Answer: False. Clay soils are no more vulnerable to compaction from traffic than lighter soils, Reeder says, contrary to earlier theories. However, soils with a high proportion of clay tend to have poor internal drainage and are apt to stay wet longer than coarse, sandy soils, Duiker says. And wet soils are easily compacted.

On all soil types, compaction increases soil bulk density, and past a certain level, restricts root growth, Duiker says. That level differs with soil texture. For example, the ideal bulk density for clay soils is smaller than for loamy or sandy soils, which have larger particles and bigger pore spaces, compared to fine-textured soils.

Penetration resistance is another way to measure the effects of soil compaction on root growth. Regardless of soil type, root growth begins to decrease with penetration resistance of 100 psi, and stops completely at 300 psi. Try to keep penetration resistance under 200, DeJong-Hughes says.

Regardless of soil type, root growth stops completely when soil penetration resistance exceeds 300 psi, as in these wheel tracks.