Soil–Water–Vine Relationships and Water Management

Abstract

Water is a prerequisite for vine growth. It is essential for photosynthesis and to maintain the hydrated conditions and cell turgor necessary for a host of other bio­chemical processes in the plant. As we saw in chapter 4, diffusion of nutrient ions to the root, and their movement by mass flow into the vine’s “transpiration stream,” both depend on water. The volumetric water content θ, defined as the volume of water per unit vol­ume of soil (section 3.3.2), indicates how much water the soil can hold. How­ever, to understand what drives water movement in the soil, we must understand the forces acting on the water because they affect its potential energy. The energy status of soil water also influences its availability to plants. There is no absolute scale of potential energy. But we can measure changes in potential energy when useful work is done on a measured quantity of water or when the water itself does useful work. These changes are observed as changes in the free energy of water, which gives rise to the concept of soil water potential. The derivation of the soil water potential ψ (psi) is given in appendix 7. Historically, the energy status of soil water has been described by a number of terms related to soil water potential, such as pressure, suction, or hydraulic head. These terms ψ and their units are explained in box 6.1. The terms and head will be used in this book. Several forces act on soil water to decrease its free energy and give rise to compo­nent potentials. These are adsorption forces, capillary forces, osmotic forces, and gravity. Adsorption Forces. In very dry soils (relative humidity, RH, of the soil air <20%), water is adsorbed onto the clay and silt particles as a monolayer in which the molecules are hydrogen bonded to each other and the surface. With an in­crease in RH, more water molecules are adsorbed by hydrogen bonding to those on the surface. The charged surfaces of clay minerals also attract cations, and the electric field of the cation orients the polar water molecules around the ion to form a hydration shell, containing 6–12 water molecules.