Salt Affected Soils in Dire Dawa, Ethiopia. A Characterization and Classification

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURE

1. INTRODUCTION

2. LITERATURE REVIEW
2.1. Definition to Describe Salt Affected Soils
2.1.1. Salt Affected soils in Ethiopia
2.1.1. Causes of Stalinization and Saline Soil
2.1.1. Saline soils
2.1. Classification of saline soil
2.1.2. Saline-Sodic Soils
2.1.3. Sodic soils
2.2. Causes of salt affected soils
2.2.1. Primary salinity
2.2.2. Secondary salinity
2.4. Impact of Salt Affected Soil on Plant
2.4.1. Osmotic effect of salt
2.4.2. Nutrition and ion toxicity of salt
2.4.3. Structure and permeability problem of salts in the soil
2.5. Adaptations of salt tolerant plants
2.6. Some salt tolerant plants
2.7. Other reclamation methods of salt affected soils

3. MATERIALS AND METHODS
3.1. Study area description
3.2. Method Of Sampling Collection and Preparation Sampling Units
3.2.2. Soil Extraction methods preparation
3.3. Soil Sample laboratory analysis methods
3.3.1. Quantify The Soil pHe and Ece From Saturated Extraction
3.3.2.Determination of soil Soluble Exchangeable base ( Na +2 , K+, Ca2+ and Mg +2)
3.3.2.1. Estimation of Soil Soluble Na + and K+ by using the flame photometer Instrument
Figure 3: A standard curve for estimating K using the flame photometer Instrument
3.3.2.2. Estimation of soil Ech. Ca2+ and Mg 2+ by using the AAS instrument
3.3.2.3. Estimation of Soil Sample SAR, ESP and ESR for Soil Sample for study site

4. SOIL SAMPLE RESULT AND DISCUSSION
4.1. Characteristics of Chemical Properties of Soil Sample at Study Area
4.1.1. Soil pH and Electrical Conductivity Result interpretation and Discussion
4.1.2. Exchangeable Soluble cations result Interpretation and Discussion
4.1.3. Sodium adsorption ratio (SAR) and Exchangeable Sodium percentage (ESP)

5. CONCLUSIONS AND RECOMMENDATION

6. REFERENCES

7. APPENDIXES

LIST OF TABLES

Table 1: Sodium Absorbance by Flame Photometer reading

Table 2: Potassium Absorbance by Flame Photometer reading

Table 3: Calcium Absorbance reading from atomic absorption spectrophotometer

Table 4: Magnesium Absorbance reading from atomic absorption spectrophotometer

Table 5: Chemical properties laboratory Analytical Result of soil sample at study area

LIST OF FIGURE

Figure 1: Study area location

Figure 2: A standard curve for estimating Na using the flame photometer instrument.

Figure 3: A standard curve for estimating K using the flame photometer Instrument.

Figure 4: A standard curve for estimating Ca using AAS instrument.

Figure 5:A standard Curve for estimating Mg using AAS Instrument.

Characterization and Classification of Salt Affected Soils in Dire Dawa, Ethiopia.

ABSTRACT

The study of soil chemical properties was made on the soils of Dire Dawa Area agricultural farmland. The aim of this study was to characterize salt-affected soils based on their salt content, quantify selectively soil chemical properties of the study area, and give suggestions appropriated reclaim of salt-affected agricultural land management practices. Soil samples were collected from the Dire Dawa Agricultural farmland area. Soil chemical analysis of the soluble salts, exchangeable salts, ECe, pHe, and ECEC was done at Haramaya main soil chemistry laboratory. Soil sample analysis in the laboratory soil test shrub confirms the presence of salt in the study area. The result of this study reveals the existence of the salt problem in the study area from the laboratory analysis of soil samples in the study area. The extent of salinity in the study area was categorized based on four main parameters of salt-affected soil such as ECe (electrical conductivity), pHe, ESP (exchangeable sodium percentage), SAR (sodium adsorption ratio). The pHe, EC, Na, K, Ca, Mg, ECEC, SAR, ESP and ESR values for soils collected from the Study area were 7.70, 4.5 dS/m, (36. 61, 8.96, 350.28, 10.33, 406.18) cmol(+)/kg, 2.73%, 10.15% and 0.087% respectively. The order of abundance of the basic exchangeable cations was Ca > Na > Mg > K. According to Classification of salt-affected soil standard guidelines the soil sample was collected from the Dire Dawa site in Saline Soil. The SAR value of this soil type was less than 13 that indicates the concentration of sodium in the soil solution is much lower than the concentration of calcium and magnesium, this value is a threshold to define saline soil. The concentration of sodium lowers so it does not affect soil's physical properties. to manage the salt-affected soils and maintain the salinity and sodicity levels of the irrigation water at or below the current level, it is essential to delineate the salt-affected areas and reclaim them using chemical amendments integrated with biological management practices. Reclamation of the study site saline soil involves the application of quality irrigation water to leach the excessively accumulated salts below the rooting zone of the soil and the development of salt-tolerant crop plant species varieties.

