Time to Evaluate Soil Compaction With a Soil Compaction Tester

The time to evaluate soil compaction with a soil compaction tester or 'penetrometer' is when the soil profile is at field capacity. Field capacity is about 24 hours after soaking rain (when free water has drained from the soil). The penetrometer simulates the resistance a root experiences. Research has shown that roots of most crops cannot penetrate a soil with resistance greater than 300 psi. This is the red area on the dial of the soil compaction tester. There is a yellow area between 200-300 psi which is when roots may be somewhat inhibited. Penetration resistance varies dramatically with moisture content – when the soil is dry it will be high, and when it is wet it will be low. The thought is that roots will start growing again when the soil moistens up as long as the penetration resistance is less than 300 psi in that moist condition.

To use the soil compaction tester, push it straight down at a rate of approximately 1 inch per second. The driving rod has gradations every 3 inches, which helps you record at what depth you reach the 300 psi threshold. The penetrometer is particularly effective in identifying a 'plow pan'. If you have a plow pan you will record high penetration resistance just beneath the usual depth of plowing, while resistance will decrease again below the plow pan. You can use this knowledge to set the depth of a tillage tool to remediate compaction. The tillage tool would be set at a depth just below the lower limit of the plow pan – deeper is unnecessary. Tillage needs to be done when the soil is in a relatively dry state, so it shatters well. We recommend a subsoiler that does little surface disturbance, but fractures soil below, to break up the plow pan but maintain the benefits of residue and limit disturbance of soil microbes and fauna. Tillage when soil is in the plastic state (when you can make a ball from the soil) will cause sidewall compaction which harms root growth, aeration and water percolation. While the soil compaction tester is helpful, it is just one tool in the diagnostics toolbox, because roots are not straight rods so they will follow pore spaces in the soil. For example, we usually find that roots follow nightcrawler burrows. It is therefore useful to check root growth by using a spade. This also allows you to visually check for compaction layers characterized by platy or massive structure. More detailed instructions for use of the compaction tester can be found in the factsheet Diagnosing Soil Compaction Using A Penetrometer.