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Salt-affected is a collective term which includes saline, sodic and saline-sodic soils. Salt-affected soils occur commonly in arid and semiarid regions of the world where annual precipitation is less than evapotranspiration. However, salt-affected soils also occur in subhumid and humid regions under conditions conducive to their development. Salt accumulation in the root zone is one of the major causes of reduced crop production. Most crops tolerate salinity to a threshold level above which yields decrease as salinity increases (Maas, 1986; Kafi and Goldani, 2001). Saline soils contain soluble salts that affect plant growth at various stages and create yield differences between crops and differences in the ion composition of crops at maturity (Sharma, 1997).
The distribution of salt-affected lands is closely related to environmental factors, in particular arid and semi-arid climates. The extent to which salinity is increasing in arid and semi-arid lands has recently become a problem of great concern in agriculture. Recent estimates shows total area of salt affected soils at some 9.5 million km2 on a world scale (Szabolcs, 1989) much of it due to inadequate irrigation practices in arid and semiarid regions, and the consequent loss of agricultural production is enormous. It is believed that about 7% of the total surface area of the world is salt affected and approximately 10% of the world's 7 Ã- 109 ha arable land surface consist of saline or sodic soils (Francois and Maas, 1994). The percentage of cultivated lands affected by salinity is even greater. Of the 1.5 Ã- 109 ha cultivated lands, 23% are considered as being saline and another 37% are sodic. It has been estimated that one-half of all irrigated lands (about 2.5 Ã- 108 ha) are seriously affected by salinity or water logging (Rhoades and Loveday, 1990). Salinity is one of the major abiotic stresses to crops, and affecting about 950 Ã- 106 ha of land worldwide (Flowers and Yeo, 1995).
Accumulation of salts in the soil root zone is a common process in arid and semiarid regions of the world where the annual precipitation is lower than the total evapo-transpiration (US Salinity Staff, 1954; Abrol et al., 1988; Sumner, 1995; Gupta and Gupta, 1997; Bohn et al., 2001; Ghafoor et al., 2004). These salts act as a source of primary salinization through weathering of parent materials and accumulate in situ or come along with waters from other areas causing secondary salinization. The soils of Pakistan mostly developed under the arid to semi arid climatic conditions under go both the process of salinization as a result of which 6.3 x 106 ha is salt-affected, of which 60% is saline-sodic and sodic (Muhammad, 1983; Ghafoor et al., 1990; Qadir et al., 2007). Because of hot and arid to semiarid climate, saline soils are slowly converting into sodic and saline-sodic soils in the country (Ghafoor, 1984; Ghafoor et al., 2004).
The problem of salinization and or sodication in recent past is because of large-scale efforts to bring additional marginal lands under irrigation (Abrol et al., 1988). The problem even go further aggravated due to the development of irrigation systems without adequate provision for drainage. The use of unscientific water management practices and inappropriate procedures for reclamation of salt-affected soils has been adding to the menace over time (Reeve and Fireman, 1967, Bohn et al., 2001). On global level, approximately 1.5 x 106 ha of irrigated land is lost to salinity and water-logging annually (Brundtland and Khalid, 1987). Another estimates show that world as a whole is losing at least ten hectares of arable land every minute, five because of soil erosion, three from soil salinization and one each from other soil degradation processes and non-agricultural uses (Abrol et al., 1988).
The world has about 955 x 106 ha salt-affected soils (Szabolcs 1994) which is equal to 20% of its irrigation land and which result in annual income loss of about US$12 billion on globe level (Ghassemi et al. 1995). In Pakistan this loss in crop productivity amounts to about 177 US $ ha-1 (Qayyum and Malik, 1988). Through wise management of salt-affected soils not only the national economy could be raised but the livelihood of the farming community living in such areas could also be uplifted (Miller and Donahue, 1992).
Categories of Salt-affected Soils
The natural and secondary processes, which determine the nature and extent of saline soils, are influenced by topography, climate, geology, soil and cultural conditions (Fitzpatrick et al., 1995). How a saline soil develops depends on which ions are dominant in the soil, the type of clay and how much precipitation the soil receives, as this affects pH changes. The earth crust contains appreciable amounts of Ca (3.6%), Na (2.8%), K (2.6%), Mg (2.1%), S (0.06%) and Cl (0.05%) (Clarke, 1924; Oilier, 1969), which upon weathering are released into the soil solution (US Salinity Staff, 1954; Brady and Weil, 1999). In humid and subhumid areas, these salts are leached beyond the root zone because of high precipitation. But in arid and semiarid areas where leaching is restricted, these salts accumulate over time reaching a level when the soil biological, physical and chemical properties are deteriorated (Rengasamy et al., 1984) and crop productivity is reduced. Besides mineral weathering the other two main natural sources of soil salinity are atmospheric precipitation/deposition and fossil salts (Bresler et al., 1982; Bohn et al., 2001).
