Mechanical Resistance as a Soil Factor Influencing the Growth of Roots and Underground Shoots

Abstract

This chapter discusses the influence of mechanical resistance as a soil factor on the growth of roots and underground shoots. Although roots and shoots may grow, mainly, through existing voids in openly structured soils, whenever these organs penetrate peds or horizons that lack wide pores, they have to deform the soil. The soil resists deformation and the growing organ is stressed mechanically, by the reaction of the soil, to the force that the organ exerts. Mechanical resistance is the reaction of the soil to the forces exerted, by the growing plant. Various types of soil deformation can be produced by the plants. The forces required to deform soils includes: tensile failure, shear failure, with and without compression, and skin friction. Mechanical resistance can be assessed, by determining the strength parameters of soil and by empirical measures. The effects of pore water pressure and void ratio on mechanical resistance are also discussed in the chapter, along with the effects of mechanical stress (uniaxial stress, plane stress, triaxial stress, and bending moments) on the growth (cell division, cell enlargement, and cell differentiation) of roots and underground shoots. The growth of roots and underground shoots at a given temperature is influenced strongly, by the physical factors: mechanical resistance, water supply, and aeration. As soil dries, pore water pressure (uw) decreases and the plant exhibits a loss of turgor. If the forces exerted by plants arise almost entirely from osmotic turgor, the ability of an organ to exert force declines as uw decreases. Also, the rigidity of the root decreases as turgor is lost. The penetrating ability of the plant roots also gets reduced when uw decreases.