525profile where it can be mobilised by groundwater. The four main sources of salt in
dryland agricultural areas are rainfall, wind deposition, seawater left over after land
has emerged from below sea level, and the weathering of rocks. Marine influences
and rock weathering are not major contributors in many environments and are not
discussed further.
All rainfall contains some salt, especially in coastal areas. Raindrops can coalesce
around tiny salt crystals in near-shore areas and around dust particles in inland
areas. Rainfall in coastal areas (salt fall ~100 kg/ha/yr) has a concentration of salt,
which may decrease tenfold with increasing distance inland (Hingston and Gailitis
1976 ). Delivery of salt with rainfall can be significant mechanism for salt accumula-
tion in the landscape (Simpson and Herczeg 1994 ). Salt concentrations in rainfall
may also be lower in tropical and sub-tropical latitudes (Hingston and Gailitis
1976 ). Coastal areas also receive more rainfall than inland areas so the salt loads
(tonnes per hectare) are proportionally even higher. However coastal areas are often
less susceptible to salinity than inland areas because they often have well-defined
rivers, and aquifers can be flushed by the higher rainfalls, which is able to return the
incoming salt to the ocean.
Wind is a common source of salts in areas with saline soils. Dunes or dry lake-
beds may act as dust sources. In inland arid regions these features may be more
important sources of salt than rainfall. As well as sodium and chloride, wind can
entrain alkali earths, carbonate and sulphates where there are gypsum or carbonate
deposits at the surface. Even when salt additions are small, they can accumulate to
problem proportions over hundreds to thousands of years if the landscape is natu-
rally poorly drained (e.g. lacks a strong river system), soils are deep and clayey
which facilitates salt storage or drainages are occluded (e.g. when they end in termi-
nal salt pans and fail to leave the region). For example, it has been estimated that
there are ~1300 years and ~33,000 years of salt-fall from rain stored respectively in
the upper 1 m and upper 50 m of two soil profiles from the Merredin area of Western
Australia (Barrett-Lennard and Nulsen 1989 ; Barrett-Lennard et al. 2016 ). The sea-
sonality and intensity of the rain can also determine the rates of runoff and recharge,
and therefore salt accumulation and removal from the regolith.
Salts accumulate in soils and landscapes where water has been evaporated or
transpired leaving the salts behind. This can take the form of a ‘salt bulge’ at a depth
at which wetting fronts reach in average years, often just below the root depth of the
deepest-rooted plants. If wetting fronts penetrate deeply into soil profiles, the salt
bulge will also be deep. Root depths may be limited through high soil strength
(often as result of sodicity), subsoil acidity, aluminium- or boron-toxicity, anoxia
caused by waterlogging or by salinity itself.
The slow accumulation of salts in deep profiles reflects the hydrological balance
of rainfall and extraction of water by plants over thousands of years. Bulges can also
be partially mobilised by extreme natural rainfall events in which case the bulge
reflects these factors rather than abilities of plant communities to extract incoming
rainfall. Typically, salt accumulation is the result of very slow processes, well
beyond management time-frames.
Salinity in Dryland Agricultural Systems: Challenges and Opportunities