil in wet conditions. As seen in Figure 10, a side-by-side comparison of a tracked and tired machine shows the improved performance of tracks at staying on top of the soil. As can be seen, the track’s footprint barely marks the ground, where the trailing tire cuts a deep rut. Severe soil compaction, like that caused by the tires in Figure 17, negatively impacts the health and performance of a farm’s soil for the long-term [32]. However, the inability to harvest crops negatively impacts the economics and viability of a farm’s business today. Although tracks are pricier than tires and may only provide limited benefits toward economically reducing soil compaction, tracks clearly outperform tires in adverse conditions.

4.2 Low inflation tires

Although the European study concluded that the use of tracks had a positive impact on soil compaction, differing studies have led to opposite conclusions [31]. The argument for tires is that correct maintenance needs to be performed to ensure that the tires are inflated correctly. A common issue is that farmers will over-inflate tires. Studies, like the one highlighted in Figure 18, reveal that incorrectly inflated tires create the most compaction [33]. In this specific experiment, correctly inflated dual tires were found to be impressively less compacting to the soil than tracks, while demonstrating that tracks could be superior to poorly maintained dual tires.

Tracks are undeniably an expensive, but great option for reducing soil compaction. However, low-pressure, properly-inflated tires are a potential option to match the benefits of tracks, at a fraction of the price. It is well documented that the depth of soil compaction is strongly correlated with tire pressure. Lower pressures cause less compaction. The limit to this practical method of reducing compaction is that the bead of the driving tires must remain on the rim. This low-pressure tire strategy helps reduce soil compaction by increasing the tire surface contact area with the ground. The increased contact area reduces the pressure exerted on the ground. Due to the limitations of decreasing the air pressure in traditional radial tires, tire companies have developed new flexion technology to allow even lower tire pressures. Increased Flexion (IF) and Very Increased Flexion (VF) tires, first introduced by Michelin during the 2000s, use a mature technology that greatly decreases the soil compaction from today’s heavy machinery. The VF and IF tires can support the same loads with 40% and 20% less air pressure than radial tires, respectively by using increased tire sidewall strength [34].

While tires do not have as extensive of a surface area as most track designs, low pressure and Flexion-style tires make-up for some of the ground pressure shortcomings on tired vehicles, and in some applications, they can be a more viable option. Tracked vehicles experience pressure spikes at each bogie, whereas tires can be more consistent in the application of load to the soil [35]. Modern row crop tractors are commonly seen with dual rear tires and even dual front tires. As new equipment becomes larger and larger, single tires are no longer viable. As the demand for Modified Front Wheel Drive (MFWD) tractors has increased, additional weight has been added to the machines, requiring further soil compaction reduction methodologies to be undertaken. The most common MFWD tractors variants today have both dual front and rear tires. As would be expected with the addition of a second set of tires, soil compaction is reduced. This is achieved by essentially doubling the contact surface area [36]. The addition of a second set of tires allows for tire pressure to be reduced even further, also decreasing the potential for soil compaction [36]. These strategies can be combined for reasonably additive results. Using Flexion-type dual tires at low tire pressures can achieve even lower soil compaction. Under certain circumstances, properly inflated duals have been shown to be more effective at reducing soil compaction than tracks. Triple tires can be seen in certain high-power applications. However, they are not commonly seen in modern agriculture. The increased width of the tractor would be a benefit in the field, but transport on the road becomes infinitely more challenging. Axle stresses multiply significantly as well.

A recent innovation in agricultural tractor tires involves changing the overall design of the tires and the rim. New Low Side Wall (LSW) tires feature a significantly reduced tire aspect ratio, which results in a wider tire with reduced side walls. These LSW tires are intended to completely replace duals on modern farm equipment. While these new tires are a more expensive initial investment, including a completely different rim and new tires, they are still cheaper than modern tracked systems. LSW tires could be a viable option for reducing soil compaction, as well as providing other benefits to the operator [37]. Tractors with a high center of gravity and LSW tires can experience reduced sway in motion, as well as better resistance to power hop [37]. LSW tires have a larger width allowing for more surface contact with the soil, as well as retaining the reduced inflation pressures similar to the Flexion-style tires [37].

Tires are typically the more attractive option for most farms, due to lower purchase and operational costs. Since the potential benefit of tracks is only for the reduction of soil compaction, tire systems, which can effectively compete with tracks in this metric, have a competitive advantage. With LSW tires closing the marginal difference in performance between tracks and tires, tires in many standard applications may be the smarter option. However, any option to reduce soil compaction will pay-out in the long run for farmers and growers. Conservation of the world’s natural resources is imperative for the continued survival of humanity, especially with the extreme population growth projected for the next fifty years. Producing more food with less resource inputs is the goal of all of agriculture. Conserving the land is the first step toward a better tomorrow that will continue to be able to feed its people from the soil.

