Agriculture Reference
In-Depth Information
restoring the pool of small aggregates (Blanchart
et al
.‚ 1997).
Other components of soil structure‚ especially the larger biopores‚ are also consis-
tently renewed through biological activity. Consequently‚ in terms of both macropores
and aggregates‚ soil structural maintenance should be considered to be largely under
biological control in all but the finest-textured soils with high concentrations of
expansive clays. Furthermore‚ such structures may persist for much longer periods
than the organisms that created them or they can be destroyed by other organisms long
before the cements and other materials binding them have lost their effects.
Apart from the biological destruction considered above‚ aggregates may be disrupted
by internal or external forces. Disruption may result from the high air pressures
associated with rapid wetting or through the dissolution of their internal “glues”.
External forces leading to aggregate breakdown include the compactive and shearing
pressures induced by tillage implements and the trampling of domestic and other large
animals. Aggregates may also be disrupted by the forces induced through drying and
freezing and by the expansion of salt crystals in saline soils.
Soil structure may be readily degraded in agricultural and other situations through
the application of external energy‚ through soil baring‚ excessive cultivation and mechan-
ical compression. Soils differ widely in their susceptibility to structural disruption.
In most soils‚ the exchange complex is dominated by and this results in strong
inter-particle bonding leading to a stable structure resistant to the disruptive forces
listed above. In contrast‚ in sodic soils (Section I.1.1.1.2)‚ inappropriate management can
quickly lead to the situation where rapid wetting by rainwater causes surface aggregates
to slake and their clays to disperse (Sumner‚ 1995). Such processes can lead to the
deposition of a layer of fine particles over the soil surface and the blocking of pores
important in water transmission and aeration. This surface sealing process may lead
to diminished plant growth‚ either directly because of the higher mechanical strength
of the surface crust (which may reduce seedling emergence) or indirectly by restricting
water entry and gas exchange.
Where its physical condition has been impaired by unsuitable land use practices‚
the soil may be rendered prone to accelerated erosion through the energy of wind or
water‚ and its capacity to support satisfactory plant growth may be reduced. This impair-
ment may be due to collapse of the surface soil structure following exposure to rain drop
impact or excessive working during tillage operations‚ particularly of very wet or very
dry soil. Agricultural operations (including the trampling and puddling activities of farm
animals) may also cause severe soil compaction‚ particularly when the soil is wet‚ and
this may cause part of the profile to become too strong for roots to penetrate.
Finally‚ oversimplification of soil fauna communities associated with conventional
tillage or cropping may result in an overdominance of either “compacting” or “de-
compacting” species with negative effects on water infiltration or resistance to erosion.
Severe compaction and significant losses of production have followed the establishment
of large populations of the endogeic earthworm
Pontoscolex corethrurus
in sweet
potato fields in New Guinea and in Amazonian pastures (Rose and Wood‚ 1980; Barros
et al
.‚ 1996). However‚ lower populations of this species normally have significant
positive effects on plant growth (see Chapter IV.4.4).
