Research has tentatively identified three criteria that may be used to recommend a limit to soil compaction. These criteria are:
(1) keep greater than 10% air-filled pore space to avoid limiting gaseous exchange with the atmosphere that may be necessary for biological activities such as root respiration.
(2) avoid pressure stresses that will shear soil aggregates and disrupt soil porosity necessary for air and water movement.
(3) avoid creating soil resistance that will limit root growth.
Unfortunately, research has not yet identified a field measurement that allows us to apply the above criteria. The problem is illustrated by the fact that a Nicollet clay loam at 1.4 gm/cm3 bulk density will have a penetration resistance of 10 kg/cm2 at 25.4% moisture, 20 at 22.5% moisture and 50 at 19.1% moisture. While the penetration resistance measurement can be taken quickly in the field, the moisture content and bulk density measurements take longer and could be different for each soil type.
Applying all three criteria to soil measurements at research sites gave a load limit of 2 kg/cm2 (28 psi) for Nicollet clay loam and 3 kg/cm2 (42 psi) for Webster clay loam with both soils at 20% water content by weight. If the water content is increased to 27% the allowable load is reduced to 13 psi on the Nicollet soil and 21 psi on the Webster soil.