4.3 Two versus four wheel drives

Four-wheel drive vehicles can produce less soil compaction than their two-wheel drive counterparts, assuming all other factors are equal. Four-wheel drive systems also encounter less slip in motion and have a more optimal weight distribution, which likewise helps reduce the soil compaction. Slip can be thought of as a horizontal component of soil compaction. Slip occurs as the soil behind the tire compacts to support the drawbar load. There is a shrinkage in the matrix of the soil [38]. When traveling off-road, all vehicles have some amount of slip. This slip is determined by the interface between the wheels and the ground. The larger the ground contact area, the less slip occurs. Four-wheel drive vehicles have less slip than two-wheel drive vehicles. While two-wheel drive vehicles may have the same number of wheels on the ground, the non-driving wheels do not provide any traction. Tracked vehicles have an advantage as the entire length of the tracks are driven, and therefore, they have reduced slip when compared to tires. Nonetheless, slip sufficient to support the forward travel and drawbar loads on the machine still occurs. The reduction in soil compaction behind four-wheel drive vehicles has been demonstrated experimentally. Figure 19 shows that the bulk density of soil was found to be 5.6% less than in rear wheel drive and 7.3% less than in front wheel drive vehicles [39].

From a practical perspective, four-wheel drive and two-wheel drive vehicles are built differently. Four-wheel drive vehicles are designed to have a different weight distribution. A rear-wheel drive vehicle has the center of mass at roughly one-third of the wheel base forward of the rear axle. A four-wheel drive vehicle has the center of mass located slightly more forward. This is advantageous, because the tractive force from the wheel depends on the normal force with the ground. Under drawbar load, the front and rear ends of the tractor are supported more equally, and the peak pressure on the ground is lower. Larger wheels have a higher area of contact with the ground, which results in lowered soil compaction. Many four-wheel drive vehicles have an articulated chassis used for steering. An articulated vehicle’s axles follow only a single pathway when turning, which also reduces the area of compaction.

Just as certain soils are more prone to soil compaction, some soil types benefit more from four-wheel drive tractors. It is more difficult to gain traction in loose soil. As Figure 20 shows, the moisture content in the soil plays a significant role in the compaction tendency of the soil [39]. Soils with a greater moisture content typically generate more slip [41]. As discussed earlier, slip is correlated with soil compaction. “Tire travel” will be significantly more in wet soil to cover the same distance.

4.4 Sensors, actuators, and special applications

Mechatronic agricultural systems are the future of agricultural machinery. One proposed means to reduce soil compaction is to utilize numerous smaller robotic machines, instead of progressively larger machines, to tend the fields. One limitation to further development along these lines is the price of a fleet of machines, while another is the human management factor. The price will likely come down as the technologies develop, but the human factor will remain stagnant until a “critical mass” of the new equipment enters the agricultural equipment market and demonstrates viability. These modern agricultural mechatronic systems will contain numerous sensors and actuators as their essential elements. Actuators perform the specific tasks directed by the vehicle’s controller. Sensors facilitate the feedback from the actuators to the tractor’s control system. The feedback data works as a performance measure for the actuators and the control system as whole. Specifically, the control system receives data on the success of the actuators’ actions, the vehicle’ position and motion, and the vehicle’s immediate environment. Driveline control systems with the feedback mechanisms can successfully address soil compaction problems in many special applications. These automatic control systems have potential for use in the envisioned swarm systems for everyday agricultural operations. A swarm of smaller automated machines could become a disruptive technology, which would shift the paradigm in the current soil compaction reduction practices for crop production systems. The utilization of real-time feedback from soil conditions has multiple previous implementations for experience to be drawn from.

One example is a unique tractor for special climates. The Gidrokhod 49,061 (“Gidro” – hydro “khod” – traveler), shown in Figure 21, is “a three-axle all-wheel drive machine with a hydrostatic driveline with an automatic control system” [42], p. 147], which combines an individually driven axle design with a feedback-based approach to vehicle control. The Gidrokhod’s driveline operates as follows: “[The] driveline is a full-flow mechanism that includes three axial-plunger controllable reversible and invertible hydraulic pumps and six axial-piston controllable and invertible hydraulic motors. Each pump is associated with two tandem hydraulic motors that set into motion the wheels of one hypothetical axle. The torques and rotational speed of the hydraulic motors are controlled individually by varying the displacements of the pumps and motors by means of an automatic control system” [42], p. 147]. The Gidrokhod’s automatic control system supplies the required power to each wheel “as a function of the current conditions of interaction with the soil” [42], p. 147]. Gidrokhod was originally designed to reduce soil compaction from human activity in tundra, where the plants and soil are particularly vulnerable to any soil loading, such as the pressure from tracks or tires. The Gidrokhod’s hydrostatic driveline also improves the vehicle’s off-road drivability by dampening returned ground shocks into the driveline. The Gidrokhod’s hydrostatic driveline is a computer-controllable, tested technology that could be transferred to off-road vehicle applications in agriculture to address the soil compaction problem.

