**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

#### **Figure 28.**

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

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].

#### **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.

#### **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 *Reducing Soil Compaction from Equipment to Enhance Agricultural Sustainability DOI: http://dx.doi.org/10.5772/intechopen.104489*

**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].*

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

*Reducing Soil Compaction from Equipment to Enhance Agricultural Sustainability DOI: http://dx.doi.org/10.5772/intechopen.104489*

#### **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].*

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 legumetype 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 covercrops with large root profiles was particularly effective at increasing the macroporosity 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

*Reducing Soil Compaction from Equipment to Enhance Agricultural Sustainability DOI: http://dx.doi.org/10.5772/intechopen.104489*

#### **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/cm<sup>3</sup> 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 cm<sup>3</sup> /cm<sup>3</sup> (WREC) and 0.27 cm<sup>3</sup> /cm<sup>3</sup> (BARC) in the surface soil (0–20 cm) and 0.29 cm<sup>3</sup> /cm3 (WREC) and 0.39 cm<sup>3</sup> /cm3 (BARC) in the subsoil (20–40 cm) (Williams & Weil [56]) – [52].*

and inexpensive tool to aid in alleviating the effects of previous compaction, costing far less than mechanical relief applied through subsoiling operations.