Salt affected soils contain soluble cations of Na, Ca, Mg, and minor amounts of K and anions Cl, SO4, HCO3, CO3, NO3 and B (US Salinity Staff, 1954, Barber, 1984; Abrol et al., 1988; Ghafoor et al., 2004). The relative amount of CO3 and HCO3 depends on the pH of a soil solution (Lindsay, 1979; Bresler et al., 1982). Appreciable amounts of CO3 can be present only at pH values of 9.5 or higher nitrate is found in some salt affected soils like in Colorado, Utah, and Washington (Kelley, 1951; US Salinity Staff, 1954). Some salt-affected soils may contain excessive amounts of B. In Pakistan, Ca2+, Mg2+, Na+ are found as dominant cations and SO42-, Cl-, CO32- and HCO3- as dominant anions (Sandhu and Qureshi, 1986; Ghafoor et al., 2004; Khattak and Khan, 2004).
The saline soils are often recognized by the presence of white crusts of salts on the surface. These soils correspond to Hilgard's (1906) "white alkali" or "Solonchaks" of the Russian soil scientists (US Salinity Staff, 1954) and are called "Thur" or "Kallar" in Urdu and "Khora" in Pushto. The sodic soils correspond to Hilgard's (1906) "black alkali" or in some cases to "Solonetz", the term used by the Russians. These soils are called "Bara" in urdu.
The main source of salt in arid and semi-arid areas includes rainfall (Rengasamy and Olsson, 1993), mineral weathering (Lindsay, 1979; Gunn and Richardson, 1979; Macumber, 1991), irrigation and various surface waters (Mehanni and Chalmers, 1986; Rengasamy and Olsson, 1993; Spore, 1995), groundwater which redistributes accumulated salts during evaporation (Macumber, 1991), chemical applications (Rengasamy and Olsson, 1993) and man activities (Dregne, 1976). These sources, coupled with environmental modifications, lead to three different classes of salinisation and sodification that are grouped so for management purposes. These classes include: saline (ECe>4dSm-1, ESP<15%, pH<8.5); saline sodic (ECe>4dSm-1, ESP>15%, pH<8.5) and sodic soils (ECe<4dSm-1, ESP>15%, pH>8.5) (Richards, 1954).
In subtropical arid climatic conditions under high evapotranspiration the upward movement of saline ground water through capillary rise or evaporation of surface water results in the formation of saline soil (Isabelo and Jack, 1993). Saline soils with lower water tables may actually have the highest levels of salinity deeper in the soil. However, the extent of salinity and vulnerability in the soil depends on various physico-chemical characteristics of the soil such as soil texture, hydraulic conductivity and permeability (Shainberg et al., 2001), clay mineralogy and salt retention capacity.
The chemical characteristics of saline soils are mainly determined by the kinds and amounts of salts present in the soil. The soil Na seldom comprises more than half of the soluble cations and hence is not adsorbed to any significant extent (US Salinity Staff, 1954, Sposito, 1989; Bohn et al., 2001). Owing to the presence of excess salts and the absence of significant amounts of exchangeable Na, the saline soils are generally flocculated and as a consequence the permeability is equal to or higher than that of similar non-saline soils. The presence of CaCl2 and MgCl2 which are hygroscopic and absorb atmospheric moisture, make the soil surface of such soil darker than non-saline soil (FAO/UNESCO, 1973).
The behavior of saline-sodic soils depends on the relative concentrations of other cations in proportion to Na on exchange complex and in soil solution. As long as excess salts are present, the appearance and properties of these soils are generally similar to those of saline soils. Under conditions of excess salts, the pH readings are seldom higher than 8.5 and the particles remain flocculated. If the excess soluble salts are leached downward, the properties of these soils may change markedly and become similar to those of nonsaline-sodic formerly called alkali soils (Mehta, 1986). Consequently the soil becomes strongly alkaline, pH rises up beyond 8.5, soil particles becomes dispersed (deflocculated) and less permeable.
The process of sodication starts with exchange of adsorbed Ca by Na on exchange sites of soil particles. The Na, Ca and Mg are readily exchangeable with each other depending on relative concentrations and chemical affinity. Whenever, excess salt are accumulated or concentrated through evaporation, salts of Ca and Mg are precipitated and Na becomes the predominant cation. The excess Na replaces Ca and Mg on exchange complex, and soil become saline-sodic or sodic. However, the affinity of soil for Ca and Mg is more as compared to Na (Lindsay, 1979; Sposito, 1989) and at equivalent solution concentrations; the amounts of Ca and Mg adsorbed are several times higher than that of Na. In general, half or more of the soluble cations must be Na before significant amounts are adsorbed by the exchange complex (US Salinity Staff, 1954). Other cations, like K and NH4 may be held at certain positions on the particles in some soils so that they are exchanged with great difficulty and, hence, are said to be fixed (Mengel and Kirkby, 1987).
