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

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.
