Soil compaction occurs below the surface, so it's hard to spot. The tester's rod should move down through the soil with steady, even pressure. Hard, compacted soils resist penetration with the rod, often abruptly at a fairly uniform depth across a field or landscape area. For trees and shrubs, comparing root growth inside and outside the root balls is a quick way to tell if compaction is a problem. Soil pores may be reduced in size or eliminated altogether, usually as a result of pressure applied to the soil surface by equipment, foot traffic, hard rain on bare soil.
Wet soil is especially vulnerable to compaction.
It's always wise to keep all traffic off wet soils. Without a normal complement of sizable pores, plant roots find it difficult or impossible to grow and penetrate into new soil. Air, water, and your fertilizers and herbicides fail to infiltrate through surface crusts or hardpan layers in compacted soils, robbing roots of essential nutrients, water and oxygen. Plants are starved because roots are not functioning to capacity. Drought symptoms may appear, even with plenty of rain, watering, or even standing water on the soil surface, because water can't get through to the roots.