As gravitational (excess) water leaves the macropores, the air that
takes its place ensures that the root zone is replenished with ‘fresh air’ .
Horticultural soils should return to at least 10 per cent air capacity in the
top half metre within one day of being saturated (see porosity). Some
soils, notably those over chalk or gravel, are naturally free draining.
However, many have underlying materials which are impermeable or
only slowly permeable to water, and in such cases artificial drainage
sometimes referred to as field drainage or under drainage, is put in to
carry away the gravitational water (see Figure 19.3). This helps the soil to
restore air content rapidly without reducing the available water content. Well-drained soils
are those that are rarely saturated within the upper
90 cm except during or immediately after heavy rain. Uniform brown,
red, or yellow colours indicate an aerobic
soil, i.e. a soil in which oxygen
is available. Poorly-drained
soils are saturated within the upper 60 cm
for at least half the year and are predominantly grey which is typical of anaerobic
soil conditions. Between these extremes, imperfectly drained
soils are those that are saturated in the top 60 cm for several months each
year. These soils tend to have less bright colours than well-drained soil;
grey and ochre colours are usually seen at 450 mm giving a characteristic
rusty mottled appearance (see Figure 17.7).
Symptoms of poor drainage
Symptoms of poor drainage include:
- grey or mottled soil colours;
- restricted rooting;
- reduced working days for cultivation;
- weed problems;
- pest and disease problems;
- excess fertilizer requirements;
- topsoils water-logged for long periods in warm conditions have a
smell of bad eggs (see sulphur cycle).
Soil pits dug in appropriate places reveal the extent of the drainage
problem and help pinpoint the cause, which is the basis of finding the
solution. The level of water that develops in the pit indicates the current
water table. Further indications of poor drainage are the presence of
high organic matter levels (see organic soils) and small black nodules of
Soil colours show the history of water-logging in the soil. Whereas free
drainage is indicated by uniform red, brown or yellow soil throughout
the subsoil, the iron oxide which gives soil these colours in the presence
of oxygen is reduced to grey or blue forms in anaerobic
i.e. when no oxygen is present. Zones of soil that are saturated for
prolonged periods have a dull grey appearance, referred to as gleying
(see Figure 17.7). Reliance on colour alone as an indication of drainage
conditions is not recommended, because it persists for a long time after
efficient drainage is established (see also soil types p301–3).
Structural damage, whether caused by water (see stability), machinery
(see cultivation), or by accumulations of iron (see natural pans), is an
obstruction to water fl ow in the soil profile. Pans or platy structures near
the surface can be broken with cultivating equipment on arable land
or spiking on grassland; but subsoiling is used to burst those deeper in
the soil. If water cannot soak away from well-structured rooting zones,
artificial drainage is required.
The low permeability of many subsoils, which create a perched
water table, is the major reason for artificial drainage in horticultural
soils. Clay, clay loam and silty clays, when wetted, become almost
impermeable as the clay swells and the cracks close; clay is ‘puddled’
to form a liner for ponds. This ‘top water problem’ is dealt with by
putting in pipes to intercept the trapped gravitational water. Straight
lines of pipes are placed at an even gradient from the highest point to
the outfall in a ditch or main drain (see Figure 19.4). The pipes are laid
below cultivation depth in a series of parallel lines across the slope to the
headland of the area to be drained. Where a valley or the lower areas lie
within the area to be drained, a herringbone pattern is used.
|Figure 19.4 Drainage
should be placed at regular intervals to help to service the
system at points where there is a change of gradient or direction (see
Figure 19.4). The spacing between the lines of pipes depends on the
permeability of the soil, a maximum of 5 metre intervals being necessary
in clay subsoils. Soil permeability and the land use dictate the depth
of the drains
which is normally more than 60 cm. Drains should be set
deeply in cultivated land where heavy equipment and deep cultivation will
not disturb the pipes. Shallow drains can be used where rapid drainage
is a high priority and the pipes are not likely to be crushed by heavy vehicles or severed by cultivating equipment, e.g. gardens and sports
grounds. Pipe drainage is usually combined with secondary treatments,
such as mole drainage or subsoiling, to achieve effective drainage at
reasonable cost. Deep subsoiling improves soil permeability and the pipes
carry the water away. Installation costs can be reduced because pipes can
be laid further apart. Similarly mole drainage over and at right angles to
the pipes enables them to be spaced 50–100 metres apart.
