The standard tool for diagnosing soil compaction is the penetrometer. It has a pressure gauge, a solid rod and a standard tip (usually of 0.5 inch diameter at the top).

It has to be pressed into the soil at approximately 1 inch per second when soil is at field capacity (a day or two after soil profile is completely soaked). Measuring at field capacity is necessary because penetration resistance will increase when soil dries out.

Root growth, however, will resume once the soils wets back up. The gauge tells you if you reach 'root limiting' compaction.

The tool was developed in completely pulverized soil packed to different densities. Seedlings were grown in these soils and it was found that roots did not penetrate soil if the penetration resistance exceeded 300 psi, and were starting to be inhibited when penetration resistance exceeded 200 psi.

In tillage literature you will find that if penetration resistance is less than 100 psi seed-to-soil contact is likely to be poor due to excessive air pockets (so soil that has been plowed has to be packed to 100 psi to get good seed-to-soil contact, that is why we use rollers and packers).

In conclusion, penetration resistance should be between 100 and 200 psi.

More information on the use of the penetrometer is found in Agronomy Facts 63 available from Penn State Extension.

While the penetrometer can give you a first assessment, it is not advisable to base your conclusion solely on it. This is especially the case in soil that has not been tilled.