What is soil compaction. In any soil there will be found both organic and inorganic particles as well as macro and micro pores. Macro and micro pores are open spaces not occupied by a solid particle. Macro pores are openings that are greater then 0.03 mm in size. Micro pores are openings of less then 0.03 mm in size. These openings provide room for gasses such as Oxygen and liquid such as water. A soil that supports good root development has both macro and micro pores. Using the picture below as an example. The large open spaces would be macro pores and the small areas would be micro pores. Brown areas are solid particles.

When a soil is compacted the macro pores are reduced. In fine soils most of the pore space in compacted soils may be just micro pores. The total pore space is greatly reduced.

Soil compaction is can have a great impact on all plant roots. The more dense a soil is the harder it is for a roots to extend into it. Many roots depend on macro pores to provide space for a root tip to expand into.Also, the more dense a soil is the fewer macro pores and micro pores found in it. This means that there is less air space in the soil.Oxygen is the major limiting factor in regards to the depth tree roots can grow. Compacted silt and clay soils will have less available oxygen then non-compacted silt and clay soils. 

In the table below are several soil types with soil densities ranging from 85 to 100 percent. Listed to the right of each row is the "Critical Bulk Density". The critical bulk density is the maximum density the a tree root can grow in for the given type of soil. Any soil density above this amount will retard tree root growth.

Data for table taken from "Soil compaction and Site Construction: Assessment and Case Studies", by John M.Lichter and Patricia A. Lindsey.

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