Quantifying root-induced soil strength, measured as soil penetration resistance, from different crop plants and soil types

A common soil mechanical property for assessing soil strength is soil penetration resistance (PR) or soil cone index (CI), which is related to the undrained shear strength of saturated and cohesive soil. Plant roots can increase soil strength, but physical conditions may confound this. Pot experiments were conducted using 70 cm soil columns, three soil types (beach sand, erosion-prone soil, and two typical arable soils), and four crop plants (maize, sorghum, Napier, and vetiver grass). We tested the hypothesis that plant roots impact soil strength, measured as soil PR, and the induced soil strength differs based on plant species. The CI and root system architecture (RSA) traits were measured. Napier grass grown in arable soils recorded higher total biomass. Together with maize, Napier grass had a more significant root length density, particularly at 25–40 cm depth. The CI increased with increasing depth, with a 57–99% increase in CI in the bottom layer compared to the top layer of the soil column. The overall CI of soils grown to Napier grass (2.0 and 2.3 MPa) and maize (1.7 and 2.2 MPa) were similar, but both were higher than the soils cultivated with the other crop plants and unplanted control. The overall CI of the SEA sand of ∼2.0 MPa was 36%, higher than that for the arable soils. Soil moisture content did not significantly increase CI, but the interaction of soil bulk density and root system traits could be implicated in increased CI of root-permeated soils. It is concluded that (i) roots growing in arable soils can increase CI and hence soil strength, possibly due to the binding effect of root systems, even when the transpiration effect of plants on soil moisture is low; (ii) crop plants contribute differently to soil strength, and (iii) Napier grass could offer a rapid growth and establishment option when considering plants for soil reinforcement and stability.