Soil compaction is one of the biggest challenges we have when trying to grow crops on the highly weathered, low organic matter, sandy soils of the Deep South.
Interestingly, it is the sandier soils that cause the worst problems. Soils high in clay and silt, such as those found in the Black Belt and Tennessee Valley region, are not as prone to compaction as are the sandier soils found in the Coastal Plain and Sandstone Plateau regions.
The reason is because sand and clay don’t mix very well and most of our sandy topsoils have a subsoil that contains some clay. When clay is mixed with sand or vice versa, the clay tends to bind the sand particles together into something resembling concrete.
The lack of organic matter makes the situation even worse. Tillage can aggravate the issue by mixing the sand and clay. Tillage when the soil is too wet really creates a problem.
The following values came from the Master Gardener Handbook but illustrates the relative compaction (force) that different activities can have on a soil.
• Person walking — 6 pounds per sq. inch;
• Crawler-type tractor — 12 pounds per sq. inch;
• Cow walking — 23 pounds per sq. inch;
• Horse walking — 40 pounds per sq. inch;
• Tractor with disk harrow — 150 pounds per sq. inch.
Recently, I’ve received a few questions about surface compaction in pastures and hayfields. Yes, it can occur, but is not as much a problem as in cultivated fields.
When a pasture is over-grazed during wet weather, I’m sure some compaction occurs from the animals. Note that trails where cattle walk frequently have nothing growing in them.
Likewise, if heavy equipment is pulled over a hayfield during wet weather, some soil compaction occurs. However, the good news is that plant roots, especially grass roots such as bahiagrass and bermudagrass do a terrific job of penetrating the soil and relieving potential compaction. As roots and rhizomes die and are replaced by new roots, organic matter is deposited and channels are opened up in the soil, resulting in a lower soil bulk density and less compaction.
Some have reported that bahiagrass is the best way of punching holes in a compacted soil.
Okay, so you suspect soil compaction is a problem in a pasture, hayfield, row crop, or garden. How can you be sure? Researchers have this device called a “pentrometer” which actually gives a measurement of soil strength when pushed into a soil.
Soil sampling tube also works
A stiff wire (surveyor’s flag), a welding rod, or a piece of rebar will work just as well. Those fortunate enough to have a soil sampling tube can use this to measure soil compaction.
The key is to do it when the soil is wet or at least moist. All dry soils have a relatively higher soil strength than wet soils. Roots do not grow into dry soils anyway so it makes sense to measure compaction when the soil is suitable for root growth i.e., when it is moist or near field capacity.
Don’t try this in mid-summer after a 3-week drought. Late fall, winter, and early spring after a good, soaking rain is the best.
Take a stiff wire (surveyer's flag) or welding rod or small rebar and try to push it into a very moist soil. Do this in several spots, especially if you have rocky or cherty soils. You should be able to push the wire or rod into the first few inches with little difficulty. If you cannot push a wire or rod into a moist soil, roots cannot grow into it.
Grass roots should naturally keep the first few inches of topsoil relatively permeable. If you don't have any vegetation, then it too, can get very compacted. If you cannot push it into the soil 3 or 4 inches, you definitely have surface soil compaction. This is a serious problem, because neither rainfall nor roots can get through this. A good, deep sod is the best solution to surface soil compaction.
Another problem arises when raindrops beat the surface of a bare soil into a thin crust such that seedlings cannot emerge through. This increases runoff and is a sure sign of poor soil quality. Crusting should be obvious without the wire or rod test.
In field crops, you can usually push the wire or rod down about 4 to 6 inches with no difficulty because this is the plow layer. However, if you encounter a stiff layer between 4 and 8 inches deep, you may have discovered a “hardpan” or “traffic pan” created by tillage equipment and tractors
With extra effort you should be able to break through this hard pan and then it will get easier to push the wire or rod on into the subsoil. A traffic pan is usually only an inch or two thick so we use subsoiling or paratilling to break up traffic pans.
Some soils have a natural "fragipan" or compacted subsoil about 18 to 24 inches or so deep. There is nothing you can do about this. An example is a Savannah fine sandy loam which is a lot more common in the Upper Coastal Plain of Alabama.
A Dickson soil has a pronounced fragipan and is found in north Alabama and Tennessee. These soils make pretty good pastures, but they tend to be wet in winter and very dry in summer.
What can I do about soil compaction?
First, find out why the soil is compacted and where the compaction is occurring and try to eliminate whatever caused it. Perhaps it was over-grazing a pasture or plowing a field when it was too wet.
Establishing a good root system is the best way to reduce soil compaction. That is one reason so many row crop farmers have switched to some form of high-residue, conservation-tillage. This system leaves old root channels in the soil to reduce soil compaction and increases surface organic matter.
Raindrops on a bare soil result in crusting, another form of soil compaction on the surface.
Rototillers, disk harrows, and turning plows are the worse machinery for creating subsoil compaction. This is another reason for the popularity of conservation-tillage or no tillage.
Subsoiling under the row and paratilling are very energy intensive operations, but these will break up traffic pans or hardpans and allow for deeper rooting of row crops.