Soil compaction is a physical degradation process where soil particles are pressed closer together, reducing the total pore space and increasing bulk density. The primary issue is the loss of interconnected large pores, which are necessary for rapid water infiltration and gas exchange. Compacted soil impedes proper root growth, limiting a plant’s ability to access water and nutrients.
Direct Mechanical Pressure
The primary cause of soil compaction is the application of heavy force upon the soil surface. This pressure is exerted by large agricultural equipment, construction vehicles, and other machinery that transmit considerable loads. The depth and severity of compaction depend on two main factors: the total axle weight and the tire contact pressure.
Heavy axle loads, particularly those exceeding 10 tonnes, can transmit stress deep into the subsoil, sometimes beyond 500 millimeters below the surface. This deep subsoil compaction is often long-lasting and difficult to alleviate because it is primarily related to the total weight of the vehicle.
Conversely, the tire inflation pressure dictates the pressure exerted on the shallow, upper layer of the soil. Higher tire pressures on a vehicle with a fixed weight concentrate the force into a smaller surface area, causing more severe compaction in the topsoil. Repeated passes by machinery can create localized, dense layers. In cultivated fields, repeated tillage at the same depth can form a highly dense, restrictive layer.
Improper Soil Moisture Management
Soil’s vulnerability to compaction changes depending on its water content when a force is applied. Moist soil is the most susceptible to densification because water acts as a lubricant, allowing individual soil particles to slide past one another under pressure. This reduced friction enables the particles to settle into a much tighter, denser arrangement than they would if the soil were completely dry.
In contrast, soil that is either extremely dry or fully saturated is less prone to immediate compaction. Dry soil resists pressure due to the strong frictional forces between the particles, making it difficult to rearrange them without significant force. When soil is fully saturated, the water fills all the macropores, and since water cannot be compressed, it resists the applied force.
However, trafficking or working a saturated soil with heavy equipment can trigger the “hydraulic ram” effect. The surface pressure forces the water out of the pores, transferring the initial stress almost instantaneously to the deeper subsoil layers. This action can cause severe, deep compaction in the subsoil even if the surface appears to only be rutted. Surface compaction can also occur from natural forces, such as the intense impact of large raindrops, which breaks down soil aggregates and causes a dense surface crust to form.
Loss of Natural Soil Structure
A soil’s resistance to compaction is linked to the stability of its natural structure. The presence of organic matter is the primary buffer against compaction because it physically binds soil particles into stable aggregates. These stable clusters maintain a high percentage of large pore spaces, making the soil structure resilient to external pressures.
Soils with low organic matter content, often below one or two percent, have weaker aggregates that are easily crushed or broken down. When this weaker structure collapses, the soil quickly loses its porosity and becomes dense, even under minimal pressure. Organic matter fosters the creation of macro-aggregates through biological binding agents, such as polysaccharides and glomalin, which are produced by microbes and fungi.
Management practices involving excessive tillage are detrimental to soil structure. Tillage physically breaks down existing soil aggregates, making the soil structure uniform and unstable. It also accelerates the decomposition of organic matter by exposing it to oxygen, removing the soil’s natural resilience. This structural degradation leaves the soil vulnerable to re-compaction.