Dry Fall Gives a Window to Alleviate Soil Compaction

Soil compaction affects soil's physical, chemical, and biological properties. It leads to reduced porosity, increased bulk density and penetration resistance, and reduced water infiltration and percolation. Reduced root growth in compacted soil leads to greater susceptibility to drought and can cause phosphorus and potassium deficiencies. Greater runoff can cause increased soil erosion and nutrient losses. Increased nitrogen losses are likely in compacted soils due to increased denitrification and greater ammonia volatilization from surface-applied manure. Soil biological activity is also inhibited in compacted soils. Now, the best way to manage compaction is to avoid it, but if you have caused it, this fall offers a great opportunity to alleviate it because soils are dry. Before you start, it pays to check if it is needed, and then select the right tool for the job.

Use soil and crop observations to diagnose whether soil compaction is severe enough to justify action. The soil compaction tester, or penetrometer is a tool to detect compaction. However, the gauge readings are meaningless at this time because soil is very 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 into the ground, and if there is a distinct layer of compaction, the resistance to push the probe into the ground will suddenly decrease when you are through the layer. Determine the depth of high resistance and run a subsoiler 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.

Once you discover that you have severe compaction, you need to select the subsoiler. If you don't have one, you may need to hire a custom operator or a neighboring farmer with a subsoiler, and they may have different types. I say subsoiler because it has the capacity to cut through the compaction instead of just scraping on top of a compacted layer. Modern subsoilers do not turn the 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 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^o 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.

Contemporary subsoiling is meant to be a one-pass operation so that crops can be planted immediately after subsoiling without secondary tillage. This is where the attachments come in. Soil tends to 'blow out' behind the shanks (especially when running at higher speeds), so attachments help to limit blow-out with wheels running next to the shanks or by having wheels or rolling baskets behind the shank to prepare a seedbed. To achieve soil conservation goals, more than 30% residue cover should be present after subsoiling and planting, so attachments should not cover residue but leave it on top. Kick-back mechanisms are another necessity on subsoilers. If not present, shear bolts will have to be replaced on a regular basis in our rocky soils, making subsoiling an arduous task. Next comes the depth to which the shanks should be set. The subsoiler should be set approximately 1 inch below a compacted layer (if present). A tractor that can pull the subsoiler needs to be available. Depending on soil conditions, you should count on at least 40-50 HP per shank.

Once subsoiling has been completed, it becomes necessary to have a plan in place to manage post-subsoiling traffic and build soil structure. The benefits of subsoiling are easily lost by recompaction with heavy equipment. In fact, the situation may be worse than before subsoiling, because the subsoiled field is more susceptible to rutting. Therefore, flotation tires should be used on all equipment and tire pressure should be reduced as much as possible to benefit from a large footprint. Do not exceed 10-ton axle loads, and limit repeated traffic to select areas of the field that can then be treated if needed. After subsoiling, plant cover crops to increase root mass in the surface and subsoil, and rotate crops with different root architectures, such as tap-rooted and fibrous-rooted crops.