Environmental Engineering Reference
In-Depth Information
Soil water is thus best regarded in terms of the energy with which it is held by the
solid phase in the soil. The smaller the water content, the more tightly the water is held
by solid particles. This force is measured in units of
suction
, i.e. the force required to
remove a certain proportion of the soil water. Suction is measured in pascals or bars or
atmospheres (10
5
pascals/one atmosphere = one bar = 1000 mbar). Table 18.2 shows the
suction with which different classes of water are held in soil. Wilting point is fifteen bars'
suction, the limit between capillary water and hygroscopic water is over thirty-one bars,
and the water in an oven-dried soil is held at over 10,000 bars. Field capacity has been
defined at various suctions in the range 0·33-0·05 bar. Table 18.2 also indicates the pore
diameter corresponding to the soil suction, the physical appearance of the soil, and the
availability of the water to plants.
The amount of available water which can be stored in any soil is influenced by soil
texture, soil structure and the organic matter content. These factors have a marked effect
on the size and distribution of the pore spaces. The influence of texture on
storage
capacity
or
available water-holding capacity
is indicated in Table 18.3. In many parts of
the world soil moisture is probably the major factor limiting crop production, so this
property of soils is of enormous economic importance. The most accurate method of
determining the water content of a soil sample is to measure the loss in weight when a
moist soil is dried in an oven at 105° C overnight. The moisture content is expressed as
the loss in weight as a percentage of the ovendried soil.
DRAINAGE AND INFILTRATION
The content of available water in soil correlates fairly closely with total pore space, i.e.
porosity
. Fine textures like clays and clay loams are able to hold considerably more
available water than coarse textures such as sands and sandy loams (Table 18.3). Excess
gravitational water will drain away from soil in macropores or transmission pores larger
than about 0·05 mm diameter. The ability of a soil to allow water to pass through in this
way is its permeability. Again it is closely linked with soil texture and soil structure. It
has no relation to total porosity; clay soils with high porosity usually have low
permeability, and vice versa with sandy soils. The important characteristic of soils with a
high permeability is their high content of large pores, wide cracks or faunal burrows and
channels (Plate 18.3). The soil physicist Darcy defined the ability of a porous medium to
transmit a fluid as the hydraulic conductivity,
K
, in units of centimetres transmitted per
hour. It is almost identical to permeability. Table 18.4 shows some typical values of
K
for
representative textures and structures.
Soils with a horizon or horizons of low hydraulic conductivity will not allow
gravitational water to drain away. This will promote waterlogging both in and above such
horizons, to the exclusion of air, with adverse effects on the respiration of plant roots,
macro-organisms and microorganisms. Waterlogging can also bring about chemical
changes through the process of
gleying
under conditions where oxygen is excluded
(
anaerobism
) (Chapter 19). Except for rice, agricultural crops are stunted or killed by
Table 18.2 Types of soil water
Suction held
(bars)
Water constant
Pore diameter
(mm)
Type Physical
state
Availability