Agriculture Reference
In-Depth Information
is ideally between 1 and 2 MPa when the soil is at its
FC
. The roots of
V. vinifera
can penetrate soil up to the top of this range. Variations in soil strength can also
affect the availability of soil water, as expressed through the nonlimiting water
range (see figure 4.8, chapter 4), an effect observed in the subsoils of a number of
drip-irrigated vineyard soils in South Australia.
Although a soil may have a desirable bulk strength, it may not form a friable
tilth on drying. The ideal condition is when the larger aggregates under moder-
ate pressure break down into many smaller, stable aggregates, as seen in the upper
sample in figure 6.4. Soil that is compacted because of too much wheeled traffic
when wet, for example, does not break down into a friable tilth as it dries but
remains massive (the lower sample in figure 6.4). A similar result can occur with
soil that is compacted through natural causes, such as sodicity that induces an
inherently unstable structure.
Soil Chemistry and Nutrient Supply
A healthy soil should provide a balanced supply of the essential nutrients (see
table 3.1, chapter 3). Deficiencies and toxicities can be identified through
soil testing. However, critical soil test values for “available” nutrients such as
Figure 6.4
Example of a friable
topsoil (upper sample) and a
compacted topsoil (lower sample).