Keywords: Salt affected soils, soil chemical analysis, Ece, SAR, ESP and, Saline Soil

1. INTRODUCTION

Soil salinity and alkalinity problems are commonly found in the arid and semi-arid regions of the earth due to insufficient annual rainfall to leach accumulated salts from the root zone (US Salinity Laboratory Staff, 1954; Heluf, 1985; Kidane et al., 2006). In other words, salt affected soils often occur in areas where soluble salts and sodium (Na) accumulate in soils through physical and chemical weathering of rocks or the pedogenic process of the soil development, atmospheric precipitation and fossil salts from marine or lacustrine environments. Moreover, heavy fertilizer application and use of poor quality irrigation water and inadequate drainage have contributed to the development of salt affected soils and productivity deterioration of many soils in irrigated arid and semi arid regions (US Salinity Laboratory Staff, 1954; Gupta and Abrol, 1990).

Salinization of land and water resources is a major landscape degradation issue worldwide, with serious salt related problems occurring in at least 75 countries (Rhoades, 1990). High concentration of salts in the root zone limits the productivity of nearly 953 million hectares (ha) of productive land in the World. Australia, followed by Asia, has the largest area under salinity and sodicity problems. According to the recent reports, the area of salt affected land coverage is estimated to be more than 60% in Australia which has continued to expand (Robertson et al., 2010). In Africa also, it covers about 81 million ha of the dry land areas (Szabolcs, 1979). Most of the salt affected soils and brackish groundwater resources are confined to the arid and semi arid regions and are the causative factors for triggering the process of desertification. Generally, in the irrigated areas, human-induced salinity and sodicity related land degradation is becoming a serious challenge for food and nutritional security in the developing world (Singh, 2009; Wahab et al., 2010).

Ethiopia is the first in Africa and the ninth country in the World having more than 11 million ha of salt affected soils (FAO, 1988) which are mainly found in the Rift Valley, Wabi Shebele River Basin and various lowlands of the country. Following the establishment of large scale irrigated farms, the problem become worse due to poor drainage system and inappropriate water management practices coupled with unsound reclamation procedures. For instance, over 2280 ha (Melka Sadi), 500 ha (Matahara), 300 ha (Asyta), 220 ha (Kebena or Yalo), 145 ha (Kesem), 100 ha (Gewane), 56 ha (Werer State Farm), 80 ha (Shoa, Kefa Dura), 20 ha (Mille) and some areas at Tangay Kuma State Farms of Ethiopia have been proved to be salt affected soils. Moreover, it is expected that the salt affected soils in these areas will dramatically increase in the few years if the current irrigation practice is allowed to continue without proper management (Girma and Endale, 1996; Kidane et al., 2006).

Debela (2017) explained that, soil salinity and alkalinity problems are particularly severe in developing countries, especially arid and semiarid regions, resulting in damage to the livelihoods of people in the short term, and with long term effects on food security of the country. Besides to these, heavy fertilizer application, use of poor quality irrigation water and inadequate drainage has contributed to rising groundwater tables leading to salinity-induced land degradation (Qureshi et al., 2013; Sarwar et al., 2015).

