Agriculture Reference
In-Depth Information
CHAPTER II
SOIL FORMATION
The rocks that outcrop at the surface and other surficial parent materials are generally
not in equilibrium with the atmosphere and the biosphere and are slowly transformed into
soil, an intimate mixture of fine mineral panicles and organic matter. As sub-components
of ecosystems, soils are open thermodynamic systems through which energy flows and
dissipates and this is reflected in the diversity of biological and physical structures present
within them.
Soils are the products of complex interactions between the lithosphere, atmosphere and
biosphere, and reflect the specific influences of each. While mainly comprising mineral
materials from the lithosphere and organic matter, the pore spaces of soils possess
distinctive atmospheres and hydrological environments. Water moves under the influence
of gravity and capillarity, transporting solutes, colloidal and larger particles, while gases
and vapours move by diffusion and mass flow. Water and gases react with the mineral
matter in both physical and chemical interactions to release chemical energy through
hydrolysis and oxidation.
Plants inject energy into the soil system partly through the growth and penetration of
roots. More importantly, organisms also add energy to the soil as the partially-degraded
products of their metabolic and digestive processes inluding such metabolites as root
exudates, leachates, microbial polysaccharides, earthworm mucus and, eventually, their
own dead biomasses.
Soil formation processes are driven by the expenditure of energy derived from
predominantly biological sources and its dissipation through a series of inter-related
physical, chemical and biological processes. Soils are dynamic entities, highly reactive,
constantly-evolving and subject to the marked daily and seasonal rhythms of their
environments but buffered from their extremes. They are also subject to longer-term
variation associated particularly with climate change; the time scales of these range from
a few years to those measured at the scales of geological processes.
Considerable spatial variation occurs in soils due to the lack of uniformity in the way
internal processes act. Soil differentiation proceeds downwards from the surface leading
to the formation of characteristic horizons and layers representing zones in the soil
dominated by specific processes. The pore space is the site of the major interactions that
occur between soil components (minerals, gases, water, living organisms and organic
matter); in the confined environment of the soil, such interactions are a major feature of
soil development and function.
Soil formation is a slow process and the formation of fully-developed soils occurs
over time scales of tens to hundreds of thousands of years. It results from a balance
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