Avoiding Compaction at Harvest
With harvest underway across Nebraska, some producers are concerned about compaction as their large combines and grain carts move across the fields. In some areas, fall rains have left the fields soft and ruts are being cut into the soil during harvest. These ruts leave the soil surface rough and have severe compaction below them. This compaction can impede the crop's roots next season and increase runoff because of reduced infiltration.
If the combines and grain carts aren't leaving a rut, don't worry about compaction from the heavy equipment. Compaction is the loss of pore space between soil particles and occurs when that space is squeezed out of the soil and reappears somewhere else, such as in the form of a rut. If a rut wasn't formed, there was enough soil structure present to support the weight without causing additional compaction.
If ruts were formed during harvest, tillage can break up compaction but the soil must be dry to fracture compaction. If the soil was wet enough to cause ruts, the odds are that it is too wet to do tillage. Tilling a wet soil causes more compaction as the soil particles are lubricated and easily slide under the weight of the tractor and tillage implement. This compaction is harder to see because the entire soil surface is compacted, even though the surface looks loosened. Deep tilling a wet soil often only cuts slots and smears the soil rather than fracturing compaction.
Regardless, tilling destroys soil structure and more tracks will be formed with future passes. Typically ruts are as deep as the soil was tilled, down to the compaction layer from the tillage. The majority of compaction is caused primarily by tillage. It breaks up the existing soil structure and packs the soil below the tillage depth. With little soil structure in the tilled layer, the next pass easily compacts the soil, either full width with tillage or in tracks with traffic.
Controlling Traffic, Controlling Compaction
Producers should practice controlled traffic to reduce the areas in the field with wheel traffic compaction. Eighty to 85 percent of soil compaction damage is done with the first pass of the tires. If additional passes are made on the same traffic lanes, little additional compaction occurs. Because once a traffic lane has been driven on and the soil has been firmed up, subsequent passes have little effect on the amount of compaction. By using the same traffic lanes year after year, the soil structure and water infiltration in the untrafficked areas greatly improve.
Controlled traffic lanes improve traction, soil load bearing, and timeliness of planting and harvesting operations while minimizing potential yield reduction from compaction. Compaction is managed, not eliminated, and the area subjected to compaction is minimized. The concept is to separate traffic zones from root zones. Controlled traffic keeps compaction where it is less detrimental to root development and uptake of nutrients and water. Fertilizer placement and furrow irrigation practices can be modified as these traffic zones are established and the traffic lanes are known.
To minimize wheel compaction at harvest time, grain carts should be following the same tracks as the combine. A lot of grain cart drivers think they should move over a few rows and spread out compaction, but this will only compact more of the field. Likewise, grain trucks shouldn't be driven in the field as the axle loads and tire pressures are not suitable for soils.
If ruts were cut at harvest, wait until the soil is dry to smooth them out to avoid causing additional compaction. This smoothing operation may be a light tillage operation next spring before planting. Deeper tillage in the spring will usually cause more compaction as the soil is wet and the tillage will break up soil structure.
To fracture the compaction in the ruts from this year's harvest, a producer may have to wait until next fall before the soil is dry enough. However, often the compaction in the bottom of the ruts extends deeper into the soil than most producers will be able to till. This is a case where prevention is far more effective than the cure. It's best to build soil structure and not drive on wet soils if possible. Controlled traffic, no tillage, and cover crops will all help build soil structure and reduce compaction concerns.