Another example of a special off-road vehicle is a small off-world exploratory rover. These machines closely resemble hypothetical swarm agricultural vehicles and are essentially miniature space tractors. The pace of modern technology suggests that humanity will start colonizing the Moon and Mars by the mid-21st century. The comparison between an off-world exploratory robot and a small, swarm agricultural robot tractor is not as outlandish as it might first appear. The sensory apparatus necessary for independent wheel suspension control works as well minimizing soil compaction as navigating unknown terrain. The external manipulators resemble plant tending tools, and the ability to remain on-station and function unmanned is similar. It is conceivable that low-compaction inducing swarm agricultural tractors may look a great deal like our exploratory robots.

Russia was the first to send a rover to the Moon and Mars. While the Mars mission was a failure, the Soviet Moon exploration program laid the foundation for the future robotized space exploration missions. Over 50 years ago, in 1970, the Soviet Lunokhod 1 (“Luna” – Moon), shown in Figure 22, had a number of soil compaction sensors, a special wheel to measure traction, and single independent drives on each of its eight wheels for improved mobility [43]. As of now, multiple Mars rovers from the United States and one from China are traveling on the surface of Mars. Modern US rovers, like Perseverance, shown in Figure 23 [44], combine the essential soil compaction sensors with the sophisticated modern drivetrain solutions, such as an advanced feedback loop from a complex sensor network, photo and video surveillance systems, as well as the use of Big Data concepts to better predict the ambient soil conditions and any possible action protocols during deployment and moving between operation areas.

The off-world researchers operating these rovers build terrain models on Earth, using transmitted data from the operating rovers, which has a 5 to 20 min signal delay. They use location information from satellites circling Mars, just like farmers on Earth do for agricultural production. Similar to how military location technologies came into the consumer world, space-based technologies will eventually find their best-use applications on Earth. One particular technology transfer path will be for the highly-accurate location technologies needed to control small robotic, low soil compaction-inducing vehicles for agricultural production. The first half of the 21st century will continue to see increasing technology transfer from electronics and space industry into everyday agricultural operations.

5. Soil compaction management in agriculture

Although the chassis and undercarriage design of tractors, combines, and harvesters is of obvious concern to engineers when trying to decrease soil compaction, agriculturalists have developed a variety of other methods and practices that contribute to the alleviation of soil compaction impact. These conservation tillage practices and alternative process design considerations are an important element in overall soil compaction reduction, as they can be applied to any and all farming operations, even those that do not have the most up-to-date equipment. Farm management practices can have a profound impact on reducing soil compaction, as well as maintaining soil organic content, reducing nutrient suppression, and decreasing time and energy spent in the field. From a sustainability standpoint, these conservation practices may even be more impactful for farm and field management at reducing soil compaction than any specific tractor or undercarriage design.

5.1 Tillage equipment and practices

The design of tillage equipment is an important and fruitful area of research for reducing soil compaction during the necessary ground-working operations. Tillage equipment design affects the ways in which tillage equipment interacts with the soil to help to alleviate long-term effects of disturbance in heavily-worked ground. Some of the core ideas within tillage implement design are load distribution, working point and shank design, working depth, the different types of soil disturbance, and the soil pulverization level. Most modern research is targeted at collecting specific information about the impacts of these conditions on soil health, compaction levels, and seed bed preparation, as well as energy use and the time spent in the field. The implications of tool design, structural loading, the types of conservation and reclamation equipment, and the impacts of soil compaction management on the energy consumption and overall performance of an agricultural venture will be examined in this section.

5.1.1 Tool design

In early agricultural practices, the moldboard plow dominated tillage as the most effective tool for turning the soil to create a seedbed. Its design was maintained in many forms of tillage equipment for years before its harmful impact on soil health, organic material, and erosion was realized. In contrast to the simplistic design of the moldboard plow, modern tillage equipment tool designs come in many shapes and sizes. The effects of tool geometry, orientation, depth, field speed, and other factors impact the level of soil disturbance and compaction. Various tool types can create a multitude of different outcomes in the upper soil layers in terms of soil aggregate size, topsoil density, porosity, and organic matter distribution. Other tools act predominately at the sub-surface level. In particular, deep cutting tines have the greatest impact on sub-soil compaction. Their shape, working depth, and spacing all affect the resulting soil compaction differently.

The effects of specific tine geometry and individual tine orientation were explored by researchers using finite element analysis (FEA) modeling [45]. Figure 24 depicts the range of geometric variation explored, including the alteration of tine width, rake angle, and tilt angle. The primary results from this study concluded that at comparable field speeds, the increase in tine width linearly increased the resulting downward vertical force, while increasing rake or tilt angle linearly decreased the downward vertical force [45]. The implications of this study affect tractor power sizing, the uniformity of transmitted force along a vertical soil profile, the soil pulverization level, and the subsoil compaction. Most certainly, the results also present a variety of design trade-offs, depending on the immediate and long-term priorities of the specific farm manager. However, from the standpoint of reducing compaction whilst maximizing surface soil pulverization, minimizing the tine width and maximizing the tilt and rake angles create the least amount of sub-surface compaction.

