3. SALINE SOILS AND THEIR MANAGEMENT

3.1 Characteristics
3.2 Reclamation and management
3.3 Crops in saline soils

3.1 Characteristics

3.1.1 Measuring salinity status
3.1.2 Salinity and plant growth

The distinguishing characteristic of saline soils from the agricultural standpoint, is that they contain sufficient neutral soluble salts to adversely affect the growth of most crop plants. For purposes of definition, saline soils are those which have an electrical conductivity of the saturation soil extract of more than 4 dS/m at 25°C (Richards 1954). This value is generally used the world over although the terminology committee of the Soil Science Society of America has lowered the boundary between saline and non-saline soils to 2 dS/m in the saturation extract. Soluble salts most commonly present are the chlorides and sulphates of sodium, calcium and magnesium. Nitrates may be present in appreciable quantities only rarely. Sodium and chloride are by far the most dominant ions, particularly in highly saline soils, although calcium and magnesium are usually present in sufficient quantities to meet the nutritional needs of crops. Many saline soils contain appreciable quantities of gypsum (CaSO₄, 2H₂O) in the profile. Soluble carbonates are always absent. The pH value of the saturated soil paste is always less than 8.2 and more often near neutrality (Abrol et al., 1980). Physico-chemical characteristics in respect of a few typical saline soil profiles are presented in Tables 5-8.

Excess salts keep the clay in saline soils in a flocculated state so that these soils generally have good physical properties. Structure is generally good and tillage characteristics and permeability to water are even better than those of non-saline soils. However, when leached with a low salt water, some saline soils tend to disperse resulting in low permeability to water and air, particularly when the soils are heavy clays. Leaching may also result in a slight increase in soil pH due to lowering of salt concentration but saline soils, as will be shown later, rarely become strongly sodic upon leaching if there is an adequate drainage system.

In field conditions, saline soils can be recognized by the spotty growth of crops and often by the presence of white salt crusts on the surface. When the salt problem is only mild, growing plants often have a blue-green tinge. Barren spots and stunted plants may appear in cereal or forage crops growing on saline areas. The extent and frequency of bare spots is often an indication of the concentration of salts in the soil. If the salinity level is not sufficiently high to cause barren spots, the crop appearance may be irregular in vegetative vigour.

Moderate salinity, however, particularly if it tends to be uniform throughout the field, can often go undetected because it causes no apparent injuries other than restricted growth. Leaves of plants growing in salt infested areas may be smaller and darker blue-green in colour than the normal leaves. Increased succulence often results from salinity, particularly if the concentration of chloride ions in the soil solution is high. Plants in salt-affected soils often have the same appearance as plants growing under moisture stress (drought) conditions although the wilting of plants is far less prevalent because the osmotic potential of the soil solution usually changes gradually and plants adjust their internal salt content sufficiently to maintain turgor and avoid wilting.

Symptoms of specific element toxicities, such as marginal or tip burn of leaves, occur as a rule only in woody plants. Chloride and sodium ions and boron are the elements most usually associated with toxic symptoms. Non-woody species may often accumulate as much or more of these elements in their leaves without showing apparent damage as do the woody species.

Table 5 CHARACTERISTICS OF TYPICAL SALINE SOILS

* pH_S - pH measured on soil saturated paste.

Depth cm	Mechanical Composition %				pHs	ECe dS/m	Composition of the Saturation Extract me/l					SAR
	Organic Matter %	Clay <2 m	Silt (2-50 m)	Sand (50 m - 2 mm)			Na+	Ca++	Mg++	Cl-	SO4 -
0 - 15	1.1	39	46	15	7.4	12	20	78	30	111	16	2.7
15 - 37	0.9	40	46	14	7.6	13	48	70	30	123	20	6.7
37 - 66	0.6	43	51	6	7.9	10	66	30	28	87	42	12.0
66 - 127	0.6	37	52	11	7.9	14	106	40	26	93	68	18.0
127 - 136	0.5	37	55	8	7.9	15	123	32	38	105	96	22.0

Table 7 TYPICAL SALINE SOIL REPRESENTING ABU-HALANA SERIES, IRAQ (Sehgal 1980)

Depth cm	Mechanical Composition %				pHs	ECe dS/m	Composition of the Saturation Extract me/l					SAR
	Organic Matter %	Clay <2 m	Silt (2-50 m)	Sand (50 m - 2 mm)			Na+	Ca++	Mg++	Cl-	SO4 -
0 - 17	1.1	49	49	2	7.0	49	320	164	178	618	26	24
17 - 57	0.7	53	46	1	7.2	49	378	160	178	648	36	29
57 - 85	0.6	48	52	1	7.4	45	366	90	166	540	68	41
85 - 108	0.5	51	48	1	7.5	39	355	70	126	468	62	35
108 - 123	0.5	45	54	1	7.7	48	488	60	188	165	96	44

Table 8 TYPICAL SALINE SOIL REPRESENTING HAMZA-KAZIN SERIES, IRAQ (Sehgal 1980)

Depth cm	Mechanical Composition %				pHs	ECe dS/m	Composition of the Saturation Extract me/l					SAR
	Organic Matter %	Clay <2 m	Silt (2-50 m)	Sand (50 m - 2 mm)			Na+	Ca++	Mg++	Cl-	SO4 -
0 - 17	1.3	38	56	6	7.5	75	690	280	320	1 140	258	40
17 - 48	0.4	37	55	8	8.0	33	330	78	32	315	100	44
48 - 75	0.3	27	66	7	8.1	26	282	42	50	216	144	41
75 - 128	0.2	14	29	57	8.2	14	141	20	20	78	96	31
128 -150	0.2	15	32	53	8.3	14	123	20	18	78	88	28

3.1.1 Measuring salinity status

The effect of dissolved salts on plant growth depends on their concentration in the soil solution at any particular time but it is extremely difficult to measure the soil solution concentration at the usual field moisture contents due to sampling problems. A simplified procedure consists of mixing a soil sample with sufficient water to produce a saturated paste and then extracting the solution for measurement of conductivity. Measuring the electrical conductivity (EC) of a saturation extract has an advantage in that saturation percentage is directly related to field moisture range. Over a considerable textural range, saturation percentage is approximately four times the moisture content held at fifteen atmospheres which closely approximates the wilting percentage. The soluble salt concentration in the saturation extract therefore tends to be about one-half of the concentration of the soil solution at the upper end of the field available moisture range and about one-fourth the concentration that the soil solution will have at the dry end of the available moisture range (Richards, 1954).

The standard unit of conductance is siemens (see Table 9) and when expressed per unit of distance, the standard unit of conductivity is siemens per metre. The conductivity of most saturation paste extracts is only a fraction of a siemens per metre. For convenience, therefore, conductivities of soil extracts are expressed in deci Siemens (mS) per metre at 25°C.

EC measurements are quick and sufficiently accurate for most purposes. To determine EC the solution is placed between two electrodes of constant geometry and constant distance of separation. When an electrical potential is imposed, the amount of current varies directly with the total concentration of dissolved salts. At constant potential, the current is inversely proportional to the solution’s resistance and can be measured with a resistance bridge. Conductance is the reciprocal of resistance and has the unit Siemens (formerly, mhos).

The measured conductance is the result of the solution’s salt concentration and the electrode geometry. The effects of electrode geometry are embodied in the cell constant and this is related to the distance between electrodes divided by their effective cross sectional area. The cell constant is commonly obtained by calibration with KCl solutions of known concentration. The conductivity of standard KCl solutions is available in handbooks.

Several empirical relationships have been developed for converting one type of analysis to another. Some of these, summarized in Table 9, are useful for routine verification of data consistency.

