We revisit soil tilth here to get a little more detail about what it means. We also want to explore how tillage and compaction affect tilth. All these concepts will come into play towards the end of the book as we uncover solutions to our soil problems. This chapter presents a basic understanding of how farm and garden operations affect soil tilth.
When the soil has a favorable physical condition for growing plants, it is said to have good tilth. Such soil is porous and allows water to enter easily, instead of running off the surface (Figure \(\PageIndex{1}\)). More water is stored in the soil for plants to use between rains, and less erosion occurs. Good tilth also means that the soil is well aerated. Roots can easily obtain oxygen and get rid of carbon dioxide. A porous soil does not restrict root development and exploration. When a soil has poor tilth, its structure deteriorates and soil aggregates break down, causing increased compaction and decreased aeration and water storage. A soil layer can become so compacted that roots can’t grow. A soil with excellent physical properties will have numerous channels and pores of many different sizes.
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Studies on both undisturbed and agricultural soils show that as organic matter increases, soils tend to be less compact and have more space for air passage, helping to conduct water into the soil and storing it for plants to use. Sticky substances are produced during the decomposition of plant residues. Along with plant roots and fungal hyphae, they bind mineral particles together into clumps or aggregates. In addition, the sticky secretions of mycorrhizal fungi—beneficial fungi that enter roots while growing thin filaments into the soil that help plants get more water and nutrients—are important binding material in soils. The arrangement and collection of individual particles as aggregates and the degree of soil compaction have huge effects on plant growth. The development of aggregates is desirable in all types of soils because it promotes better drainage, aeration, and water storage. The one exception is for some wetland crops, such as rice, where you want dense soil that keeps fields flooded. (Although newer rice-growing systems show that high yields can be obtained with less flooding, thereby saving water.)
Organic matter, as residue on the soil surface or as a binding agent for aggregates near the surface, plays an important role in decreasing soil erosion. As with leaves and stems of living plants, surface residues intercept raindrops and decrease their potential to detach soil particles. These surface residues also slow water as it flows across the field, giving it a better chance to infiltrate into the soil. Aggregates and large channels greatly enhance the ability of soil to conduct water from the surface into the subsoil. Larger pores are formed in a number of ways. Old root channels may remain open for some time after the root decomposes. Larger soil organisms, such as insects and earthworms, create channels as they move through the soil. The mucus that earthworms secrete to keep their skin from drying out also helps to keep their channels open for a long time.
Most farmers can tell that one soil is better than another by looking at them, seeing how they work up when tilled, or even by sensing how they feel when walked on or touched. What they are seeing or sensing is good tilth. Digging a bit into the soil can give a sense of its porosity and extent of aggregation.
Since erosion tends to remove the most fertile part of the soil, it can cause a significant reduction in crop yields. In some soils, the loss of just a few inches of topsoil may result in a yield reduction of 50%. The surface of some soils low in organic matter may seal over, or crust, as rainfall breaks down aggregates and as pores near the surface fill with solids. When this happens, water that can’t infiltrate into the soil runs off the field, carrying away valuable topsoil.
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 content, the soil may behave like a liquid because it has little internal cohesion. 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, the soil has somewhat more cohesion, but it can still be easily molded and is said to be plastic. Upon further drying, the soil will become friable: it will break apart rather than mold under pressure.
The point between plastic and friable soil, the plastic limit, has important agricultural implications. When 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. 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. 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 compacted soil tills up very cloddy and then requires extensive secondary tillage and packing trips to create a satisfactory seedbed. 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 because few sturdy aggregates are present to prevent the soil from dispersing. The result may be 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. The 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. 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 the 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.