It is important to note that the tilt angles can be non-uniform both on individual tines and on the overall tine set-up for an entire tillage unit. Many times, a compromise between minimizing draft forces, decreasing compaction, and managing soil upheaval can be achieved by applying a diverse range of different geometric and orientation values throughout a single tillage implement [46]. This becomes even more applicable the larger the implement is, due to the increased number of rows and columns of working points. Besides the considerations outlined above, two other vital components of tillage implement design are tool spacing and working depth in relation to the “critical depth”. Critical depth is generally considered to be the point below which soil disturbances are concentrated near the working point and not distributed throughout the soil. Figure 25 shows both the effects from operating below a critical depth and the dramatic increase in soil compaction as a result of tillage below this level [46].

Unfortunately, critical depth is not uniform by any means. It varies significantly with multiple variables, and it can be heavily impacted by moisture level, soil type, and the presence of a cover crop. This makes determining an operational depth a challenging task, particularly for inexperienced operators. Often initial passes are needed to estimate ideal working depths. There has been some research done regarding the use of strain gauges on subsoiler tines in conjunction with depth sensors, which can utilize a closed loop response system automatically adjusting height to maintain the desired draft and vertical forces [47]. These systems still require a degree of experience and skill to determine the expected shank loading at, above, and below the critical depth, in order to set the necessary system limits prior to operation. Although the practical difficulties with feedback-based systems are numerous, increased implementation of the above described depth adjustment mechanisms will provide a wealth of data regarding forces at and around critical depth. This information will only make these systems more effective in the future [48]. Figure 26 illustrates the effects of tine spacing on overall soil disturbance. When tine spacing exceeds 1.5 to 2.0 times the working depth, an interesting phenomenon takes place, where the soil disturbance only occurs locally and results in a non-uniform subsoil profile and soil surface [46].

This outcome is likely to be troublesome for planting, as different row unit depth wheels will be penetrating the surface soil to different depths. The lack of consistency in seed depth, because of this poorly prepared seedbed, will result in emergence and germination issues. Although not initially obvious, the lack of uniform soil disturbance also affects compaction levels. Firstly, the lack of a uniform soil disturbance cross-section that occurs when using widely-spaced tines, illustrated in Figure 19-a, results in some subsoil being undisturbed. This soil remains compacted over time. When using a tillage implement with a wider tine set-up, it is easy for an operator to exceed the critical depth in order to achieve a cleaner surface profile, but in doing so, the subsoil compaction has been increased throughout the field. Using a narrow tine design dramatically decreases the chances that an operator will need to exceed critical depth in order to achieve the desired seed bed quality.

5.1.2 Structural loading

While magnitude of downward vertical force for tillage equipment simply does not compare to tractor units, it is still important to consider how the soil reacts with the implement loading as it moves through the field and what factors play into determining the optimal number of tines and the structure of tillage equipment. There are three primary ways in which soil reacts to the loads and forces placed on it by cultivation implement: brittle loosening disturbance, compressive disturbance, and tensile disturbance [43]. Brittle loosening occurs when the implement load compresses the soil and causes a sliding or slipping during the operation. The effects of the sliding and slipping are such that the soil aggregates, clumps, and masses move relative to one another. The overall volume of soil masses is increased, cracked, and spread-out. Contrary to compressive disturbances, a large quantity of the soil is actually decompressed or loosened as a result of brittle loosening. This is the kind of soil response that occurs primarily under ideal loading and working depth conditions.

Compressive disturbance also occurs under compressive loading, but without the exposure to masses sliding relative to one another. In this case, without sliding, the soil is more likely to experience high degrees of compression and increases in density. This process is more common using heavier implements, when there is a low draft force. Tensile disturbance is virtually the same as brittle loosening and has similar results, such as decreased density and alleviated compaction. The difference lies in the fact that tensile disturbance occurs when soil aggregates are pulled-away from one another and forced to spread-out. This kind of disturbance is more likely to occur under high moisture conditions, where the load is cushioned and absorbed to a greater extent, thus negating the compressive impact of the load.

Each of the three kinds of soil matrix disturbances can be modified and impacted by the working depth, operation speed, and the weight of the implement. Table 1 provides the basic tendencies for determining the design of the implement, based on the power of the tractor unit, and for potentially determining necessary engine power or anticipated working depth, based on the tine and structural design of the tillage implement and its working points. Table 1 can be used for reclamation projects in which the soil has experienced long-term compaction and where aggressive subsoiler action is needed to prepare the soil for further tillage and planting preparation [46].

Tractor size		Capability
Engine power (hp/kW)	Ballasted weight (tonnes)	Working depth range (cm)	Number of tines
30/23	1.5	20–30	1
60/45	3.0	30–40	1
75/56	3.75	35–45	1
		25–30	2
100/75	5.0	40–50	1
		30–35	2
		25–30	3
125/95	6.25	45–55	1
		35–40	2
		25–30	3
150/110	7.5	50–60	1
		35–45	2
		30–35	3
		25–30	4
200/150	10.0	40–50	2
		35–40	3
		30–35	4
		25–30	5
250/185	12.5	45–55	2
		40–45	3
		35–40	4
		30–35	5
		25–30	6

Table 1.

Wheeled tractor capability for operating loosening tines in compacted soil (Spoor, 2006) – [46].

For crawler tractor in same horsepower range, increase number of tines by 50% or working depth by 20%.

