Agriculture Reference
In-Depth Information
Table 12.4 Potential transpiration rates. The calculated water loss (mm) from a crop grown in moist soil with a
full leaf canopy based on weather data collected in different areas in Britain and Ireland
Area
April
May
June
July
Aug
Sept
Ayr
50
80
90
80
65
40
Cheshire
50
75
80
90
75
45
Channel Isles
50
85
90
100
85
45
Glamorgan
50
80
85
85
75
45
Hertfordshire
50
80
90
95
80
45
Northumberland
45
65
80
75
60
35
N Ireland (coast)
45
70
80
80
70
40
water loss from an area covered with leaves is not
as rapid as from a wet bare soil surface, it is now
being lost from the whole root zone. The losses from
ground covered by foliage for different lowland areas
are shown in Table 12.4. Compare your local rainfall
figures with the losses of water from full crop cover
that occur in your area and you can see what sort of
soil moisture deficits you might have in your area over
the summer months.
As the drying of the soil in the root zone continues,
it becomes more and more difficult for plants to
extract and the plant's cells begin to lose turgor
(see p. 120). The leaves begin to look 'stressed'
and they wilt. Growth is affected because flaccid
(wilted) leaves are less effective at intercepting
sunlight. It also leads to the stomata closing to
reduce the water loss (see p. 123), which, in turn,
affects photosynthesis (see p. 112). The inability
of the plant to keep up with water loss may occur
even in ideal soil conditions with the roots taking
water up efficiently. A temporary wilt of a plant
commonly occurs when there are very strong drying
conditions, especially hot and windy conditions.
Typically in Britain and Ireland this occurs on
summer afternoons; then the plants recover as the
temperature drops and the rate of transpiration falls.
However, as more water is lost from the root zone,
temporary wilt becomes more frequent. If water is
not added to the soil, there comes a point when no
more water can be taken up by the plant so there
is no recovery of turgor overnight. This is when the
permanent wilting point of the soil (PWP) has
been reached. At this point most sandy soils have
virtually no water left in them whereas clay loams
may have 15% left inside the clay particles and in
the very smallest pores (see Table 12.3).
Available water
The water that plants are able to take from a soil is
that held between field capacity and at the permanent
wilting point (see Table 12.3). The amount of water is
made available for plants by increasing:
X root exploration by eliminating poor structure
X rooting depth
X organic matter levels.
Roots remove the water at field capacity very easily.
Even so, plants can wilt temporarily and any restriction
of rooting (e.g. caused by soil pan, phosphate
deficiency, root disease) makes wilting more likely.
Water uptake is also reduced by high soluble salt
concentrations such as excess use of fertilizers (see
osmosis, p. 120) and by the effect of some pests (see
vine weevil p. 242) and diseases (e.g. vascular wilt
diseases).
Available Water Content (AWC) is the water
held in the soil between field capacity and the
permanent wilting point.
It is after about half the available water content has
been removed that temporary wilting becomes
significantly more frequent. Watering the soil before it
reaches this point helps to maintain growth rates.
Adding water
For most deep-rooted plants (trees and shrubs) there
is usually no need for water after they are established.
Ideally, once seeds and plants have been 'watered in'
there should be no need for more unless significant
amounts of water are lost from the soil, that is, about
half the available water in the root zone. Plants with
roots down to 50 cm in loamy soils will have access
to an available water content of about 100 mm (see
Table 12.3); half this is about 50 mm of water. This is
a soil moisture deficit of 50 mm, that is, the amount
of water to add to return the soil to field capacity.
The permanent wilting point (PWP) is the water
content of the soil when a wilted plant does not
recover overnight.
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