6.3: Soil Tilth and Compaction
- Page ID
- 25022
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)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. It can occur either at or near the surface (shallow compaction, which includes surface crusting) or deeper down in the soil (subsoil compaction). See Figure 6.8.
Shallow Compaction
Shallow compaction, which is compaction of the surface layer or plow layer, occurs 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 forces exerted by the weight of field equipment. The first two result in reduced supplies of sticky binding materials and a subsequent loss of aggregation. Livestock can damage pastures through their hoof action during times when soils are susceptible to compaction.

Compaction of soils by heavy equipment and tillage tools is especially damaging when soils are wet. 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 6.9) because it has little internal cohesion (Figure 5.10, left). 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, but it can still be easily molded and is said to be plastic (Figure 6.9). Upon further drying, the soil will become friable: it will break apart rather than mold under pressure (Figure 6.9).

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 trafficked because soil aggregates are pushed together into a smeared, dense mass. This may be observed when you see shiny, cloddy furrows or deep tire ruts in a field (Figure 6.10). The soil is more resistant to deformation 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 it is much lower when the soil is adequately dry. A soil’s consistency is strongly affected by its texture (Figure 6.9). For example, as coarse-textured sandy soils drain, they rapidly change from being plastic to being 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.
Soils are thus less susceptible to compaction when they are dry, which may be a better time to run heavier equipment. Similarly, when soils are frozen and the soil particles are fused by ice, the soil becomes solid and resistant to compaction.

Surface sealing and crusting. This problem is also caused by aggregate breakdown but specifically occurs when the soil surface is unprotected by crop residues or plant canopies. The energy of raindrops disperses wet aggregates, pounding them apart so that particles settle into a thin, dense layer. The sealing of the soil reduces water infiltration, and the surface forms a hard crust when dried. Crusting generally occurs after tillage and planting when the soil is unprotected, and it can 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 by maintaining strong soil aggregation. Sometimes, farmers break crusts with a harrow, but that only treats the symptom, not the cause.
Intensive tillage. Shallow compaction is especially common with repeated soil disturbance. Tillage operations often become part of a vicious cycle in which a compacted soil tills up very cloddy (Figure 6.11a) and then requires extensive secondary tillage and packing trips to create a satisfactory seedbed (Figure 6.11b). 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 6.11c) because few sturdy aggregates are present to prevent the soil from dispersing. The result may be a dense soil with 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
Subsoil compaction occurs deeper in the soil and is sometimes referred to as a plow pan, although it is commonly caused by more than just plowing. Subsoil is prone to compaction 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. Another challenge is that the subsoil is by definition buried and therefore compaction is invisible unless you dig down or push a rod into the soil.Subsoil compaction occurs when farmers run heavy vehicles, especially those with poor weight distribution. The load exerted on the surface is transferred into the soil along a cone-shaped pattern (Figure 6.12). 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 deep in the soil is minimal. But when the load is high from heavy equipment, like with a heavy manure spreader or combine, 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, which causes even more compaction damage. Clearly, the most severe compaction in subsoils occurs with the combination of heavy vehicle traffic and wet soil conditions.
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.
Another major cause of subsoil compaction is the pressure of a tillage implement, especially a plow or disk, pressing on the soil below (hence the term plow pan).

Plows cause compaction because the weight of the plow plus the lifting of the furrow slices results in high downward forces from the plow share (bottom) onto the soil layer immediately underneath. Disks also have much of their weight concentrated at the bottom of the disk and can cause shallow 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. Overall, these pans are very common in soil that has been plowed, sometimes even many years after the field was converted to no-till.


