Agriculture Reference
In-Depth Information
(or catenas) that form down slopes and the vertical succession of horizons that occur
within soil profiles are pre-eminent features. Micro-relief structures occur at a smaller
scale. Finally, within specific soil horizons, macro-aggregates are subdivided into a
diminishing hierarchy of smaller macro- and micro-aggregates which comprise the
subsequent levels of the aggregate structure. Common soil properties such as clay
and organic matter concentrations often have clearly-defined spatial patterns that vary
depending on the scales at which they are considered (Figures 1.47-1.49).
4.2.2
TEMPORAL HETEROGENEITY
Chemical and physical soil transformations may occur over a broad range of time scales.
In terms of profile formation processes, these range from the near-geological times of
the
ca.
years required for the formation of a fully-developed oxisol profile to less
than 100 years which is the approximate time needed for the formation of a fully-developed
A horizon (Birkeland, 1984). Biological processes may operate on time scales that are
effective over the hundreds to thousands of years needed for succession in certain plant
communities to the annual and seasonal scales, to those of a day or an hour, the genera-
tion time of fully active bacteria.
4.2.3
FUNCTIONAL HETEROGENEITY
Soils comprise a wide variety of components and structures varying in nature from biotic
to abiotic and intermediate (the necromass). Any one of these units, from the simplest
inorganic molecule to an aggregate may form part of larger composite functional entities
and, therefore, potentially have many different effects. For example, soil nutrients
sequestered within a pool of resistant organic matter with a slow turnover rate will have
a much different impact on plant growth than the same nutrients incorporated into labile
compounds with rapid rates of turnover. As indicated above (section 1.1.3), soil organic
matter may be divided into pools of differing qualities, turnover times and abundances.
Similarly, the effects of the broad biological systems of regulation (for example, the
rhizosphere and the drilosphere, Chapter IV) and the histories of the soil components
within which they are included (
e.g.,
successional processes taking place in the
rhizosphere, ageing process within invertebrate faecal pellets) influence the present
and future of these molecules.
Some of the difficulties associated with the study of soil processes arise from the
variety of factors involved. Each phenomenon results from the multiple interactions of a
diversity of components and sub-processes operating at different scales of time and
space. It is necessary to understand the structure of this heterogeneity in order to incor-
porate the systems of interaction into a coherent intellectual framework. An hierarchical
approach has been developed in Chapter IV to provide a more integrated approach to the
study of soil processes (Mesarovic
et al.,
1970; Allen and Starr, 1982).
