Agriculture Reference
In-Depth Information
Different species build their epigeal mounds with characteristic proportions of
inorganic materials. They may be composed predominantly of soil materials, as are the
mounds of many grass-harvesting and litter-feeding termites. Some have a hard earthen
capping with a softer, more-organic interior while others possess a largely-organic
structure with little or no included soil material. The largely-earthen parts of the mounds
are normally of finer texture than the surrounding soils since termites exclude the larger
particles from their nests (Lee and Wood, 197la). As discussed below, the mound
materials of many grass-harvesting and litter-feeding termites are substantially enriched
in nutrient elements compared with the local surface soils; this is ascribed to the transport
of harvested materials to their nests where they may be partly digested and their
breakdown products incorporated in mounds or galleries as linings or infillings.
5.2.2.2
Galleries
Termite galleries vary in shape from circular to narrowly-elliptical and, in size, from
less than 1 mm to more than 20 mm in their greatest diameters (Grassé, 1984) (see also
Table I.10): they are therefore all in the large macropore size range. Gallery lengths
of up to 7.5 km (Darlington, 1982; McKay and Kladivko, 1985; McKay and Malcolm,
1988) have been estimated for soils associated with the mounds of
Macrotermes
michaelseni
although Wood (1988) felt that this figure could perhaps have been doubled
to account for the galleries of the subterranean termite species also present. The equivalent
of 90 000 storage chambers per hectare also occurred in the soils of Darlington's
study area. Assuming that these structures were largely confined to the top 20 cm of
the soil and applying Wood's multiplier, it may be calculated that termite galleries
and related structures formed voids occupying approximately 0.4 % of the soil volume
to this depth. Some Macrotermitinae may open access holes at the surface which they
use for foraging at night; the overall surface of these openings has been estimated at
2-4 in a dry savanna of Kenya (Lepage, 198la).
It is clear that the sizes, types of wall construction, depth distributions and extents
of termite gallery systems differ markedly, depending on the ecological strategies
of the species involved and the characteristics of the soils present. The galleries of
soil-feeding termites, for example, are usually more superficial than those of the Macro-
termitinae and their walls are dark and plastered with fluid excreta. In contrast, those of
the Macrotermitinae are smooth and massive due to compression of the soil and the
clay-saliva mixture used to line them (Kooyman and Onck, 1987).
In certain Australian soils (alfisols) with coarse-textured A and massive, clay-rich
B horizons, termite galleries occur principally in the A horizon and descend into the B
horizon largely in old root channels and adjacent to rocks where local changes in the soil
matrix have occurred (A.V. Spain, unpublished). Where termite populations are substan-
tial, their gallery systems are considered to strongly influence soil physical properties
(Lee and Wood, 197la). While few quantitative data are available, as shown below
(Section IV.5.3.1.2), their effective penetration of surface crusts is of agronomic relevance.
