Agriculture Reference
In-Depth Information
he
AWC
measures the amount of water held between
FC
and the permanent
wilting point. The product of
AWC
and soil depth (in meters) determines the
potential
PAW
for a given soil profile. As shown in table 4.4, chapter 4, sandy clay
loams and clay loams have the most available water per centimeter of depth and
consequently have the largest potential
PAW
. Achieving this potential, however,
depends on the effective rooting depth of the vines. For this reason, deep-rooting
vines, such as on Ramsey rootstock, are likely to be the most drought-tolerant
when grown under low rainfall on deep sandy clay loams.
For irrigated vines, the depth of soil having a large proportion of readily avail-
able water (
RAW
) and deficit available water (
DAW
) can be more important than
the potential
PAW
, depending on the variety and whether regulated deficit irriga-
tion (RDI) is applied. For example, a white variety such as Chardonnay does not
benefit from water stress and does best on a deep soil with large
RAW, DAW,
and
PAW
. On the other hand, a vigorous red variety such as Shiraz (Syrah) is better
grown on a relatively shallow soil of smaller
RAW
and
DAW
, so that RDI can be
used more effectively to manipulate berry size and grape quality (see “Managing
Soil Water” later in this chapter).
In addition to these aspects of structure and water supply, soil aggregation
and aggregate stability are important for optimum soil condition. Figure 4.3 in
chapter 4 shows an ideal topsoil structure of predominantly crumb-like aggre-
gates, best formed when organic matter is naturally high (>1.7% organic carbon
[C] or about 3% organic matter) and well humified. However, the role of organic
matter is less significant in the subsoil where the type of clay mineral and the
exchangeable cations are more important. Iron (Fe) oxides (identified by their
red color) and aluminum (Al) oxides contribute to stable subsoil structures, as
do clay minerals that have calcium ions (Ca
2+
) as the dominant exchangeable cat-
ion. Figure 4.4 in chapter 4 shows the ideal subsoil structure of a Terra Rossa in
the Coonawarra region, South Australia, which consists of large, porous aggre-
gates stabilized by Fe oxides, with well-defined cracks between the aggregates. The
emphasis on exchangeable Ca
2+
is necessary because too much sodium ion (Na
+
)
destabilizes a soil's structure. A soil's sodicity is defined by its exchangeable Na
+
content, expressed as a percentage of the cation exchange capacity, with a value of
more than 6% being undesirable.
Soil Strength
Soil strength influences how easily roots can push through a soil and determines
what load can be borne without structural damage, especially when wet. Soil
strength, bulk density, and aggregate consistence or cohesion are interdependent.
For example, bulk density (inversely related to porosity) increases as a soil becomes
more compacted and, at the same time, its bulk strength, as measured by a pen-
etrometer, increases. Soil strength for easy root penetration but good load bearing
