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6.1 Hazards associated with water quality
6.2 Salinity problems
6.3 Management practices for efficient use of high salinity water
6.4 The sodicity (alkali) problem
6.5 The toxicity problem
6.6 Management practices

Crop production in the arid and semi-arid regions is dependent on irrigated agriculture. The hot and dry climates of these regions require that the irrigation water does not contain soluble salts in amounts that are harmful to the plants or have an adverse effect on the soil properties. Water of such quality is usually not available in sufficient quantities to satisfy the water requirements of all the crops grown. Under these conditions the farmers are obliged to use irrigation water with high quantities of dissolved salts, invariably accompanied by yield reductions of most crops. Indiscriminate use of such water can often lead to crop failures and to the development of saline or sodic soils which, in turn, require expensive treatment to make them productive again. On the other hand, when saline water is skilfully used, it can contribute to the successful production of a variety of crops.

The original source of salts in irrigation water is the rock that forms a part of the earth’s crust - it is constantly subject to weathering which releases salts to be carried away by water. When the soil becomes truly saline, the visible surface evidence might be a white crust or dark, moist, oily looking patch. However, salt accumulation begins to affect crop yields long before visible signs of its presence appear.

6.1 Hazards associated with water quality

6.1.1 Salinity hazard
6.1.2 Sodicity (alkali) hazard
6.1.3 Toxicity hazard

There are three principal problems that can arise from the quality of irrigation water delivered to the agricultural fields.

6.1.1 Salinity hazard

This is directly related to the quantity of salts dissolved in the irrigation water. All irrigation water contains potentially injurious salts and nearly all the dissolved salts are left in the soil after the applied water is lost by evaporation from the soil or through transpiration by the plants. Unless the salts are leached from the root zone, sooner or later they will accumulate in quantities which will partially or entirely prevent growth of most crops.

6.1.2 Sodicity (alkali) hazard

This is another problem often confronting long-term use of certain water for irrigation and relates to the maintenance of adequate soil permeability so that the water can infiltrate and move freely through the soil. The problem develops when irrigation water contains relatively more sodium ions than divalent calcium and magnesium ions while the total concentration of salts is generally not very high. Accumulation of sodium ions on to the exchange complex results in a breakdown of soil aggregates responsible for good soil structure needed for free movement of water and air through the soils. As in the case of sodic soils, accumulation of sodium on the exchange complex can be reduced by applying appropriate quantities of amendments, e.g. gypsum.

6.1.3 Toxicity hazard

A third problem results from the existence, in some water, of such toxic substances as boron or heavy metals. Boron, though an essential element for plant growth and nutrition, is required only in very small amounts. A high concentration of boron in the irrigation water can have a toxic effect on the growth of many plants. Similarly, certain other ions, e.g. chloride, sodium, etc., could prove toxic to specific crops if present in excessive quantities.

There are other factors which could influence the suitability of water for irrigation but one or more of the above factors are of concern in most situations and are discussed below.

6.2 Salinity problems

A salinity problem related to water quality occurs if the total quantity of salts in the irrigation water is such that the salts accumulate in the root zone to the extent that crop yields are adversely affected. The salinity level of an irrigation water can be determined directly by evaporation of a known quantity of water and measuring the residue of dissolved salts that remain. The results are often expressed in parts of salt per million parts of water (mg/l). An indirect and a more common method of determining the salt content of an irrigation water is to measure its electrical conductivity (EC). The greater the conductivity, the greater is its salt content. EC of irrigation water is expressed in deci Siemens per metre at 25 °C (dS/m), superseding the old millimhos per centimetre (mmho/cm). Irrigation water has a wide range of total salinity. Most surface irrigation water, whose source is snow-fed rivers, has a total salinity of less than about 0.5 to 0.6 dS/m. Groundwater in the semi-arid and arid regions has generally higher salinity and may vary from less than one dS/m to more than 12 to 15 dS/m. Sea water is highly saline with an average total soluble salts content of about 35 g/l corresponding to an electrical conductivity of about 50 dS/m. The higher the total salinity of an irrigation water, the higher is its salinity hazard for the crops if the soil and climatic conditions and the cultural practices remain the same. Soil, crop, climatic and cultural factors which promote accumulation of soluble salts in the root zone are inimical to the utilization of high salinity water for irrigation. Similarly, factors that promote leaching of salts from the root zone through periodic leaching favour the utilization of high salinity water for irrigation. Under favourable conditions groundwater with salinity of more than 10 dS/m has been used for the production of semi-tolerant crops like wheat in coarse textured soils (Paliwal, 1972; Manchanda, 1976) with only slight yield reductions. On the other hand unfavourable soil and climatic conditions and/or poor management have resulted in serious salinity problems even with the use of water of as low salinity as 0.4 to 0.5 dS/m. Ayers and Westcot (1985) reviewed the existing information on the subject and developed practical guidelines for evaluating water quality for irrigation. These guidelines, reproduced in Table 43, are intended to be of help for preliminary evaluation of the suitability of a water supply for irrigation. In arriving at these guidelines the authors made certain basic assumptions which must be kept in view while evaluating any irrigation water for its suitability. These assumptions are:

