Agriculture Reference
In-Depth Information
5.3.1
SOIL PHYSICAL CHARACTERISTICS
The presence of substantial termite populations in a landscape affects most aspects
of soil organisation and function, both at coarse and fine spatial scales (Figure IV.64).
Over long periods, much of the soil mass may be modified directly by termite activity
through the faunal pedoturbation (Hole, 1961) processes discussed in Chapter II.
These activities include the continuing cycle of void formation (nests, galleries, storage
structures) in the soil. It also includes the construction of above-ground structures togeth-
er with their subsequent destruction by natural processes of erosion and the infilling of
soil voids through biological activity, gravity- and water-mediated sediment transport.
In soils where termites are populous, their effects on profile morphology are clearly
apparent in the form of the lower parts of epigeal mounds, subterranean nests, systems
of small and large galleries radiating from nests, and food stores associated with
the galleries. Their activities may lead to the formation of new horizons (Nye, 1954) or,
in some instances, interfere with profile differentiation. At a landscape scale, the surface
expression of faunal pedoturbation contributes to toposequence formation (Nye, 1954,
1955abc) and at even larger scales to the formation of biomantles (Johnson, 1990)
(Chapter II.3.3.2).
At shorter time scales, termite activity influences most aspects of soil function
including water movement, gas exchange, organic matter processing and biological
nutrient flows. Even the spatial dispositions of the biota (micro-organisms to higher
plants) and the stocks of organic matter, nutrient and other elements are affected.
Fourteen years after experimental eradication of termites from a North American desert
rangeland through pesticide application, soil N concentrations had increased although
water
infiltration
and
storage
had
diminished
in
comparison
with
untreated
plots
(Whitford,
1991).
5.3.1.1
Termites and soil textural relationships
Termite distributions and activity both influence and are affected by soil texture.
As considered in Chapter III, mound-building termites are largely excluded from certain ver-
tisols in northern Australia through the instability induced by the expansive smectitic clays.
Shortages of clay in deep sands may equally limit the colonisation of certain species.
Termites substantially affect surface processes by transporting soil from various depths in
the profile to the surface, where it is built into epigeal mounds and other structures. Surface
structures erode continuously particularly under the influence ofintense rainfall leading to the
redistribution of soil materials in the landscape. In locations with dispersive B horizon clays,
the fine-textured materials eroded from termite mounds may contain sufficient clay and silt
to form a thin surface seal.
Because termites generally select the smaller particles from within the profile, materials
brought to the surface are commonly finer in texture and may have different
elemental and clay mineral compositions than those predominating at the original
surface (Boyer, 1973b, 1982). Thus, as discussed in Chapter II.3.3.2, abstraction of finer par-
ticles from various depths in the profile and their deposition on the surface often lead to the
gradual build up of a fine-textured surface soil which may sometimes be underlayed by stone
