Agriculture Reference
In-Depth Information
A horizon
Figure 1.8
A deep Podzol
soil formed on sandstone
near Sydney, Australia. Iron
complexed with organic matter
has been translocated from the
bleached horizon, to be deposited
as orange-red iron oxide in the
subsoil. The pale soil on top
is spoil from the pit. See color
insert.
B horizon
inhospitable subsoils and poor drainage can inhibit this rooting. However, the
constraint of poor drainage can be managed by deep ripping with or without
applications of gypsum, as discussed in “Preparing the Site by Cultivation and
Ripping,” chapter 2.
Soils on Igneous Rocks
Igneous rocks, produced from volcanic activity deep in the earth's crust (intru-
sive or plutonic rocks) or at the earth's surface (extrusive rocks formed from
lava flows and ash showers), are the ultimate source of all rocks on the planet
(see table 1.1). They range from silica-rich, “acidic” rocks such as granite and
rhyolyte to rocks of intermediate basicity (dacite and andesite) to low-silica
“basic” rocks such as basalt, gabbro, and dolerite. This range of rock types natu-
rally gives rise to a variety of soils. For example, in the Granite Belt region of
southeast Queensland, Australia, the landscape is old and much reworked by
erosion and deposition: hence the soils formed are acidic (pH(H
2
O) <6) and
of variable depth but consistently coarse-textured, reflecting the high quartz
content of the granitic parent material (figure 1.9). Such soils are generally well
drained.
