Agriculture Reference
In-Depth Information
moisture regimes, climate determines the phase of the soil water and the intensity of
water fluxes. It thereby controls the transport of solid particles and dissolved materials
within developing soils, over their surfaces and laterally in the landscape.
2.2
The parent material
This is the basic inorganic material from which the soil will be formed and, depending
on its physical, chemical and mineralogical composition, strongly influences the
composition and texture of the resulting soil.
The chemical and mineralogical composition and the physical structure of the parent
material set the initial conditions of the incipient soil. Thus, soils formed from basaltic
rock parent materials usually contain higher concentrations of phosphorus, alkaline earth
elements and iron than those formed from other parent materials because of the higher
concentrations of such elements in these compared with most other rock types.
The forms in which they occur and the concentrations of these elements affect the
ability of the soil to supply biological nutrient and other elements and thereby condition
the way in which the profile develops.
The grain size of the parent rock is important in that coarse grained igneous rocks
usually weather more rapidly than do those of finer grain size (Birkeland, 1984). It also
sets the size of the units that are initially detached from the parent rock by weathering
processes. Thus, soils derived from fine-grained basaltic parent materials have a finer
texture than those derived from coarse-grained granitic parent materials.
The type of the parent rock affects soil formation particularly through (Brewer, 1954):
(i) The amount of clay that may be potentially formed by
in situ
weathering. This
varies with the nature of the parent rock and intensity of water fluxes. It may vary from
quite small amounts to large: for example, 12 % kaolinite from a sandstone to 33-55 %
from biotite granite to 99 % kaolinite from an amphibolite-pyroxine schist (Macias and
Chesworth, 1992);
(ii) The amounts of alkali (notably Na, K), alkaline-earth (largely Ca, Mg) metals and
iron that can be released by weathering;
(iii) The ease of release of the above metals;
(iv) The permeability of the parent rock.
While rocks are the most common parent materials, soils also develop on a diverse
range of materials transported by wind, water, gravity and vulcanism, and on the trun-
cated profiles of previously eroded soils. Agriculturists profoundly modify soil chemical
and physical properties through such practices as drainage, flooding and the addition
of organic and inorganic ameliorants; these are also new parent materials for further
soil development.
In addition to the parent materials considered above, a range of incipient soils are
currently developing on parent materials provided through man's activities. These include
such materials as urban organic and inorganic wastes and materials dredged from
wetland environments. A variety of overburden materials are deposited at the surface
from mining operations and these, together with waste materials from initial mineral
processing, manufacturing and power-generating industries also provide a diversity of