Frequently saline soils can turn into saline-sodic or sodic soils when irrigated with water that contains high amounts of nutrients and salts. The change occurs as soluble salts, such as NaCl, and CaCl2, are leached and exchangeable Na+ accumulates in the soil (Naidu and Rengasamy, 1993; SalCon, 1997).
Effect of Salts on Soils
In semi-arid zones there is intense evaporation which tends to accumulate salts in the upper soil profile, especially when it is associated with an insufficient leaching or where soluble salts move upward in the soil profile from a water table instead of downward (Isabelo and Jack, 1993). Such accumulation of salts in the soils may alter its physical and chemical properties, including soil structure and hydraulic conductivity (Rengasamy et al., 1984; Mullins et al., 1990). Excessive exchangeable sodium and high pH decrease soil permeability, available water capacity and infiltration rates through swelling and dispersion of clays as well as slaking of soil aggregates (Läuchli and Epstein, 1990). These modifications may further compromise the yield of crops growing on such soils (Voorhees, 1992).
Accumulation of salts in the soil alters its physical, chemical and biological properties. The soil physico-chemical properties are deteriorated with increase in soil electrical conductivity (Rengasamy et al., 1984; Yasin et al., 1987). Excessive exchangeable Na decreases soil permeability, available water capacity and infiltration rates through swelling and dispersion of clays as well as slaking of soil aggregates (Lauchli and Epstein, 1990; Qadir et al., 2006). Compaction and bulk density is increased and soil infiltration capacity and porosity is reduced with use of saline-sodic water irrigation (Gupta and Gupta, 1997; Al-Nabulsi, 2001). The poor soil properties and imbalanced ionic concentrations of soil solution under sodic soils conditions induce deficiency or toxicity of elements important in plant nutrition that ultimately affect the growth and yield of most crops (Sumner 1993; Curtin and Naidu, 1998; Grattan and Grieve, 1999; Bohn et al., 2001).
Saline soils contain, from an agricultural point of a view, relatively large amounts of neutral soluble salts (Abrol et al., 1988), which can be present in soils as chlorides and sulphates of Na+, Ca2+ and Mg2+ and sometimes as nitrate (NO3-) (SalCon, 1997; Anzecc, 2000; Fitzpatrick et al., 2001). The pH and the concentration and nature of salts in the soil solution determine which ions are dominant in the soil. In most regions salinity is usually combined with high soil pH, because of calcium enrichment in the upper soil layer (Sardinha et al., 2003).
The effects of elevated soluble salt concentrations are such that they prevent soil colloids from dispersing and promote flocculation of soil particles. For example exchangeable Ca2+ has the ability to flocculate or clump individual clay particles and as a consequence creating larger pore spaces in the soil, which facilitates root growth and better movement of water and air through the soil (Bell, 1993). Plant growth in saline soil is generally not constrained by poor infiltration, aggregate stability and aeration, but instead by high salt levels which are detrimental to plants (Abrol et al., 1988; Brady and Weil, 2002; Warrence et al., 2002).
Saline soils can turn into saline-sodic or sodic soils when irrigated with low quality water, due to the accumulation of exchangeable Na+ ions relative to Ca2+ and Mg2+ ions in soil and water (SalCon, 1997; Anzecc, 2000). For example, if irrigation water has a high exchangeable sodium percentage (ESP) and is low in soluble salts, the exchangeable Na will replace the soluble salts present at the cation-exchange sites, and thus saturating the soil with exchangeable Na+ ions (Fetter, 2001). The soil structure is destroyed as Na+ weakens the bonds between the clay particles (Rengasamy and Walters, 1994; SalCon, 1997; Fetter, 2001). These small clay particles, as they move down through the soil profile, are able to clog pore spaces, thus reducing water infiltration and, consequently, causing temporary water logging (Naidu and Rengasamy, 1993; Sumner, 1995; Qadir et al., 2001; Warrence et al., 2002). Over time the soluble salts accumulate in the subsoil if they cannot be dissolved, and leached (Rengasamy and Olsson, 1991), resulting in the upper soil layer becoming sodic and the subsoil becoming saline (Rengasamy, 2002).
Saline or salt affected soils will often not show symptoms of sodicity, even when excess Na+ is present (Rengasamy and Walters, 1994), as excess salt can prevent clay particles from dispersing. Only if these soils are leached of the salt will the symptoms of sodicity start to appear (Rengasamy and Walters, 1994). Even a small amount of adsorbed Na+ (about 6%) at the exchange sites is enough to cause the decline in soil structure (Naidu and Rengasamy, 1993).