5.1.3 Soil loosening equipment

There is a big difference between common tillage equipment used for routine crop cultivation, associated with planting and harvest, and machinery used to rejuvenate the soil from excess compaction. Robust subsoilers are utilized when efforts are made to restore long-term compacted soil. These subsoilers must be capable of decreasing soil density and effectively disturbing the mid-subsoil level to make the land workable under normal cultivation protocols. As seen in Figure 27, these reclamation subsoilers typically utilize a three-point hitch attachment for depth adjustment, rather than a drawbar attachment and trailing configuration [46]. One issue with these subsoilers is the need to operate below the critical depth to create an adequate soil disturbance to restore the soil profile. Unfortunately, this process can cause further, deeper subsurface soil compaction, despite alleviating the compaction in the upper subsurface soil levels.

5.2 Controlled traffic farming

Since compaction is inevitable in agricultural operations, its minimization through operational management is critical to long-term sustainability. The essential principles of compaction management are the reduction of both tillage and field traffic. Modern best practices decrease these elements in crop production processes to the smallest feasible levels. Compaction mitigation techniques are reviewed in this subsection.

5.2.1 Low-till

The first management practice that can be used to reduce soil compaction is the low-tilling method. There are several aspects to low-till that help reduce erosion and soil compaction collectively. Low-till involves planting with a seed drill after a minimally-disturbing tillage operation. The soil is not as exposed to and penetrated by wind and water under this protocol. Low-till keeps an estimated 30% minimum of crop residue on the soil surface. This allows for more organic material to remain present in the topsoil, increasing the soil stability [45]. With low-till, water erosion is inhibited, due to the higher surface trash coverage and lower general depth of water penetration. Low-till farming protocols are an extremely popular choice currently, creating a nice compromise between conventional agricultural practices and more extreme conservation processes.

5.2.2 No-till

No-till farming is extremely effective at helping soil health in multiple different ways. With this method, only the soil surrounding the seed trench is tilled by the row crop planter. No additional tillage operations are performed. Besides being extremely cost effective in fuel consumption, no-till operations have very quick positive results, when compared to other methods. In as short as 2–5 years, soil compaction will naturally be reduced in the topsoil, as microorganisms and organic material increase and expand in the soil. The increased biomass will have a longer lasting effect, as the crushing strength of the soil will be dramatically increased. In clay soils, these results may be more pronounced. Compacted clay soils create the tightest restriction of all soil types. Allowing for root penetration and added biomass expands clay soil until it is much less compactable. With the no-till method, the higher vegetative density alone can help absorb impact from smaller implements. With the proper planting equipment, the no-till method is a very simple and effective method for reducing and reversing soil compaction [49].

6. The causes of soil compaction

Soil compaction is the phenomenon associated with the collapse of soil media to support the loads imposed upon it. All agricultural operations on the surface of the ground cause soil compaction. Heavy axle loads, wet soil operations, livestock grazing, and materials stored directly on the surface can all result in unwanted compaction. The details of these agricultural process root causes of soil compaction will be explored in this section.

6.1 Operation of equipment with heavy axle loads

An axle load is the total load supported by a single axle, usually across two points of contact on either side of the vehicle. Although most agricultural equipment uses two axles for load distribution, each point of contact carries harmful loads into the soil. A large agricultural vehicle weighing 20 ton, creates 10 ton of force on each axle and causes the soil beneath each point to compact, until it can support the imposed load. The biggest factor to consider in reducing soil compaction is large axle loads. For two vehicles with the same weight distribution, the bigger the vehicle’s contact area with soil, the lesser the pressure is applied to the soil surface. Figure 28 illustrates an advantage of tracks over tires by the contact area parameter [22]. Research has shown that having an axle load of 10 ton can cause deep (more than 45 cm) subsoil compaction under moist conditions [8]. Grain carts and other heavy trailing implements behind the power units add to the problem of soil compaction, since axle load is determined by the total weight of the vehicle divided by the number of axles. Reducing single axle loads below five ton or less will diminish subsoil compaction, and only cause topsoil compaction [8]. Using heavy machinery under wet or moist conditions always increases soil compaction dramatically over use under dry conditions for most soil types [23]. The relationship among pressure applied, water content and bulk density varies across different soil types as particles rearrange with changing water contents [24].

Figure 8.

Benjamin Holt testing the first prototype gasoline-powered track-type tractor in 1908 (Caterpillar, Inc., 2021) – [27].

6.2 Operation during non-optimal soil conditions

Under non-optimal soil conditions, field farm operations should be considered with great reluctance, due to the potential for severe damage to the soil matrix. As farm equipment crosses through a wet field, ruts are formed from soil compaction around the tire path. Tillage is a common practice to relieve soil compaction due to poor soil management. However, tilling breaks-apart the soil structure and causes further traffic, in addition to deeper compaction in the field. A tilled soil is more easily compacted, since the subsoil beneath the tillage line is now in a more vulnerable state for soil compaction [25]. Under good soil conditions, the integrity of the soil is reasonably strong and minimizes the loss of pore space from heavy equipment travel. When soil conditions are non-optimal, the structural integrity of the soil is significantly reduced, and this results in the elimination of pore space with vehicle traffic. As shown in Figure 29, when the same pressure is applied in a loam soil, the bulk density significantly increases with increasing soil water content, thus, leaving the soil susceptible to compaction [24]. Additionally, water within the soil matrix reduces the coefficient of friction between neighboring soil particles and promotes the ease of displacement and flowability of the soil.

