After a very wet 2018 and a wet spring in 2019, we are faced with dry soil conditions this fall that may offer opportunity to alleviate soil compaction by subsoiling.
Soil compaction affects soil physical, chemical and biological properties. It leads to increased bulk density, reduced porosity, increased 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 possible due to denitrification in soils that sit wet for too long. Ammonia volatilization of applied manure and urea is possible because the nitrogen does not soak into the soil as quickly. Soil biological activity is also inhibited. Last year was probably one of the worst years I have seen in Pennsylvania for soil compaction followed by a wet spring. It is very possible, therefore, that you still have remnants of compaction in your fields from field operations in 2018 and in Spring 2019.
Before you get enthusiastic about tillage, make sure to diagnose if you have a compaction problem first. To identify soil compaction use soil and crop observations. The soil compaction tester, or penetrometer is one means of compaction detection, but the absolute numbers on the gauge are meaningless since soil conditions are too dry. Nonetheless, this probe can help you detect if you have a compacted layer and at what depth that ends. You would 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 would suddenly decrease when you are through the layer. Determine at what depth that is so you can run a subsoiler just below that to remediate this compaction. But don’t rely on the penetrometer alone. Also, take a shovel and dig out some crop roots. If you see distinct thin layers or a massive structure and roots that grow horizontal instead of vertical 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 need to be treated while the rest of the field may be left untouched.
There are different types of subsoilers. With the recognized benefits of surface residue preservation, modern subsoilers do not turn soil over. They 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.
Contemporary subsoiling is meant to be a one-pass operation so that crops can be planted immediately after subsoiling without secondary tillage. The choice of attachment is therefore important because it determines surface residue reduction to a large degree. Soil tends to ‘blow out’ behind the shanks (especially when run at higher speeds) so attachments are available to push soil back to create a suitable seed bed. 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 re-compaction with heavy equipment. In fact, the situation will be worse than before subsoiling, because the subsoiled field is more susceptible to rutting. Therefore, use flotation tires on all equipment and reduce tire pressure as much as possible to benefit from a large footprint. Do not exceed axle loads of ten tons 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.
Source : psu.edu