Ed Barrett-Lennard

Dryland salinity is caused by a build-up of salts in the root zone of plants in non-irrigated areas to the extent that it affects plant growth. Salinity can have direct adverse effects on agricultural systems, but the mobilisation of salt from affected land often causes downstream impacts on water resources as well as the loss of associated infrastructure, environmental and social values. Dryland salinity is a problem in areas where internal (leaching) and external (runoff) drainage is unable to remove salts, which may come from several sources, commonly from rainfall or dryfall (wind-borne). Primary salinity occurs as a result of pedogenesis and within the context of geologic processes, while secondary salinity results from human-induced land uses change, such as clearing for agriculture. Dryland salinity is often associated with sodic soils and waterlogging. The combined effect of these problems on plants is often much greater than the sum of their individual impacts so addressing the problem that is most tractable can provide a partial solution. For example, it may be better to reduce surface waterlogging rather than to drain subsurface saline groundwater, especially if disposing the drainage water has additional impacts on rivers and downstream water resources. Dryland salinity results from increased recharge leading to the mobilisation of salts by groundwater at multiple-scales, something that occurs mainly at the local scale in irrigation salinity. A drying climate in parts of southern Australia in recent decades has reduced the perception of risk of dryland salinity, so that it has become a much lower priority for Government and some land managers. With further reductions in rainfall-recharge due to continued poleward shift of weather systems, this trend is expected to continue. This chapter outlines types and causes of dryland salinity as it affects agricultural land, its interactions with waterlogging, sodicity and other factors, and methods of management. The chapter has a focus on Australia which has over 2 M ha of affected land, more than half of which is in Western Australia, an area that has been experiencing a drying climate since about 1975. This experience is likely to be informative of areas not yet affected by climate change. The impacts of dryland salinity on stream salinity is an important topic but is not within our scope.

In Australia large areas of agricultural land are currently affected by dryland salinity, with this area expected to reach 17 million hectares by 2050. Many of these soils are also subject to periods of waterlogging. The commonly sown pasture and fodder legumes in southern Australia are sensitive to these conditions. A recent series of field experiments across southern Australia found that of 33 self-regenerating annual legumes, the undomesticated species, Melilotus siculus (Turra) Vitman ex B.D. Jacks, was the only one productive and persistent beyond the first year on waterlogged, saline (ECe levels in summer > 8 dS/m in the top 0–10 cm) sites. The salinity and waterlogging tolerance of M. siculus in the vegetative phase has been confirmed by glasshouse experiments, while recent studies have shown mechanisms for salinity tolerance and avoidance in germinating seedlings. Recent work has identified suitable rhizobia able to nodulate regenerating plants on saline soils. Evaluation of M. siculus genotypes will now commence, with the aim of developing a new fodder legume cultivar suitable for saline soils prone to waterlogging.

Salinity is a major desertification process affecting the agricultural productivity of irrigated and non-irrigated land resources. Human induced salinity presently occurs on about 80 Mha, but will affect substantially greater areas in the future. The limited available information suggests that salinity is highly damaging to economic prosperity and morale in agricultural communities, especially in developing countries. This paper argues that saltland is a resource capable of significant production. Experience from within Australia and Pakistan suggests that profitable new agricultural industries can be based around the growth of salt tolerant plants. It is argued that an urgent cooperative effort is required by technologists and affected communities to research, develop and implement new saline agricultural industries.

Results of a Western Australian bio-economic model (MIDAS) suggest that increasing the productivity of forage shrubs growing on salt-affected farmland can substantially increase whole farm profitability. Agronomic experimentation has shown that such improvements in productivity are achievable by deep ripping soils to reduce subsoil compaction, adding low levels of fertiliser, planting on less saline land and choosing more appropriate genotypes.