Agriculture Reference
In-Depth Information
Box 6.4
Estimating the Pore-size Distribution of a Soil
h
g
) is inversely proportional to
the radius of the largest pores holding water at that potential. Using this
relationship, the radius of pores
r
Equation 6.1 shows that the potential
(
r
(delta r, where
r
is a small change in
r
)
that release water for a small decrease in potential,
m
, is known. The volume of
these pores can be calculated from the change in
of the soil. Thus, the slope
d
m
of the soil water retention curve, as shown in figure 6.3b, can be used to
define the soil's pore-size distribution (provided that shrinkage of the soil as it
dries is small). The larger the value of
d
/
d
m
, the greater the volume of pores
holding water within the size range defined by
/
d
r
. The curves in figure 6.3b show
that the sandy loam soil has a majority of large pores (macropores) that release
water at low suctions, compared with the clay soil, which has mainly small pores
within aggregates (micropores) that release water at high suctions. Macropores
promote drainage and aeration, whereas pores within aggregates are more likely to
hold water in the “available range” (section 6.2.2).
tionship. The volume of water held between these two points defines the soil's
available water capacity, AWC
, which is water nominally available for plant growth.
The upper limit of
AWC
is set by the field capacity
FC
(section 3.3.3). This point
corresponds approximately to the water content at which the macropores are
drained, but micropores within aggregates remain filled with water. Measurements
with tensiometers show that soils attain
FC
at
m
values between
5 and
10
kPa. The value of
10 kPa is accepted for soils in Australia.
The lower limit of
AWC
is set by the
value at which plants wilt, that is,
the
permanent wilting point
,
PWP
. Wilting indicates that the rate of transpiration
exceeds the rate of water movement to the roots, and the plant loses turgor. For
many plants, the critical leaf water potential,
l
, at which wilting occurs is about
1500 kPa (
1.5 MPa). The onset of permanent wilting depends not only on
l
to maintain a favorable
gradient for water flow, and hence rate of water flow, from the soil to the root.
Thus, both
FC
and
PWP
depend to some extent on the dynamics of water move-
ment in the soil, as discussed in section 6.3.
in the soil, but also on the plant's ability to lower
Dynamics of Soil Water
6.3
Infiltration and Runoff
Water entry into soil is called
infiltration
. The water arrives as rain or irrigation,
but the vigneron has little control over the former. Whether all the rain that falls
is absorbed depends on the condition of the soil surface and the rate of rainfall.
The latter is called the
rainfall intensity
(box 6.5). If the surface soil aggregates are
stable, the soil will have a high initial infiltration rate, especially when it is dry.
This is usually the case under a grass cover crop in the vineyard. But where the
inter-rows are clean-cultivated, structural changes can occur in the surface fol-
lowing raindrop impact. Aggregates may slake and disperse so that small particles
6.3.1
