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Below the Surface: Some Facts About Soil Compaction
 
 
 
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Soils: September 2003
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by Wade Thomason

Compaction occurs when soil particles are pressed together, reducing pore space between soil particles and pushing out the air normally located there. Soil compaction can be a serious problem for agricultural producers. As a rule of thumb, it is assumed that air and water make up about 50 percent of the total soil volume, with the other 50 percent being soil particles. However, this can change dramatically as soil particles are pressed together to squeeze out air. The largest spaces are generally eliminated first, taking away the path of least resistance for air and water movement as well as for root penetration.

Compaction may displace water, but total water volume in the soil doesn’t change. Additionally, water makes the surface of soil particles slicker, so they will slide closer together when wet. Soils with a mixture of textures (some sand, silt and clay) are more susceptible to compaction than those with homogenous texture when exposed to the same compactive force. Plants that are growing on compacted areas are often stunted and have slower root penetration rates. Uneven emergence and slow early-season growth are also often experienced in these areas. Premature drought stress in compacted areas is also likely. Soil compaction has three main causes — equipment and animal traffic, tillage and rainfall.


Avoiding wet fields can reduce compaction.
Photo: Kiesling/Northeast Research & Extension Center

Rainfall intensity is out of our hands except that surface residue protects the soil from the full impact of the drops.

Tillage compaction is caused by heavy equipment that exerts all the mass on one point, like the bottom edge of a disk, or when tilling soil that is too wet. This problem is exacerbated by some of today’s larger equipment and the ability to plow too close to wet areas without getting stuck.

Remedies:
  • Plan tillage operations to avoid wet fields.
  • Reduce tillage.
  • Confine traffic to specific paths.
  • Encourage drainage to reduce areas of standing water.
  • Rotate crops with different root growth habits.

Approximately 80 percent of the total compaction from equipment traffic is produced in the first pass, so driving in the same row middles or on tramlines can help reduce whole field compaction. Adding duals does not help the problem as you actually compact twice the area of surface soil and add weight to the equipment. The factor with the greatest effect on deep compaction is equipment weight. The heavier the axle weight, the deeper the compaction. Compaction in the upper 6 inches is largely related to the inflation pressure (psi) of the tire. Radial tires exert a pressure of one to two pounds higher than their inflation pressure. For example, if a radial tire is inflated to eight psi, the tire exerts a pressure of nine to 10 psi on the soil. Since tracks and tires carry similar loads and have low soil pressure, they both exert similar stress onto the soil. Duals that allow more flotation may also contribute to the problem of tilling when the soil is too wet as well as adding more axle weight.

Remedies:
  • Use only as much weight on the tractor as necessary.
  • A long and narrow track, accomplished by larger diameter tires, is preferable to short and wide (duals).
  • Use radial tires with low pressure if possible.

High stocking densities increase the amount of foot traffic on a given area, especially when wet.

Remedies:
  • Remove animals from fields when soils are wet.
  • Reduce stocking rates on areas prone to compaction.
  • No-till planting into sods, like bermudagrass, gives increased resistance to compaction.

One caution about soil compaction … Do not just assume that an area is compacted. Dig down and see what’s going on below the surface.


  
         
       
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