The four electrode technique for measuring bulk soil electrical conductivity has been developed (Rhoades, 1976) for use on irrigated soils and on dry land saline seeps in the field. This is a relatively new technique that has great potential for measuring soil salinity in the field without soil sampling and subsequent laboratory analysis. The technique can be used to great advantage for diagnosis and monitoring salinity changes due to season or cultural practices including cropping, etc.

3.1.2 Salinity and plant growth

Excess soil salinity causes poor and spotty stands of crops, uneven and stunted growth and poor yields, the extent depending on the degree of salinity. The primary effect of excess salinity is that it renders less water available to plants although some is still present in the root zone. This is because the osmotic pressure of the soil solution increases as the salt concentration increases. Apart from the osmotic effect of salts in the soil solution, excessive concentration and absorption of individual ions may prove toxic to the plants and/or may retard the absorption of other essential plant nutrients.

Table 9 SOME USEFUL CONVERSION FACTORS

Note: The SI unit of conductivity is ‘Siemens’ symbol ‘S’ per metre. The equivalent non-SI unit is ‘mho’ and 1 mho = 1 Siemens. Thus for those unused to the SI system mmhos/cm can be read for dS/m without any numerical change. Conductivity 1 S cm-1 (1 mho/cm) = 1000 mS/cm (1000 mmhos/cm) 1 mS/cm-1 (1 mmho/cm) = 1 dS/m = 1000 mS/cm (1000 micromhos/cm) Conductivity to mmol (+) per litre: mmol (+)/1 = 10 × EC (EC in dS/m) for irrigation water and soil extracts in the range 0.1-5 dS/m. Conductivity to osmotic pressure in bars: OP = 0.36 × EC (EC in dS/m) for soil extracts in the range of 3-30 dS/m. Conductivity to mg/l: mg/l = 0.64 × EC x 103, or (EC in dS/m) mg/l = 640 × EC for waters and soil extracts having conductivity up to 5 dS/m. nmol/l (chemical analysis) to mg/l: Multiply mmol/l for each ion by its molar weight and obtain the sum.

There is no critical point of salinity where plants fail to grow. As the salinity increases growth decreases until plants become chlorotic and die. Plants differ widely in their ability to tolerate salts in the soil. Salt tolerance ratings of plants are based on yield reduction on salt-affected soils when compared with yields on similar non-saline soils. Soil salinity classes generally recognized are given in Table 10.

Table 10 SOIL SALINITY CLASSES AND CROP GROWTH

Soil Salinity Class	Conductivity of the Saturation Extract (dS/m)	Effect on Crop Plants
Non saline	0 - 2	Salinity effects negligible
Slightly saline	2 - 4	Yields of sensitive crops may be restricted
Moderately saline	4 - 8	Yields of many crops are restricted
Strongly saline	8 - 16	Only tolerant crops yield satisfactorily
Very strongly saline	> 16	Only a few very tolerant crops yield satisfactorily

3.2 Reclamation and management

3.2.1 Salt leaching
3.2.2 Drainage

3.2.1 Salt leaching

The amount of crop yield reduction depends on such factors as crop growth, the salt content of the soil, climatic conditions, etc. In extreme cases where the concentration of salts in the root zone is very high, crop growth may be entirely prevented. To improve crop growth in such soils the excess salts must be removed from the root zone. The term reclamation of saline soils refers to the methods used to remove soluble salts from the root zone. Methods commonly adopted or proposed to accomplish this include the following:

Scraping: Removing the salts that have accumulated on the soil surface by mechanical means has had only a limited success although many farmers have resorted to this procedure. Although this method might temporarily improve crop growth, the ultimate disposal of salts still poses a major problem.

Flushing: Washing away the surface accumulated salts by flushing water over the surface is sometimes used to desalinize soils having surface salt crusts. Because the amount of salts that can be flushed from a soil is rather small, this method does not have much practical significance.

Leaching: This is by far the most effective procedure for removing salts from the root zone of soils. Leaching is most often accomplished by ponding fresh water on the soil surface and allowing it to infiltrate. Leaching is effective when the salty drainage water is discharged through subsurface drains that carry the leached salts out of the area under reclamation. Leaching may reduce salinity levels in the absence of artificial drains when there is sufficient natural drainage, i.e. the ponded water drains without raising the water table. Leaching should preferably be done when the soil moisture content is low and the groundwater table is deep. Leaching during the summer months is, as a rule, less effective because large quantities of water are lost by evaporation. The actual choice will however depend on the availability of water and other considerations. In some parts of India for example, leaching is best accomplished during the summer months because this is the time when the water table is deepest and the soil is dry. This is also the only time when large quantities of fresh water can be diverted for reclamation purposes.

It is important to have a reliable estimate of the quantity of water required to accomplish salt leaching. The initial salt content of the soil, desired level of soil salinity after leaching, depth to which reclamation is desired and soil characteristics are major factors that determine the amount of water needed for reclamation. A useful rule of thumb is that a unit depth of water will remove nearly 80 percent of salts from a unit soil depth. Thus 30 cm water passing through the soil will remove approximately 80 percent of the salts present in the upper 30 cm of soil. Similarly, to reduce the salt content of the surface 60 cm of soil to about 20 percent of the original value would require the passage of about 60 cm of water through the soil. For more reliable estimates, however, it is desirable to conduct salt leaching tests on a limited area and prepare leaching curves. Leaching curves (Figure 1) relate the ratio of actual salt content to initial salt content in the soil (Sa/Sb) to the depth of leaching water per unit depth of soil (Dw/Ds). Results of leaching tests on three soils in Iraq (Dieleman, 1963) presented in Figure 1 show the effect of soil type on the quantity of water required to achieve the same extent of leaching. Results of one such test (Khosla et al., 1979) are presented in Figure 2 (a and b) and some of the soil characteristics of the experimental site are given in Table 11. Figure 2a shows the actual salt distribution following the passage of different quantities of water while Figure 2b relates the depth of water per unit soil depth to the fraction of salts leached (leaching curve). Information on these aspects is important when planning reclamation of large areas.

Figure 1 Typical leaching curves for soils in Iraq (Dieleman, 1963)

Figure 2a Effect of passage of different quantities of water on salt distribution (Khosla et al., 1979)

Figure 2b The leaching curve using data from Figure 2a. Subscript ‘O’ indicates leaching, ‘eq’ represents equilibrium value of electrical conductivity in existing irrigated conditions

Table 11 SOIL CHARACTERISTICS OF THE LEACHING SITE (Khosla et al., 1979)

Depth cm	Texture	Bulk Density	ECe dS/m	pHS	Saturation Extract Composition me/I			SAR
		(g/cm3)			Na+	(Ca+Mg)2+	Cl-
0-15	SL*	1.49	63.8	8.1	975	125.7	645	123
15-30	SL	1.61	19.3	7.6	246	48.5	131	50
30-45	SL	1.55	14.0	7.5	164	39.1	74	37
45-60	SL	1.56	12.9	7.6	150	38.1	64	34
60-75	SL	1.55	12.1	7.9	180	38.6	74	41
75-90	SL	1.58	12.2	8.1	142	28.2	67	38

* SL = Sandy Loam

Results from several laboratory experiments (Miller et al., 1965; and Nielsen and Biggar, 1961) and some field trials (Biggar and Nielsen, 1962; Nielsen et al., 1966; and Oster et al., 1972) have shown that the quantity of salts removed per unit quantity of water leached can be increased appreciably by leaching at soil moisture contents of less than saturation, i.e. under unsaturated conditions. In the field unsaturated conditions during leaching were obtained by adopting intermittent ponding or by intermittent sprinkling at rates less than the infiltration rate of the soil. Nielsen et al. (1966) for example, showed that 25 cm of sprinkled water reduced the salinity of the upper 60 cm of soil to the same degree as 75 cm of ponded water.

