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Wet conditions in 2019 have left ruts in fields across the region. The ruts can lead to long-term compaction problems if not properly dealt with. (NDSU photo)

Managing ruts and compaction during a wet harvest

Excessive moisture and ponding on farmer's fields has caused major difficulties during this fall's harvest and tillage operations.

Field work on excessively wet soils increase the risk of reversing soil aggregation and health, creating ruts and subsoil compaction, and smearing topsoils. When these risks turn into reality, the effects set up poor soil conditions for not only next year's crop, but likely for the next several years.

The difficult choices being made now will either exacerbate these risks or help to minimize them to future crops.

What you can expect:

• Ruts will cause compaction down into the lower parts of the topsoil and the upper parts of the subsoil. This deep compaction will reduce crop yields by 10% to 30% during the next one to three cropping seasons.

• Filling in ruts when soils are still wet will cause further damage. This happens for two reasons. First, the additional traffic will cause additional deep compaction unless operators drive in the existing ruts. Secondly, the tillage implement used to fill the ruts will cause soil smearing both vertically along the discs/shanks and horizontally along the bottom edge of the tillage depth. These smeared zones reverse soil aggregate and cause very poor physical conditions proper drainage and adequate crop root growth. A smeared topsoil only adds insult to injury for soils with deep compaction.

• The depth of ruts are not a good indicator of the depth or severity of the underlying compaction. A 6-inch rut made by high axle loads and over-inflated tires can cause twice as much underlying soil to compact as a 6-inch rut made by the same equipment with properly adjusted tire pressures. Deep compaction from ruts extends both vertically and horizontally under the tires. As compaction propagates deeper, the affected zone also becomes wider.

• The winter freeze will only help to alleviate compaction in the top few inches. Freeze-thaw cycles can break up some compaction. However, several to dozens of these cycles are needed to accomplish this. Soils in our region typically experience only one or two cycles below a 6-inch depth each winter. Additionally, soil moisture must be very high (more than 85% of all pores filled with water) before freezing will do anything to help alleviate compaction. If weather improves and allow soils to dry, then the odds decrease for freezing and thawing to be helpful in alleviating any compaction.

Recommendations:

• Wait for soils to dry or freeze before continuing field operations, such as harvest, tillage, or filling in and leveling ruts. Continuing operations on wet and rutted fields will further reduce next year's yields. This may mean leaving some of these operations until the spring and/or considering alternative practices for next year's crop. These may include reducing tillage passes or going without tillage for a year, adjusting planters for high residue fields, or a change in crop choice for next year.

• Wait until spring to fill in and level ruts. Use a tillage implement that is as deep or shallower than the ruts. If soils are still wet in the spring, drive in the old ruts to avoid causing additional deep compaction.

• Let deep compaction be naturally alleviated by soil cracking during next summer. Many soils naturally crack during the dry summer months. This cracking can help alleviate soil compaction several feet below the soil surface and is much more effective than freeze-thaw cycles or mechanical methods. Attempts to mechanically alleviate deep soil compaction by ripping is unreliable with oftentimes poor or detrimental results. In general, deep ripping only affects crop yields 50% of the time and place. When it does have an impact on crops, half of the time it improves yields whereas the other half of the time it further decreases yields due to smearing and breaking down soil structure.

(Aaron Daigh is an assistant professor of soil physics at North Dakota State University.)

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