What makes soil compact

Soil water acts as a lubricant increasing compaction when a load is imposed on the soil. If near saturation, however, the load is likely to exceed the soil strength and bearing capacity, resulting in excessive wheel slippage and rutting as well as soil mixing and smearing.

Compaction usually results in less plant root proliferation in the soil and lowers the rate of water and air movement. Because of the root restriction the amount of water available to the crop is often decreased. Slower internal drainage results in poorer subsurface drain performance, longer periods of time when the soil is too wet for tillage following rainfall or water application, increased denitrification and decreased crop production.

Increased compaction also adds to the energy consumption by tractors for subsequent tillage. Most effects of compaction are detrimental. However, in some cases, slight compaction near seeds can aid germination and improve plant growth in times of low soil moisture caused by low rainfall or low water-holding capacity soils. Deep soil compaction is excessive soil compaction below the normal annual tillage depth, usually 15—20 cm.

It is of greater concern than near surface compaction because it is a difficult problem to solve and may take many years, or even decades, under vigorous pasture to rehabilitate. Research on Tasmanian soils has shown that deep sandy soils are prone to deep soil compaction at 20—40 cm depth figure 1. This has been associated with the use of heavy pea and potato harvesters when the soils are moist or wet.

Near surface compaction 0—5 cm occurs on other soils, which results in the formation of clods and restricted root growth. Near surface compaction can be relieved relatively easily and is likely to be short lived. The volume of soil compacted by a wheel pass varies with soil type, soil moisture, tire size, pressure and total load. Pressures are transmitted deeper into wet soil than in dry soil by the same tyre size and wheel load.

Figure 1: Plot showing the difference in penetration resistance between a long term pasture green and an intensely cropped purple deep sand. Dig a small pit crossing the crop root zone, use a knife to examine the rooting pattern and test the resistance to penetration. Note denser zones of soil and see if they coincide with a reduction in root growth. By examining the soil profile it is possible to identify a compacted soil layer because it is physically stronger harder and more dense than the soil above or below it.

Compacted layers often have distinct massive or blocky appearance and are a clearly defined horizontal layer that occurs between 10 and 40 cm. This method measures compaction by the resistance encountered as you push down through the soil. It is useful for detecting dense soil layers that may resist root penetration or growth.

Hand probes are basically steel rods that are pushed into the soil by hand. Compacted layers are more difficult to push through, and easier once past the compacted zone. Water content plays a very important role in soil compaction. The maximum dry density is only achieved when the water content is at an ideal level. This point is known as optimum moisture content or OMC. Optimal moisture content and maximum dry density are determined in a laboratory and are then used as targets for on-site operations.

If the soil is too dry, water trucks can be deployed to spread water in order to raise the water content within the acceptable range of optimal moisture content. Conversely, overly wet soils present their own set of problems.

Recent rain, spring thawing or soils that retain moisture can be handled in a number of ways. Soil compaction rollers come in a variety of styles with different options such as single or double drums, vibratory mechanisms or dozer blades. Soil compaction sometimes involves compacting multiple lifts, or layers of soil, until the overall desired thickness is achieved.

The stability of each lift depends on the one below it so compaction of each layer is critical and must be monitored. Establishing the correct lift thickness is important to find the balance between layers that are too small or too large.

A lift that is too large could result in poor compaction and compromise stability whereas a lift that is too small would result in excessive costs and time. Lift thickness typically range from 8 to inches depending on specifications.

Contact pressure between the soil and equipment used for compaction is also important to realize. Contact pressure is effected by the overall weight of the compaction equipment and the area of soil contacted by the equipment. The higher the contact pressure the more compaction that is achieved. When discussing the speed of soil compaction there is a dichotomy to be considered. Faster compaction speeds will allow more area to be compacted. However, if compaction is conducted too quickly there may not be enough time for the necessary deformations to take place.

In this case, additional passes would be required to complete the compaction process. Slower equipment traveling speeds are often deemed necessary, especially when using vibratory equipment. Slower speeds of vibratory equipment allow more time for additional vibrations at a given point resulting in better compaction. Compaction equipment usually have travel speeds between 5 and 15 mph.

Smooth drum rollers typically move 5 to 7 mph and Padfoot rollers move 5 to 15 mph. Pneumatic rollers can operate at speeds of almost 15mph. At a high level, the number of passes needed to achieve desired compaction depends on the contact pressure and speed of the equipment. Factors, such as soil type, moisture level, lift thickness and the type of compactor are also important.

