Subsoiling in Fall to Alleviate Compaction

Soil compaction affects soil physical, chemical, and biological properties. Field traffic earlier in the year, when the soil was wet, may have caused detrimental soil compaction, and now that we have hit a drier stretch, it seems a great opportunity to do something about it. But before you get too excited, let's first check if remediation is justified, and, if so, what the right equipment and settings are.

Check soil compaction using soil and crop observations. The soil compaction tester, or penetrometer is a tool to detect compaction. The gauge readings are meaningless when soil is dry – the readings are valid after the soil profile has been thoroughly wetted 24 or 48 hours prior to measurement. At that moisture content, the readings correlate to the ability of roots to penetrate that soil. But this soil compaction tester can still help you detect if you have a compacted layer and at what depth. You detect that by pushing the probe in the ground, and if there is a distinct layer of compaction, the resistance to push the probe in the ground will suddenly decrease when you are through the layer. Determine the depth of high resistance, and run a subsoiler or other tillage tool just below that to break through the compaction layer. But don't rely on the penetrometer alone. Also, take a shovel and dig out some crop roots. If you see distinct platiness or a massive structure and roots that grow horizontally instead of vertically, you have another indicator of compaction. If roots are clearly restricted at a certain depth, this may call for action. Try to determine if compaction is limited to certain areas of the field – it is common that only headrows have severe compaction, but the rest of the field does not.

The depth of compaction will determine which tool to use to alleviate it. If the compaction is limited to 2 inches, a no-till drill may be as effective as anything else. The benefit of the drill is that you put cover crops in the soil, and their roots can help to alleviate soil compaction biologically, improve soil structure, and recycle nutrients at the same time. This may be all that is needed to manage soil compaction.

If the compacted layer extends 6-8 inches deep, a heavy chisel plow may do the job. Soil management principles we use suggest you do minimum inversion tillage to preserve surface organic matter and mulch cover, so the straighter the points, the better. Also, make it into a one-pass operation by having some type of spike-harrow and rolling basket behind the chisel plow.

If the soil compaction extends below 10-12 inches, you may need to revert to a true subsoiler. Modern subsoilers do not turn soil over, so that residue cover is preserved. This is important for erosion control and soil health maintenance. These subsoilers have narrow shanks that are not parabolic and have attachments that help to keep residue in place. Some have large winged points that heave the soil and cause much fracturing of the soil, even between shanks. Others have narrow tips that are meant to only create a vertical slot for deep root penetration and water percolation while doing less fracturing between the shanks.

Paratill subsoilers have bent-leg shanks. The shanks come down straight, then curve sideways on a 45-degree angle, whereas the tip is again positioned downwards. Research at the Soil Dynamics Lab in Alabama has shown that paratill shanks do maximum fracturing below the surface, take less power per shank than straight shanks, and do minimum surface residue disturbance. Make sure you have the right attachments on the subsoiler (berm tuckers besides or rolling baskets or press wheels behind) to make this a one-pass operation. Besides compaction, soil moisture content and depth of tillage determine how much horsepower it takes to pull a subsoiler through the field – but count on at least 50 hp per shank.

Although it is tempting to rely on tillage to manage compaction, it is much more beneficial to avoid compaction using the tools of the trade, such as using tires inflated to low pressures, tracks, moderate axle loads, monitoring soil moisture conditions before accessing the field, limiting field traffic as much as possible, rotating annual crops with perennials, and trying to keep a living root in the soil year-round. Our research has shown that on our well-drained soils, we can manage soil compaction in permanent no-tillage without doing any tillage at all, which is truly the better option.