A soil becomes more compact, or dense, when aggregates or individual particles of soil are forced closer together. Soil compaction has various causes and different visible effects. Compaction can occur either at or near the surface (surface compaction, which includes surface crusting as well as plow layer compaction) or lower down in the soil (subsoil compaction). See figure 6.6.
Plow layer compaction—compaction of the surface layer—has probably occurred to some extent in all intensively worked agricultural soils. It is the result of a loss of soil aggregation that typically has three primary causes—erosion, reduced organic matter levels, and force exerted by the weight of field equipment. The first two result in reduced supplies of sticky binding materials and a subsequent loss of aggregation.
Surface crusting has the same causes as plow layer compaction but specifically occurs when the soil surface is unprotected by crop residue or a plant canopy and the energy of raindrops disperses wet aggregates, pounding them apart so that particles settle into a thin, dense surface layer. The sealing of the soil reduces water infiltration, and the surface forms a hard crust when dried. If the crusting occurs soon after planting, it may delay or prevent seedling emergence. Even when the crust is not severe enough to limit germination, it can reduce water infiltration. Soils with surface crusts are prone to high rates of runoff and erosion. You can reduce surface crusting by leaving more residue on the surface and maintaining strong soil aggregation.
Compaction of soils by heavy equipment and tillage tools is especially damaging when soils are wet. This combination of factors is the primary cause for subsoil compaction and one of the causes for plow layer compaction. To understand this, we need to know a little about soil consistence, or how soil reacts to external forces. At very high water contents, a soil may behave like a liquid (figure 6.7), because it has little internal cohesion (figure 5.10). On a slope it can simply flow as a result of the force of gravity—as with mudslides during excessively wet periods. At slightly lower water contents, soil has somewhat more cohesion (figure 5.10, middle), but it can still be easily molded and is said to be plastic (figure 6.7). Upon further drying, the soil will become friable—it will break apart rather than mold under pressure (figure 6.7).
The point between plastic and friable soil, the plastic limit,has important agricultural implications. When a soil is wetter than the plastic limit, it may become seriously compacted if tilled or traveled on, because soil aggregates are pushed together into a smeared, dense mass. This compaction may be observed when you see shiny, cloddy furrows or deep tire ruts in a field (figure 6.8). When the soil is friable (the water content is below the plastic limit), it crumbles when tilled and aggregates resist compaction by field traffic. Thus, the potential for compaction is strongly influenced by the timing of field operations as related to soil moisture conditions.
A soil’s consistency is strongly affected by its texture (figure 6.7). For example, as coarse-textured sandy soils drain, they rapidly change from being plastic to friable. Fine-textured loams and clays need longer drying periods to lose enough water to become friable. This extra drying time may cause delays when scheduling field operations.
Surface crusting and plow layer compaction are especially common with intensively tilled soils. Tillage operations often become part of a vicious cycle in which a compacted soil tills up very cloddy (figure 6.9a), and then requires extensive secondary tillage and packing trips to create a satisfactory seedbed (figure 6.9b). Natural aggregates break down, and organic matter decomposes in the process—contributing to more compaction in the future. Although the final seedbed may be ideal at the time of planting, rainfall shortly after planting may cause surface sealing and further settling (figure 6.9c), because few sturdy aggregates are present to prevent the soil from dispersing. The result may be a soil with a dense plow layer and a crust at the surface. Some soils may hard-set like cement, even after the slightest drying, thereby slowing plant growth. Although the soil becomes softer when it re-wets, that moisture provides only temporary relief to plants.
Subsoil compaction—dense soil below the normally tilled surface layer—is usually referred to as a plow pan, although it is commonly caused by more than just plowing. Subsoil is easily compacted, because it is usually wetter, denser, higher in clay content, lower in organic matter, and less aggregated than topsoil. Also, subsoil is not loosened by regular tillage and cannot easily be amended with additions of organic materials, so compaction in the subsoil is more difficult to manage.
Subsoil compaction is the result of either direct loading or the transfer of compaction forces from the surface into deeper layers. Subsoil compaction occurs when farmers run heavy vehicles with poor weight distribution. The load exerted on the surface is transferred into the soil along a cone-shaped pattern (figure 6.10). With increasing depth, the compaction force is distributed over a larger area, thereby reducing the pressure in deeper layers. When the loading force at the surface is small, say through foot or hoof traffic or a light tractor, the pressure exerted below the plow layer is minimal. But when the load is high from heavy equipment, the pressures at depth are sufficient to cause considerable soil compaction. When the soil is wet, the force causing compaction near the surface is more easily transferred to the subsoil. Clearly, the most severe compaction damage to subsoils occurs with the combination of heavy vehicle traffic and wet soil conditions.
CHECK BEFORE TILLING
To be sure that a soil is ready for equipment use, you can do the simple “ball test” by taking a handful of soil from the lower part of the plow layer and trying to make a ball out of it. If it molds easily and sticks together, the soil is too wet. If it crumbles readily, it is sufficiently dry for tillage or heavy traffic.
Direct loading is also caused by the pressure of a tillage implement, especially a plow or disk, pressing on the soil below. Plows cause compaction because the weight of the plow plus the lifting of the furrow slices results in strong downward forces. Disks have much of their weight concentrated at the bottom of the disk and thereby cause pans. Subsoil compaction may also occur during moldboard plowing when a set of tractor wheels is placed in the open furrow, thereby applying wheel pressure directly to the soil below the plow layer.