Agriculture Reference
In-Depth Information
2.2.2.2
The energy status of water in soils
Water falling on the soil surface enters the pores and soil aggregates under the influence
of gravitational and capillary forces. Part of the applied water will run off over the surface
if the rate of input per unit area of surface is greater than its infiltration capacity‚ the rate
at which the soil surface will accept water.
The size-frequency distribution of the soil pores determines the maximum retentive
capacity of soils which is defined as the maximum volume of water that may be retained
in the absence of drainage. At this stage‚ water potential is close to zero. When water
input ceases‚ part of the soil water drains to the water table under the influence of
gravity and a new state of approximate equilibrium‚ field capacity‚ is reached.
Field capacity is defined as the water content of a drying profile after drainage from it
is arbitrarily judged to have become negligibly small. At this stage‚ capillaries less
than a given diameter remain full of water and the forces retaining water in the soil
approximately balance those of gravity. Matric potentials at field capacity depend on
texture and may vary from -0.01 MPa (pF 2.0) for a uniform sandy soil to -0.05 MPa
(pF 2.7) for one of fine texture. Because of this‚ the diameters of the capillaries
remaining full of water at field capacity range from approximately 6 to 30
As the soil continues to dry‚ a further characteristic stage is attained below which
many mesophytic crop plants can extract no further water from the soil: this is termed
the permanent wilting point. The capillary water extractable by plants (
i.e
.‚ from pores
greater than
ca.
0.15 in diameter) is exhausted and the water potential is -1.5 MPa
(pF 4.2). However‚ plants are capable of drying soils to potentials far below -1.5 MPa.
Root-zone matric potentials of -4.5 MPa (pF 4.7) have been recorded under winter wheat
crops (Papendick
et al
.‚ 1971) and -7.0 MPa (pF 4.9) from soils supporting the North
American xerophyte‚
Artemisia tridentata
(sagebrush) (Campbell and Harris‚ 1977).
Some few extreme xerophytes (“resurrection plants”) may survive drought almost
independently of soil water status and can withstand the near-complete desiccation
of their tissues (Gaff‚ 1981).
With further drying below -1.5 MPa‚ water only remains in progressively smaller
pores (Table I.12) until effectively no liquid water remains‚ although the soil atmosphere
still remains almost completely saturated with water vapour. The fall in the relative
humidity of the soil air with increasing water potential is slow and it is not until
potentials of -20 MPa are reached that it falls much below 90
per cent.
If‚ during the progressive drying of a wet soil‚ soil water potential is plotted against
the equilibrium gravimetric water content over a wide range of potentials‚ the resulting
characteristic draining curve describes how much water will be held in that section of
the profile. The nature of the characteristic draining curve differs from soil to soil
depending on soil texture‚ with fine textured soils retaining more water at any given
potential than those of coarse texture (Figure I.25). Soil structure also plays a part but
mainly at high water potentials. The shape of this curve differs when the soil is taking
up as opposed to losing water‚ a phenomenon known as hysteresis. This results from
several causes including the non-uniformity of pores‚ differential changes in structure on
drying and wetting‚ differences in the radii of curvature of advancing versus retreating
menisci in the pores‚ and the presence of entrapped air bubbles that may block pores