The pipes are made of clay or plastic. The diameter
of the pipe depends
on the gradient available and the amount of water to be carried when
wet conditions prevail. Tiles (clayware) are usually 300 mm pipes either
75 mm or 100 mm in diameter (see Figure 19.1). These lead into a ditch
or a larger main drain. The tiles are butted tightly together to allow entry
of water, but not soil particles. It is recommended
that they are covered with permeable fill, usually
stones or clinker, to improve water movement into
the drains. Plastic pipes consist of very long lengths
of pipe perforated by many small holes and usually
covered with a rough felt to keep out soil particles.
An outlet into a ditch is very vulnerable to damage
and so it should consist of a strong, long pipe set
fl ush in a concrete or brick headwall so that it is
neither dislodged by erosion in the ditch nor by
people using it as a foothold. Outlet pipes should
be glazed to prevent frost damage. Vermin traps
should be fitted to prevent pipes being blocked by
nests or dead animals (see Figure 19.4).
is very much cheaper than pipe
drainage. A mole plough draws a 75 mm ‘bullet’,
followed by a 100 mm plug, through the soil at a
depth of 500–750 mm from a ditch up the slope of a
field or across a pipe drain system with permeable
backfill (see Figure 19.4). The soil should be plastic
at the working depth so that a tunnel to carry water
is created. The soil above should be drier so that
some cracks are produced as the implement is
drawn slowly along. These cracks improve the soil
structure and conduct water to the mole drain. Sandy
and stony areas are unsuitable because tunnels are
not properly formed or collapse as water fl ows.
Tunnels drawn in clay soils can remain useful for
10–15 years, but in wetter areas their useful life may
be nearer 5 or even as little as 2 years.
is used on sports grounds to remove
water from the surface as quickly as possible.
It involves cutting narrow trenches at frequent
intervals in the soil and infilling with carefully
graded sand that conducts water from surface to a
free draining zone under the playing surface.
French drains can be placed around impermeable surfaces, such as
concrete hard standings and patios, to intercept the run-off.
Maintenance of drainage systems
|Figure 19.5 Ditch. This is open
elements and easily
Artificial drainage is very expensive to install and must be serviced to
ensure that the investment is not wasted. Ditches
need regular attention
because they are open to the elements (see Figure 19.5). Weed growth
should be controlled; rubbish cleared out and collapsed banks repaired,
because obstructions lead to silting up or undercutting of the bank. The
design of the ditches depends on the soil type and should be maintained
when being repaired. The batter (the slope of the sides) on sandy soils
has to be less steep than on clays.
are a particularly vulnerable part of the drainage system,
especially if not set into a headwall. They should be marked with a
stake (holly trees were traditionally used in some areas) and inspected
regularly after the soil returns to field capacity. Blockages should be
cleared with rods and vermin traps refitted where appropriate.
need to be cleaned out regularly to prevent accumulated soil
being carried into the pipes. Wet patches in the field indicate where a
blockage has occurred. The pipe should be exposed and the cause of the
obstruction removed. Silted-up pipes can be rodded, broken sections
replaced, or dislodged pipes realigned.
At all times it should be remembered that the drains only carry away
water that reaches the pipes. Every effort must be made to maintain
good soil permeability and to avoid compaction problems. Subsoilers should be used to remedy subsoil structural problems. Once
drainage has been installed the soil dries more quickly, leading to better
soil structure because cracks appear more extensively and for longer
periods. Deeper layers of the soil are dried out as roots explore the
improved root environment, which adds to the improving cycle.
(‘bottom water’) problems occur where the water table
is too high and drains at the desired depth are of no use, because there
is nowhere low enough to discharge the water. An artificially low water
table can be created by pumping water out of the ditches up into a
network of waterways. A line of windmills that provided the required
energy was a familiar sight in lowland areas such as the Fens, where
such land reclamation was undertaken. Goundwater problems are a
common feature of many gardens where attempts to introduce drainage
systems are thwarted by there being nowhere suitable (or legal) to
discharge the water.