Influence of growth media on the biomechanical properties of the fibrous roots of two contrasting vetiver grass species

Different artificial (non-soil) growth media have been used to cultivate plants for measuring their root biomechanical properties and estimating root reinforcement to soil. However, the effect of growth media on root tensile strength and Young's modulus has rarely been investigated. We studied the effects of three contrasting growth media (hydroponic, rice husk ash, and lateritic soil) on the biomechanical properties of the fibrous roots of two contrasting vetiver species (Chrysopogon nemoralis and Chrysopogon zizanioides). After growing in the different media for 7 weeks, the tensile strength and Young's modulus of both species were measured via uniaxial tensile tests. Roots grown in the ash were the strongest (10.21 ± 0.8 and 9.46 ± 1.1 MPa for C. nemoralis and C. zizanioides, respectively) and stiffest (275.98 ± 20.6 and 172.74 ± 26.3 MPa, respectively), followed by those grown in the soil (9.1 ± 0.9 and 6.18 ± 0.5 MPa for strength; 182.3 ± 16.9 and 81.88 ± 5.3 MPa for modulus) and then those grown hydroponically (5.99 ± 0.5 and 5.87 ± 0.5 MPa for strength; 107.06 ± 10.8 and 82.13 ± 7.8 MPa for modulus). Irrespective of the growth treatments, root strength and modulus were correlated with root diameter by a negative power law model (R² ranged between 0.11 and 0.68; p-value <0.05). The shape and scale factors of the strength–diameter and modulus–diameter correlations were different among the growth treatments. We caution the use of the tensile properties of roots grown in artificial media and laboratory conditions for predicting root reinforcement to natural soil on slopes.