Agriculture Reference
In-Depth Information
unusual parent materials, some with properties that may be inimical to soil development
processes. For this reason, composite soils are often formed by topdressing with a layer
of fully-developed soil transported from elsewhere to provide a favourable base for
the growth of stabilising vegetation.
2.3
Time
Soils undergo extended and complex series of reactions and processes during formation
from their parent materials. The net effects of these eventually lead to the differentiation
of fully-developed profiles and, at a landscape scale, of suites of related soils. Depending
largely on parent materials and environment, the development of fully-differentiated
profiles occurs at widely variable rates. Chesworth (1992) considered that in humid
temperate climates, the formation of a metre of temperate soil from aluminosilicate
parent materials is likely to take approximately 20,000 years and half that on carbonate-
rich parent materials. The formation of a similar depth of ferrallitic soil (oxisol) in
the Côte d'Ivoire may take up to 77,000 years (Leneuf and Aubert, 1960). In contrast,
an Andisol with a distinct AC horizon structure may form in Japanese volcanic ash in
as little as 500 years although more than 1000 years is required for a fully differentiated
ABC profile to develop (Yamada, 1968 in Wada, 1985).
Duchaufour (1982, 1997) divides soils into those forming over short and long cycles.
The short-cycle soils develop over periods ranging from less than to years.
Some soils in this group owe many of their properties to those of their organic matter
inputs; they include such groups as the peats (histosols) and the spodosols. Other soils
in this category include those whose pedogenetic differentiation has progressed little
(entisols, inceptisols), those forming on rapidly weathering parent materials (for
example, certain mollisols forming on carbonate-rich parent materials) and those whose
pedogenetic development requires relatively small changes to the parent materials
(vertisols). The long-cycle soils require periods of to years for development and
are typified by the highly-weathered soils of the humid tropics, the ultisols and oxisols.
Over long periods of time, soils continue to change with on-going inputs and weather-
ing while climatic change and mechanical disruption may influence profile differentiation.
In the absence of disturbance, most changes that occur are smooth (Bockheim, 1980b)
although exceeding a threshold in some critical process may invoke relatively rapid
change in the direction of development, as illustrated in Section II.4.1 (Birkeland, 1992).
As discussed later in this chapter (Section II.4.4.3), extended weathering leads
eventually to soils that are highly depleted in plant nutrient elements and possess
characteristic suites of the simpler clay minerals, typically with low and pH-related
cation exchange capacities and therefore very limited capacities for nutrient retention.
Chesworth (1973a) points out that differences between soils formed on various parent
materials are greatest in younger soils and diminish but rarely disappear completely with
time. The chemical compositions of old, highly-weathered soils become remarkably
uniform with chemical compositions increasingly dominated by the elements Al, Fe and
Si, irrespective of their original parent materials (Chesworth, 1973b).
