Sodic below 4 above 13 above 15 dispersed
Saline-sodic above 4 above 13 above 15 flocculatedTable 2.1 The physicochemical properties of salt-affected soil
2.4 Salt Ions and Compounds
Soluble salt ions most common in salt-affected soils are Ca+2, Mg+2, Na+, K+, Cl-, SO4-2, HCO3-,NO3-, and CO3-2 (this last one at pH > 9.0). It is the magnitude and balance of these ions (especially imbalances and dominance on the CEC sites or in the soil solution) that are the basis for the various salinity stresses in a particular landscape. These ions arise from the following:
• Dissolution of minerals in the weathering process
• Salt additions by saline irrigation water
• Salts in standard application of fertilizers and soil amendments
• Salts transported into the root zone by a rising water table (e.g., saltwater intrusion)
• Capillary rise from deeper in the soil
• Seepage zones where saline water moves to another site
• Subsoil migration to lower-topography areas due to gravity
• Flooding, such as in coastal areas
• Saltwater spray
• Use of primary or secondary salinized dredged soils on a site (Robert et al.2012).
Inorganic salts can be present in the soil not just as soluble ions but also as compounds that vary in solubility from relatively insoluble (lime) to moderately soluble mineral forms (gypsum dihydrate). Relatively soluble minerals that can dissolve to release soluble salt ions include (a) various sulfate compounds such as Na2SO4, K2SO4, CaSO4?2H2O (gypsum dihydrate), and MgSO4; (b) chloride compounds, such as KCl, NaCl, and CaCl2; and (c) carbonate or bicarbonate compounds with high solubility such as Na2CO3 and NaHCO3. Examples of insoluble salts would be CaCO3, MgCO3, dolomite (CaCO3?MgCO3), anhydrite of gypsum (CaSO4-i.e., without the hydrated water), and soil minerals such as apatites, while dihydrate gypsum (CaSO4?2H20) is a moderately soluble salt. While the insoluble and moderately soluble mineral forms can influence soils and plants over time, it is the soluble salts that have the most direct and rapid impact on soils and plants, due to their mobility and innate ability to accumulate in soils and plants. When comparing various salt ions versus salt compounds in the soil, individual salt ions can react with other ions to produce salt compounds varying in solubility. (Robert et al., 2012)
The degree of mobility of individual salt ions in response to water movement in the soil is based on whether they reside in the soil solution, as a component of a soluble compound, or on the soil CEC sites that are associated with negative charges on clay and organic matter. Highly mobile ions are Cl-1, SO4, and K+1, while Na+1, Ca+2, and Mg+2 are less mobile in the soil. Ion and nutrient concentrations and mobility in plants are also important in salt-challenged sites. For example, an immobile ion when applied to the turf grass foliage will not move to lower tissues, including the root system. Thus, if Ca is required in the root zone of a sodic soil to maintain root viability (to limit Na displacing Ca in root cell walls and causing roots to deteriorate), application to the shoot tissues would not have any effect. Carrow et al. (2001) reported ion mobility within plants as (a) mobile (N, P, K, Mg, and Na), (b) somewhat mobile (S, Zn Cu, Mo, and B), and (c) immobile (Ca, Fe, Mn, and Si).
2.5 Extent of salinity problem
Globally, it is estimated that there are 76 million hectares (Mha) of human-induced salt affected land, representing 5% of the world’s cultivated land, (Ghassemi el aI., 1995). Salt affected lands are those where crop yields are reduced or where less desirable crops must be grown because of the salinity. This human induced salinization is termed secondary salinization, in contrast to regions that were saline in their native condition. This value underestimated the extent of salinity because it does not include large areas where land could be potentially cultivated if not for the native salinity.
Salinity problems are more prevalent in irrigated lands relative to the total cultivated acreages. This is not surprising as irrigated lands are concentrated in more arid regions, where salinity is more prevalent. Also, irrigation results in land application of more water, thus imposing additional drainage needs to the natural hydrologic system. Of the world’s 227 Mha of irrigated lands, it is estimated that 45.4 Mha, ur 20% are adversely impacted by secondary salinization (Ghassemi el al., 1995).
Salinity is a major threat to current irrigation projects and to the remaining near-surface fresh water supplies in arid regions. The extent of the secondary salinization problem has not stabilized; instead, it is estimated that as much as 2 Mha of irrigated land, representing approximately 1% of the total, is lost from production due to salinity each year (Umali, 1993, in Postel, 1997). Most of the world’s salt affected, cultivated lands are in Asia and Africa, where population densities and economic conditions make the problem proportionately more severe, it is estimated that Egypt, Iran and Pakistan had 33, 30 and 26% respectively of their irrigated land impacted by secondary salinizatiun (Ghassemi el al., 1995). However more developed countries are not immune to these salinity problems, it is also estimated that over 20% of irrigated land in the U.S. is salt-affected (Postel, 1999), a value comparable to the global average.