Figure 3 Effect of method of irrigation and water redistribution following irrigation² and evaporation on the salt concentration³ profiles (Bresler and Hanks, 1969) (flooding)

Figure 3 Effect of method of irrigation and water redistribution following irrigation² and evaporation on the salt concentration³ profiles (Bresler and Hanks, 1969) (sprinkler)

The salt concentration profiles in a flooded and a sprinkler irrigated soil are demonstrated in Figure 3. In both irrigation methods, at the end of irrigation, upper parts of the soil profiles have low concentration of salts and these will depend on the salt concentration of the applied irrigation water. The salt in the profile increases to a maximum value close to the wetting front and drops to its initial value below the wetting depth. Because of a slower wetting rate under sprinkling, the zone of complete leaching at the end of irrigation extends more deeply into the profile than under flood irrigation. When the soil is subjected to evaporation, water carrying salts moves simultaneously in the upward and downward directions. Thus some salts continue to move down with the redistributed water and, at the same time, salts near the surface move towards the soil surface where they accumulate. The amount of salts which move to the surface depend on the amount of salts present in the upper soil layers from where the water can flow upwards. Thus only a small fraction of salts move up during evaporation from the soil previously irrigated by sprinklers. In flooded soils, on the other hand, more salts move upward and accumulate in the soil surface.

iii. Amendments

Whether an amendment (e.g. gypsum) is necessary or not for the reclamation of salt-affected soils is a matter of practical importance. Saline soils are dominated by neutral soluble salts and at high salinities sodium chloride is most often the dominant salt although calcium and magnesium are present in sufficient amounts to meet the plant growth needs. Since sodium chloride is most often the dominant soluble salt, the SAR of the soil solution of saline soils is also high (Table 11). Figure 4 shows the effect of leaching such a soil with a low electrolyte (EC 0.25 dS/m) water on the resulting SAR of soil solution. The data in this Figure demonstrate that an increasing passage of water resulted in desalinization and simultaneous desodication, i.e. reduction in soil solution SAR although, compared to desalinization, a somewhat greater quantity of water was required to attain the same degree of desodication. When a soil solution is diluted by a factor X, the reduced ratio

will decrease by a factor . This implies that desodication will always accompany desalinization. A favourable calcium to sodium ratio of the irrigation water and any supply of inherent calcium from the soil is likely to further accelerate the desodication process.

Figure 5 shows the distribution of salts and SAR changes when leached with, and without, application of gypsum. It is seen that the salt displacement was about the same in the two treatments, the depth of water applied being 35 cm. While the upper soil layers had nearly the same resultant SAR after leaching, the SAR of the soil solution of deeper soil layers was somewhat lower in the case of gypsum treatment. But since the desalinization and desodication processes proceed simultaneously, it is expected that the SAR of the profile resulting from leaching with gypsum could also be achieved with leaching alone if more water was passed through the soil. The desodication curve (Figure 4) indicates that with an additional 16 cm of leaching water, the SAR of the soil profile could be reduced to the level of the SAR of gypsum treated soil.

Dieleman (1963) and Leffelaar and Sharma (1977) also reported that an amendment may not be needed for reclamation of saline soils having high SAR. The effect of gypsum application on the infiltration rate of a saline soil upon leaching shows in Figure 6 a higher cumulative intake when gypsum was applied. These studies indicate that the application of an amendment, per se, might not be essential for either desalinization or desodication but could hasten the process by maintaining a higher infiltration rate by continuously supplying soluble calcium to the leaching water. Thus, the decision to use an amendment for the reclamation of saline soils having excess neutral soluble salts and a high SAR of soil solution (the so called saline-sodic soils) would depend on soil infiltration characteristics and the electrolyte level of the irrigation water. Light textured soils and those having a favourable infiltration rate are not likely to respond to gypsum application. In heavy textured soils, and where such soils are leached with low electrolyte water, application of an amendment is desirable to hasten reclamation. When any large-scale reclamation is undertaken, the need for application of amendments and their quantities must be established by trials on an experimental scale.

Figure 4 Changes in SAR in relation to depth of leaching water per unit depth of soil. Subscript ‘O’ indicates before leaching and ‘eq’ represents equilibrium value of SAR under existing soil-irrigation water conditions (Khosla et al., 1979)

Figure 5 Effect of gypsum and leaching on salt displacement and SAR (Khosla et al., 1979)

Figure 6 Effect of gypsum application on cumulative infiltration in a saline soil. a - with, b - without gypsum.

3.2.2 Drainage

Irrigation is the most effective means of stabilizing agricultural production in areas where the rainfall is either inadequate for meeting the crop requirements or the distribution is erratic. Before the introduction to an area of large quantities of water through irrigation, there exists a water balance between the rainfall on the one hand and stream flow, groundwater table, evaporation and transpiration on the other. This balance is serously disturbed when additional quantities of water are artificially spread on the land to grow agricultural crops, introducing (Plates 3, 4a, 4b) additional factors of groundwater recharge from seepage from canals, distributors and field channels, most of which are unlined, and from the irrigation water let on to the fields over and above the quantities actually utilized by the crops, etc. As a result of these, the groundwater table rises. There are numerous instances throughout the world, where consequent upon the introduction of canal irrigation, the water table has risen considerably within 10 years to less than 2 m. Once the groundwater table is close to the soil surface, due to evaporation from the surface, appreciable movement of the groundwater takes place resulting in the accumulation of salts in the root zone. A schematic relationship between depth of groundwater and evaporation from the soil surface is shown in Figure 7. This relationship is significant and shows that there is a critical depth of water table above which there is a sharp increase in the evaporation rate and therefore soil salinization. In general, the critical depth of water table ranges between 1.5 to 3.0 metres depending on soil characteristics, root zone of crops, salt content of groundwater, etc: To ensure a salt-free root zone, evaporation from the groundwater must be prevented thus keeping the groundwater table below the depth that will cause rapid soil salinization. Provision of adequate drainage measures is the only way to control the groundwater table. Subsurface drainage problems may also arise due to the presence, at some soil depth, of a clay barrier, a hardpan, bed rock, or even a subsoil textural change.

In many areas drainage problems also arise because of the accumulation and stagnation of rainfall or excess irrigation water on the soil surface. Surface drainage problems usually arise due to slopes that are too flat or to slow water penetration because of structural instability of the soils or to uneven land. Temporary water stagnation in standing crops results in problems of aeration, disease, weed control and nutrient supply. Proper land shaping and provision of surface drains are needed to solve the problems of surface water stagnation. The experience of some countries in tackling drainage problems and the nature and properties of various drainage materials are described in two FAO publications (FAO, 1971a; 1972).