Accumulation of Na+ can be a predominant feature in the heavy cracking clays (Vertosols) that swell upon wetting, thus preventing deep penetration of irrigation water (Rengasamy and Olsson, 1991). In some clayey soils (i.e. Vertosols), these swelling forces can give rise to the upward movement of clay to form large mounds and depressions, or gilgai (Fitzpatrick et al., 1995). Repeated wetting and drying solidifies sodic soils over time, producing cement like soil with little or no structure (Sumner, 1995; Warrence et al., 2002). A relatively thin (up to 10 mm thick) but very dense crust can form upon wetting, which then acts as a natural barrier for emerging seedlings.
Another important factor is the low organic matter level in sodic soils. The high levels of Na+, low or high pH and low biological activity found in these sodic soils are not conducive to the accumulation of organic matter and its mineralization (Naidu and Rengasamy, 1993; Peineman et al., 2005). In fact Na+ should first be replaced by divalent cations, for example in form of gypsum, to enable the formation of stable linkages between particles by organic matter (Rengasamy and Olson, 1991).
Effect of Salts on Plants
Apart from adverse effects on soil physical conditions like poor aeration, dispersion, and lower infiltration, the hazardous effects of salinity and/or sodicity on crop growth arises from: (1) increasing the osmotic potential and thereby making the water in the soil less available for plants, and (2) specific ion effects i.e toxic concentrations of Na, Cl, and B, and (3) interactive effects of cations with nutrient elements that reduce the nutrients bioavailabilities or elevate them to toxic levels (US Salinity Staff, 1954; Muhammad, 1996; Bonn et al., 2001; Yamaguchi and Blumwald, 2005). Imbalance in plant nutrient concentration is developed which affect the plant metabolism (Kramer, 1983; Garg and Gupta, 1997). Such changes in crop nutrition and metabolism ultimately decrease crop growth and yield under saline conditions (Mer et al., 2000; Qadir et al., 2001; Qadir et al., 2006), and excessive concentrations of salts kill the growing plants (Donahue et al., 1983). Garg and Gupta (1997) reported that salinity causes reduction in leaf area as well as in rate of photosynthesis, which together results in reduced crop growth and yield with stunted growth of roots.
The responses of plant to salinity are complicated depending upon the duration, type of salts, development stage of the plant, time of the day and many other factors (Maas and Hoffman, 1977; Cramer et al., 2001). Salinity reduces plant growth (Garg and Gupta, 1997; Mer et al., 2000) and in case of excessive salts concentrations plants completely failed (Donahue et al., 1983). Salt tolerant crops also show retarding efficiency in case of salinity above threshold level (Maas and Hoffman, 1977). Ramoliya (2004) reported reduced emergence and shorter stems and roots of acacia seedlings with stunted biomass under increasing salinity stress. It is reported that soil salinity suppresses shoot growth more than root growth (Maas and Hoffman, 1977; Ramoliya and Pandey, 2003). These adverse effects of salinity on plant ultimately reduce the over all performance of crops under saline condition which could be mitigated through effective reclamation strategies.
Reclamation of Salt-affected Soils
The maintenance of adequate soil physical chemical properties in sodic environments may be achieved by using good quality water, proper choice of and/or combination of soil ameliorants, good drainage and appropriate cultural practices (Grattan and Oster, 2003). In this respect, the development of the most suitable reclamation technology or a combination of technologies may be critical to optimize farm management and better crop yields in a sodic soil. Leaching has been shown to be the most effective method for removal of soluble salts from the rhizos- phere (Abrol et al. 1988). The high quality fresh water is intensely irrigated onto the soil surface and allowed to flush the soluble salts down as it infiltrates, and is usually incorporated with an effective drainage system (Jury et al. 1979). On the other hand, removal of exchangeable sodium necessitates application of chemical amendments to remove the sodium from the soil's cation exchange sites (Sahin et al., 2002). Gypsum (CaSO4.2H2O) is the most commonly used amendment due to its availability at low cost.
The amelioration of saline-sodic soils, thus, requires both leaching and application of gypsum (Abrol et al., 1988). However, this approach fails to improve the physical and biological properties of the already degraded soil suffering from low hydraulic conductivity caused by dispersion. Indeed, during the leaching-gypsum treatment, the soil is kept soaked for such a prolonged time which can even cause soil aggregates to lose their stability.
