Agriculture Reference
In-Depth Information
and activity of the soil biota through their influences on water relationships, the distri-
butions of soil organic matter and nutrient elements or by providing a favourable or shel-
tered habitat for other organisms.
Micro-relief features created by physical processes include the frost polygons
characteristic of the soils of polar regions. These structures may form extensive
semi-regular patterns of low mounds defined by systems of polygonal cracks. In the cold
desert of Antarctica, snow preferentially entrapped in such cracks melts to provide
a locally-favourable micro-habitat for Acari, Collembola and their supporting microbial
food base (Wise and Spain, 1967). Gilgai formations commonly comprise patterns of
alternating low hollows and mounds at scales of a few metres and are common in soils
containing expansive clays. Biotic micro-relief structures include such features as
coppice mounds formed through the trapping of wind-borne soil materials around
the bases of shrubs in desert regions, the mounds that form at the bases of trees through
bole expansion and the termite and ant mounds abundant in many savanna, tropical
forest and other landscapes (Chapter IV.5.2.2.1).
At smaller scales than those of micro-relief, most soils possess some degree of
structural development. Soil structure may be defined as the grouping of the primary soil
particles into larger compound units (aggregates or peds) of different origins, sizes and
shapes. These units are separated from those adjacent by pore spaces which permit water
movement and gas exchange with the atmosphere. The pore space comprises voids of
many types ranging from large planar cracks formed by the contraction of clay-rich soils