Plate 3 An unlined field channel; such channels are highly conducive to water loss through seepage

Figure 7 A schematic relationship between the depth of groundwater and relative evaporation rate from soil surface

Plate 4a Lining a field channel to reduce water loss

Plate 4b A canal lined to reduce water loss

i. Surface drainage

In surface drainage, ditches are provided so that excess water will run off before it enters the soil. However the water intake rates of soils should be kept as high as possible so that water which could be stored will not be drained off. Field ditches empty into collecting ditches built to follow a natural water course. A natural grade or fall is needed to carry the water away from the area to be drained. The location of areas needing surface drainage can be determined by observing where water is standing on the ground after heavy rain. Field ditches and collection or outlet ditches should be large enough to remove at least 5 cm of water in 24 hours from a level to a gently sloping land. The capacity of a drainage system should be based on the amount and frequency of heavy rains. How quickly water runs into ditches depends on the rate of rainfall, land slope and the condition of the soil surface including the plant cover. The area that a ditch can satisfactorily drain depends on how quickly water runs into the ditch, the size of the ditch, its grade or slope and its irregularity. The latter is measured by the roughness and the contents of debris and growing vegetation in the ditch. In relatively level areas (slope < 0.2%) a collecting ditch may be installed along one side and shallow v-shaped field ditches constructed to discharge into this collecting ditch. Field ditches used to discharge water into collecting ditches should be laid out parallel to each other 20 to 60 m apart. They should be 30 to 45 cm deep depending upon the depth of the collecting ditch. Care should be taken to avoid sharp curves in the ditches to lessen erosion of the banks. Before planning a detailed surface drainage of an area a standard handbook on the subject should be consulted (for example, ILACO, 1981).

ii. Subsurface drainage

If the natural subsurface drainage is insufficient to carry the excess water and dissolved salts away from an area without the groundwater table rising to a point where root aeration is affected adversely and the groundwater contributes appreciably to soil salinization, it may be necessary to install an artificial drainage system for the control of the groundwater table at a specified safe depth. The principal types of drainage systems may consist of horizontal relief drains such as open ditches, buried tiles or perforated pipes or in some cases pumped drainage wells (Plate 5).

a. Open ditches: Open drainage ditches are advantageous for removing large volumes of either surface or subsoil water from land and for use where the water table is near the surface and the slope is too slight for proper installation of tile drains. Where subsurface tile drains are uneconomic or physically impossible, as in many heavy clay soils and where the topography is nearly flat, open drains may be the only practical means of draining the land. Open ditches also serve as outlets for tile drains where their depth is sufficient and other conditions are favourable. The chief disadvantage of open drains is that they occupy land that might otherwise be put to cultivation; open ditches across cultivated fields also obstruct farming operations and are a danger to the livestock and are more costly to maintain than the subsurface covered drains. Open drains become ineffective due to growth of weeds, collapse of banks resulting in partial filling with soil material, etc., and must be periodically cleaned.

b. Mole drains: these are channels left by a bullet shaped device pulled through the soil, they have been used successfully for shallow subsurface drainage of heavy clay soils in many, relatively humid, parts of Europe but have been found impractical with soils of coarser texture. Mole drains are generally cheaper to install than tile or plastic tubings but may last only for two or three years. In addition to being temporary, mole drains are generally shallow and have not been used extensively where salinity build up from the groundwater table is a major problem.

c. Other subsurface drains: These include any type of buried conduit with open joints or perforations that collect and convey excess water from the soil. The conduits may be made from clay, concrete, plastic or other synthetic material but clay and concrete tiles have been the most widely used. Clay tiles are generally manufactured in 30 and 60 cm lengths and have an inside diameter of 10 to 25 cm. They are made from surface clay or shale, which is pulverized, extruded through a die, dried and then burnt in a kiln. Clay tiles are not affected by acid or sodic soils but those made from surface clay or poorly burnt tiles are subject to deterioration by freezing and thawing action. Good quality clay tiles have been found to last indefinitely in the soil. Concrete tiles are made from sand and gravel aggregate and steam or water-cured to obtain the desired strength. Concrete tiles are resistant to freezing and thawing but may be subject to deterioration in acid and sodic soils. For such soils the tiles should be made with cement having a special chemical composition. Water enters the tiles at the butt joints or spaces between adjacent sections. Both clay and concrete tiles may have fitted ends and be perforated for easier entry of water. All drain tiles should meet standard specifications.

Since the nineteen sixties, thermoplastic tubing has become a common drain material. High density polyethylene and polyvinylchloride are the two most common materials. The plastic tubing is corrugated and, unlike clay or concrete tiles, flexible and will deflect vertically when soil is backfilled in the trench. As it deflects, the sides of the tubing move outward horizontally into the surrounding soil. The circular tubing changes to a slight oval shape, which becomes stabilized because the soil on the sides of the tubing resists the further outward movement. Corrugations in the tubing provide sufficient stiffness to resist the initial soil load. They also reduce the amount of plastic required to make the tubing as well as provide flexibility which permits the smaller size tubing to be coiled into a compact package. Plastic drain pipes are generally available in 8 to 30 cm diameters and usually come in rolls of 75 to 80 m long depending on the diameter of the tubing. Corrugated plastic tubing weighs only 1/25 of a concrete or clay tile, resists practically all soil chemicals and can be installed in continuous lengths. Compared to concrete and clay tiles, greater care is required in placing soil around plastic- tubing because of deflection. Water enters the drain tube through sawed slits or cut holes spaced uniformly around and along the tube.

iii. Filter materials

These are sometimes placed around subsurface drains primarily to prevent the inflow of soil into the drains which may cause failure, and/or to increase the effective diameter or area of openings in the drains which increases inflow rate. Two types of materials are generally used:

- thin sheets such as of fibre glass or spun nylon, and
- sand and gravel envelopes or other porous granular materials.

The thin sheet filters may be sealed on to the plastic tubing at the manufacturing site or they may be installed above and/or below the drains as they are being laid. Granular materials should be placed above or below the drains during installation. Such materials must have the proper gradation of sizes to prevent the inflow of soil.

The principal factors affecting the costs involved in installing subsurface drainage systems in large areas are the spacing and depth of drains. Many mathematical equations have been developed to arrive at the optimum depth and spacing for drains but in practice these have found limited application because of the difficulty and high cost of obtaining soil hydraulic conductivity data and related soil and crop interactions. For these reasons the depth and spacing of drains are based largely on experience and judgement (Schwab et al., 1966).

Subsurface tile or plastic drains are relatively permanent when correctly installed and protected. Large-scale subsurface drainage systems have been in operation in the western United States for nearly fifty years. Extensive installations for water table and salinity control are now being made in many countries including Iraq, Egypt, Australia, etc.

iv. Pump drainage

The chief drawback of gravity drainage systems is their inability to lower the water table to an adequate depth. Pumping groundwater in areas where a suitable permanent aquifer exists is often an effective means of lowering the water table. A decision to pump groundwater for drainage is generally favoured by adequate depths and permeabilities of the water bearing formations, by high values of pumped water for irrigation and by low power costs (see section 4.8.1 on drainage of sodic soils). To determine if pumping would be effective, pumping tests have to be carried out in test wells to determine the feasibility and area of influence by measuring water levels in adjacent observation wells or piezometers. Spacing, depth, and capacity of the pumped wells and other operational details also need to be evaluated from these tests.

v. Maintenance of drainage systems

After a subsurface drainage system has been installed, a suitable map should be made and filed with the property deed. The map should show the location of all ditches and subsurface drains, tile size and grade, depth and spacing. Any subsequent changes should also be recorded on the map. The record is of considerable value to the present and future land owners when the drainage system might need repairs or maintenance.

A subsurface drainage system normally requires little maintenance if it is properly designed and installed. The outlet ditch should be kept free of the sediment and the tile outlet should be protected against erosion and undermining.

If a drain line becomes filled with sediment or roots the line should be uncovered at some point downstream to locate the obstruction. If the line is not completely clogged and water is available the sediment can sometimes be flushed out. A suitable plug, swab or a rigid rod can be used to remove the blockage. A high pressure water jet may be needed to clean out some lines. Often it is more economic to replace the entire plugged section.

Roots of nearby trees can also block subsurface drains. For this reason shrubs and trees growing adjacent to a tile line should be removed. If the tile lines become filled with roots, it is best to dig up and replace the clogged section and remove the troublesome trees at the same time.

The maintenance of open collecting ditches is most important and it is difficult. Weed growth must be controlled and the caving in of the sides requires continuous attention in order that the entire drainage system continues to work efficiently.