Figure 9.

A 1986 Caterpillar challenger 65^® rubber-tracked tractor (TractorData.com, 2016) – [3].

Figure 10.

A 1997 Case IH Steiger Quadtrac^® tractor (Case IH, 2022) – [28].

Figure 11.

Costs of operating tractors on tires and tracks (Case IH, 2022) – [28].

Figure 12.

Soil compaction study findings for a beat harvesting machine on tires (left) and the same machine on tracks (right) (Lamandé et al., 2018) – [29].

Figure 13.

Bogie wheel track design (Case IH, 2022) – [28].

Figure 14.

European soil load carrying capacity map, showing the maximum load (kN) that can be carried by a 1050/50R32 tire without inducing permanent soil deformation at 0.35 m depth (Lamandé et al., 2018) – [29].

Figure 15.

European soil load carrying capacity map, showing the maximum load (kN) that can be carried by a rubber track without inducing permanent soil deformation at 0.35 m depth (Lamandé et al., 2018) – [29].

Figure 16.

European soil load carrying capacity map, showing the maximum load (kN) that can be carried by a rubber track with perfectly even stress distribution without inducing permanent soil deformation at 0.35 m depth (Lamandé et al., 2018) – [29].

Figure 17.

Rut comparison of tracks vs. tires in muddy conditions (Elmers manufacturing Inc., 2019) – [30].

Figure 18.

Soil compaction comparison study findings (NTS Tire supply team, 2019) – [31].

Figure 19.

Bulk density of soil following a tractor pass with different drive systems (Abu-Hamdeh et al., 1995) – [37].

Figure 20.

Effects of moisture on soil compaction between multiple vehicles (Abu-Hamdeh et al., 1995) – [37].

Figure 21.

Gidrokhod 49,061 (Vantsevich & Blundell, 2015) – [39].

Figure 22.

Lunokhod 1 moon rover (Kassel, 1971) – [40].

Figure 23.

Perseverance Mars rover (Wikipedia, 2021) – [41].

Figure 24.

(A) Six single sideway-share subsurface tillage implements with the same rake and tilt angles of 10° and 15° with different cutting widths; (B) dual sideways-share subsurface tillage implements with rake angle of 15° with different tilt angles; and (C) five dual sideways-share subsurface tillage implements with share tilt and rake angles of 10° and 15° with different shank rake angles (Hoseinian et al., 2022) – [42].

Figure 25.

Varying level of soil disturbance with narrow tine: (a) above critical depth; (b) below critical depth (Spoor, 2006) – [43].

Figure 26.

Influence of tine spacing on the soil disturbance profile: (a) wide spacing; (b) narrow spacing (Spoor, 2006) – [43].

Figure 27.

Reversible subsoiler and its impact (Spoor, 2006) – [43].

Figure 28.

Tracks versus tires load distribution areas (Mellgren, 1980) – [22].

Figure 29.

Water content, pressure applied and bulk density diagram (left) and compression curve for a loam – Typic Haplaquept soil (right) (Smith, Johnston, & Lorentz, 1997) – [23].

Figure 30.

NEXAT system for controlled traffic farming (Misser Uitgeverij B.V., 2021) – [47].

Figure 31.

Soil penetration resistance measured shortly after tillage operations in may 1998 and 1999. PAC: Compacted; PUD: Intensive rotary cultivation (Munkholm & Schjonning, 2004) – [49].

Figure 32.

Relative tensile strength (REF = 100) of air-dried soil aggregates (average of the four size fractions) at the different times of sampling. PAC: Compacted; PUD: Intensive rotary cultivation (Munkholm & Schjonning, 2004) – [49].

Figure 33.

Minirhizotron images showing canola roots growing in may (left) and soybean roots observed in July and august (right) following the channels made by the preceding canola cover crop at 38.2 cm (at WREC) (top) and 18 cm (at BARC) (bottom) depth. The bulk density was 1.55 and 1.61 g/cm³ and penetration resistance was 2247 and 2176 kPa for the upper and lower soils, respectively (Calonego et al., 2017) – [51].

Figure 34.

Penetration resistance (kPa) with depth (cm) at Beltsville agricultural research Center (BARC) and wye research and education Center (WREC). The average volumetric water content at time of penetration resistance measurement was 0.22 cm³/cm³ (WREC) and 0.27 cm³/cm³ (BARC) in the surface soil (0–20 cm) and 0.29 cm³/cm³ (WREC) and 0.39 cm³/cm³ (BARC) in the subsoil (20–40 cm) (Williams & Weil [56]) – [52].