In general, a lighter piece of equipment that has less contact with the soil would need a higher number of passes over the same soil to achieve the desired density versus heavier equipment with a larger contact area.

Operating a heavy compactor very slowly is not necessarily the most efficient option. Typically, a test section can be used to determine the roller pattern that works for the mentioned variable above.

Particle size and critical water values play a large role in soil compaction. Different soil types react differently to compaction efforts. Soil types are classified by their particle size and, in fine-grained soils, by their Atterberg limits.

Particle size is determined in a laboratory by separating a representative sample on a series of sieves, or screens, ranging from 4. Distribution of soil particles are either well graded, poorly graded or gap graded. Well graded soils that contain a wide range of particles are preferred in construction applications because they can be easily compacted, eliminating voids, interlocking the particles and resisting moisture absorption allowing the soil to support heavier loads as a very dense soil.

Poorly graded soils contain a narrow range of particle sizes and are less conducive for construction purposes as shear strength is not associated with the non-interlocking particles because of their similar sizes. Gap graded soils contain a break in the overall distribution of grain sizes. There are several methods used to compact soil. Static force uses the pressure of a weight to physically and continuously compact soil. If a soil is tilled after compaction, the infiltration rate will be high because the soil is cloddy and rough.

Seedbed preparation to shatter clods includes several passes with a tractor over the field. This will decrease surface roughness, but compacted soil that has been tilled has coarser aggregates than the same soil that was not compacted. So, the infiltration rate may still be rather high in the compacted soil immediately after tillage. The action of raindrops on the soil surface and subsequent trips over the field destroy much of this apparent advantage.

This is visible in the field as stagnating water in wheel tracks Figure 14, see next page. It is common for runoff and erosion to start in these wheel tracks, especially if they run up and down the slopes. Soil compaction causes reduced infiltration. Root growth in compacted soils is restricted because roots can develop a maximum pressure above which they are not able to expand in soils. As mentioned above, the maximum penetration resistance measured with a standard cone penetrometer that roots can overcome is psi.

In many cases, cracks and fissures will be available for roots to grow through, so a total lack of root growth is not likely. Instead, roots will concentrate in areas above or beside compacted zones in the soil Figure Aside from the effect of penetration resistance, roots also suffer from increased anaerobic conditions in compacted soils. A reduction of root growth will limit root functions such as crop anchoring and water and nutrient uptake. In addition, soil compaction has been found to reduce nodulation of leguminous crops such as soybean, which may limit nitrogen nutrition of these crops.

Roots occupy a larger soil volume in uncompacted soil left than in compacted soil right. Adapted from Keisling,T. Batchelor, and O. Soil compaction affects nutrient uptake. Nitrogen is affected in a number of ways by compaction: 1 poorer internal drainage of the soil will cause more dentrification 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.

In humid temperate climates--as in Pennsylvania--soil compaction primarily increases denitrification loss and reduces nitrogen mineralization. In one study on a loamy sand in a humid temperate climate, nitrogen mineralization was reduced 33 percent and the denitrification rate increased 20 percent in a wet year.

In a study with ryegrass, the nitrogen rate had to be more than doubled on the compacted soil to achieve the same dry matter yield Figure Thus, compaction results in less-efficient use of nitrogen and the need to apply more for the same yield potential.

Nitrogen response curve of ryegrass on a clay loam soil in Scotland in compacted and uncompacted soil. Douglas, J. Compaction strongly affects phosphorus uptake because phosphorus is very immobile in soil.

Extensive root systems are necessary to enable phosphorus uptake. Because compaction reduces root growth, phosphorus uptake is inhibited in compacted soil Figure Potassium uptake will be affected in much the same way as phosphorus.

Phosphorus uptake and concentration in grain and straw are decreased due to soil compaction. Lipiec, J. The primary aim of this fact sheet has been to review effects of soil compaction on soil properties and crop growth.

Soil compaction increases soil density, reduces porosity especially macroporosity , and leads to increased penetration resistance and a degradation of soil structure. This degradation is enforced when tillage is used to break up compacted soils. Soil biota suffers from compaction. For example, earthworm numbers and activity will be reduced in compacted soils; water infiltration and percolation are slower in compacted soils; root growth will be inhibited due to soil compaction, leading to reduced uptake of immobile nutrients such as phosphorus and potassium; and increased nitrogen losses can be expected because of prolonged periods of saturated conditions in compacted soils.

