The potential restriction to root growth in structurally weak tropical soils

Abstract

The penetration resistance was measured in undisturbed cores equilibrated at two matric potentials for three weakly structured soils from Nigeria. Within and between sites differences provided a wide range of dry bulk densities and water contents. Penetration resistance was inversely related to water content and significant whilst the relationship with dry bulk density was not significant. At −6 kPa matric potential (field capacity) penetration resistances were less than 1.5 MPa while at −100 kPa matric potential they were near to or greater than 2 MPa. The results suggest that potential restriction to root growth is likely to occur when such soils dry to a matric potential of < − 100 kPa. Across soils the variation in effective stress accounted for 62% of the variation in penetration resistance. For cores equilibrated at −100 kPa matric potential the penetration resistance was strongly correlated to unconfined compressive strength and indirect tensile strength of cores 23 mm in diameter and 40 mm in length (referred to as minicores). This suggests that the minicorer technique may be useful for rapidly assessing the root restricting behaviour of such soils. Critical values of unconfined compressive and indirect tensile strengths corresponding to a penetration resistance of 2 MPa, lower limit of restricted root growth in these soils were 28 kPa and 7.7 kPa, respectively.