Harvest Compaction: What to do and not to do.
With recent rainfalls getting us closer to average September precipitation, wetter field conditions could lead to compaction during harvest. This blog post will cover types of compaction, conditions conducive to compaction, and tips for avoiding or minimizing compaction.
Types of compaction
There are two main types of compaction that we will be dealing with. The first is surface compaction. Surface compaction is compaction that is less than 12 inches deep. This type of compaction is caused by high amounts of point contact, like narrow tires with a lot of weight. Surface compaction can lead to drastic yield reduction but can rebound in a few years timespan. This is particularly true if the field is in no-till. Studies show that no till fields have high amounts of biological activity leading to a reduction in prior compaction.
The second type of compaction is subsurface compaction. This is compaction that occurs over 12 inches deep. This will depend less on the amount of contact and much more on the overall axle load. If you have axle loads greater than 10 tons in wet conditions, you are likely causing subsurface compaction. It is much more difficult to get rid of subsurface compaction. Tillage equipment isn’t deep enough to break it up, and freeze-thaw cycles have relatively little impact.
Overall, a combination of surface and subsoil compaction will result in about a 15% yield reduction during the first year. This reduction in yield results from reduced root growth, low water infiltration due to decreased pore size, increased runoff and erosion, and reduced plant water and nutrient availability. Plants cannot grow well if access to water, nutrients, and air is restricted. Yield losses can still be around 3% from the initial compaction event for as long as 10 years. This is why we care about compaction.
Conditions favorable for compaction
Soils at or near field capacity are much more prone to compaction. This combination of water and air allows for the pore space to be compressed. When fields are at saturation, they are much less likely to be compacted, because the water is not able to be compressed like air, and pore space is maintained. They are however, much more likely to rut or for surface smearing which can inhibit emergence and plant growth the next spring. This graphic from the University of Wisconsin Extension diagrams the outcomes of compaction of wet soil at harvest.
Controlled traffic or repeated travel patterns are an excellent way to reduce compaction and restrict the impact of compaction across the whole field. This will contain damage to specific, repeatable locations within the field to maintain the undisturbed nature of the soil. This keeps traffic zones separate from rooting zones. Even if not practicing controlled traffic regularly, it is still wise to practice it seasonally. Make sure the grain cart follows in the combine tracks as much as possible. It may be tempting to “spread out the load”, but 85% of compaction occurs in the first pass. If you don’t follow in the combine tracks, a larger field percentage is now being compacted. However, if you do follow in the tracks, since 85% of the damage has already been done, further compaction is reduced and restricted to just that area.
If controlled traffic isn’t an option, utilizing duals or tracks can help reduce the point contact to decrease possible surface compaction. Similarly, reducing tire psi will reduce the possible field compaction by increasing flotation. Additionally, smaller load sizes will reduce both the surface and subsurface compaction. While this would be an inconvenience, it should be considered perhaps for portions of the field that are particularly wet, or in situations where a reduced field efficiency can be tolerated. Finally, always keep trucks outside the field edge. Move all grain to the trucks via grain cart to reduce surface compaction from smaller truck tires.
Tillage can also be used to manage surface compaction. However, make sure the soil is dry, as wet soil smears and is unable to fracture the compaction. It is important to note, that when tilling, 25% of the tractors power is compacting the soil the tractor is driving over. It takes that much again by the tillage tool to repair what the tractor just compacted. Essentially half of the power requirements of a tractor tilling is creating and breaking up its own wheel tracks.
This list from the University of Nebraska can be very helpful in outline steps to take to reduce or avoid compaction.
“10 Tips to Avoid Compaction on Wet Soils at Harvest Time
Wait until the soil dries enough to support the combine.
Don't use grain bin extensions or fill the combine as full.
Use wide tires with lower inflation pressures.
Keep trucks out of the field. Consider unloading at the ends of the field, not on the go.
Grain cart should track the same rows as the combine.
Don't turn around in the middle of the field.
Don't fill the grain cart as full, unload more often.
Establish a grain cart path and stay on it.
Don't till wet soils as they are easily compacted.
Use cover crops to help build soil structure.”