3.3 Crops in saline soils

3.3.1 Rice in saline soils
3.3.2 Factors influencing tolerance of crops to salinity
3.3.3 Water management
3.3.4 Nutrient availability and uptake by plants

Crop plants differ a great deal in their ability to survive and yield satisfactorily when grown in saline soils. Information on the relative tolerance of crops to a saline soil environment is of practical importance in planning cropping schedules for optimum returns. There are situations where farmers have to live with salinity problems, for example, in areas having saline water as the only source of water for irrigation. In other situations where good quality water is available for reclamation of saline soils, it is often helpful to grow crops simultaneously with reclamation efforts to make reclamation economic.

There is much literature on the relative tolerance of different crops to soil salinity obtained under a vast range of soil, climatic and salinity conditions. As will be discussed in section 3.3.2, tolerance to salinity is not a fixed property of a species but varies with the growth stage of the crop, climatic conditions and within the same species for different varieties of the crop. These factors render the task of evaluating crop salt tolerance data difficult. Also the methodologies adopted by different workers for studying tolerance have varied from water culture experiments on the one extreme to field studies with little control on the root zone salinity on the other. Maas and Hoffman (1977) and Maas (1984) compiled and reviewed available data to arrive at the best assessment of the relative salt tolerance of agricultural crops. The information presented by these workers has now been extensively quoted and used for practical purposes. Salt tolerance data for several crops, as presented by Maas and Hoffman are reproduced in Figure 8 (a to h). These figures show that, in general, crop yields were not reduced significantly until a threshold salinity level was exceeded, and then the yields decreased approximately linearly as the salinity increased beyond the threshold. The salt tolerance curve for each crop was obtained by calculating a linear regression equation for the yield beyond the threshold point. From the curves presented in these figures, (EC) relative yield (Y) (percent) at any given soil salinity can be calculated by the equation:

Y =	100 (ECo - ECe)		where EC100 is
		ECo - EC100

the salinity threshold value (EC_e where Y = 100) and EC the salinity at zero yield (EC_e where Y = 0). Values of EC₁₀₀ and EC_o for a given crop can be taken from the appropriate figure. Taking cotton as an example, EC₁₀₀ =8 dS/m and EC_o =27.0 dS/m (Figure 8b). Therefore, the relative yield at an EC of say, 10 dS/m will be:

Y	= 100 (27.0 - 10.0)/(27.0 - 8.00)
	= 100 (17.0)/(19.0)
	= 89 percent.

The shaded areas in Figures 8a to 8h indicate a qualitative salt tolerance rating for each crop. The five areas represent respectively, from left to right, sensitive, moderately sensitive, moderately tolerant, tolerant and unsuitable for crops.

It was emphasized by Maas and Hoffman (1977), Maas (1984) and we wish to repeat that the relative tolerance of crops as depicted in Figures 8a to 8h does not represent the absolute salt tolerances independent of other factors. Only a guide is furnished to the relative tolerance of crops. Whereas actual tolerance will vary with climate, cultural practices and other variables, relative tolerance should apply under most conditions.

Very often tolerance to saline and sodic conditions is not adequately differentiated and this can lead to inappropriate conclusions. The data in Figures 8a to 8h are for saline conditions and do not apply for sodic conditions. For example, barley is known to be a tolerant crop to saline conditions (Figure 8a) but is not a tolerant crop for sodic conditions. Similarly cotton, while tolerant of saline conditions, is only moderately so (or even sensitive at some growth stages) to sodic conditions. On the other hand, rice, though considered only moderately sensitive to saline conditions (Figure 8a), is highly tolerant of sodic conditions. Information on tolerance to sodic conditions is summarized in a subsequent section.

Figure 8 (8a-h) Salt tolerance of crops (Maas and Hoffman, 1977) (a)

Figure 8 (8a-h) Salt tolerance of crops (Maas and Hoffman, 1977) (b)

Figure 8 (8a-h) Salt tolerance of crops (Maas and Hoffman, 1977) (c)

Figure 8 (8a-h) Salt tolerance of crops (Maas and Hoffman, 1977) (d)

Figure 8 (8a-h) Salt tolerance of crops (Maas and Hoffman, 1977) (e)

Figure 8 (8a-h) Salt tolerance of crops (Maas and Hoffman, 1977) (f)

Figure 8 (8a-h) Salt tolerance of crops (Maas and Hoffman, 1977) (g)

Figure 8 (8a-h) Salt tolerance of crops (Maas and Hoffman, 1977) (h)

3.3.1 Rice in saline soils

Although rice is not tolerant to excess salinity, it is a crop favoured in saline soils and, in fact, is preferred over other tolerant crops during the initial stages of reclamation of many saline soils. This is chiefly due to the system of lowland rice culture that is advantageous to the crop rather than to the tolerance of the crop to soil salinity. The system of lowland rice culture involving maintenance of standing water almost throughout the growing season brings about a significant reduction in the root zone salinity by leaching and dilution of the salts. Thus the crop is at no stage subjected to the salinity stress that might be indicated by the initial soil analysis. Rice is an important crop in many coastal regions and is grown during the rainy season. Although initially the soil salinity may be high, after one or two rains salinity is reduced in the upper few centimetres enabling planting of seedlings grown in a relatively good soil. Salinity is usually a greater constraint in the dry season when the evaporative demand is high and supply of good quality water restricted. Under these conditions when groundwater of high salinity must be used, salinity becomes a major constraint to obtaining satisfactory crop yields.

Reclamation requires that the soluble salts from the profile are leached and drained through a suitable system of drainage, but good quality water is often a major constraint in arid regions. Therefore, leaching alone for prolonged periods is not justifiable and so a rice crop is conveniently grown during reclamation. Rice gives satisfactory yields even when the electrical conductivity of the saturated soil extract is 20 to 25 dS/m in the upper layers (Van Alphen, 1975; Yadav and Girdhar, 1981). Even on soils with low infiltration rates the accumulated depth of water percolating through the soil profile in one rice season may be 100 to 200 mm. Table 12 gives data on salinity changes due to cropping with rice. Although leaching under continuously ponded conditions has the disadvantage of being less efficient for salt leaching compared to intermittent irrigation, the benefit of simultaneous crop production makes rice an ideal crop during reclamation of saline soils.

Table 12 CHANGES IN SOIL SALINITY DURING THE RECLAMATION OF A HIGHLY SALINE SOIL BY GROWING A RICE CROP (Van Alphen, 1975)

Soil	dS/m
Depth cm	Initial	Before 1st rice crop	After 1st rice crop	After 2nd rice crop	After 3rd rice crop
0-10	169.0	34.0	20.1	16.9	12.1
10-20	130.0	45.2	22.1	15.9	12.1
20-40	75.1	54.0	31.5	21.2	16.1
40-60	42.2	46.6	33.4	26.4	20.
60-80	33.8	42.2	35.5	29.0	22.7
80-100	30.2	40.9	36.5	30.4	24.2

Data in Table 13 gives the change in soil pH and SAR of the saturation extract during reclamation. It can be seen that there was a continuous reduction in SAR and pH of the soil without an indication of the soil becoming truly sodic.