As reported by Murphy (1968), the Rift Valley Zone of Ethiopia as a whole is potentially a very valuable agricultural area. Moreover, Zinabu (2003) indicated that the greatest concentration of water bodies in Ethiopia is located in the Rift Valley. Thus, there is a tendency to consider the use of these waters for irrigation as a solution to alleviate the problem of the very unreliable rainfed agriculture and to the determinant for agricultural development and self-sufficiency with respect to food production. Realizing this options and opportunities, Ethiopia, which suffered from repeated droughts, famine, low soil fertility, low productivity of the rain fed agriculture and high population pressure in the highlands (Lakew et al., 2000) is currently increasing the need to extend the agricultural production using irrigation to the vast areas of the potentially irrigable lands in the arid and semi-arid lowlands of the country at which rain-fed production is difficult.

To understand how improved the soil fertility may assist in reaching these needs, it requires knowledge of salinity and sodicity related soil chemical properties the soils of the lands agriculture. Such knowledge is believed to help the producers and production managers to understand about and make the necessary modifications in the soil-salt-water balance.

To resolved the problem of the Dire Dawa study site agricultural problem it should be characterizing the nature of salinity of the area bade on laboratory soil test and soil chemical properties. The aim of this study was:

- To identify the nature and quantify selectively soil chemical properties of the study area.
- To characterized and categorized salinity soil based on their salt content of the study area.
- To give suggestion appropriated reclaim of salt affected agricultural land management practices.

2. LITERATURE REVIEW

2.1. Definition to Describe Salt Affected Soils

According to Gonzalez et al., (2004) and Qadir and Schubert (2002) salt soil classifications currently used in all countries of the world were the one which were divided into three main groups: saline, sodic and saline sodic.

2.1.1. Salt Affected soils in Ethiopia

Ethiopia is reported to possess over 11 million hectares of unproductive naturally salt affected wastelands (Tadelle 1993). The naturally salt affected areas are normally found in the arid and semi-arid lowlands and in Rift valley and other areas that are characterized by higher evapotranspiration rates in relation to precipitation (PGRC 1996). With the development and expansion of irrigated agriculture, however, man’s activities are greatly contributing to the buildup and spread of salinity problems. Close to 3 million ha of potentially irrigable land in the different river basins and in the rift valley areas where existence of salinity problems is well documented (Tadelle 1993).]

The arid like climate in the irrigated area allow limited leaching by favoring concentration and accumulation of soluble salts in the soil. Poor water management practices (excess application of irrigation water contribute to recharge of ground water) and lack of adequate drainage facilities have greatly contributed to the conversion of large productive lands in to an unproductive waste land in a short period of time. Rising ground water leads to water logging of the root zone area and ultimately to Stalinization. Stalinization processes continue to affect production and productivity since drainage facilities are absent and water management practices remain inefficient and quite wasteful (Tadelle 1993).

High salinity in the Ethiopian ground waters is apparent in some parts of the Rift because of the influence of saline geothermal waters. In the southern parts of the Rift, sodium and bicarbonate (high alkalinity) are the dominant dissolved constituents. High concentrations of dissolved salts in the ground waters from the sedimentary formations are also common, as a result of reaction of the often-abundant evaporite minerals. In these, high salinity may be manifested by high concentrations of sodium, chloride and/or sulphate in particular. Observed increased salinity in many ground waters from sediments in the south, southeast and northeastern parts of the country arises from the dissolution of evaporate minerals (the products of evaporation) in certain horizons of the sediments (BGS 2001). Tamirie (1994) has revealed that 44 million ha (36% of the country’s total land areas) are potentially susceptible to salinity problems. Out of the 44 million ha, 33 million ha have dominantly salinity problems, 8 million ha have combined salinity and alkalinity problems, and 3 million ha have dominantly alkalinity problems.

2.1.1. Causes of Stalinization and Saline Soil

In many places in the world, the productivity of soil has deteriorated because of an excess of salt has accumulated in the soil around the plant root zone. Large-scale soil salinization has mostly occurred in arid and semi-arid regions. Soil affected by salt also widely exists in sub-humid and humid (i.e. high rainfall) regions. Saline soil is particularly frequent in coastal areas since the soil in those areas is exposed to seawater. Even if the water is low in salinity, the salinity in the soil will increase if the water is used for irrigation for a long time because the trace amount of salt gradually accumulates.

