Soil compaction around a small penetrating cylindrical body and its consequences

Penetrometer probes as well as roots and earthworms push soil particles (grains or small aggregates) radially during penetration and/or thickening due to growth. By this, the adjacent soil within a concentric layer is compacted. The degree of compaction depends on soil texture and soil physical properties that are influenced by soil moisture status. In the present study an estimate of the mean but constant increase of the initial bulk density and of the outer radius of this concentrically compacted zone, which are interdependent, is given. For instance, an increase from 1.2 g/cm³ to 1.6 g/cm³ yields a compacted zone of radius 0.5 mm when a body of radius 0.25 mm penetrates into a soil. This estimation is less laborious, but also less exact than that of Dexter (1987: Comparison of soil around roots. Plant Soil, 97: 401–406). This study shows that the distance between adjacent penetrations, when measuring resistance to penetration, should be ⪖ 10 times the probes radius. It is moreover supposed that the compaction produced by roots and earthworms makes it easier for them to anchor within (loose) soil. However, it possibly diminishes infiltration and exchange of soil solution.