Salinization is the accumulation of water-soluble salts in the soil solum or regolith to a level that impacts on agricultural production, environmental health, and economic welfare. Salt-affected soils occur in more than 100 countries of the world with a variety of extents, nature, and properties. No climatic zone in the world is free from salinization, although the general perception is focused on arid and semi-arid regions. Salinization is a complex process involving the movement of salts and water in soils during seasonal cycles and interactions with groundwater. While rainfall, aeolian deposits, mineral weathering, and stored salts are the sources of salts, surface and groundwaters can redistribute the accumulated salts and may also provide additional sources. Sodium salts dominate in many saline soils of the world, but salts of other cations such as calcium, magnesium, and iron are also found in specific locations. Different types of salinization with a prevalence of sodium salts affect about 30% of the land area in Australia. While more attention is given to groundwater-associated salinity and irrigation salinity, which affects about 16% of the agricultural area, recent investigations suggest that 67% of the agricultural area has a potential for "transient salinity", a type of non-groundwater-associated salinity. Agricultural soils in Australia, being predominantly sodic, accumulate salts under seasonal fluctuations and have multiple subsoil constraints such as alkalinity, acidity, sodicity, and toxic ions. This paper examines soil processes that dictate the exact edaphic environment upon which root functions depend and can help in research on plant improvement.

Salts can be deposited in the soil from wind and rain, as well as through the weathering of rocks. These processes, combined with the influence of climatic and landscape features and the effects of human activities, determine where salt accumulates in the landscape. When the accumulated salt in soil layers is above a level that adversely affects crop production, choosing salt-tolerant crops and managing soil salinity are important strategies to boost agricultural economy. Worldwide, more than 800 million hectares of soils are salt-affected, with a range of soils defined as saline, acidic-saline, alkaline-saline, acidic saline-sodic, saline-sodic, alkaline saline-sodic, sodic, acidic-sodic and alkaline-sodic. The types of salinity based on soil and groundwater processes are groundwater-associated salinity (dryland salinity), transient salinity (dry saline land) and irrigation salinity. This short review deals with the soil processes in the field that determine the interactions between rootzone environments and plant responses to increased osmotic pressure or specific ion concentrations. Soil water dynamics, soil structural stability, solubility of compounds in relation to pH and pE and nutrient and water movement all play vital roles in the selection and development of plants tolerant to salinity.

Despite the fact that most plants accumulate both sodium (Na+) and chloride (Cl–) ions to high concentration in their shoot tissues when grown in saline soils, most research on salt tolerance in annual plants has focused on the toxic effects of Na+ accumulation. There have also been some recent concerns about the ability of hydroponic systems to predict the responses of plants to salinity in soil. To address these two issues, an experiment was conducted to compare the responses to Na+ and to Cl– separately in comparison with the response to NaCl in a soil-based system using two varieties of faba bean (Vicia faba), that differed in salinity tolerance. The variety Nura is a salt-sensitive variety that accumulates Na+ and Cl– to high concentrations while the line 1487/7 is salt tolerant which accumulates lower concentrations of Na+ and Cl–. Soils were prepared which were treated with Na+ or Cl– by using a combination of different Na+ salts and Cl– salts, respectively, or with NaCl. While this method produced Na+-dominant and Cl–-dominant soils, it unavoidably led to changes in the availability of other anions and cations, but tissue analysis of the plants did not indicate any nutritional deficiencies or toxicities other than those targeted by the salt treatments. The growth, water use, ionic composition, photosynthesis, and chlorophyll fluorescence were measured. Both high Na+ and high Cl– reduced growth of faba bean but plants were more sensitive to Cl– than to Na+. The reductions in growth and photosynthesis were greater under NaCl stress and the effect was mainly additive. An important difference to previous hydroponic studies was that increasing the concentrations of NaCl in the soil increased the concentration of Cl– more than the concentration of Na+. The data showed that salinity caused by high concentrations of NaCl can reduce growth by the accumulation of high concentrations of both Na+ and Cl– simultaneously, but the effects of the two ions may differ. High Cl– concentration reduces the photosynthetic capacity and quantum yield due to chlorophyll degradation which may result from a structural impact of high Cl– concentration on PSII. High Na+ interferes with K+ and Ca2+ nutrition and disturbs efficient stomatal regulation which results in a depression of photosynthesis and growth. These results suggest that the importance of Cl– toxicity as a cause of reductions in growth and yield under salinity stress may have been underestimated.