Excessive salinity of the soil surface and the root zone are typical properties of saline soils. The main source of salts in soil is exposed bedrock in geologic strata in the Earth's crust. Salts are gradually released from the bedrock after becoming soluble through physical and chemical weathering such as hydrolysis, hydration, dissolution, oxidation, and carbonation. The released salts dissolve into the surface water or groundwater. As the water with dissolved salts flows from humid regions to less humid or arid regions, salts in the water are gradually concentrated.

The most dominant ions at the place where salts become soluble by weathering are carbonate and bicarbonate of calcium, magnesium, potassium and sodium, if carbon dioxide exists. At first, the salinity of the water is low, but as the water flows from a humid area to a less humid area, it becomes higher as the water evaporates. As the salts in the water are further concentrated, salts with lower solubility start to precipitate. In addition, due to other mechanisms such as ion exchange, adsorption, and the difference of mobility, the concentrations of chemical substances dissolved in the water gradually shift; this always results in increased concentration of chloride and sodium ions in water and soil.

2.1.1. Saline soils

All soils contain some water-soluble salts, but when these salts occur in amounts that are harmful for germination of seeds and plant growth, they are called saline (Conway 2001; Denise 2003). The soluble salts that occur in soils consist mostly of various proportions of the cations calcium (Ca++), magnesium(Mg++) and sodium (Na+), and the anions chloride (Cl-), and sulfate (SO4 =). Constituents that ordinarily occur only in minor amounts are the cation potassium (K+) and the anions bicarbonate (HCO3 -), nitrate (NO3-) and carbonates (CO32) but soluble carbonates are almost invariably absent. The salts types found in saline soils are mostly sulfates and/or chlorides of calcium and magnesium (Bonnie et al., 2002; BPMC 1996; Joe 2002).

Owing to the presence of excess soluble salts and the absence of significant amounts of exchangeable sodium, saline soils generally are flocculated often are in normal physical condition with good structure; and, as a consequence, the permeability is equal to or higher than that of similar nonsaline soils (BPMC 1996; Conway 2001; Jim 2002).

Saline soils are often recognized by the presence of white crusts of salts on the soil surface called “White alkali” (soluble salts) and irregular plant growth. Electrical conductivity of these soils when a solution extracted from saturated soil is greater than or equal to 4.0 mmhos/cm at 250C. The pH is generally less than 8.5, sodium makes up less than 15 percent of the exchangeable cations and the sodium adsorption ratio (SAR) is less than 13 (Conway 2001; Jim2002; Michael & Paul 2002).

2.1. Classification of saline soil

Saline soil is usually categorized into the following three types, saline, sodic, and alkaline sodic soil. Saline soil contains a lower amount of sodium absorbed onto soil particles. This type of soil is often seen in sandy soil containing lower amounts of clay and organic matter. Saline soil in deserts is usually of this type. Sodic soil contains a large amount of sodium absorbed onto soil particles. This type of soil is often seen in soil that contains large amounts of clay. Alkaline sodic soil is a type of sodic soil that is highly alkaline with the pH value more than 8.5. This type of soil contains higher amounts of carbonate and bicarbonate which can be hydrolyzed to alkalize the products. This soil type has also been called “alkaline soil”. Excessive amounts of carbonate and bicarbonate salts may be brought into soil with groundwater by capillary effect or by irrigation water, or may be formed from soil particles themselves.

2.1.2. Saline-Sodic Soils

Saline Sodic soils contain large amounts of total soluble salts and exchangeable sodium (BPMC 1996). As long as excess soluble salts are present, the good physical properties (stable soil structure), the whitish appearance, the EC which is greater than 4 mmhos/cm at 250C and the pH which is less than 8.5 are generally similar to those of saline soils. But they differ by the fact that more than 15 percent of the exchangeable cations are sodium and the sodium adsorption ratio is greater than 13 (Conway 2001; Jim 2002). If the excess soluble salts are leached downward, the properties of these soils may change markedly and become similar to those of sodic soils with high pH above 8.5, the dispersion of particles and the soil becomes unfavorable for the entry and movement of water and for tillage (BPMC 1996; Denise 2003). Saline-sodic soils sometimes contain gypsum. When such soils are leached, calcium dissolves and the replacement of exchangeable sodium by calcium takes place concurrently with the removal of excess salts (Conway 2001).