Various organic amendments such as mulch, manures, and compost have been investigated for their effectiveness on remediation of saline-sodic soils (Diez and Krauss 1997; Wahid et al., 1998). In general, organic amendments have a very little effect on improving soil salinity and sodicity when they are applied alone (Madejon et al., 2001). On the other hand, their effectiveness in improving many soil properties is well documented in literature (Cheny and Swift, 1984; Uson and Cook, 1995; Gao and Chang, 1996; Prihar et al., 1996; Singh and Singh, 1996; Entry et al., 1997; Giusquiani et al., 1995; Ibrahim and Shindo, 1999; Mamo et al., 2000; Naeini and Cook, 2000). Even small amounts of organic matter addition to the soil have a positive effect on the physical and biological soil properties including water stable aggregates, water-holding capacity, cation exchange capacity, and plant nutrition elements (Hanay et al., 1992).
Reclamations involve all those management strategies that can reduce the salinity levels of the soil (Abrol et al., 1988). These reclamation methods may be chemical, physical or biological adopted alone or in integration. Excessive soluble salts in saline soil may be removed though leaching with good quality water under adequate drainage system but reclamation of saline-sodic soil needs both leaching of excess salts and removal of exchangeable Na from exchange complex. Replacement of Na on exchange sites improves physical condition of the soil making it conducive for leaching of salts. The reclamation of non-gypsiftrous saline-sodic soil through leaching with irrigation water alone is not recommended. This may further aggravate the problems by causing sodicity which also deteriorates the soil physical condition and make management strategies more complex. Amelioration of saline sodic and sodic soils with variety of chemical amendments has now an established technology (Abrol et al., 1988; Shainberg et al., 1989; Gupta and Abrol, 1990; Ghafoor et al., 2004; Qadir et al., 2006, 2007).
Amendments are materials that directly or indirectly through chemical or microbial action furnish divalent cations (usually Ca2+) for replacement of exchangeable Na+ (Muhammad, 1996; Qadir et al. 2001). The replaced Na is leached from root zone with excess irrigation. Many saline-sodic soils may contain a source of Ca, i.e. calcite (CaCO3) at varying depths (Kovda et al., 1973) but because of its extremely low solubility is less effective in ameliorating processes (Qadir et al., 2007).
The choice of amendment depends upon the characteristics of the salt affected soils including soil pH and presence of CaCO3 and MgCO3, level of ESP, desired rate and extent of replacing exchangeable Na, soil type and the cost and availability of the amendment. All chemical amendments share a common characteristic when applied under appropriate soil conditions that is the supply of soluble Ca that replaces Na on exchange sites (Keren and Miyamoto, 1990; Nadler et al., 1996; Bohn et al., 2001). Some of amendments are briefly reviewed in the following sections.
Pressmud (PM), waste product of sugar mills, possess both the qualities of organic and inorganic fertilizers and can be used for the reclamation of saline-sodic soil (Patel and Singh, 1993; Zerega, 1993; Rai et al., 1999; Yaduvanshi and Swarup, 2005). It is the residue obtained from sedimentation of the suspended materials such as fiber, wax ash, soil and other particles from the cane juice. There are two types of PM, one that contains high amount of SO4 and the other that contains high amount of CO3. In Pakistan carbonation is the only method by which sugar is extracted from sugar cane and sugar beet juice in sugar industry, and therefore it contains high amount of lime. The PM usually contains about 70% lime, 15-20% Organic matter and 2-3% sugar (Khattak and Khan, 2004). The PM also contains variable amounts of nutrients. The organic matter fraction of PM is highly soluble and readily available for enhancing the microbial activity (Gaikwad et al., 1996; Tompe and More, 1996a; Rangaraj et al., 2007). The higher [CO2] in soil solution liberated by microbial activity helps to dissolve lime and promote the process of saline-sodic soil reclamation (Robbins, 1986a; Nadler et al., 1996; Qadir et al., 2001, 2006). This advantage of the readily available amount of organic mater makes the PM efficient ameliorant both in non-calcareous and calcareous saline-sodic soils having sparingly soluble lime.
Though the pressmud contains Ca of low solubility mostly in calcite form, but its higher organic fraction and other nutrients may produce desirable effects in saline-sodic and sodic soils especially in soil low in organic matter. Many researchers used pressmud, applied alone or with other amendments to test its effect on soil conditions and crop yield in problem soils. It was found effective in removing leachable Na, Ca and Mg under percolated conditions (Patel and Singh, 1993) and successfully used for reclamation of saline-sodic soils (Zerega, 1993). Its application improves the physical condition of the soil; supplies trace nutrients, and may assist retention of inorganic fertilizers (Singh et al., 1991; Yadav et al., 1995). It increases the permeability and air porosity (Cairo et al., 1994), and increases water holding capacity and decreases bulk density of soil (Tompe and More, 1996a). Because of lime, the soil pH may increase but in low pH soils.
