539that had not been especially screened for salt tolerance. However, it appeared that
this micro-organism was not surviving on saline soils over the summer when the
salinity of the soil solution probably increased to levels approaching saturation.
Best successful regeneration of the plants occurred only using a rhizobium (SRDI
554) selected from a naturally saline environment in South Australia. This new
rhizobial selection had increased saprophytic competence than the previously used
commercial rhizobium (WSM 1115), and average plant nodulation increased from
11 % (with the commercial line) to 74 % (with SRDI 554) (Bonython et al. 2011 ).
As noted above, when soils become exceptionally saline, they are only suited to
the growth of halophytes, so there have been a range of efforts to develop industries
around the domestication of the highly salt tolerant plants (Barrett-Lennard et al.
2003 ; Glenn et al. 2013 ; Panta et al. 2014 ). Not surprisingly, halophytes seem best
suited to agriculture when they fit smoothly into existing production systems. This
has been an important part of the thinking in adapting halophytic forages (notably
Atriplex species) as forages into grazing systems on salt affected farms in southern
Australia. The main idea is that grazing systems in southern Australian mixed farms
generally have periods of ‘feed-gap’ when fodder is not available and farmers often
hand feed stock with grain at these times. Salt tolerant forages have a premium
value at these times because their utilisation by sheep can be used to offset feeding
with grain, so that grain can be sold increasing farm profits (Barrett-Lennard et al.
2003 ).
One key paper catalysing the development of salt land pastures has been an eco-
nomic analysis by O’Connell et al. ( 2006 ), which showed that the economic gains
to farmers could be doubled if the nutritive value of the forage available to grazing
sheep could be increased by 10 %. This led to a new focus on increasing nutritive
value in salt land pastures by incorporating: (a) Atriplex nummularia subsp.
Nummularia as the key fodder shrub in the system (this had higher digestible
organic matter in the dry matter than A. amnicola – reviewed by Norman et al.
2013 ), and (b) high nutritive value annuals into the under-storey (Norman et al.
2010 ). The potential of this system recently became apparent when in an experi-
ment with A. nummularia and an improved salt tolerant under-storey (Trifolium
michelianum, T. subterraneum, Medicago polymorpha and Lolium multiflorum),
Norman et al. ( 2010 ) achieved higher rates of grazing (~7 growing sheep/ha for ~8
months of the year) on mildly saline land with ~330 mm annual rainfall than the
district average stocking rate for pastures that were not salt-affected.
Finally, in more saline areas, halophytes can be used to moderate the impact of
groundwater driven soil salinity and change the potential for other plants to estab-
lish and produce. In a paired plot study (25 ha) at Yealering, Bennett et al. ( 2012 )
documented a one-decade study showing how narrow spaced alleys of saltbush have
been used to lower water tables from less than 1 m to about 1.5 m (i.e. by more than
0.5 m) and in so doing restrict seasonal access of the capillary fringe to the soil
surface. This in turn limited the flux of salt into the root zone and reduced salt flux
from the site (88 % less for the treated site). As a result, more salt sensitive plants
established in the inter-rows and productivity increased from < 1 T ha−^1 to up to 7 T
ha−^1. Of importance here is that in broad valleys with relatively deep water tables
Salinity in Dryland Agricultural Systems: Challenges and Opportunities