Agriculture Reference
In-Depth Information
2.2.4
A BROAD CLASSIFICATION OF SOIL WATER
While the amount of water retained in a soil is a continuous function of the energy
required for its extraction (Figure I.25), it is convenient‚ and meaningful in terms of
the behaviour of certain crop plants‚ to consider three broad categories of soil water‚
based on their energies of retention within the soil matrix:
1.
Gravitational water fills the larger pores (minimum diameter range 6 to >30 )and
is retained in the soil with little energy. It drains rapidly (from pores greater than 50
diameter) or slowly (from pores of 10 to 50 diameter) towards the water table
under the influence of gravity. This fraction of the soil water is that held at
potentials of greater than approximately -0.03 MPa and is indicated as region A
in Figure I.25.
Capillary water corresponds to the water held in soil pores or on surfaces with
potentials in the range -0.03 to -3.1 MPa (region B in Figure I.25). It also corresponds
to water held in pores between the approximate diameters of 0.1 and 6.0
2.
Below
potentials of -3.1 MPa‚ little further capillary water remains.
Hygroscopic water forms a thin‚ tightly-adsorbed (matric potential less than -3.1
MPa) layer held on the surfaces of the soil particles‚ particularly the clay minerals and
is represented as region C in Figure I.25. This category includes the inter-layer water
bound to the surfaces of clay particles by covalent and hydrogen bonds (Figure I.24)
and inter-particle water adsorbed within small tactoids formed from aggregated clays.
3.
The mass of water held in a given volume of soil between the approximate potential
levels of -0.03 MPa (field capacity) and -1.5 MPa (the permanent wilting point) corre-
sponds to the plant-available water. This is the reserve (R) of water available to
certain mesophytic agricultural crop plants and is expressed in millimetres of water
using the formula:
where F and W are the percentages of water per unit mass retained at‚ respectively‚
field capacity and wilting point‚ D is the depth of soil and BD is the bulk density
(Duchaufour‚ 1997).
At a profile scale‚ the total water reserve may be calculated by summing estimates
made for each horizon. Such reserves may range from 50 to 60 mm for a shallow
( ca. 30 cm deep) soil to more than 400 mm for the water available to strongly
rooting plants in a deep silty soil (Mériaux‚ 1979). The store of “plant-available” water
calculated in this way will underestimate that actually available because the roots of
many plant species are infected with mycorrhizal fungi. Water held at potentials
substantially lower than -1.5 MPa may be made available to their host plants through
the medium of these fungi (Chapter IV.3.2.1.2). Further water may also be extracted
from the weathering zone beneath the soil (the R layer) by deeper-rooted species.
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