Soil biological properties improve with PM addition. The rhizosphere microbial population especially bacteria and actinomycetes were increased after 30 d of PM application (Gaikwad ct al., 1996). In another study Tompe and More (1996a) reported that application of pressmud increased the bacterial, fungal and azotobacter population, whereas combined PM plus recommended dose of fertilizer showed the highest actinomycetes population at all growth stages of sunflower. Application of 12.5 Mg ha-1 PM to soil resulted in more number of colonies of bacteria, fungi and actinomycete compared to the same level of FYM (Rangaraj et al., 2007).
The PM is an excellent material for adding organic matter, Ca, P and K to soil (Orlando et al., 1991; Piedra et al., 1992; Duran, 1993; Zerega, 1993, Yadav et al., 1995; Gaikwad et al., 1996). It is an important potential source of organic matter in arid and semi-arid countries such as Pakistan or India (Badole et al., 2001). Zeerega (1993) summarized that PM is used in many countries as a source of nutrients to improve soil physical properties and ameliorate the salt-affected soils. However, the use of filter cake is limited mainly because of high moisture content (75-80%) at time of supply from the factory. This increases the transport cost and makes its use very expensive. Another limitation on filter cake is its high C:N ratio, which retards crop growth when it is incorporated to the soil at planting time. It was recommended that these limitations could be prevented by dehydrating the filter cake and enriching it with N before applying it to the soil.
Gypsum has long been recommended for the amelioration of sodic and saline sodic soils (Shainberg et al., 1982; Elshout and Kamphorst 1990; Bohn et al., 2001; Qadir et al., 2006; 2007). The solubility of gypsum is relatively low, 0.2% (0.2 g in 100 mL water) that may hinder its effectiveness in reclaiming sodic soil (Carter et al., 1977). The solubility can be enhanced with application of fine ground material and with application methods. Application of gypsum in standing water is more efficient than application over soil surface (Chaudhry et al., 1986) because of rapid dissolution in case of standing water. Similarly the more powder is the gypsum the more it is efficient in reclaiming sodic soils (Chaudhry et al., 1986; Chaudhry and Ihsanullah, 1989; Ghafoor et al., 1989; Chaudhry, 2001). Dut et al. (1971) estimated that 52 to 72 cm water is required to dissolve 16.5 to 23.9 Mg ha-1 gypsum applied on soil surface. The solubility of gypsum increases by 10 folds under sodic soil condition. Moreover, mixing of gypsum and fast removal of Na from the soil solution will speed up the exchange process (Frenkel et al., 1989). However, if the soil is dense and has poor drainage, little or none of the exchange will be removed and gypsum application will largely be ineffective rather it can increase the soil salinity. Ilyas et al. (1997) observed higher Na, Ca+Mg, and EC values with gypsum application that were mainly attributed to poor soil permeability where the replaced Na remained in the soil solution. However, alter one yr the EC and Na started to decline. Under soil conditions deep ploughing will facilitate the process of reclamation to allow leaching of Na salts.
Gypsum application improves physical and chemical properties of the soil (Ayers and Westcot, 1985), soil porosity (Oster 1982; Gal et al., 1984; Shainberg ct al., 1989) and soil hydraulic conductivity (Scotter, 1985; Greene et al., 1988). A marked decrease in soil bulk density was observed when treated with surface applied phospogypsum (Southard et al., 1988). Ghafoor et al. (1985) observed a dramatic increase in grain yield of wheat with gypsum CaSO4.2H2O) application.
Applications of organic materials improve soil fertility (Tang and Yu, 1999; Tang et al., 1999; Ferreras et al., 2006; Flavel and Murphy, 2006; Grace et al., 2006) along with favorable effects on soil physico-chemical and biological properties (Usirri et al., 2006; Clark et al., 2007; Abbas and Fares, 2009). The higher structural stability indicated an improvement in pore size distribution due to an increased number of soil macropores (Marinari et al., 2000).
Application of organic materials reduces the deteriorative effects of salinity. It increases the CEC of soil and replacement of Na by Ca (Rengasamy and Olsson, 1991; Samin et al., 1999). Moreover, it has the potential to mobilize native soil Ca from CaCO3 and other Ca-bearing minerals (Minhas et al., 1995; Choudhary et al., 2002). Kahlown and Azam (2003) reported that application of farmyard manure improved infiltration rate by 88.9% and decreased sodicity by 41.3%. Because of improvement of soil aggregation through its effects on soil water potential, temperature, aeration and mechanical impedance, organic materials influence root development and seedling emergence (Ferreras et al., 2000) and plant growth (Marinari et al., 2000). With increase in FYM application rate the sugar beet yield was increased reaching up to 42% as compared with control in salt-affected soils of Charsadda, Mardan and Swabi district, NWFP, Pakistan (Haq, 2005).