Table 13 THE SAR VALUE MEASURED IN THE SATURATION EXTRACT AND THE pH (1:1) PRIOR TO AND DURING RECLAMATION (Van Alphen 1975)

Depth cm	Initial		After 1st rice crop		After 2nd rice crop		After 3rd rice crop
	pH	SAR	pH	SAR	pH	SAR	pH	SAR
0-10	7.4	90	7.8	25	7.3	13	7.3	14
10-20	7.5	85	7.9	29	7.5	17	7.3	14
20-40	7.8	73	7.9	40	7.7	28	7.6	24
40-60	8.0	57	8.0	48	7.7	38	7.5	33
60-80	8.1	52	7.9	54	7.6	45	7.5	35
80-100	8.0	46	7.8	53	7.6	47	7.5	39

3.3.2 Factors influencing tolerance of crops to salinity

Tolerance of plants to soil salinity is not a fixed characteristic of each species or a variety but may vary with the environmental conditions. The tolerance to salinity may even vary with the stage of crop growth of the same species.

i. Growth stage

Although some crops seem to tolerate salinity as well during seed germination as during later growth stages, germination failures are most commonly responsible for poor and spotty stands and bare spots in otherwise cultivated fields. Frequently this is not the result of crops being especially sensitive during germination, but rather is caused by exceptionally high salt concentration in the shallow surface zone where seeds are planted. These high salt concentrations result from the salt that is left behind as the upward moving water is evaporated near the soil surface.

Most plants are more sensitive to salinity during germination than at any other growth stage. However, there are large variations in the sensitivity of germinating seeds to salinity as is depicted in Figure 9 where percentage germination of four plant species is plotted against the electrical conductivity of soil saturation-paste extract. It is seen that beans and sugarbeet are more sensitive to salts at germination than are alfalfa and barley.

Information on the relative tolerance of rice to salinity at various growth stages was reviewed by Ikehashi and Ponnamperuma (1978). Most published work (Pearson, 1959, 1961; Kaddah and Fakkry, 1961) tends to indicate that while rice can tolerate a high concentration of salts at germination (up to 30 dS/m) it is sensitive to salinity in the early growth stages and that the tolerance increases with age during the tillering phase of growth. Tolerance to salinity at germination is usually not of much significance since in most rice growing areas rice is grown in a nursery in a good soil and then transplanted when the seedlings are 20 to 40 days old. The tolerance of rice decreases from panicle formation stage to flowering stage such that salinity stress at this stage invariably results in reduced grain yields. It has been observed that the straw weight and total number of tillers is generally less affected than the grain yield and the number of productive tillers and that with increasing salinity the inflorescence was progressively delayed and the number of sterile spikelets increased (Kaddah, 1963; Murthy and Rao, 1967; Pearson et al., 1966).

Figure 9 Percent germination of four crops, as related to conductivity of the saturation extract of the soil, under laboratory conditions (Ayers and Hayward, 1949)

Maas and Hoffman (1977) reviewed data on tolerance of crops in relation to growth stage and showed that the tolerance pattern of barley, wheat and maize was nearly the same as that of rice. Sugarbeet and safflower, on the other hand, were sensitive during germination while the tolerance of soybean could either increase or decrease between germination and maturity depending on the crop variety. Table 14 depicts the large variations that exist in the tolerance to salinity of crops at two growth stages.

Crop	Germination stage	Established stage
Barley	Very good	Good
Corn (maize)	Good	Poor
Wheat	Fairly good	Fair
Alfalfa	Poor	Good
Sugarbeet	Very poor	Good
Beans	Very poor	Very poor

ii. Environmental factors

Climatic conditions greatly influence plant responses to salinity. Table 15 illustrates the effect of climatic conditions in a given saline environment. The data were obtained by growing plants in large outdoor sand cultures supplied with complete nutrient solutions adjusted to varying salinities. The basic nutrient solution had an electrical conductivity of about 1 dS/m. The salinity level at which a yield reduction of 25 percent was observed compared to control was determined graphically. All the three species studied showed greater tolerance to salinity at the location where the environment was coot and humid than at the location where the environment was hot and dry. Furthermore, the order of tolerance changed from onions > beets > beans at the cool location to beets > onions > beans at the hot location.

Crop	Solution salinity at which 25% yield reduction was observed dS/m
	Cool location	Hot location
Bean pods	4.0	3.0
Garden beetroots	11.1	6.6
Onion bulbs	12.5	3.3

In some parts of India rice is grown both during the rainy season (kharif) and during the dry season (rabi). Data in Table 16 are the relative average yields of eight rice varieties grown in kharif and rabi seasons at four salinity levels (Murthy and Janardhan, 1971). The data clearly indicate that the yield reduction with increasing salinity was much more in the dry than in the wet season.

Salinity of root zone dS/m (approximate range)	Relative	yield
	Wet Season	Dry Season
Control (non saline)	100	100
2-4	93	81
4-8	63	53
10-12	39	11

Note: Relative yields are comparable only within the same season.

Sinha and Singh (1974, 1976) studied the effect of transpiration rate on the accumulation of sodium and chloride ions near the root surface of maize and wheat crops under controlled conditions. Their studies showed that the sodium and chloride contents of the soil closely adhering to the roots were linearly related to the total amount of water transpired by the plants as well as the water transpired per unit root length. Based on these studies it was pointed out that the stress to which plants are subjected in saline soils would be determined by the evaporative demand during growth and could be much greater than that indicated by the electrical conductivity of the bulk soil. These results explain the observed differences in plant responses to salinity in different climatic conditions reported by several workers.

Apart from the atmospheric evaporative demand, some workers (Hoffman et al., 1975) have shown that air pollution may increase the apparent salt tolerance of many crops. For example, with alfalfa, grown at ozone concentratons often prevalent in several agricultural areas, yields were highest at moderate salinity levels that normally reduced growth. Because some crops are affected more by air pollutants when grown under non-saline than under saline conditions, they may appear more salt tolerant in air polluted areas.

iii. Varietal differences in salt tolerance

Differences in varietal tolerance to salinity and other adverse soil conditions have been known to exist for decades but it is only in the latest decades that serious efforts have been initiated to exploit the genetic potential of salt-tolerant crop varieties through different breeding programmes.

Rice has long been grown in the coastal regions of India and other countries where salinity is a perpetual problem due to inundations from the sea, and intrusion of sea water through rivers, estuaries, etc. Screening of a large range of rice germ plasm collected at different saline areas in India led to the identification of several genotypes that are extremely salt tolerant (Bhattacharya, 1976) (see Table 17).

State	Varieties
Andhra Pradesh	MCM 1; MCM 2
Kerala	Pokhali
Maharashtra	Kala Rata; Bhura Rata
Orissa	SR 26 B
West Bengal	Matla, Hamilton
Tamil Nadu	PVR I
CSSRI, Canning	Damodar, Dasal, Getu

Though most of these rice varieties are highly tolerant of salinity, all the varieties are tall indica and photosensitive types and have a low yield potential compared to the dwarf high-yielding types. In recent years systematic breeding efforts have been made and some of the tolerant genotypes used extensively in a hybridization programme with high yielding lines to act as donors for salinity tolerance. Some of the cultures which have now been released for large scale cultivation in the saline areas possess good agronomical traits in addition to tolerance to salinity.

Apart from hybridization, mutation breeding approaches were tried and some promising cultures, Mut-1 (CSR4), evolved from the widely cultivated high-yielding variety IR-8 (Sinha and Borah, 1980). In recent years intensive efforts have been made at the International Rice Research Institute at Los Banos in the Philippines to breed varieties for tolerance to various adverse soil conditions and many advanced lines in IRRI’s breeding programme show tolerance for one or more adverse soil factors (Ponnamperuma, 1977; Ikahashi and Ponnamperuma, 1978).

Researchers at the University of California at Davis are breeding barley for culture with sea water irrigation (Epstein, 1976). Lines have been developed which survive and set seed (yields in the order of 1188 kg/ha) under irrigation with undiluted sea water. Similar breeding is underway with wheat.

The same researchers screened for salt tolerance in tomato cultivars with little success. However a wild tomato, Lycopersicon cheesmannii, collected from sites close to the shore on the Galapagos Islands was able to survive in saline culture equivalent in salinity to sea water. While the fruit of the wild species is too small for commercial use, F-progeny of crosses of the wild species and commercial cultivars include segregates with acceptable fruit size (similar to cherry tomatoes) and tolerance to a salinity equivalent to one-third that of sea water (Epstein, 1976).