6.3 Livestock grazing

Livestock grazing can affect soil stability and functionality if not managed properly. The severity of soil damage due to livestock grazing is related to the soil type, texture, and moisture content. Pugging, the formation of soil around the hoof of the livestock, can result in increased soil compaction and a reduction in soil surface water infiltration rates [26]. When water does not infiltrate through the soil surface during rainfall or irrigation, puddling occurs in fields. The trampling and pugging from livestock onto soil surfaces damages the subsurface soil integrity. The density of the livestock per unit of area in a pasture impacts the level of soil compaction due to pugging. This effect also negates the value of winter grazing on crop land to glean harvest losses. The long-term damage from soil compaction to the crop ground greatly outweighs the value of the “free” feed gained.

6.4 Other

Aside from intensive farming and grazing practices common in modern agriculture, there are other factors, some environmental and some man-made, that can have a noticeable effect on soil compaction. Depending on the region of agricultural production, the type of soils, as well as natural and artificial drainage, some fields can be subject to prolonged ponding of water in localized areas. Over time, the weight of the water ponded on the soil surface causes the soil pores to collapse further, slowing the movement of water through the soil and increasing the weight of water on top of the soil surface during future precipitation events. Water ponded on the soil surface adds 10 kPa of pressure per m of depth. Additionally, slowed water movement through the soil increases the risk of farming operations occurring during non-optimal soil conditions. Another non-conventional contribution to soil compaction is the relatively new practice of storing grain in large plastic bags that are laid-out on the soil surface. Producers using this method of temporary grain storage have noted significant soil compaction on the surface due to the weight of the grain.

6.4.1 Dedicated tramline equipment

The newest realm of controlled traffic farming incorporates unified implements that minimize in-field travel in a variety of ways. NEXAT GmbH is a leader in this field. The company has developed a single equipment carrier, known as a beam tractor, capable of planting, soil cultivation, crop treatment, and harvesting. They refer to this as the NEXAT System [50]. This fascinating piece of equipment, pictured in Figure 30, manages to minimize the required crop production machinery, is fully integrated, and does not require additional equipment or chassis components. It can keep-up with the advancing digital age of electronic controls and even has autonomous guidance. However, its most impressive feature is an ability to reduce the land driven-on from 60 to 80% to less than 5% by only traveling on dedicated drive lanes. NEXAT-like systems are crucial to the continuing effort of reducing soil compaction through the minimization of machinery footprint on arable land.

6.4.2 Tillage timing

Even the simple aspect of the timing of the tillage in a field can play a major factor in soil compaction. Early season tillage is often performed to reduce the weed density late in the season. However, early season tillage often is the wrong choice for both soil compaction and weed control during the growing season. Late-season tillage allows for more organic material to be added to the soil, while actively and drastically reducing the number of weeds present in the crop’s growth cycle. Early tillage during the wet spring times increases the soil’s tendency toward compaction. Heavy equipment and traffic through the fields amplify the destruction of the soil’s internal structure. Decreased pore space and limited soil and water volume can result from wet soil tillage during the early parts of the crop production season [51].

The impact of tillage operations during non-optimal, wet conditions is a common concern for farm managers, and research into the actual implications of these kinds of operations is rather common. Figures 31 and 32 below detail the results of a study looking into the change in resistance to soil penetration following tillage during wet conditions and the progression of the soil aggregate strength throughout the growing season for these soils [52]. Figure 26 shows that after non-optimal cultivation, penetration resistance increased slightly compared to a reference soil that was not tilled but that this resistance was still significantly lower than heavily compacted soil. The true consequences of non-optimal tillage operations are exposed in Figure 27, in which it is demonstrated that the tilled soil is unable to recover during the following growing cycle. As a result, the tilled soil maintains a very high soil aggregate tensile strength over time, further decreasing the soil’s productivity and the long-term sustainability of agricultural operations in such soil.

6.5 Cover-cropping and crop rotation

The final aspects of farm management that impact soil compaction and soil health are the decisions that farm managers make regarding crop rotation and cover cropping. Both have specific impacts for nutrient availability and storage, organic material availability and control, weed control, and erosion prevention. However, both cover cropping and crop rotation can also impact the prevention of soil compaction. This section will review the impacts of cover cropping versus crop rotation, an outline cover crop selection to achieve maximum compaction prevention and maintain the necessary levels of erosion prevention, and the impact of pre-planting cultivation and its effects on seed bed, germination, and root development.

6.5.1 Cover-cropping vs. crop rotation

Cover-cropping is the practice of planting legume and grass varieties after the primary harvest, in the late fall, winter, or early spring before planting. Typically, these cover-crops are planted to instill nutrients into the soil, increase the organic material in the topsoil layer, and to better hold the soil together during tillage to prevent erosion issues. In addition to promoting yield advantages, cover cropping can also be used to improve the soil profile and decrease existing compaction through the creation of pores and reduction of soil bulk density.

