Soil compaction can be a serious form of soil degradation resulting in decreased crop production and increased risk of soil erosion. Soil compaction can reduce water infiltration into soil, crop emergence, root penetration, crop nutrient uptake and water uptake — all of which can reduce crop yields.
Soil compaction is caused by tillage equipment during soil cultivation or from the heavy weight of field equipment. Soil compression can cause soil particles to become compacted closer together into a smaller volume. As particles are compressed together, the space between soil particles (pore space) is reduced, reducing the space available in the soil for air and water. Soil compaction can have a number of negative effects on soil quality and crop production including:
- causing soil pore spaces to become smaller;
- reducing water infiltration rate into soil;
- decreasing the rate that water will penetrate into the soil root zone and subsoil;
- increasing the potential for surface water ponding, water runoff, surface soil waterlogging and soil erosion;
- reducing the ability of a soil to hold water and air, which are necessary for plant root growth and function
- compaction force causing crushing of soil aggregates
- reducing crop emergence as a result of soil crusting
- impeding root growth, limiting the volume of soil explored by roots and penetration of roots into subsoil; and,
- restricting root exploration decreases the ability of crops to take up nutrients and water efficiently from soil
All of these factors result in increased crop stress and yield loss.
Importance of Soil Porosity
Soils consist of organic matter, various-sized soil particles referred to as soil texture (proportion of sand, silt and clay particles) and pore spaces that contain air and water. The connectivity of soil pores coupled with the size and number of pores is very important for water infiltration, water and nutrient movement within soil and the ability of the soil to hold water. Larger, inter-connected soil pore spaces enhance water infiltration into soil, water percolation into the root zone and subsoil, and air exchange with the atmosphere.
Many important biological and chemical processes occur in soil pores that require both water and air. Reduced pore size and number will affect processes such as the reduced nutrient cycling and release of plant available nutrients.
One way to quantify the change is by measuring soil bulk density. This procedure is done by carefully taking a soil core and measuring the diameter and length to determine the volume of the core, then oven-drying the core to determine the dry soil weight.
Soil bulk density is the dry weight of soil divided by the volume of the soil. It is usually expressed in grams per cubic centimeter (g/cm3). As the pore space is decreased within a soil, the soil bulk density is increased.
Normally, loam to clay loam soils have a bulk density of about 1.3 to 1.4 g/cm3, and sandy loam to loamy sand soils have a bulk density of 1.4 to 1.6 g/cm3. Naturally dense horizons in a Solonetzic soil will have bulk densities of 1.6 g/cm3 or greater, and root growth will be hindered. Disced or cultivated surface soils will have bulk densities in the range of 1.0 to 1.2 g/cm3.
Large soil pores are the most effective in moving water through the soil. When large pores are absent, the hydraulic conductivity of soil (rate water will move through soil) will be greatly reduced.
In addition, the exchange of gases in soil with the atmosphere slows down in compacted soils, causing an increase in the likelihood of aeration-related problems. Soil compaction increases soil strength, which means plant roots must exert greater force to penetrate the compacted layer.
Identifying Types of Soil Compaction
Soil compaction can occur at the soil surface in the form of soil crusting, or it can occur in the subsoil. Soil compaction is sometimes blamed for reduced crop productivity, but it is important to correctly diagnose the cause or causes of reduced crop production. Poor plant growth can be caused by a number of factors, including soil compaction.
The first step is to correctly diagnosis if a soil compaction problem exists, and then develop short- and long-term management practices to prevent further damage.
Soil compaction can occur at different times of the year through different mechanisms. Careful observations can help diagnose the problem. If the answer to these questions is “yes,” you may have a soil compaction problem.
- Is there poor crop growth in all years, with all crop types in the same area of the field?
- Is there a spatial pattern to the crop growth (associated with wheel tracks, windrows, equipment widths, haul trails)?
- Does the soil surface appear smooth and crusted?
- Has there been a change in equipment size, weight or operations?
- Are there soil types in the field with naturally dense horizons such as eroded knolls?
- If you scrape away the surface soil with a shovel or trowel, can you see dense layers and/or horizontal root growth?
Compaction of agricultural soils can be caused by various farming practices. Here are three of the most common.
- Soil tillage that removes the protective residue from the soil surface, leaving the soil prone to natural environmental forces, or excessive soil tillage that causes surface soil aggregates to break down or degrade. This can lead to soil crusting, causing the surface soil layer to become hard and compacted.
- Soil tillage implements can induce soil compaction just below the depth of tillage, particularly when soils are wet. This type of compaction is sometimes referred to as a hard pan or plow layer.
- The weight of large farm equipment (tractors, seed carts, combines, trucks, manure spreaders) can cause wheel traffic compaction to a considerable depth within the root zone. As soil moisture content increases, so too does the depth of soil compaction.
In my next article on this subject in Grainews I’ll discuss the specific causes of soil compaction and how each should be managed.