Watch for Soil Compaction Threat

Posted: May 4, 2017

Soil compaction threat increased in much of the state after significant rainfall over the past days. Avoiding compaction has been shown to be much more cost effective than repairing it.

Significant rainfall hit much of the state over the past couple of days causing soils to be saturated or at field capacity. In saturated soils all pores are filled with water, while in soils at field capacity large pores have emptied of water. The critical moisture point for soil compaction is the ‘plastic limit’, when soil can be easily molded. This is usually a bit dryer than field capacity. The plastic limit is the point when soil is most compactable. You can determine if your soil is at the plastic limit with the clod test – grab a handful of soil and mold it in your hand – if you can form a ball, your soil is at the plastic limit. Soil that is wetter than the plastic limit is sensitive to rutting and pugging. For all practical purposes, if your soil is at the plastic limit or wetter, it is better to stay out of the field. Remember that avoiding compaction is much more effective than causing it and then having to repair it. Here are some other things you can do to avoid compaction:

  1. Increase the resistance of your soil to compaction. Increasing soil organic matter is the first thing you can do. Research has shown that a soil high in organic matter cannot be compacted to the same density as the same soil low in organic matter. Having live roots in your soil at all times is another way to make your soil resist compaction – the live roots act like a geo-textile under the soil. Using no-tillage has been found to increase soil resistance to compaction because you have a firm matrix interspersed with macropores, which remain open for water percolation, aeration, and root penetration.
  2. Reduce the likelihood of surface compaction by using tires inflated to low pressures or increase vehicle footprint by using tracks. Surface compaction is caused by the stress put out on the surface and this corresponds very closely to the inflation pressure in completely flexible tires. Refer to your tire manual to check the lowest inflation pressure you can use for the load that tire has to carry. There are tires nowadays that can run at 12 psi pressures which is not likely to cause much surface compaction, even if soil is at plastic limit.
  3. Reduce the likelihood of subsoil compaction by reducing axle load. Axle load determines subsoil compaction. Our research suggests that if axle load remains below 10 Tons you are unlikely to cause compaction below 12 inches. However, manure trucks or grain carts may have higher axle loads than that.
  4. Keep your grazing animals in the shed when soil is too wet. It is much more costly to feed animals in the shed than in the pasture (at least twice as expensive), but this is the time you don’t want to cut corners because it can hurt you for a long time if your soil is compacted by your grazing animals. Fortunately, soil compaction caused by grazing animals has been shown not to cause subsoil compaction (ie it does not go deeper than about 8 inches).
  5. Make your soil more resilient. Resilience is the power to come back. A resilient soil can come back from compaction quicker and better than a non-resilient soil. Resilience has much to do with the life in the soil. If you have many earthworms and other tunneling animals (such as the lowly dung beetle, see last week’s FCN), mycorrhizal and detrital fungi, and living, growing roots in the soil, the soil is able to repair itself. These organisms are helped by not disturbing the soil, leaving crop residue at the soil surface, using diverse crop rotations, and using cover crops during fallow periods in your rotation.

Contact Information

Sjoerd Willem Duiker
  • Associate Professor of Soil Management and Applied Soil Physics
Phone: 814-863-7637