Agriculture Reference
In-Depth Information
As shown in Chapter IV, roots are important in the formation of both aggregates and
biopores. During growth, they compress the surrounding soil to an approximate distance
of one diameter (Dexter, 1991) and they cause the surrounding soil to shrink as they
absorb water; they also act to bind small aggregates together mechanically. Living roots
secrete a range of exudates which, in combination with those of the rhizosphere micro-
organisms, cement soil particles together. Further, mycorrhizal fungi acting in conjunc-
tion with the roots are important in both physical binding and the cementation of soil
particles. It has been found that monocotyledonous plants are more effective than
dicotyledons at stabilising aggregates and that grasses are more efficient than cereals
(Oades, 1993). Following root death and disintegration, a biopore remains in the soil
providing a route for water entry and gas exchange and a preferential pathway for future
root growth. Also, aggregates may form around nuclei of decaying root materials.
Soil animals, particularly earthworms, termites and ants strongly influence soil struc-
ture through the formation of both biopores and aggregates (Figure I.17) resulting from
their different feeding and burrowing behaviours (Lee and Foster, 1991). Warner
et al.
(1989) reported the presence of faunally-created cavities at depths down to 560 cm in
a Nonh American mollisol supporting pasture; these were attributed to the activities
of ants and earthworms although surface evidence of their activity was not necessarily
visible. Structures created by soil invertebrates, especially the large “ecosystem engineers”
(see II.2.4 and Chapter IV) may significantly affect gas diffusion. Kretzschmar and
Monestiez (1992), for example, have demonstrated that earthworm burrows
significantly increase relative diffusivity of gases, especially at high matric potentials
where a low proportion of porosity is filled with air.
These faunal structures may persist in the soil for long periods (years to centuries),
long after the invertebrates that created them have disappeared (Wielemaker, 1984;
Eschenbrenner, 1986; Blanchart
et al
., 1997). The influences of these organisms are
considered further in Chapter IV.
In agricultural terms, a well-developed soil structure implies the presence of porous,
water-stable aggregates and is considered important both in protecting the soil surface
against erosion and in promoting a satisfactory level of plant growth. The favourable
effects of well developed structure on plant growth are achieved by facilitating aeration
and water flow into and through soils. A high level of porosity is advantageous since, as
considered in the next section, it is desirable that aerobic processes dominate within
