Soil Conservation
Soil Compaction
Soil compaction occurs when soil particles are compressed together—especially when the soil is wet—destroying soil structure, reducing porosity, limiting water and air infiltration, increasing resistance to root penetration, and often resulting in reduced crop yield. Agricultural practices, such as increased number of field operations, earlier planting schedules, and larger equipment, have made soil compaction more common on many fields. Significant compaction can also be caused by tillage and livestock. Aggressive tillage increases the susceptibility of a soil to compaction because tillage reduces aggregate stability and reduces soil strength. The trend toward growing continuous row crops increases the potential for soil compaction. Grazing cattle under very wet conditions can cause soil puddling (trampling of soil) due to overgrazing, resulting in structural breakdown at the soil surface and subsequent crust formation when the soil dries out. Most farmers are aware of compaction problems, but the significance is often underestimated.
Forms of Soil Compaction
Surface Soil Compaction
Surface compaction is when the dense soil layer occurs at the surface of the soil. This type of compaction may be seen in the early growing season, especially with clean-tilled soil, and in the fall and spring after a summer crop. Surface compaction happens when the surface soil aggregates are broken down through the impact of falling raindrops, runoff, or standing water during irrigation. The aggregates usually aren’t very stable, and once broken down, the small particles fill in the pore space between the larger particles, making very dense crusts that restricts water and air entry. Keep in mind that this may not happen every year.
Subsoil Compaction
The second type of compaction is subsoil compaction in which the compacted layer is found deeper within the soil. This type of compaction may be natural or human-caused. Subsoil is prone to compaction because it is usually wetter, denser, higher in clay content, lower in organic matter, and less aggregated than topsoil. When soil aggregates become weaker from loss of the organic matter, silt and clay particles can wash down and settle in the subsoil pores, thereby creating a dense layer.
Types of Subsoil Compaction
Extensive soil modification is not always necessary to prepare a site for planting. Some soils are naturally deep and relatively uniform in texture and structure and do not require soil modification. However, some soils need modification because they have physical limitations commonly referred to as stratified soils, claypan soils, plow pan soils, hardpan soils, and clay-rich soils.
Stratified Soils
Stratified soils have horizons or layers with abrupt changes in soil texture. The layers interfere with the uniform drainage of water, causing zones of poor aeration that may restrict root growth. Modifying stratified soils to improve productivity requires mixing the soil layers with slip plows, moldboard plows and disk plows.
Claypan Soils
Claypan soils have an abrupt increase in clay content within a very short vertical distance (1–2in, 0.5–2cm) in a soil horizon. This increase in clay content restricts water movement downward, thus restricting aeration and root growth in the subsoil. Since claypan soils typically occur near the surface, the moldboard plow and the disk plow are effective methods of thoroughly mixing the claypan with the rest of the soil profile.
Plow Pan Soils
Soils with plow pan soils can be found in some orchard or vineyard sites. Plow pan soils are caused by tilling a soil at the same depth repeatedly or by repeated heavy traffic, particularly when soils are relatively wet. A soil with a plow pan does not necessarily have different textural layers like those of stratified, claypans, or hardpan soils.
Hardpan Soils
Hardpans soils are similar to claypans, except the soil particles are cemented together by hard mineral matter (silica) that do not soften, even when wet. This hardened layer is usually an absolute barrier to root growth and water percolation.
Clay-Rich Soils
Clay-rich soils may be found in some orchard or vineyard sites. These soils typically contain 35 percent clay and appear relatively uniform throughout the profile with subtle changes in soil texture, structure, and hardness.
Soil Susceptibility to Compaction
Soil moisture is critically important to the process of compaction. Dry soils are difficult to compact because the friction between particles prevents them from moving. On the other hand, very wet soils are difficult to compact because pores are filled with water, which must be squeezed out before compaction can occur. Moist soils are the most susceptible to compaction because there is sufficient moisture to allow particles to move and the macropores, which are most susceptible to compaction, are not filled with water to keep them from compressing. Texture (the percentage of sand, silt and clay) is important in determining a soil’s susceptibility to compaction.
Effects of Soil Compaction
Depending on the level of external force applied to the soil, compaction often leads to fewer large and medium pores and more smaller pores. The most notable changes are in soil bulk density, soil strength, porosity, and hydraulic properties such as infiltration rate and hydraulic conductivity.
Hydraulic Conductivity
Changes in hydraulic conductivity affect the water and air movement through the soil. When soil is compacted due to implement traffic the major change will be in soil bulk density. When this change occurs, the total porosity is reduced due to the compaction of soil particles, which leads to reduction in the space that the water and air can occupy. Loss of large pores between aggregates is particularly harmful for fine- and medium-textured soils that depend on those pores for good infiltration and percolation of water, as well as air exchange with the atmosphere.
Nutrient Uptake
Soil compaction affects uptake of nitrogen. Nitrogen is affected in a number of ways by compaction: (1) poorer internal drainage of the soil will cause more denitrification losses and less mineralization of organic nitrogen; (2) nitrate losses by leaching will decrease; (3) loss of organic nitrogen (in organic matter) and surface-applied nitrogen fertilizer may increase; and (4) diffusion of nitrate and ammonium to the plant roots will be slower in compacted soils that are wet, but faster in those that are dry.
