Agriculture Reference
In-Depth Information
6
Soil-Water-Vine Relationships and Water Management
Water Quantity and Water Energy in Soil
6.1
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 bio-
chemical 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
, defined as the volume of water per unit vol-
ume of soil (section 3.3.2), indicates how much water the soil can hold. How-
ever, 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
(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 and their units are explained in box 6.1. The terms
and
head
will
be used in this topic.
Components of the Soil Water Potential
Several forces act on soil water to decrease its free energy and give rise to
compo-
nent potentials
. These are adsorption forces, capillary forces, osmotic forces, and
gravity.
6.1.2
141
