. Agriculturally, salinity is the concentration of dissolved mineral salts in water and soil-water as a unit of volume or weight basis (Ghassemi et al., 1995).
Salinity problems become visible when salt concentrations in the soil solution exceed crop threshold levels. Crops can tolerate low concentrations of salt throughout the root zone. Productivity declines above the threshold concentration. The salt tolerance thresholds for crops vary between species. Maas and Hoffman (1977) summarised previous published work and carried out a comprehensive review of crop salt tolerance data, which was subsequently updated by Maas (1990). However, the data indicates that some crops can tolerate a high level of salinity (e.g. 7 dS /m for barley). In addition, the decline of crop yield occurs gradually above the salinity threshold level. Such crop behaviour allows for crop selection and management for irrigation with different water qualities. However, salt tolerance data has inherent uncertainties concerning plant responses to spatial and temporal variations in root zone salinity (Hopmans and Bristow, 2002; Meiri and Plaut, 1985).
Sodicity describes the relative concentration of sodium (Na+) compared with the divalent cations, mainly calcium (Ca+2) and magnesium (Mg+2) in the soil solution. Sodicity problems manifest at higher relative Na+ concentration and lead to degradation of soil structure. Sodicity problems are usually inherent with salinity in irrigated clayey soils having significant sodium content. Sodicity is common also in soils irrigated with water containing considerable bicarbonate concentrations. This is because bicarbonate anions raise soil pH and can result in precipitation of divalent cations and an increase in the relative sodium concentration. High levels of sodium in irrigation water typically result in an increase of soil sodium levels, which affect soil structural stability, infiltration rates, drainage rates, and crop growth potential.
The interrelation between sodicity and salinity levels in irrigation water introduces a dual problem in terms of crop response, soil structure degradation, and irrigation management. An increase of water salinity is shown to have a positive consequence on the sodicity effect (Goldberg et al., 1991)
Sodicity has less impact at higher electrolyte concentrations at any particular level.
Nevertheless, continuous use of saline irrigation water might lead to an accumulation of salt above the threshold level of crops. On the other hand, low water salinity and high levels of sodicity can cause soil degradation and a reduction in soil permeability. Such degradation results in aeration and waterlogging problems, which negatively affect the crop yield (Goldberg et al., 1991).
Consequently, waterlogging and low permeability might also induce salt accumulation within the root zone. Rising salinity associated with an increase of relative Na+ concentration presents two thresholds values to be considered: the lower level is the salinity threshold above which the soil structure remains stable, and the higher salinity threshold level is the salt tolerance threshold of the grown crop.
Sodicity-salinity effects on the physical and hydraulic properties of the soil are very complicated processes that can be influenced by many factors. The main factors that control sodicity problems are soil type (Felhendler et al., 1974; Quirk and Schofield, 1955), clay type and content (Goldberg et al., 1991), pH of the soil solution (Suarez et al., 1984; Sumner, 1993), the manner of application of irrigation water, the initial water content in the soil (Dehayr and Gordon, 2005), and organic matter. Therefore, the soil structure degradation due to rising sodicity is unique for a given soil and its condition (Evangelou and McDonald, 1999).
2.8 The Effect of Salinity and Sodicity on Soil and Water Movement
2.8.1 Clay Minerals and Dispersion
Brady (1990) categorised clay types in four major groups of colloids present in soils: layer silicate clays, iron and aluminium oxide clays, allophone and associated amorphous clays, and humus. All the groups have general colloidal characteristics; however each group has some specific characteristics. Silicate clay minerals are the most prominent clay minerals in soils of temperate areas and tropical soils (Brady, 1990). The most important property of this group is the clarity of their crystallines, which are layer-like structured. The silicate clay fraction in general consists of many plate-like minerals. Crystalline particles are made up of two basic units which are tetrahedral silica and octahedral aluminium hydroxide in alternating layers, as shown in Figure 1 Due to imperfections in the crystals the Si+4 is substituted with aluminium (Al+3) ions, and some Al+3 ions are replaced by magnesium (Mg+2) ions. Silicate clays commonly have permanent negative charges which enable clay fractions to attract cations. The silicate clays fall into three subcategories, which are 1:1, 2:1, and 2:1:1 type minerals.
