526
4.2 Salt Mobilisation
Changes to native vegetation, especially clearing, intensive grazing and/or increased
burning can all affect the hydrological balance, resulting in increased recharge and
catchment runoff. Salt accumulations are no longer stable under the changed hydro-
logical regime and can be mobilised within the landscape to locations such as plant
root zones, small and large river valleys, wetlands and floodplains. The way that
salinity is expressed at the soil surface often defines the name of the problem (hill-
side-, valley-salinity etc).
The time-scale with which salts may be leached depends on a number of
factors:
(i) The degree to which the hydrological regime has been altered, especially the
change in recharge rates;
(ii) The hydraulic conductivity of the material containing the salts, with some clay
regoliths containing relatively easily mobilised salt in cracks as well as within
clay matrices which require slow diffusion to a preferred pathway to be
mobilised;
(iii) The nature of the salt, with sodium chloride being more soluble than sulphates
which are in turn more soluble than carbonates; and
(iv) The distance that the salt needs to travel from its area of origin to where it can
impact on plants or on water quality (surface water, soil water or groundwater).
4.3 Salinity Types: Primary Versus Secondary
Primary salinity is a natural condition of world-wide soils and may also be referred
to as sodic, alkaline or solonchak and solenetzic (Richards 1954 ). These soils are
saline as a result of natural pedo-genesis or environmental change and may occur
where geological sources interact with soils, where evaporation exceeds rainfall, or
in association with salt lakes (playas) and coastal soils.
Szabolcs ( 1989 ) also recognised that both North America and Australia had a
major problem of salinisation created by watertable induced salinisation he called
‘active solonchaks’. In the Australian context these soils had previously been
referred to by Northcote and Skene ( 1972 ) as ‘dryland salting’ and thereafter
adopted as ‘dryland salinity’ (Peck 1978 ).
Secondary dryland salinity is the term used to describe degradation that results
in the decline in yield and productivity of agricultural systems due to the accumula-
tion of excessive salts in the root zone. The degradation is driven by a change in the
catchment water balance that leads to development of shallow groundwater and
evaporation and accumulation of salts where water tables are within 2 m (Nulsen
1981 ). Dryland salinity is usually first noticed when agricultural and native plants
senesces and yields of farm crops and pastures are reduced by more than 20 %. In
severe cases, bare areas or salt scalds develop. Where aquifer pressures and high
D.J. McFarlane et al.