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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.

The sustainability of irrigated agriculture depends on the quality of irrigation water used. The electrolyte concentration (EC) of irrigation water may lead to the accumulation of salts in the root zone layers and affect the physiological functions of the crop by osmotic and ion toxicity effects. Further, the cationic and anionic composition of the water may alter the exchangeable cation composition of the soil as well as its pH. Because of the dominance of sodium salts in many sources of irrigation water, parameters related to sodium such as exchangeable sodium percentage (ESP) of soils and sodium adsorption ratio (SAR) of soil solutions have been commonly used to study the effects of sodium in irrigation water on soil structural stability. Quirk and Schofield’s concept of ‘threshold electrolyte concentration’ (TEC) has shown the importance of electrolytes in preventing the effects of sodium on soil structure. Based on this concept, several models have been proposed to relate ESP or SAR with EC to predict the possible impacts of irrigation water on soil structural stability. However, many research reports indicate that this relationship varies with soils, and a given model is not suitable for all types of soils. Further, the effects of potassium and magnesium in the processes leading to clay dispersion are disregarded in these models. This essay analyses all the factors involved in the structural failure of soils with different cationic composition, identifies the defects in these TEC models, and re-defines TEC on the basis of new insights on dispersive and flocculating charges of soils. This review does not deal with EC effects on crops nor the role of contaminant ions not involved with soil structural stability.

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 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 Ca 2+ 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 culute, and that the importance of the different mechanisms of salt damage varies according to the system under which the plants were grown.

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.

Phenotyping for soil constraints can be done under controlled conditions, but to be relevant, screening should replicate the effects of the conditions in which the plants grow in the field (Sadras, 2019). [...]when evaluating germplasm for improved tolerance to sodicity it is important to understand the properties of sodic soils and use methods that mimic the constraints of sodic soils. [...]a soil defined as sodic based on its exchangeable sodium percentage (ESP) may be non-dispersive if the flocculating charge is greater than the dispersive charge (Figure 1). [...]it is perhaps not surprising that the importance of individual soil constraints to yield can vary considerable over sites and seasons (McDonald et al., 2012). Another limitation to this method is the use of University of California (UC) potting mix, consisting of coarse sand and peat moss (Genc et al., 2016).

Plant growth in saline soils may be increased by fertilisation, but little is known about the effect of different forms of N on wheat growth in soils with different salinity levels. The aim of this study was to investigate the response of wheat (Triticum aestivum L., cv Krichauff) to (NH₄)₂SO₄ or KNO₃ or NH₄NO₃ at 0 (N0), 50 (N50), 100 (N100) and 200 (N200) mg N kg⁻¹ soil in a saline sandy loam. Salinity was induced using Na⁺ and Ca²⁺ salts to achieve three ECe levels, 2.8, 6.6 and 11.8 dS m⁻¹ denoted S1, S2 and S3, respectively, while maintaining a low SAR (>1). Dry weights of shoot and root were reduced by salinity in all N treatments. Addition of N significantly increased shoot and root dry weights with significant differences between N forms. Under non-saline conditions (S1), addition of NO₃ − N at rates higher than N50 had a negative effect, while N100 as NH₄ − N or NH₄NO₃ − N increased shoot and root dry weights. At N100, shoot concentrations of N and K were higher and P, Ca, Fe, Mn, Cu and Zn were lower with NO₃ − N than with NH₄ − N nutrition. The concentration of all nutrients however fell in ranges did not appear to be directly associated with poor plant growth with NO₃ − N. At all N additions, calculations indicated that soil salinity was highest with N addition as NO₃ − N and decreased in the following order: NO₃−N > NH₄−N > NH₄NO₃−N. Addition of greater than N50 as NO₃ − N, compared to NH₄ − N or NH₄ − NO₃, increased soil salinity and reduced micronutrient uptake both of which likely limited plant growth. It can be concluded that in saline soils addition of 100 mg N kg⁻¹ as NH₄ − N or NH₄NO₃ − N is beneficial for wheat growth, whereas NO₃ − N can cause growth depression.

Success in breeding crops for yield and other quantitative traits depends on the use of methods to evaluate genotypes accurately under field conditions. Although many screening criteria have been suggested to distinguish between genotypes for their salt tolerance under controlled environmental conditions, there is a need to test these criteria in the field. In this study, the salt tolerance, ion concentrations, and accumulation of compatible solutes of genotypes of barley with a range of putative salt tolerance were investigated using three growing conditions (hydroponics, soil in pots, and natural saline field). Initially, 60 genotypes of barley were screened for their salt tolerance and uptake of Na+, Cl–, and K+ at 150 mM NaCl and, based on this, a subset of 15 genotypes was selected for testing in pots and in the field. Expression of salt tolerance in saline solution culture was not a reliable indicator of the differences in salt tolerance between barley plants that were evident in saline soil-based comparisons. Significant correlations were observed in the rankings of genotypes on the basis of their grain yield production at a moderately saline field site and their relative shoot growth in pots at ECe 7.2 [Spearman’s rank correlation (rs)=0.79] and ECe 15.3 (rs=0.82) and the crucial parameter of leaf Na+ (rs=0.72) and Cl– (rs=0.82) concentrations at ECe 7.2 dS m−1. This work has established screening procedures that correlated well with grain yield at sites with moderate levels of soil salinity. This study also showed that both salt exclusion and osmotic tolerance are involved in salt tolerance and that the relative importance of these traits may differ with the severity of the salt stress. In soil, ion exclusion tended to be more important at low to moderate levels of stress but osmotic stress became more important at higher stress levels. Salt exclusion coupled with a synthesis of organic solutes were shown to be important components of salt tolerance in the tolerant genotypes and further field tests of these plants under stress conditions will help to verify their potential utility in crop-improvement programmes.