2.1.3. Sodic soils

Sodic soils are low in soluble salts than saline or saline-sodic soils but high in exchangeable sodium (Jim 2002; Pam 2002). The soil solution of sodic soils, although relatively low in soluble salts, has a composition that differs considerably from that of normal and saline soils. These soils contain (HCO32-), and CO32-as the dominant anion (Qadir & Schubert 2002). At high pH readings and in the presence of carbonate ions, calcium (Ca++) and magnesium (Mg++)are precipitated and hence, the soil solutions of sodic soils usually contain only small amounts of these cations but high amount of sodium (Na+) being the predominant one (Joe 2002; Qadir & Schubert 2002). In addition, when the plants extract the water from the soil; the salts remain and become concentrated. This concentration causes the calcium to precipitate as calcium carbonate, while much of the sodium remains in the soil water (Conway 2001).

These soils have exchangeable sodium percentages of more than 15. This means that sodium occupies more than 15 percent of the soil cation exchange capacity (CEC) and the sodium adsorption ratio (SAR) is greater than 13. The electrical conductivity is less than 4 mmhos/cm at 250C and the pH readings usually range between 8.5 and 10.

2.2. Causes of salt affected soils

Salts in the soil occur as ions (electrically charged forms of atoms or compounds). These ions are released from two main sources: primary or natural sources and secondary or salinization caused by human factors (Michael & Paul 2002).

2.2.1. Primary salinity

Primary salinity is caused by naturally occurring salt deposits mostly in arid and semiarid areas. Some salt soils are released from weathering saline parent rocks. Also when precipitation is insufficient to leach ions from the soil profile and transport the salts away from the root zone and when there is high evaporation rate salts accumulate in the soil and soil salinity can result this way too (Ashraf & Harris 2005; Salliah & Pathmarajah 2003).

Poor drainage is another factor that usually contributes to the salinization of soils, it is the most common cause, and it may involve the presence of a high ground-water table or low permeability of the soil. The high ground-water table is often related to topography. Therefore, there are drainage basins that have no outlet to permanent streams. The drainage of salt-bearing waters away from the higher lands of the basin may raise the ground-water level to the soil surface on the lower lands, may cause temporary flooding, or may form permanent salty lakes. Under such conditions, upward movement of saline ground water or evaporation of surface water results in the formation of saline soil (Ashraf & Harris 2005). Low permeability of the soil causes poor drainage by impeding the downward movement of water. Low permeability may be the result of an unfavorable soil texture or structure or the presence of indurated layers (Ashraf & Harris 2005; Salliah & Pathmarajah 2003).

2.2.2. Secondary salinity

Natural salinity has been intensified by change in land use, which means change from more water use plant to less water use plant cause water table rise, when irrigation water quality is marginal or worse (Selliah & Pathmarajah 2003). In addition, when the drainage of the soil may not be adequate for irrigation, the ground-water table may rise from a considerable depth to a few feet of the soil surface in a few years due to irrigation. When the water table rises to 5 or 6 feet of the soil surface, ground water moves upward into the root zone and to the soil surface. Under such conditions, ground water, as well as irrigation water, contributes to the salinization of the soil. Deforestation, overgrazing, or intensive cropping, fertilizer and amendments applied to soils are another secondary salinity causes (Ashraf & Harris 2005; Selliah & Pathmarajah 2003). 2.3. Tests to quantify and qualify soil saltiness There are several tests to quantify or qualify soil saltiness. It is important to understand the differences among these tests because each provides a specific type of information (Joe 2002).

Electrical conductivity (EC) describes the amount of electrical current conducted by extracting the solution (water containing ions) from a saturated soil sample at a fixed temperature (Joe 2002). The higher the moisture content, the easier it will be to obtain the extract. The greater the concentration of ions or soluble salts in the saturation extract, the more electricity the solution will conduct, the greater the EC reading, and the greater the toxicity to plants. This test does not distinguish between one type of salt and another; it simply provides an overall measure of water-soluble salts. The electrical conductivity of the saturation extract is often referred to by the abbreviation ECe (Conway 2001). Units of measurement include decisiemens per meter (dS/m or dS m-1) and millimhos per centimeter (mmhos/cm or mmhos cm-1), which are synonymous (1dS m-1 = 1mmhos cm-1) (BPMC 1996; Conway 2001; Joe 2002; Marx et al., 1996).

