Soil water is lost from the soil surface by evaporation and from the
rooting zone by plant transpiration.
The rate of water loss from the soil by evaporation
depends on the
drying capacity of the atmosphere just above the ground and the water
content in the surface layers. The evaporation rate is directly related
to the net radiation
from the sun which can be measured
with a solarimeter. Evaporation rates increase with higher air temperatures and wind speed or lower humidity levels. As water
evaporates from the surface, the water films on the soil particles
become thinner. The surface tension forces in the film surface become
proportionally greater as the water volume of the film decreases. This
leaves water films on the particles at the surface with a high surface
tension compared with those in the films on particles lower down in
the soil. The increased suction gradient causes water to move slowly
upwards to restore the equilibrium. Whilst the surface layers are kept
moist by water moving slowly up from below, the losses by evaporation,
in contrast, are quite rapid. Consequently the surface layers can become
dry and the evaporation rate drops significantly after 5 to 10 ‘mm of
water’ is lost. Evaporation virtually ceases after the removal of 20 ‘mm
of water’ from the soil. Maintaining a dry layer on the soil surface helps
conserve moisture in the soil below. Evaporation from the soil surface
is almost eliminated by a leaf canopy
that shades the surface, thus
reducing air fl ow and maintaining a humid atmosphere over the soil. Mulches
can also reduce water loss from the soil surface.
As a leaf canopy covers a soil the rate of water loss becomes more
closely related to transpiration rates. The potential transpiration rate
represents the estimated loss of water from plants grown in moist soil
with a full leaf canopy. It can be calculated from weather data (see
Table 19.2 ).
As roots remove water it is slowly replaced by the water film
equilibrium, but rapid water uptake by plants necessitates root growth
towards a water supply in order to maintain uptake rates. At any point
when water loss exceeds uptake, the plant loses turgor and may wilt.
This tends to happen in very drying conditions, even when the growing
medium is moist. Wilting is accompanied by a reduction in carbon
dioxide movement into the leaf, which in turn reduces the plant’s growth
rate (see photosynthesis). The plant recovers from this temporary wilt
as the rate of water loss falls below that of the uptake, which usually occurs in the cool of the evening onwards. Continued loss of water
causes the soil to reach the permanent wilting point because roots can
extract no more water within the rooting zone.
When the soil has reached its permanent wilting point (PWP)
is still water in the smallest of the soil pores, within clay particles and
in combination with other soil constituents, but it is too tightly held to
be removed by roots. Typical water contents of different types of soil at
their permanent wilting point (PWP) are given in Table 19.1.
|Table 19.1 Soil water holding capacity: the amount of water in a given depth of soil at field capacity can be
calculated by simple proportion
|Table 19.2 Potential transpiration rates. The calculated water loss (mm) from a crop grown in moist soil with
a full leaf canopy, over different periods of time and based on weather data collected in nine areas in the
Roots are able to remove water held at tensions up to 15 atmospheres
within the rooting zone, and gravitational water drains away.
Consequently the available water for plants is the moisture in the rooting
depth between field capacity and the permanent wilting point. The
available water content (AWC) of different soil textures is given in Table
19.1 . Fine sands have very high available water reserves because they
hold large quantities of water at FC and there is very little water left
in the soil at PWP (see sands). Clays have lower available water
reserves because a large proportion of the water they hold is held too
tightly for roots to extract (see clay).
Roots remove the water from films at field capacity very easily. Even
so, plants can wilt temporarily and any restriction of rooting makes
wilting more likely. Water uptake is also reduced by high soluble
salt concentrations (see osmosis) and by the effect of some pests and
diseases (see vascular wilt diseases). As the soil dries out, the water
films become thinner and the water is more difficult for the roots to
extract. After about half the available water content has been removed,
temporary wilting becomes significantly more frequent. Irrigating
before available water falls to this point helps maintain growth rates.
Plants grown under glass are often irrigated more frequently to keep the
growing medium near to field capacity. This ensures maximum growth
rates since the roots have access to ‘ easy ’ water, i.e. water removed by
low suction force.
The number of days each year that are available for soil cultivation
depends on the weather, but more specifically on soil consistency
(sometimes referred to as the workability of the soil). It also infl uences
the timing and effect of cultivations on the soil.
It is assessed in the field by prodding and handling the soil. A very wet
soil can lose its structure and flow like a thick fluid
. In this state it has
no load-bearing strength
to support machinery. As the soil dries out it
, then plastic. When plastic
the soil is readily moulded.
In general, the soil is difficult to work in this condition because it still
tends to stick to surfaces, has insufficient load-bearing strength, is
readily compacted and is easily smeared by cultivating equipment. As
the soil dries further it becomes friable
. At this stage it is in the ideal state for cultivation because it has adequate load-bearing strength,
but the soil aggregates readily crumble. If the soil dries out further
to a harsh
(or hard) consistency
the load-bearing strength improves
considerably, but whilst coarse sands and loams still readily crumble in
this condition, soils with high clay, silt or fine sand content form hard
resistant clods. The friable range
can be extended by adding organic
matter (see humus). At a time when bulky organic matter is more
difficult to obtain it is important to note that a fall in soil humus content
narrows the friable range. This allows less latitude in the timing of
cultivations and increases the chances of cultivations being undertaken
when they damage the soil structure.
Whereas many sands and silts can be cultivated at field capacity, clays
and clay loams do not become friable until they have dried out to well
below field capacity, i.e. heavier soils need more time for evaporation to
remove water through the soil surface.