Sorption is an important process for retention of organic carbon (C) in soils. The effect of Na⁺ and Ca²⁺ on sorption of organic C has been studied in salt-affected soils, but little is known about the effect of K⁺ and Mg²⁺ ions on sorption of water-extractable organic C (WEOC). The effect of Na⁺, K⁺, Ca²⁺ and Mg²⁺ ions on sorption of WEOC and its decomposition were investigated in a loamy sand (7.5 % clay) and a sandy clay loam (34.4 % clay). Salinity was developed with NaCl, KCl, MgCl₂ or CaCl₂ to obtain different concentrations of exchangeable Na⁺, K⁺, Ca²⁺ and Mg²⁺ at an electrical conductivity in a 1:5 soil/water extract (EC₁:₅) of 1 dS m⁻¹. Water-extractable organic C was derived from wheat straw, and microbial activity after sorption was quantified by measuring CO₂ emission from the soils for 27 days. The concentration of sorbed C was higher in the sandy clay loam than in the loamy sand and decreased in the treatment order Ca²⁺ > Mg²⁺ > K⁺ > Na⁺, but cumulative CO₂-C emission after sorption was highest from the Na⁺ and lowest from the Ca²⁺ treatments. The strong binding in the Ca²⁺ and Mg²⁺ treatments can be explained by the low zeta potential and the high covalency index of cation binding with C, whereas zeta potential was high and the covalency index was low in the Na⁺ treatments. Although K⁺ is also monovalent, WEOC was more strongly bound in the K⁺ than in the Na⁺ treatment. The weak binding with Na increased the accessibility of the sorbed C to soil microbes and, thus, microbial activity. Our results suggest that monovalent cations may enhance decomposition and leaching of WEOC in saline soils with Na⁺ having a greater effect than K⁺. Divalent cations, particularly Ca²⁺, enhance the binding of organic matter and thus organic C stabilization, whereas Mg²⁺ ions have a smaller effect.

Limitations to agricultural productivity imposed by the root-zone constraints in Australian dryland soils are severe and need redemption to improve the yields of grain crops and thereby meet world demand. Physical, chemical and biological constraints in soil horizons impose a stress on the plant and restrict plant growth and development. Hardsetting, crusting, compaction, salinity, sodicity, acidity, alkalinity, nutrient deficiencies and toxicities due to boron, carbonates and aluminium are the major factors that cause these constraints. Further, subsoils in agricultural regions in Australia have very low organic matter and biological activity. Dryland salinity is currently given wide attention in the public debate and government policies in Australia, but they only focus on salinity induced by shallow groundwater. However, the occurrence of transient salinity in root-zone layers in the regions where water tables are deep is an important issue with potential for larger economic loss than water table-induced seepage salinity. Root-zone constraints pose a challenge for salinity mitigation in recharge as well as discharge zones. In recharge zones, reduced water movement in sodic horizons results in salt accumulation in the root zone resulting in chemical and physical constraints that reduce transpiration that, in turn, upsets salt balance and plant growth. High salinity in soil and groundwater restricts the ability of plants to reduce water table in discharge zones. Thus plant-based strategies must address different kinds of limitations in soil profiles, both in recharge and discharge zones. In this paper we give an overview of plant response to root-zone constraints but with an emphasis on the processes of salt accumulation in the root-zone of soils. We also examine physical and chemical methods to overcome subsoil limitations, the ability of plants to adapt to and ameliorate these constraints, soil modification by management of agricultural and forestry ecosystems, the use of biological activity, and plant breeding for resistance to the soil constraints. We emphasise that soil scientists in cooperation with agronomists and plant breeders should design site-specific strategies to overcome multiple soil constraints, with vertical and lateral variations, and to develop plant-based solutions for dryland salinity.

The yield response of various crops to salinity under field conditions is affected by soil processes and environmental conditions. The composition of dissolved ions depend on soil chemical processes such as cation or anion exchange, oxidation-reduction reactions, ion adsorption, chemical speciation, complex formation, mineral weathering, solubility, and precipitation. The nature of cations and anions determine soil pH, which in turn affects crop growth. While the ionic composition of soil solution determine the osmotic and ion specific effects on crops, the exchangeable ions indirectly affect the crop growth by influencing soil strength, water and air movement, waterlogging, and soil crusting. This review mainly focuses on the soil chemistry processes that frustrate crop salinity tolerance which partly explain the poor results under field conditions of salt tolerant genotypes selected in the laboratory.