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Sports turf typically gets much heavier use than commercial or residential turf areas but each may suffer the effects of compaction that results from traffic. Soil particles are squeezed together with the upper 3/4 to 1-1/2 inches of soil becoming the most compacted. This condition resists water penetration and limits the effectiveness of top-applied fertilizers.

Mergenthal's plan to adapt army surplus equipment for turf aeration appeared to work until he noticed an excessive buildup of iron in the soil.

The problem goes much further: the relationship between air, soil and water is upset. Roots need oxygen for healthy growth but compacted soil holds built-up carbon dioxide and prevents oxygen infusion. Roots lose their ability to absorb the water and nutrients that may still be available in the packed soil. This is especially problematic in short-rooted turf grasses that must rely on the top few inches of soil for nutrition.

Compaction and Thatch
When a soil compaction condition is accompanied by excessive thatch buildup, as is almost always the case in poorly maintained turf, each condition contributes to the effect of the other. When thatch exceeds about 1/2 inch of undecomposed material, it acts as a barrier to water and air infiltration into the soil below and will provide an environment encouraging turf diseases and harmful insects. Compacted soils, on the other hand, are subject to greater temperature extremes than loose soils, because of limited air movement; microbial activity necessary to thatch decomposition is reduced or halted.
Water that cannot penetrate the soil runs off or accumulates in low spots where it harbors fungus growth.
Alleviating either condition will help, but only when thatch is kept under control and the soil is properly aerified will turf have the best chance for healthy, vigorous growth and disease resistance.

What To Do
Dethatching and aerifying should be done as part of the same process. Once the soil has been deeply, slowly watered to about a 6-inch depth to soften the compaction then:

• mow with a dethatching mower
• rake off the thatch
• aerify
• test soil (pH, etc.)
• fertilize as needed
• irrigate.

Dethatching mowers use a horizontal, rotating bar with vertically arranged cutters that slice the interwoven network of dead and living grass plants. The resulting short lengths of this material are loosened and brought up from the base of the live grass stalks where they can be raked away.
Aerifying—known commonly as aerating—may be done with a variety of devices. What they should all have in common is the ability to draw cores from the turf. Generally, the core depth is 3 to 4 inches and the core diameters vary between 1/4 and 5/8 inch. The core holes in the turf should be spaced about 2 inches apart. Perhaps the best machine for working large areas is a piston driven aerator that thrusts the core cutters vertically. Direct up and down coring leaves a clearly defined hole. Drum-type roller aerators will work but may cause tearing damage to the remaining grass since this type of cutter enters the turf at one angle, moves in an arc with the drum movement, and is withdrawn at a different angle.
Aerators must pull a core from the soil. Merely piercing the soil does not have the same beneficial effect. The benefits of aerifying turf include:

• improved water absorption
• exchange of CO2 and oxygen
• better placement of fertilizer
• loosened soil texture
• decomposition of remaining thatch particles
• improved germination of reseeded and overseeded turf
• revitalized, deeper, healthier root systems
• better success at introducing top dressings

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