In general, only the 2:1 clay minerals exhibit swelling during the wetting process. Most swelling clay minerals for this group are smectite minerals, such as montmorillonite (Churchman et al., 1993).
The increase of relative concentration of a specific cation in the soil solution can increase the adsorption ratio of that cation on the colloid surface. The order of strength of adsorption on the clay surface, when the cations are present in equivalent quantities in the soil solution is Aluminium (Al+3) Calcium (Ca+2) Magnesium (Mg+2) Potassium (K+) = Ammonium (NH4+) Sodium (Na+) (Brady, 1990). Clay particles do not have a very strong preference for which cations are adsorbed to compensate for their built-in negative charges (Van de Graaff and Patterson, 2001). The relative concentration of the cations in the soil solution might determine which is the dominant cation being adsorbed. For example, increasing the Na+ cations in the soil solution will replace gradually the Ca+2 and Mg+2 cations (Figure 2.2). However, it is easy to replace Na+ on the exchange complex by increasing the divalent cations such as Ca+2, because Na+ is less effective in neutralising the negative charges, and clay fractions have preference for cations with more than one positive charge (Van de Graaff and Patterson, 2001).
Therefore, when excessive irrigation water is applied, it is most likely that the cations adsorbed on the negative charges are closely related to the relative concentrations of cations in the added water.
Figure 2.1 basic molecular and structural components of silicate clays (Source: Brady, 1990).
Sodicity is manifested when the sodium concentration in the soil solution increases and the structural stability of soil aggregates degrades significantly. Quirk and Schofield (1955) explain that soil structural degradation caused by sodicity in soils is due to swelling and dispersion processes. Swelling is the increase of aggregate size as a result of water and sodium cations entered between the platelike structure, while dispersion describes the process of separating and moving the clay layers with percolated water.
According to the diffuse double layer theory (DDL), both swelling and dispersion processes stem from the balance between repulsive forces (as a result of osmotic pressure) in diffuse double layer and Van Der Waals forces of attraction on clay fraction surfaces (Sumner, 1993). Swelling is a reversible and continuing process and depends on the threshold concentration of ambient solution and the degree of sodicity. Dispersion is not a continuing process and may occur even at low SAR, as long as soil salinity cannot prevent dispersion. Dispersion is an irreversible process because flocculation by increasing concentration above the threshold level does not restore the original particle associations and orientations (Levy, 2000).
The clay mineral crystal layers in soils are closely associated with each other to form structures known as “domains” or “tactoids” (Quirk, 2001). In such systems, dispersion can only occur if the individual mineral layers separate.
2.9 Managing Soil Salinity
2.9.1 Managing Saline Soils
Numerous secondary problems or challenges arise from the primary salinity stresses of high total soluble salts, ion toxicities and problem ions, and nutritional levels and imbalances. A number of articles have been written on the management of salinity in agricultural soils (Abrol et al., 1988; Rhoades and Loveday, 1990; Chhabra, 1996; Keren, 2000; Qadir et al., 2000; Rengasamy, 2002; Qadir and Oster, 2004). Carrow and Duncan (1998) and Duncan et al. (2009) have addressed salinity management in turf grass situations. While osmotic stress is a consistent factor in all salinity sites, nutrient imbalances and possible toxic ions may vary with soil pH. Thus, management would require adjustment for any nutrient imbalances or excesses. Rengasamy (2010a) summarized saline soil problems as:
• Acidic pH 6.0: osmotic effects; root toxicities of Fe, Mn, and SO4-2
• Near-neutral pH of 6.0 to 8.0: osmotic effects; possible toxicity from any dominant cation or anion, especially under higher salinity
• Alkaline pH 8.0: osmotic effects; excessive HCO3-1 and CO3-2; and Fe, Mn, Al, and OH-toxicities at pH 9.0
Reclamation of saline soils is not by chemical amendments (e.g., gypsum), but by the removal of the excess total soluble salts from the plant root zone. The three most important salinity management aspects are (a) the leaching of total soluble salts by the application of sufficient water volume to