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There is a view that divalent ions (e.g., Mg2+, Ca2+ and SO 42 −) have less severe
effects on plants than monovalent ions (e.g., Na+ and Cl−). These claims need to be
carefully examined and may occur simply because soil salinity is widely measured
based on the electrical conductivity of soil extracts, which is the product of the
valency and concentration of ions. Solutions of monovalent and divalent salts at
equal electrical conductivity would therefore differ twofold in concentration.
Waterlogging is a major potential hazard to susceptible commercial crops and
grasses in saline environments because salinity is caused by the presence of a shal-
low watertable and/or by major decreases in the hydraulic conductivity of soil
caused by sodicity. In each case seasonal rainfall can cause waterlogging, which
results in decreases in soil oxygen concentrations within 1–2 days (Barrett-Lennard
et al. 1986 ). The presence of oxygen is essential for energy production in roots
(Barrett-Lennard 2003 ). In the absence of oxygen, energy production decreases by
~95 %, and there is a breakdown in the processes that ensure Na+ and Cl− are
excluded and K+ is taken up by plants. This decreases plant growth and yield
(Barrett-Lennard 2003 ; Barrett-Lennard and Shabala 2013 ).
Boron is a micro-nutrient that can be present at toxic concentrations in soils
affected by salinity, particularly in sodic soils affected by transient salinity
(Rengasamy 2002 , 2006 ). One important point is that tolerance to boron toxicity is
not necessarily associated with tolerance to salinity. For example, barley is gener-
ally regarded as being more salt tolerant than wheat, but there are many moderately
saline soils where wheat will be the preferred crop selection than barley because
wheat is generally more tolerant to boron toxicity than barley. One way to increase
crop production on salt lands may therefore be to introgress genes for B toxicity
tolerance into crops (Barrett-Lennard and Setter 2010 ).
4 Dryland Salinity
Three processes combine to cause dryland salinity; salt accumulation in the rego-
lith, the mobilisation of these salt stores, and their expression at the land surface and
streams.
4.1 Salt Accumulation
Salt accumulation is typically a slow process, operating over millennial time-scales,
with evidence that salinisation has occurred in phases in response to the onset of
aridity across the last 1–2 Ma (George et al. 2008 ). Depending on the hydraulic
conductivity of the soil the accumulated salt can be at shallow soil depths (e.g.
within the root-zone of annual crops and pastures – Barrett-Lennard et al. ( 2016 ))
or in more permeable soils, at considerably greater depths (> 10–100 m) in the soil
D.J. McFarlane et al.