Root Growth
Excessive compaction can reduce seedling emergence rates as well as stunt plant height throughout the growing season. Crops in their early growth stages are very susceptible to compaction (because roots are still shallow), and the plants may go through a noticeable period of stunted growth on compacted soils.
Prevention of Soil Compaction
The best way to minimize soil compaction is to avoid field activities that have the potential to damage the soil. Some management practices that can prevent subsurface soil compaction include tillage practices, building soil organic matter, controlling traffic, load distribution, timing of field operations, improved soil drainage, rotational grazing, and managed grazing.
Reduced Tillage Practices
OReduced tillage practices can minimize the incidences of soil compaction in agricultural fields by reducing the farm traffic and/or number of passes.
Build Soil Organic Matter
Organic matter (plant debris and residues) promotes the development of good soil structure and decreases soil bulk density. Organic matter makes the soil more resilient to soil compaction by making the soil more elastic, thus limiting the ability of an applied load to compact the soil.
Controlled-Traffic Farming
Controlled-traffic farming utilizes the same traffic lanes year after year, thereby sacrificing a small portion of the field in favor of having no wheel traffic in the majority of the field.
Timing of Field Operations
Avoid soil tillage operations when the soil is wet. Tilling when the soil is wet leads to a quicker breakdown of soil structure. When soil structure breaks down, soil particles become reconsolidated and are arranged closer to each other to form a compaction layer
Load Distribution
Tractor tires distribute forces into the soil, which can initiate the compaction process. A higher tire inflation pressure will cause more ground pressure resulting in greater shallow compaction than a lower inflation pressure. Some tire configurations allow lower inflation pressures on equipment.
Managed Grazing
Overgrazing livestock can result in the near complete removal of surface residues, which can be very damaging to the soil surface allowing for crusting which prevents water infiltration.
Crop Rotations
Using diverse crop rotations, which include forage, cereal, oilseed, and pulse crops that vary in rooting depth and type (fibrous versus taproot), combined with good agronomic management practices, such as direct seeding will help reduce soil compaction. Growing deep-rooted cover crops are capable of penetrating compact layers within the soil. An added advantage of crop-based mitigation strategies is that they add organic matter to the soil, which will lead to better soil structure in addition to breaking up the compaction layer.
Remediation of Soil Compaction
After compaction has been identified, the next step is to determine remediation options. The options are dependent on the depth of the compaction, tillage system, and soil type. Some management practices that can mitigate subsurface soil compaction include tillage, subsoiling, crop rotations, improved soil drainage, and growing deep-rooted cover crops.
Tillage
A short-term emergence solution to soil crusting after seeding might be a light harrowing or rolling with packers to gently fracture the soil crust after seeding, to aid in seedling emergence through the crust. If compaction is less than 10 inches deep, a chisel plow can be used to break up the compacted layer. Tillage is more effective on sandy soils rather than heavier soils. Compacted surface layers tend to be extremely cloddy when tilled.
Subsoiling
Subsoiling (also referred to as ripping or deep tillage) can alleviate deep soil compaction. Subsoiling increases rooting depth and soil drainage by applying an upward force that shatters the compacted soil. Subsoiling creates larger pores that increase rooting and infiltration. The benefits of subsoiling depend upon many factors including soil type, soil moisture level, soil management, and implement management.
Suitable Conditions for Subsoiling. Soils should be mostly dry and friable. If the soil is too wet, subsoiler shanks will slide through the ground without breaking up the soil. The shank can actually glaze the soil and compact it even more. If the soil is extremely dry, getting the subsoiler into the ground can be difficult, requiring larger, more powerful tractors to pull the shanks through compacted areas.
Subsoiling Equipment. Common tillage tools (disk, chisel plow, etc.) are too short and not built to be pulled through densely compacted soil. There are two major types of subsoilers: (1) tools with parabolic shanks, often equipped with wings on the shanks and multiple disk gangs, and (2) tools with straight shanks and a single coulter designed to cut through residue. The first example is more aggressive and disrupts a considerable portion of the soil volume.
Using Subsoilers. Shank spacing will vary depending on soil moisture, soil type, degree of compaction, and the depth of the compacted layer. Spacing should be adjustable so the worked area can be fractured most efficiently. Shank depth will depend upon the type of implement used but should not exceed one and a half (1.5) times the working depth. Before subsoiling, it is important to know the depth and extent of the compaction layer.
Improved Soil Drainage
Fields that do not drain in a timely manner often have more severe compaction problems. Sometimes, the fields are plowed when the bottom of the plow layer is still too wet, causing smearing and plow pans.
Growing Deep-Rooted Cover Crops
Some cover crops are capable of penetrating compact layers within the soil. These crops could be annual or perennials. In cases where the land does not need to be used immediately, such crops can be planted to overcome the compaction problem. Particular cover crop species such as cereal rye and radishes, if allowed enough time to grow, often root more deeply than summer cash crops such as corn and soybeans.
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