Shannon (1978) screened for salt tolerance 32 accessions of tall wheatgrass, Agropyron elongatum (Host) Blauv., a forage grass used on the western rangelands of the United States. He classified plants by ability to recover from salt stress and identified seven tolerant genotypes from diverse geographic origins for continued selection.

A long-term programme of evaluation and selection of avocado rootstocks for tolerance of salinity has resulted in the development of very successful avocado orchards in regions with saline irrigation water in Israel (Kadman and Ben-Ya’Acova, 1976).

Efforts are also being made in different parts of the world to induce tolerance to salinity in other field crops. Rana et al. (1980) indicated the promising role of polyploid breeding in evolving crop varieties suited to problem soils. It is apparent that breeding crop varieties tolerant to salinity offers significant opportunities for better management of areas where salinity is a perpetual problem.

Figure 10 Effect of increasing salinity level on the chloride content of leaves of six citrus root stocks (Cerda et al., 1977)

iv. Rootstocks and salinity tolerance

Most fruit crops are more sensitive to salinity than are field, forage or vegetable crops (Figure 8h). Grapes, citrus, stone fruits, pome fruits, berries and avocados are all relatively sensitive to salinity. However, certain stone-fruits, citrus and avocado rootstocks differ in their ability to absorb and transport sodium and chloride ions and have, therefore, different salt tolerance. Cerda’ et al. (1977) studied the effect of sodium chloride in the irrigation water on the foliar contents of chloride and sodium of six citrus rootstocks, viz., Sour orange (Citrus aurantium L.), Troyer citrange (Poncirus trifoliata x Citrus sinensis), Cleopatra mandarin (Citrus reticulata Blanco), Allemow (citrus macrophylla Wester), Nanshodaidai (Citrus taiwanica) and Kinnow mandarin (Citrus nobilis Loureiro x Citrus deliciosa Tenore). Their results showed that mandarin as a group was characterized by a marked capacity to exclude chloride ions while the sour orange and Troyer citrange varieties, in general, accumulated high amounts of chloride ions (Figure 10). Similar results were earlier reported by Cooper (1961) and other investigators. Bernstein (1965) pointed out that for many fruit crops damage to the plants could be related to the concentration of specific ions, e.g. chloride or sodium in the soil solution and/or plant leaves rather than to the total soil salinity. Thus, the specific injury symptoms appeared before any effect of total salt concentration was observed. For instance, a chloride level of 10 mmol/l in a saturated paste extract is considered toxic to sensitive rootstocks although the same rootstock can tolerate a higher total salinity if it is not due to chloride salts. For this reason, classification of fruit crops with respect to specific salinity according to varieties and rootstocks is important. Such a tolerance classification was presented by Bernstein (1965) and is reproduced in Table 18.

Crop	Rootstock/Variety	Limit of tolerance to chloride in soil saturation extract mmol/l
Rootstocks
Citrus (Citrus spp.)	Rangpur lime, Cleopatra mandarin	25
	rough lemon, tangelo, sour orange	15
	sweet orange, citrange	10
Stone fruit (Prunus spp.)	Marianna	25
	Lovell, Shalil	10
	Yunnan	7
Avocado (Persea americana Mill.)	West Indian	8
	Mexican	5
Varieties
Grape (Vitis spp.)	Thompson seedless, Perlette	20
	Cardinal, Black rose	10
Berries (Rubus spp.)	Boysenberry	10
	Olallie blackberry	10
	Indian Summer raspberry	5
Strawberry (Fragaria spp.)	Lassen	8
	Shasta	5

Most fruit crops are also sensitive to other toxic elements, particularly boron. This ion is present in most irrigation water and in saline soils. It is toxic to many plants at a concentration only slightly in excess of that required for optimum growth. Small quantities of boron absorbed by the roots are accumulated by the leaves and values above 250 ppm result in typical leaf burns. A grouping of plants according to their relative tolerance to boron is presented in Table 45. The data show that fruit crops, in general, are more sensitive to boron in irrigation water and soils compared to field crops. Significant reductions in yield of most field crops due to excess boron alone under field conditions have rarely been reported.

3.3.3 Water management

i. Irrigation frequency

Modifying water management through appropriate irrigation practices can often lead to increased crop yields under saline soil conditions. Most plants require a continuous supply of readily available moisture to grow normally and produce high yields. After an irrigation the soil moisture content is maximum and the salt concentration or the osmotic pressure of the soil solution is minimal: favourable for crop growth. As the soil progressively dries out due to evapo-transpirational losses the concentration of salts in the soil solution and, therefore, its osmotic pressure increases making the soil water increasingly difficult to be absorbed by the plants. Thus infrequent irrigation aggravates salinity effects on growth. On the other hand, more frequent irrigations, by keeping the soil at a higher soil moisture content prevent the concentration of salts in the soil solution and tend to minimize the adverse effects of salts in the soil. For these reasons crops grown in saline soils must be irrigated more frequently compared to crops grown under non-saline conditions so that the plants are not subjected to excessively high soil moisture stresses due to combined influence of excess salts and low soil water contents. Figure 11 depicts changes in the total soil moisture stress to which the growing plants are subjected in a non-saline soil compared to a saline soil. Several studies have shown that growth of plants was reduced nearly proportionally to the areas under the curves. Thus, when the areas under two such dissimilar stress curves as A and B were equal, the growth of plants was found to be reduced to nearly the same level. If the saline soils were irrigated infrequently plants would be subjected to very high soil moisture stresses with consequent yield losses.

Figure 11 Changes in total moisture stress in a saline and a non-saline soil in the interval between two irrigations

ii. Irrigation method

Irrigation method can play an important role in controlling salts in the root zone. It has been discussed that frequent irrigations are helpful in saline soils in maintaining adequate availability of soil water. Sprinkler irrigation is an ideal method for irrigating frequently and with small quantities of water at a time. Leaching of soluble salts is also accomplished more efficiently when the water application rates are lower than the infiltration capacity of the soil and such a condition cannot be achieved by flood irrigation methods. In a field experiment (Nielsen et al., 1966) flood irrigation required three times as much water as sprinkling to reduce soil salinity by the same increment. Sprinkler irrigation also has the advantage that small local differences in the level of the field will not cause non-uniform water application and salt leaching.

In the trickle or drip irrigation method water is supplied continuously at a point source and in the immediate vicinity of plant roots. The method is suitable for perennial or seasonal row crops; it has been found particularly useful when irrigating with water of high salinity. The method has the advantage that it keeps the soil moisture continuously high in the root zone, therefore maintaining a low salt level. The roots of the growing plants tend to cluster in the high soil moisture zone near the tricklers and therefore avoid the salts that accumulate at the wetting front. Results of field trials to compare sprinkler and drip irrigation methods using water of two qualities are presented in Table 19. The good quality water had an electrical conductivity of 0.4 dS/m and the saline water an electrical conductivity of 3 dS/m.

Irrigation method	Electrical conductivity of water dS/m
	0.4	3.0
Drip	66.7	65.0
Sprinkler	52.0	39.2

The yield difference between the two methods of water application was greater when saline water was used. Further, the yield obtained by the drip method with saline water was almost equal to that produced when the high quality water was applied by this method. A more favourable distribution of salts in the soil profiles with drip irrigation in comparison with the sprinkler and furrow methods was also shown at the end of the growing season on a sweet corn plot (Figure 12), although in the drip irrigation method appreciable salt accumulation is likely to occur between the rows depending on the inter and intra row space between the drip points. Although sprinkler and trickle irrigation methods are highly efficient, both from the view of water use and salinity control, their high initial costs often preclude their use in regions where transport infrastructure and markets are not highly developed.

