Agriculture Reference
In-Depth Information
Figure 1.7
A shallow soil formed on
a gravel and boulder alluvium in the
Marlborough region, New Zealand. The
scale is 10 cm.
soil profiles of uniform texture similar to that shown in figure 1.8 can form.
Under native vegetation, complexes of iron (Fe) and aluminum (Al) with organic
compounds released from decomposing plant litter are gradually leached into the
subsoil (B horizon) where they are transformed into Fe oxides (orange-red col-
ors) and humified organic matter (black color). The A horizon above the subsoil
is conspicuously bleached due to the removal of Fe and organic matter. Although
such soils are extremely well drained because of their coarse texture, they are not
much used for viticulture because of their poor water holding capacity and very
low fertility.
Where the sedimentary rocks have a greater proportion of particles smaller
than sand size—the siltstones and mudstones—duplex soil profiles similar to that
shown in figure B1.1.1 can form. Many vineyards in southeast Australia, particu-
larly in the cooler regions, have been established on such soils, which also occur
in the Stellenbosch region of South Africa on granitic and sedimentary parent
materials. A characteristic of many duplex soils, especially under a cool and wet
winter climate, is gleying and mottling in the subsoil where, because of the high
clay content, drainage is impeded. The biochemical processes of gleying and mot-
tling are explained in box 1.3. Although grapevines are naturally deep rooting,