Organic matter releases substantial nutrients upon decomposition. However, there are evidences that rate of C and N mineralization is either decreases (Malik and Haider, 1977; Wichem et al. 2006) or increases by salinity (Laura, 1973; Laura, 1976) depending upon the C:N ratio and nature of organic matter. Abdou et al. (1975) found that salinity and sodicity decreased the rate of mineralization from added straw early in the incubation. Mineralization is negatively correlated with salinity (Wichern et al., 2006). Organic matter decomposition depends on clay mineral type, percentage of clay, presence of divalent cations and the soil structure, which all affect the availability of water and aeration, and directly or indirectly microbial activity (Baldock and Skjemstad, 2000; VonÂ·Lutzow et al., 2006).
Microbial inoculants also known as soil inoculants are agricultural amendments that use beneficial microbes (bacteria or fungi) to promote plant health. Many of the microbes involved form symbiotic relationships with the target crops where both parties benefit (mutualism). While microbial inoculants are applied to improve plant nutrition, they can also be used to promote plant growth by stimulating plant hormone production (Bashan & Holguin, 1997; Sullivan, 2001). Use of beneficial and effective microorganisms as microbial inoculants in agriculture is a promising new technology (Shao et al., 2001). It has been shown to be effective in improving soil health and quality, and raising the growth yield and quality of crops (Li et al., 1999). The associations of plants with fungal inoculation have strong influences on the responses of microbial activities to nutrient fertilization, due to the fact that these fungi are able to enhance nutrient availability to and uptake by plants inoculated with mycorrhiza (Smith and Read, 1997). Following inoculation, AM fungi influence microbial population and activity and consequently nutrient dynamics in the soil through the release of organic compounds. Fungal inoculation may directly or indirectly contribute to soil C and N dynamics. The activity of fungal inoculation in the mycorrrhizosphere could be a source of different soil enzymes required for biochemical reactions. There are reports indicating that soil enzymatic activities, such as phosphatases, dehydrogenases are increased by inoculation (Kothari et al., 1990). Mycorrhizosphere development process by the inoculation of mycorrhiza has been reported to modify the quality and abundance of rhizospheric microflora and alter overall rhizosphere microbial activity which may be responsible for the bioremediation in the contaminated soil (Khan, 2006). Moreover, they influence the physiology of their host plant making them less vulnerable to pathogens, soil pollution, salinity, drought and a number of other environmental stress factors.
There has been a large body of literature describing potential uses of plant associated bacteria as agents stimulating plant growth and managing soil and plant health Bashan and Holguin, (1998). Plant growth-promoting bacteria (PGPB) are associated with many, if not all, plant species and are commonly present in many environments (Kloepper et al., 1999). The most widely studied group of PGPB is plant growth-promoting rhizobacteria (PGPR) colonizing the root surfaces and the closely adhering soil interface, the rhizosphere (Kloepper et al., 1999). PGPR can contribute to plant growth by increasing nitrogen uptake, synthesis of phytohormones (auxin, cytokinin), solubilization of minerals, and iron chelation (Bowen and Rovira, 1999). Much remains to be learned from nonsymbiotic bacteria that have unique associations and apparently a more pronounced growth-enhancing effect on host plants (Döbereiner and Pedroza, 1987; Ping and Boland, 2004). Most of the focus has been on free-living rhizobacterial strains, especially to Azotbacter. Of the several species of azotobacter, A. chroococcum happens to be the dominant inhabitant in arable soils capable of fixing nitrogen (2-15 mg N2 fixed g-1 of carbon source) in culture media. The bacterium produces abundant slime which helps in soil aggregation (Mishra and Dadhich, 2010). Significant increase in wheat grains quality and productivity upon Azotobacter inoculation (Abou-Zeid et al., 2003). Azotobacter may supply most of the fixed N required for plant growth or produce several compounds: acetoacetyl-CoA, a precursor for poly-Î²-hydroxybutyrate (PHB) synthesis, and in Î²-oxidation Mishra and sen, (1996), exopolysaccharide (Meenakshi et al., 1995) pantothenic acid and thiamine (Martinez-Toledo et al., 1996), Fertility-promoting metabolites (Fiorelli et al., 1996), alginates (Sabra and Zeng, 2009) and influences nitrogen transformation in the rhizosphere soils (Das and Saha, 2000), leading to a greater crop yield (Mishra and Sen, 1996). They also facilitate the mobility of heavy metals in the soil and thus enhance bioremediation of soil from heavy metals, such as cadmium, mercury and lead (Chen et al., 1995). Use of Azotobacter as biofertilizer help crop plants' uptake of nutrients by their interactions in the rhizosphere when applied through seed or to soil. They accelerate certain microbial processes in the soil which augment the extent of availability of nutrients in a form easily assimilated by plants (Mishra and Dadhich, 2010).