A soil factor of considerable importance in relation to growth of plants is the location of salts in relation to root zone or seed placement. Irrigation practices can often be modified to obtain a more favourable salt distribution in relation to seed location or growing roots. It is well known that salts tend to accumulate in the ridges when using furrow type irrigation. The direction of movement of applied water and dissolved salts (arrows) is shown in Figure 13. With each irrigation salts leach out of the soil under the furrows and build up on the ridges. Where soil and farming practices permit, furrow planting may help in obtaining better stands and crop yields under saline conditions.

Figure 12 Salinity profiles in sweet corn under drip, sprinkler and furrow irrigation methods (Goldberg et al., 1976)

Figure 13 Direction of salt flow and salt accumulation in furrow irrigation. The zone of maximum salt accumulation is in the top of the ridges

Figure 14 The pattern of salt build-up depends on bed shape and irrigation method. Seeds sprout only when they are placed so as to avoid excessive salt build-up around them (Bernstein et al., 1955)

Certain modifications of the furrow irrigation method including planting in single/double rows or on sloping beds, are helpful in getting better stands under saline conditions. Typical patterns of salt accumulation under different types of beds are shown in Figure 14. With double beds, most of the salts accumulate in the centre of the bed leaving the edges relatively free of salts. Sloping beds may be slightly better on highly saline soils because seed can be planted on the slope below the zone of salt accumulation.

iii. Mulching

During periods of high evapotranspiration between the two irrigations and during periods of fallow there is a tendency for the leached salts to return to the soil surface. Soil salinization is particularly high when the water table is shallow and the salinity of groundwater is high. Any practices that reduce evaporation from the soil surface and/or encourage downward flux of soil water will help to control root zone salinity. Sandoval and Benz (1966) and Benz et al. (1967) studied soil salinity changes as effected by bare fallow and straw mulch on fallow over a three years period. Their results showed that on bare fallow a soil mulch should be maintained to induce salinity reduction. Under straw mulch there was a significant reduction in soil salinity which resulted in an increased wheat yield of 25 to 50 bushels per hectare in an area where the normal wheat yields were about 62 bushels per hectare. Fanning and Carter (1963) reported significant reduction in root zone salt concentration of plots where cotton-burr mulch had been applied at the rate of 90 tons per hectare. These workers also reported that periodic sprinkling of mulched soils resulted in greater salt removal and therefore higher leaching efficiency than did flooding or sprinkling of bare soil (Carter and Fanning, 1964).

iv. Other practices

Crops vary not only in their tolerance to salinity but also in their water requirements, optimum growth season, rooting depth and moisture extraction pattern and cultural requirements. Thus, in the absence of proper water and soil management practices, salinity of the soil may be affected differentially under various crop rotations. Cropping sequences which include crops such as rice, berseem and those requiring frequent irrigations reduce salinity effectively, where drainage is adequate. Therefore knowledge of the expected salt balance of the root zone under various crop rotations will be extremely helpful in planning the best cropping sequences during and after reclamation (Massoud, 1976).

Changes in the micro relief in the order of a few centimetres can result in increasing the salt content on the raised spots and better leaching in the dips. Proper land shaping before cropping can help to correct these elevation differences. Land levelling that results in the formation of shallow profiles or an exposure of an impervious layer close to surface may enhance salinization. Since this operation is executed at an early stage in new surface irrigation projects, it should be carefully evaluated as a possible cause of salinization.

Tillage is another mechanical operation that is usually carried out for seed bed preparation and soil permeability improvement but if it is improperly executed it might form a plough layer or turn a salty soil horizon and bring it closer to the soil surface. Proper monitoring of changes in the soil will help the timely adoption of corrective mesures for the control of salinity that might otherwise be accentuated.

3.3.4 Nutrient availability and uptake by plants

Apart from the effect on water availability to plants and the possible toxic effect of some constituents, excess neutral soluble salts in soils may also interfere with the normal nutrition of crops in saline soils. At a given level of salinity, growth and yield of crops are likely to be depressed more when nutrition is disturbed than when it is normal. At moderate salt concentrations in the soil solution, plants generally try to exclude unwanted ions, as far as possible, and promote the uptake of nutrients. With increasing salt concentration, the uptake of sodium and chloride ions increases sharply. This luxury consumption of ions is essential for the plants to compensate for the increased outside osmotic pressure but is responsible for growth retardation. Excessive uptake of certain ions, in turn, often results in reduced uptake of some essential plant nutrients causing nutrient imbalances and deficiencies. Thus, although the available status of a nutrient in a soil might not be in a deficient range per se, its application might compensate for the decreased uptake by plants resulting from the antagonistic effect of excess uptake of certain ions. Results of several studies tend to show that deficiencies of the elements K and Ca appear to play an important role in the observed growth depressions in many saline soils (Finck, 1977). Proper fertilization of soils of low or medium salinity should serve to:

- supplement nutrients that are present in insufficient amounts;

- supplement nutrients that, although present in sufficient amounts, are not taken up in adequate amounts due to antagonistic effects, e.g. K or Ca; and decrease the uptake of harmful ions, e.g. K against Na or phosphate against chloride (Rankovitch and Porath, 1967; Chhabra et al., 1976).

Figure 15 Schematic diagram showing the effect of fertilization for correction of an unbalanced and insufficient nutrient supply to plants in saline soils; the columns indicate the plant contents (Nutrient supply without fertilization) (Finck, 1977)

Figure 15 Schematic diagram showing the effect of fertilization for correction of an unbalanced and insufficient nutrient supply to plants in saline soils; the columns indicate the plant contents (Nutrient supply with fertilization) (Finck, 1977)

There have been only a limited number of studies on the effect of salinity on the nutrition of crops in respect of micronutrients. A disturbed balance in the uptake and composition of major nutrients is bound to influence the plant composition of micronutrients. Besides the generally known toxic effects of boron there is a need to understand better the behaviour of Fe, Mn, Zn, Cu, etc., in relation to soil salinity particularly with a view to establishing limiting values - so far only developed for normal soils. Figure 15 schematically demonstrates how a well-adjusted fertilization could improve the yields of crops (Finck, 1977).

Figure 16 shows the inverse relationship observed between available soil phosphorus and the chloride content of wheat straw in pot studies (Singh et al., 1979). Fine and Carson (1954) observed that the application of P increased yields of crops markedly and alleviated salt injury symptoms in oats and barley. They observed a 400 percent increase in yield with a saline soil in spite of its having high available P. Ferguson and Berlin (1963) reported that much higher responses of applied phosphorus occurred on a moderately saline than on a non-saline soil of comparable available P status. Dregne and Mojallali (1969) reported that the beneficial effect of applied P to wheat and barley crops was limited up to an ECe of 9 dS/m. These observations show that higher plant responses to applied P occur on moderately saline than on non-saline soils. Responses to applied P-fertilizers in saline soils cannot be explained on the basis of soil test values alone as the saline soils, even when containing high amounts of extractable P have shown positive responses to applied phosphorus. This is because in saline soils the availability of P is more a function of plant root length and area (which is restricted due to salinity) and the negative effect of excess chlorides on P absorption by roots. Application of judicious quantities of P-fertilizers in saline soils helps to improve crop yields by directly providing phosphorus and by decreasing the absorption of toxic elements like Cl.

Figure 16 Effect of available soil phosphorus on the chloride content of wheat straw (Singh et al., 1979)

On moderately saline soils, the application of potassic fertilizers may increase the crop yields (Dregne and Mojallali, 1969) either by directly supplying K or by improving its balance with respect to Na, Ca and Mg. However under high salinity conditions it is difficult to exclude Na effectively from the plant by use of K-fertilizers.