Use of effective stress to predict the penetrometer resistance of unsaturated agricultural soils

• Published in: Soil & tillage research 2005-11, Vol.84 (1), p.18-27
• Main Authors: Whalley, W.R., Leeds-Harrison, P.B., Clark, L.J., Gowing, D.J.G.
• Format: Article
• Language: English
• Subjects: agricultural soils; Agronomy. Soil science and plant productions; Biological and medical sciences; Bulk density; clay loam soils; Effective stress; Fundamental and applied biological sciences. Psychology; loamy sand soils; Mechanical impedance; Penetrometer resistance; penetrometers; resistance to penetration; sandy soils; soil compaction; Soil science; soil strength; soil water content; Soil water status; Tensile strength; unsaturated conditions
• ISSN: 0167-1987; 1879-3444
• DOI: 10.1016/j.still.2004.08.003

One of the most important functions of soil is to provide an environment for root growth. Ideally, roots will elongate with minimal impedance and acquire water and nutrients so that crops may grow to their full potential. In practice, physical stresses in the root environment restrict root growth: one of the most commonly-cited physical stresses that affect root growth is “mechanical impedance”. When soils are strong, because they are compact or dry, roots elongate more slowly and this in turn has a detrimental effect on plant growth. In this paper, we examine the relative importance of compaction and soil drying as factors that lead to strong soil. We measured soil strength with both a tensile test and a penetrometer. We compared pressures of rotating and fixed (non-rotating) penetrometers. The utility of the rotating penetrometer is that it is thought to give a resistance to penetration similar to that experienced by elongating plant roots. For three soils, we confirmed published work reporting that effective stress can be used to define a relationship between tensile strength and soil water status that is common for a range of soils. Then, for loamy sand and silty clay loam soils at two dry bulk densities, we considered the relationship between penetrometer pressure and effective stress at matric potentials greater than −100 kPa. We found that for both soils, penetrometer pressure increased with effective stress and for the loamy sand, but not the silty clay loam soil, penetrometer pressure increased with density for a given effective stress. To examine the relationship between soil density and effective stress more fully, we measured the penetrometer pressures of five soils at two densities following equilibration at −100 kPa. We suggest that effective stress can be used to predict penetrometer pressure (provided the soil is compressible) and therefore the resistance to root penetration offered by the soil. Rotation decreased penetrometer pressure in the soils tested. Data obtained from the rotating penetrometer suggests that relatively moist soils (matric potential as high as −100 kPa) can provide high mechanical impedance to root elongation.