Crop rotation aims more at cycling specific nutrients within the soil matrix to promote a greater yield for specific crop types during different cyclic years. A good example of this is the common corn and soybean rotation, in which soybeans are rotated-in, when soil nutrient sampling indicates low nitrogen levels. Soybeans are utilized in this way, due to their nitrogen fixing attributes. This locks excess atmospheric nitrogen beyond what is needed for the soybean crop into the soil, to be used by corn in the following years. Crop rotation can additionally impact topsoil and subsurface soil compaction, because of the differences in root penetration profiles. This can aid in moisture uptake and retention.

One study looked at the difference between cover-cropping and crop rotation and then compared the impact on yield results, as well as the resulting soil nutrients [53]. The findings were such that in the short term, there was little evidence to say that cover-cropping alone could result in an adequate yield improvement, but a combination of cover-cropping and crop rotation promoted increased crop yields and retained the benefits of using cover-crops. On the other hand, when examining the effects on soil compaction, the long-term consequences of cover-cropping helped to dramatically negate long-term compaction issues. Cover-cropping plays an essential role in decreasing soil compaction through the reduction of soil bulk density, the alteration of soil aggregate size, the creation of root channels, and improving the aeration and pore space within the soil. Specific cover-cropping can also help to combat long-term compaction by promoting subsoil disturbances via root channels.

6.5.2 Cover-crop selection

One of the primary ways in which cover crops can impact soil compaction is through the creation of pore space and root channels. These openings help to decrease the soil’s bulk density, break-up previously compacted volumes, and promote water infiltration, all of which further aid in this endeavor. Figures 33 and 34 depict the results of studies on the effects of root profiles and root penetration resistance, which indicate compaction relief from cover-cropping [54, 55]. In particular, the studies investigated the differences in channels created by soybean and canola plant roots, as well as the effects on soil nutrient and water content from a variety of other legume-type cover crops [54, 55]. Cover-cropping with radish and legume type crops aided in decreasing the soil penetration resistance during later planting, and it marginally disrupted soil compaction. In addition, cover-cropping had added benefits for nutrient content and water availability. The data from WREC in Figure 34 showed how cover cropping impacted soil with historically high compaction [55]. Utilizing cover-crops with large root profiles was particularly effective at increasing the macro-porosity and facilitating aggregate break-up in both topsoil and subsoil [54]. The latter is particularly important for soil types with increased risk of compaction, such as those with a high clay content. In addition to its other benefits, cover-cropping is a useful and inexpensive tool to aid in alleviating the effects of previous compaction, costing far less than mechanical relief applied through subsoiling operations.

7. Conclusion

This chapter provided a comprehensive review of the soil compaction problem. The historical aspects, the mechanism, and the environmental implications of soil compaction were discussed. A large off-road vehicle maneuvering with a draft load causes the soil beneath each point to compact, until it can support the imposed load in both the vertical and horizontal directions. Different vehicle designs have advantages and disadvantages in addressing the soil compaction problem. The track versus tires debate continues to this day, and the farmer’s choice should depend on their specific situation. Sophisticated farm management practices can significantly reduce soil compaction in the mid-term. Farmers risk losing 50% of their expected profits, when the soil compaction is not addressed in a sustainable way. Farmers and policymakers are encouraged to work toward reducing and reversing soil compaction for sustainable management of agricultural lands.

The issue of soil compaction is not one that will ever cease to exist. It will continue to cause trouble for those in agriculture, construction, mining, and other industries that deal with soil and ground working. As such, it is important that an understanding of the impact of soil compaction is continually being disseminated into these industries, as well as the basic management practices that can help to prevent an extensive spread of the problem. For design engineers in these fields, soil compaction offers the potential for the continued improvement in equipment design. Looking specifically at agriculture, the on-going trend of increasing equipment size and capacity in order to improve fuel and energy sustainability indicates that there will be a continued demand to further reduce the equipment loading and improve soil interaction of crop machinery, in order to maintain an adequate level of soil compaction minimization. The design of tillage equipment has already come a long way from the moldboard plow, specifically in terms of minimizing soil disturbed unnecessarily, while maximizing the implement’s capacity to pulverize the soil aggregates within the seed-bed. Moving forward, tillage equipment’s most likely challenge will be ensuring adequate wheel support during operation, without causing additional soil loading and compaction forces.

Farm managers must recognize that the prevention of unnecessary soil compaction is of paramount interest in the long-term productivity of their resources. They need to adopt a continuous improvement attitude and do whatever is feasible to minimize compaction. The seemingly small benefits of tillage cycling, crop rotation, and cover cropping should not be overlooked, since these practices continue to prevent soil compaction and reduce equipment traffic in the fields. As with many other aspects of off-road vehicle and machine design, committing to improving the performance of all factors will increase the effectiveness of the soil compaction control and prevention areas. Because the ability to sustainably grow food is critical to humanity’s future, agricultural engineers of the 21st century with a working knowledge of soil compaction phenomena will continue to be in high demand.

Acknowledgments

We would like to acknowledge our fellow classmates from the Fall 2021 Design of Off-Road Vehicles class at Purdue University’s School of Agricultural and Biological Engineering for their contributions to the structure and content of this technical chapter. Robert M. Stwalley IV is acknowledged for his work on the figures within the document.

Conflict of interest

The authors declare no conflict of interest.