A penetrometer is a gauged probe type tool which is used to measure the level of compaction at various depths in the soil profile.
As soil becomes more compacted, it loses its ability to absorb and retain moisture. The soil pressure in the aerobic zone (with oxygen) should not exceed 300 PSI (pounds per square inch). This is where the feeder roots of most plants exist and they are unable to penetrate higher levels of pressure. The anaerobic zone (without oxygen) is where the soil loses its ability to drain water and can no longer pull fresh oxygen in.