Agriculture Reference
In-Depth Information
Table 12.3 Soil water-ho lding capacity of different soils
Soil texture
Water held in 300 mm soil depth (mm)
At field capacity (FC) i.e. water-
holding capacity (WHC)
At permanent wilting
point (PWP)
Available water (AW)
Coarse sand
26
1
25
Fine sand
65
5
60
Coarse sandy loam
42
2
40
Fine sandy loam
65
5
60
Silty loam
65
5
60
Clay loam
65
10
55
Clay
65
15
50
Peat
120
30
90
soils in the lowland areas of Britain and Ireland hold
about an average month's rainfall (60 mm) in a topsoil
of 30 cm. Coarse sandy and gravelly soils hold far less
and peaty soils hold much more.
Draining soils
Wet soil problems should be tackled according to
the cause or causes. Many of the symptoms of poor
drainage occur when there are compacted layers in
the cultivation zone. This is essentially a soil structure
problem that should be dealt with by appropriate
cultivation to remove the obstruction. If the problem is
as a result of water being held back by the conditions
below cultivation depth, then drainage may be
installed to advantage.
Improving poor drainage by removal of gravitational
('excess') water trapped in the root zone should be
undertaken before any other remedial work on a soil. It
is normally achieved on large areas by laying drainage
pipes, such as 'clays' (Figure 12.14) or perforated
plastic pipes, in parallel lines under the ground about a
metre deep and taking the drainage water to a ditch.
In domestic situations, the most common problem
is the large amount of water running off buildings
and hard areas such as patios and driveways on to
borders and lawns. A French ('rubble' or 'interceptor')
drain can be placed across where the run-off occurs.
A trench is dug where it can intercept run-off and
lined with woven fibre. A line of clay (tile) drains,
perforated plastic pipe or even 'rubble' is laid with a
slope and the trench is back filled with gravel. It can
be planted up if the trench is part filled with gravel
and 'blinded' with sand or woven fibre before putting
on a layer of soil. The pipe then leads the water away
for disposal in a ditch or a soakaway. Note that 'bye-
laws' normally prevent you discharging water into
one of the many pipes servicing your house. Finding
a discharge point below the level of the drainage
pipe is a problem in a garden surrounded by other
gardens. Most people have to resort to a soakaway
which works very well if the bottom of the hole
is permeable; much less so if it just a large water
reservoir.
Gravitational water is the water that can be
removed from the soil by the force of gravity.
Drainage
The continued addition of water eventually brings
all the root zone to field capacity. In many soils the
gravitational ('excess') water can drain naturally into
the lower depths or because some form of artificial
drainage has been installed. This ensures that
saturation in the root zone is temporary. However,
when more water is added to soils overlying impervious
material such as non-porous rocks and some subsoils
(e.g. wet clay and clay loams), it leads to poorly drained
soils. In these air only gets into the top layers of the soil
during the drier parts of the year. A water table marks
the level below which the soil is saturated with water.
This usually varies over the year with the water table
falling to a minimum in the summer. A hole dug in the
ground will fill up with water to the water table.
Drainage is the removal of gravitational (excess)
water.
Saturated ( waterlogged ) soils can be recognized by:
X standing water and surface run-off
X grey or mottled soil colours (Figure 12.13)
X smell of hydrogen sulphide - 'bad eggs'
X indicator plants (such as rushes, mosses)
X weed problems
X restricted rooting
X reduced working days for cultivation
X pest and disease problems, e.g. clubroot (see p. 256).
 
Search WWH ::




Custom Search