Thus, limiting soil compaction is necessary. Below are some tips to manage compaction. More information is available in the fact sheet Avoiding Soil Compaction. Let's Stay Connected. By entering your email, you consent to receive communications from Penn State Extension. View our privacy policy. Thank you for your submission!

Home Effects of Soil Compaction. Effects of Soil Compaction. Soil compaction is the reduction of soil volume due to external factors; this reduction lowers soil productivity and environmental quality. Introduction The threat of soil compaction is greater today than in the past because of the dramatic increase in the size of farm equipment Figure 1.

Effects of Compaction on Crop Yields Soil Compaction Effects on Forages The effect of traffic on alfalfa and grass sod is a combination of soil compaction and stand damage. Soil Compaction Effects on Tilled Soils Tillage is often performed to remove ruts, and farmers assume that it takes care of soil compaction. Soil Compaction Effects on No-Till Crop Production No-till has a lot of advantages over tillage--reduced labor requirements, reduced equipment costs, less runoff and erosion, increased drought resistance of crops, and higher organic matter content and biological activity.

Effects of Soil Compaction on Soil and Crop Health In this section we will review the effects of soil compaction on soil physical, chemical, and biological properties, as well as on crop growth and health.

Soil Density The most direct effect of soil compaction is an increase in the bulk density of soil. Table 1. Ideal and root-restricting bulk densities. Soil quality test kit guide. Washington, D. Penetration Resistance Root penetration is limited if roots encounter much resistance. Soil Structure Soil compaction destroys soil structure and leads to a more massive soil structure with fewer natural voids Figure Strongly developed structure, crumb Weakly developed structure, crumb Soil material with many old root and earthworm channels Weakly developed structure, cloddy Plow pan, compacted, few root or earthworm channels Soil material with root channels Broken plow pan with some large air pockets Weakly developed structure Figure Soil Biota Soil contains a tremendous number of organisms.

Table 2. Effects of soil compaction on earthworm counts in Australia average of 5 years. Compaction treatment Earthworms per acre Adapted from Radford, B. Wilson-Rummenie, G. Simpson, K. Bell, and M. No compaction Water Infiltration and Percolation Soil compaction causes a decrease in large pores called macropores , resulting in a much lower water infiltration rate into soil, as well as a decrease in saturated hydraulic conductivity.

Table 3. Effects of compaction on macropore volume, air permeability, and infiltration rate in a grassland study. Uncompacted 0. Root Growth Root growth in compacted soils is restricted because roots can develop a maximum pressure above which they are not able to expand in soils. Nutrient Uptake Soil compaction affects nutrient uptake.

Managing Soil Compaction The primary aim of this fact sheet has been to review effects of soil compaction on soil properties and crop growth. Avoid trafficking wet soil. Only wet soil can be compacted. Fields should not be trafficked if they are at or wetter than the plastic limit. To check if soil is at the plastic limit, start by taking a handful of soil. If you can easily make a ball by kneading this soil, conditions are suboptimal for field traffic.

Artificial drainage can help increase the number of trafficable days on poorly drained soil. Keep axle loads below 10 tons. Subsoil compaction is caused by axle load and is basically permanent. To avoid subsoil compaction, keep axle loads below 10 tons per axle--preferably below 6 tons per axle.

Decrease contact pressure by using flotation tires, doubles, or tracks. Topsoil compaction is caused by high contact pressure. To reduce contact pressure, a load needs to be spread out over a larger area. This can be done by reducing inflation pressure. A rule of thumb is that tire pressure is the same as contact pressure. Tires inflated to psi such as truck road tires should be kept out of the field.

To be able to carry a load at low inflation pressure, bigger or multiple tires are needed, hence the need for flotation tires and doubles. Large-diameter tires also help to increase the tire footprint. Tracks help to spread the load over a large area, but having multiple axles under the tracks is necessary to avoid high spikes of pressure.

Tracks have the advantage over doubles of reducing contact pressure without increasing the area of the field trafficked. Decrease trafficked area by increasing swath and vehicle width or by decreasing number of trips. Reduce the area of a field that is subject to traffic by increasing swath width of manure spreaders or the spacing between wheels so individual wheel tracks are more widely spaced.

Using larger equipment and no-tillage can reduce the number of trips across the field.