Since Na toxicity to plants is severe, and the effects of Na on soil pH and structure are significant, two tests have been devised to describe the relative amounts of Na present in the soil. The exchangeable sodium percentage (ESP) provides a measure of the amount of exchangeable Na relative to the total cation exchange capacity of the soil expressed as a percentage. As the ESP goes up, more exchangeable Na is available, and the greater the potential to be toxic to plants and soil. Its unit is percent (BPMC 1996; Joe 2002; Marx et al., 1996). The Sodium Adsorption Ratio (SAR) describes the ratio of the concentration of soluble sodium ion in the soil solution to the square root of the dissolved calcium and magnesium ion concentration that moderate the adverse effects of sodium (Joe 2002). It is calculated from concentration of cations (me/l) in the saturation extract. This measure is important, because magnesium and calcium provide a buffering effect on the impacts of high sodium that means where Na+ ions favor dispersion and Ca2+ and Mg2+ favor flocculation of soils. However, the presence of high magnesium has the same problem with sodium (Qadir & Schubert 2002). Excess exchangeable Mg2+ alone or in combination with excess exchangeable Na+ may behave like Na+ in soil degradation (Qadir & Schubert 2002). The greater the SAR, the more Na relative to Ca and Mg, the greater the toxicity to plants and indicates a sodic soil. SAR is unitless. At equivalent solution concentrations, the amounts of calcium and magnesium adsorbed are several times that of sodium (BPMC 1996; Conway 2001; Marx et al., 1996; TDEC 2005).

Soil pH is another important soil measure even though it does not directly measure saltiness. It is a measure of the hydrogen ion concentration in soil solution. This is an important indication of the chemical status of the soil. Since soluble salts affect soil pH and vice versa, it is often included in evaluations and discussions of soil saltiness. A main implication of changing the soil pH is plant nutrient availability (Conway 2001; Joe 2002; Marx et al., 1996).

Cation-Exchange-Capacity (CEC) is the capacity of a soil to adsorb and exchange cations. It is commonly expressed in milli equivalents per 100 gm. of soil (Joe 2002; Michael & Paul 2002; Robert & Robert 2001). Cation adsorption being a surface phenomenon is identified mainly with the fine silt, clay, and organic matter fractions of soils and it occurs as a consequence of the electrical charges at the surface of the soil particles. In general, it should be noted that the soil CEC values will increase in the following order of textural classes: sand, loamy sand, loam, silty loam, silty clay loam, clay loam and clay (Mostafa et al., 2001; TDEC 2005; Yormah & Egbenda 2005). Sodium, calcium, and magnesium cations are always readily exchangeable. Other cations, like potassium and ammonium, 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 (Fabrice & Michael 2003; Marx et al., 1996; Robert & Robert 2001). Salt affected soils having pH in alkaline range (pH>7) do not have Aluminum ion on the exchangeable site (Qadir & Schubert 2002). It is thus possible to use Effective Cation Exchange Capacity (ECEC) like CEC in soil which have pH value >7. The term ECEC refers to the sum of the four cations such as Sodium, Calcium, and Magnesium and Potassium (Robert & Robert 2001)

2.4. Impact of Salt Affected Soil on Plant

Strong salt affected soils can affect plant growth both physically (osmotic effect) and chemically (nutrition effect and/or toxicity). Due to these plant growth and yield is reduced, and the quality or value of agricultural production is lowered (BPMC 1996; Denise 2003; Gonzalez et al., 2004).