2.6 Chemical Properties
2.6.1 Soil Reaction-pH
The pH value of an aqueous solution is the negative logarithm of the hydrogen-ion activity. The value may be determined potentiometrically, using various electrodes, or calorimetrically, by indicators whose colors vary with the hydrogen-ion activity. Soil characteristics that are known to influence pH readings include: the composition of the exchangeable cations, the nature of the cation-exchange materials, the composition and concentration of soluble salts, and the presence or absence of gypsum and alkaline-earth carbonates.
A statistical study of the relation of pH readings to the exchangeable-sodium-percentages of soils of arid regions have been made by Fireman and Wadleigh (1951).
(1) Experience and the statistical study of Fireman and Wadleigh (1951) permit the following general .statements regarding the interpretation of pH readings of saturated soil paste pH values of 8.5 or greater almost invariably indicate an exchangeable-sodium-percentage of 15 or more and the presence of alkaline-earth carbonates;
(2) The exchangeable-sodium-percentage of soils having pH values of less than 8.5 may or may not exceed 15;
(3) Soils having pH values of less than 7.5 almost always contain no alkaline-earth carbonates and those having values of less than 7.0 contain significant amounts of exchangeable hydrogen.
2.6.2 Soluble Cations and Anions
Analyses of saline and alkali soils for soluble cations and anions are usually made to determine the composition of the salts present. Complete analyses for soluble ions provide an accurate determination of total salt content. Determinations of soluble cations are used to obtain the relations between total cation concentration and other properties of saline solutions, such as electrical conductivity and osmotic pressure. The relative concentrations of the various cations in soil-water extracts also give information on the composition of the exchangeable cations in the soil.
The soluble cations and anions commonly determined in saline and alkali soils are calcium, magnesium, sodium, potassium, carbonate, bicarbonate, sulfate, and chloride. Occasionally nitrate and soluble silicate also are determined. In making complete analyses, a determination of nitrate is indicated if the sum of cations expressed on an equivalent basis significantly exceeds that of the commonly determined anions. Appreciable amounts of soluble silicate occur only in alkali soils having high pH values.
2.6.3 Exchangeable Cations
When a sample of soil is placed in a solution of a salt, such as ammonium acetate, ammonium ions are adsorbed by the soil and an equivalent amount of cations is displaced from the soil into the solution. This reaction is termed “cation exchange,” and the cations displaced from the soil are referred to as “exchangeable.” The surface-active constituents of soils that have cation-exchange properties are collectively termed the “exchange complex” and consist for the most part of various clay minerals and organic matter. The total amount of exchangeable cations that a soil can retain is designated the “cation-exchange-capacity,” and is usually expressed in meq/100 g.soil. It is often convenient to express the relative amounts of various exchangeable cations present in a soil as a percentage of the cation-exchange-capacity. For example, the exchangeable-sodium-percentage (ESP) is equal to 100 times the exchangeable-sodium content divided by the cation-exchange-capacity, both expressed in the same units.
Determinations of the amounts and proportions of the various exchangeable cations present in soils are useful, because exchangeable cations markedly influence the physical and chemical properties of soils. (Richards, 1954).
2.7 Salinity and Sodicity Problems in Irrigation
The term salinity refers to the concentration of ions in water (Burger and Celkova, 2003). The salinity level for water to be considered as saline depends on the purpose of the water use. Guidelines have been provided for different water uses, including drinking, agriculture and industry. 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.
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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 allow net downward movement of salts; (b) the adjustment of soil fertility programs to correct nutritional deficiencies, imbalances, or toxicities; and (c) to select turf grasses that can tolerate.
The salinity levels expected when saline irrigation water is used on a regular schedule.
During dry periods, the background soil salinity will not be any lower than what the salinity level in the irrigation water is, and usually accumulated soil salinity is higher. Some common management considerations on saline soils are as follows (D.A. Horneck et al 2007)
• Irrigation system distribution uniformity and efficiency become very important since the non-uniformity of irrigation water will affect spatial distribution on not just the water but also soluble salts. Design issues (spacing between sprinklers and wind issues) will become more pronounced, and salinity leaching more difficult.
• Irrigation scheduling, to avoid normal and physiological drought stress and for salt leaching, is critical.
• Factors to enhance water infiltration (into soil surface), percolation (through the root zone), and drainage past the root zone “Cultivation, Topdressing, and Soil Modification.
• Traffic control programs to avoid total salt-induced wear injury must be carefully designed.
• Fertilization must be adjusted not just to meet plant growth requirements but also to maximize salt tolerance mechanisms and to correct imbalances. As irrigation water salinity increases, so do the nutritional interaction issues and these can become dominant management challenges under high salinity?
• Proactive soil and water quality testing should be performed, as well as tissue testing if needed, to monitor salinity impacts.
• Drought and high-temperature stresses are more common on saline sites since high-soluble salts cause physiological drought and may injure roots, and plant tolerance to these stresses is reduced by high salinity.