Soil salinity affects large areas of the world's cultivated land, causing significant reductions in crop yield. Despite the fact that most plants accumulate both sodium (Na+) and chloride (Cl–) ions in high concentrations in their shoot tissues when grown in saline soils, most research on salt tolerance in annual plants has focused on the toxic effects of Na+ accumulation. It has previously been suggested that Cl– toxicity may also be an important cause of growth reduction in barley plants. Here, the extent to which specific ion toxicities of Na+ and Cl– reduce the growth of barley grown in saline soils is shown under varying salinity treatments using four barley genotypes differing in their salt tolerance in solution and soil-based systems. High Na+, Cl–, and NaCl separately reduced the growth of barley, however, the reductions in growth and photosynthesis were greatest under NaCl stress and were mainly additive of the effects of Na+ and Cl– stress. The results demonstrated that Na+ and Cl– exclusion among barley genotypes are independent mechanisms and different genotypes expressed different combinations of the two mechanisms. High concentrations of Na+ reduced K+ and Ca2+ uptake and reduced photosynthesis mainly by reducing stomatal conductance. By comparison, high Cl– concentration reduced photosynthetic capacity due to non-stomatal effects: there was chlorophyll degradation, and a reduction in the actual quantum yield of PSII electron transport which was associated with both photochemical quenching and the efficiency of excitation energy capture. The results also showed that there are fundamental differences in salinity responses between soil and solution culture, and that the importance of the different mechanisms of salt damage varies according to the system under which the plants were grown.

More than 60% of the 20 million ha of cropping soils in Australia are sodic and farming practices on these soils are mainly performed under dryland conditions. More than 80% of sodic soils in Australia have dense clay subsoils with high sodicity and alkaline pH (>8.5). The actual yield of grains in sodic soils is often less than half of the potential yield expected on the basis of climate, because of subsoil limitations such as salinity, sodicity, alkalinity, nutrient deficiencies and toxicities due to boron, carbonate and aluminate. Sodic subsoils also have very low organic matter and biological activity. Poor water transmission properties of sodic subsoils, low rainfall in dryland areas, transpiration by vegetation and high evaporation during summer have caused accumulation of salts in the root zone layers. This transient salinity, not influenced by groundwater, is extensive in many sodic soil landscapes in Australia where the watertable is deep. ‘Dryland salinity’ is currently given wide attention in the public debate and in government policies, but only focusing on salinity induced by shallow watertables. While 16% of the dryland cropping area is likely to be affected by watertable-induced salinity, 67% of the area has a potential for transient salinity not associated with groundwater and other subsoil constraints and costing the Australian farming economy in the vicinity of A$1330 million per year. A different strategy for different types of dryland salinity is essential for the sustainable management and improved productivity of dryland farming. This paper discusses the sodic subsoil constraints, different types of salinity in the dryland regions, the issues related to the management of sodic subsoils and the future priorities needed in addressing these problems. It also emphasises that transient salinity in the root zone of dryland agricultural soils is an important issue with potential for worse problems than watertable-induced seepage salinity.

Sodic soils are widespread in Australia reflecting the predominance of sodium chloride in groundwaters and soil solutions. Sodic soils are subject to severe structural degradation and restrict plant performance through poor soil-water and soil-air relations. Sodicity is shown to be a latent problem in saline-sodic soils where deleterious effects are evident only after leaching profiles free of salts.A classification of sodic soils based on sodium adsorption ratio, pH and electrolyte conductivity is outlined. Current understanding of the processes and the component mechanisms of sodic soil behaviour are integrated to form the necessary bases for practical solutions in the long term and to define areas for research. The principles of organic and biological amelioration of sodicity, as alternatives to costly inorganic amendments, are discussed.

A scheme is proposed, together with a procedure suitable for routine laboratory use, for the prediction of dispersive behaviour of surface layers of red-brown earths and their classification into one of six classes. Each class is defined on the basis of predictive relationships established between dispersion (spontaneous and mechanical), sodium adsorption ratio (SAR) and total cation concentration (TCC). These relationships were established experimentally using 138 samples representing both surface and subsurface layers from 69 red-brown earth profiles. Preliminary studies including samples from red clay and black earth profiles indicated that the proposed scheme is not suitable for these soils. Neither can it be used for soils containing free lime. The procedure proposed enables the prediction of the probable dispersive behaviour of the surface layer of red-brown earths, including exposed subsoils. It provides a rational basis for the formulation of appropriate management strategies for the manipulation of the surface structure of individual red-brown earths used for dryland or irrigated agriculture. Application of the proposed scheme to the estimation of the minimum level of residual gypsum required to maintain aggregate stability via the electrolyte effect is discussed, with special reference to low-sodic soils (i.e. with a SAR below 3, e.g. Classes 2a and 3c).

Sodium salts tend to dominate salt-affected soils and groundwater in Australia and therefore, sodium adsorption ratio (SAR) is being used to parameterize soil sodicity and the effects of sodium on soil structure. Recent reports, however, now draw attention to elevated concentrations of potassium and/or magnesium in some soils naturally and also as a result of increasing irrigation with recycled water in Australia. Therefore, there is a need to derive and define a new ratio of these cations in place of SAR, which will indicate the effects of Na and K on clay dispersion and Ca and Mg on flocculation. Rengasamy and Sumner (1998) derived the flocculation power of these cations and on this basis Rengasamy (unpublished) defined the cation ratio of soil structural stability (CROSS). This paper gives the results of an experiment conducted on ten soil samples on hydraulic conductivity using a number of artificially prepared irrigation waters, containing different proportions of the cations Ca, Mg, K and Na. The relative changes in hydraulic conductivity of these soils reflected the flocculating power of the cations, compared to the control treatment of using CaCl 2 solution. Clay dispersion was found to be highly correlated to CROSS rather than to SAR.