2.4.1. Osmotic effect of salt

Water is absorbed into plants because of a gradient that exists between the soil solution and the cell sap of the interior root cells. High concentration of neutral salts in the soil solution tends to narrow the gap between the soils (external) and plant cell (internal) water potentials (BPMC 1996). This means that salts increase the energy with which water is held in the soil. Then the soil water potential become more negative, making water movement to the root cells more difficult. If the soil solution potential becomes negative enough, water may actually migrate out of the plant cells and into soil solution (Joe 2002; Silvertooth, & Norton 2000). There may be ample available soil moisture for plant growth; it is just that the plant cannot extract it because of the strong negative potential. The effect is essentially the same as drought. The plant cannot get enough water to maintain proper growth, or it takes so much plant energy extracting the water that growth suffers. The situation is exacerbated under water stress conditions or drought, particularly on fine-textured soils where it takes more pull for the plant to remove water at a given soil moisture level (Conway 2001; Gonzalez et al., 2004).

High salinity (high EC) causes plant cell dehydration, reduced plant growth and possibly death in less tolerant plants, while tolerant ones survive in a number of physiological ways (Joe 2002). All of which show similar visual symptoms. The first visual symptoms appear the same as those of moisture stress from dry conditions. Plants may be stunted, leaves may cup, and overall plant health and color are affected. The symptoms progress to brown and brittle leaf tips, leaf margins, the overall leaf and, finally, the entire plant (Denise 2003).These symptoms may occur within a few days of planting young seedlings or after several weeks. With older plants, water deficit may present as a rapid dying off or browning of leaves at the top of the plant (BGS 2001)

2.4.2. Nutrition and ion toxicity of salt

In addition to the osmotic effect, certain ions are directly toxic to plants (Joe 2002). Specific ions such as sodium, chloride and boron have toxic effects on plants: reducing plant emergence and growth or causing damage to cells and membranes. Plants sensitive to these elements may be affected at relatively low salt levels if the soil contains enough of the toxic element (BPMC 1996; Fisseha 1998; Gonzalez et al., 2004).

Moreover, ions such as Na can influence soil chemistry and biology to such a degree as to limit plant nutrient availability by disrupting the uptake and utilization of other minerals needed by the plants (Joe 2002). This can cause disorders in mineral nutrition. Because many salts are also plant nutrients, high salt levels in the soil can upset the nutrient balance in the plant or interfere with the uptake of some nutrients (Conway 2001).For example, high sodium concentrations may cause deficiencies of other elements, such as potassium and calcium, and high levels of sulfate and chloride diminish the rate of nitrate absorption (BPMC 1996). In sodic soils, higher pH generally occurs, which also can affect nutrient availability. Soils with pH above 7 can have fewer nutrients availability (Denise 2003). The nutritional deficiencies and toxicities of plants can be characterized by necrosis (tip burning or marginal scorch browning and dieback of leaves), Chlorosis (turning yellow in color), and abscission (premature dropping) (BPMC 1996).

2.4.3. Structure and permeability problem of salts in the soil

Lastly, certain ions negatively influence soil structure and permeability characteristics, there by retarding plant growth (Joe 2002). Sodic and saline–sodic soils show structural problems created by certain physical processes (slaking, swelling and dispersion of clay) and specific conditions (surface crusting and hard setting). Such problems may affect water and air movement, plant-available water holding capacity, root penetration, seedling emergence, runoff and erosion, as well as tillage and sowing operations in sodic and saline-sodic soils (Qadir et al., 2003).

If a sodic clay layer occurs near the surface of sodic soils it often acts as a barrier to roots. Hence, most roots are restricted to the topsoil above the clay pan, because movement of water, nutrients is restricted (Fitzpatrick et al., 2003; Pam 2002). Plants on sodic soils usually show a burning or drying of tissue at leaf edges, progressing inward between veins. General stunting is also common (Fitzpatrick et al., 2003; Pam 2002).

Besides plant growth, salinity may also reduce seed germination, thereby affecting the ability to revegetate these sites. According to Conway (2001), a high salinity level restricts seed germination, but does not affect seed viability. Usually the plant stages that are most sensitive are germination or the early growth stages. However, some plants, known as halophytic (salt-loving) plants, are more tolerant (Denise 2003). Salt tolerance refers to the ability of a plant to maintain growth and metabolic function (including photosynthesis) when exposed to a high concentration of salt in the soil or in irrigation water (Duncan & Carrow 2000; Marcum 2001).

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