The combination of strains of Plant Growth Promoting Rhizobacteria has been shown to benefit rice (Nguyen et al., 2002) and barley (Belimov et al., 1995a). The main benefit from dual inoculants is increased plant nutrient uptake, from both soil and fertiliser (Bashan et al., 2004; Belimov et al., 1995a). Interestingly, multiple strain inoculants have also been demonstrated to increase total nitrogenase activity compared to single strain inoculants, even when only one strain is diazotrophic (Lippi et al., 1992; Khammas & Kaiser, 1992, Belimov et al., 1995a).
PGPR and arbuscular mycorrhizae in combination can be useful in increasing wheat growth in nutrient poor soil (Singh & Kapoor, 1999) and improving nitrogen-extraction from fertilised soils (Galal et al., 2003). In salinised soils, Rabie (2005) found that inoculating AM-infected Vicia faba plants with Azospirillum brasilense amplified the beneficial effects of AM inoculation.
Effect of Salts on Soil Microbial Properties
The soil's microbes are essential for plant life. They are the primary living beings which through their microbial activity extract vitamins, minerals and nutrients from agricultural inputs in the soil's atmosphere; and take nitrogen, oxygen and carbon dioxide from the atmosphere and make these necessary ingredients available to plants for healthy growth. Saline and sodic soils exhibit soil structural problems, due to changes in physical and chemical properties (Qadir et al., 2007). Presence of excess sodium in salt affected soil leads to the development of poor physical conditions by dispersing the soil aggregates.
The chemistry of the soil is influenced by physical problems such as water logging and compaction, which can lead to changes in the nutrient ion formation, rendering them unavailable to plants (Naidu and Rengasamy, 1993). Osmotic stress and high levels of Na+ can cause imbalances in plant nutrition, causing ion deficiencies or toxicities (Sheldon et al., 2004; Qadir et al., 2007). These physical and chemical changes reduce the activity of plant roots and crop growth as well as of soil microbes (Rietz and Haynes, 2003). Furthermore, low SOM combined with high salt concentrations and high or low pH, will generally also show low biological activity (Naidu and Rengasamy, 1993; Sardinha et al., 2003).
Salinity and sodicity in soils has a detrimental effect on the microbial activity, i.e. reduce microbial biomass, which in turn are then less efficient in using available C resources and consequently showing a decrease in soil respiration (Rietz and Haynes, 2003; Sardinha et al., 2003; Wichern et al., 2006; Yuan et al., 2007). Sardinha et al. (2003) found that the combined effects of salinity and acidity depressed microbial communities more than those of heavy-metal pollution under acidic conditions. They attributed this to the possibility that a decline in vegetation reduces the root and litter debris, which is an important energy resource for microbial organisms.
A decline in microbial activity in saline soil may lead to an accumulation of non-decomposed OM, due to slower transformation of organic substances (Rasul et al., 2006; Wichern et al., 2006). This will negatively affect the subsequent release of nutrient for plant growth (Yuan et al., 2007). Some investigators (Rietz and Haynes, 2003; Sardinha et al., 2003; Yuan et al., 2007) reported no evidence of SOM accumulation in salt affected soils with low levels of both soil microbial activity and N mineralization. Yuan et al., (2007) found that in soils with highest salinity, the organic C content was lowest. Rietz and Haynes (2003) reported declines in soil C due to lower OM inputs, as plant growth was greatly reduced in these soils.
Increases in salinity have been shown to decrease soil respiration rates and the SMB (e.g., Laura, 1973, Laura, 1976, Pathak & Rao, 1998) and was attributed to stresses placed on the microbial population due to changes in osmotic potential (Batra & Manna, 1997). Similarly, increasing sodicity levels have had slight negative correlations with C mineralization (Nelson et al., 1997), and caused a decrease in the amount of SMB (Chander et al., 1994). Conversely, increasing sodicity has increased C mineralization, possibly due to increased solubilisation of organic matter (Nelson et al., 1996).
The soil microbial biomass is a labile pool of organic matter and comprises 1-3% of total soil organic matter (Jenkinson and Ladd, 1981). The soil microbial biomass acts as a source and sink of the plant nutrients ( Singh et al., 1990) and regulates the functioning of the soil system. Plant cover through its effects on the quantity and quality of organic matter inputs influences the levels of soil microbial biomass (Wardle, 1992). In saline and alkaline soils, excessive amounts of salts have an adverse effect on biological activity including soil enzyme activity, nitrogen mineralization (McClung and Frankenberger, 1985) and soil microbial biomass (Kaur et al., 2000). The biological activity of alkaline soils has been found to improve under a crop, grass or tree cover (Rao and Ghai, 1985). Nitrogen mineralization is an essential function of the soil microbial system (Ellenberg, 1971).