The nature of Australian soils - Soil Essentials: Managing Your Farm's Primary Asset - page 2

Air is the other major factor in soil formation, with wind carrying soil particles
and depositing them often large distances from their original source.
For Australian soils there is a further major inf luence in soil formation: the
fact that most of Australia has been under the sea at least once over eons of time.
The immense pressure of deep water over the soil has turned old soil to rocks such
as sandstone, siltstone and mudstone, and skeletons of marine organisms have
formed limestone. When these rocks emerged from under the sea the erosion cycle
started again, forming new soils from old. The salt content of Australian soils is
largely due to the marine inf luence, but also wind-borne salt blown inland from
ocean salt spray.
This very incomplete thumbnail sketch of the formation of our soils is enough
information to enable an observer in a landscape to get a rough idea of where the
soil they are standing on came from, and possibly a glimmering of the sometimes
immense geological forces involved in making our landscape what it is today.
Past cataclysmic events such as volcanoes, land upheaval and massive rainfall
events have also played a major part in forming our soils.
Effect of climate
Climate plays a major part in soil formation, chemistry, microbiology and
structure.
Compared to cooler, moister, European conditions, our soils have to contend
with much harsher effects. Remembering that fertile soils have a large living
component, periods of hot dry summer conditions and cold wet winter conditions
drastically change the quantity and species composition of living soil organisms.
Many of the micro-organisms cannot thrive in hot dry conditions, and go into
survival mode until soil temperature and moisture become more favourable. This
is in contrast to European conditions when soil organisms often thrive throughout
the year.
Waterlogged soil, on the other hand, has the effect of favouring some types of
organisms, thus changing the ratios of organisms present at that time. Often these
organisms can be pathogenic to plants and cause disease. As soil warms up and
becomes moist (as distinct from saturated) organisms that have been suppressed
by cold, wet conditions thrive again and, in turn, plants growing in that soil also
thrive.
Soil chemistry is also altered by seasonal conditions, with soil pH, for example,
changing as soil moisture and temperature changes. Periods of high rainfall can
leach soluble nutrients out of the upper layers of the soil, often to below the depth
where plants can access them. This is especially so in light sandy soils, but is not
the case in heavier soils such as clays.