Yield Potential: Full production capability of all crops, without the use of special practices, is assumed when the guidelines indicate no restrictions on use. A ‘restriction on use’ indicates that there may be a limitation in choice of crop, or special management may be needed to maintain full production capability. A ‘restriction on use’ does not indicate that the water is unsuitable for use.

Site Conditions: Soil texture ranges from sandy loam to clay loam with good internal drainage. There is no uncontrolled shallow water table present within 2 metres of the surface. The climate is semi-arid to arid and rainfall is low. Rainfall does not play a significant role in meeting crop water demand or leaching requirement. (However, in a monsoon climate or in areas where precipitation is high for part or all of the year, infiltrated water from rainfall is effective in meeting all or part of the leaching requirement; in these cases, the restrictions are less severe.)

Methods and Timing of Irrigations: Normal surface or sprinkler irrigation methods are used. Water is applied infrequently, as needed, and the crop utilizes a considerable portion of the available stored soil-water (50 percent or more) before the next irrigation. At least 15 percent of the applied water percolates below the root zone (leaching fraction [LF] ³15 percent). The guidelines are too restrictive for specialized irrigation methods, such as localized drip irrigation, which results in near daily or frequent irrigations, but are applicable for subsurface irrigation if surface applied leaching satisfies the leaching requirements.

Water Uptake by Crops: Different crops have different water uptake patterns, but all take water from wherever it is most readily available within the rooting depth. On average about 40 percent is assumed to be taken from the upper quarter of the rooting depth, 30 percent from the second quarter, 20 percent from the third quarter, and 10 percent from the lowest quarter. Each irrigation leaches the upper root zone and maintains it at a relatively low salinity. Salinity increases with depth and is greatest in the lower part of the root zone. The average salinity of the soil-water is three times that of the applied water and is representative of the average root zone salinity to which the crop responds. These conditions result from a leaching fraction of 15-20 percent and irrigations that are timed to keep the crop adequately watered at all times.

Salts leached from the upper root zone accumulate to some extent in the lower part but a salt balance is achieved as salts are moved below the root zone by sufficient leaching. The higher salinity in the lower root zone becomes less important if adequate moisture is maintained in the upper, ‘more active’ part of the root zone and long-term leaching is accomplished.

Restriction on Use: The ‘Restriction on Use’ shown in Table 43 is divided into three degrees of severity: none, slight to moderate, and severe. The divisions are somewhat arbitrary since change occurs gradually and there is no clear-cut breaking point. A change of 10 to 20 percent above or below a guideline value has little significance if considered in proper perspective with other factors affecting yield. Field studies, research trials and observations have led to these divisions, but management skill of the water user can alter them. Values shown are applicable under normal field conditions prevailing in most irrigated areas in the arid and semi-arid regions of the world.

Ordinarily no soil or cropping problem due to water quality would be experienced or recognized when using water containing less than the values shown for no restriction on use in Table 43. On the other hand, if water is used which exceeds the values shown for the ‘severe’ restriction on use, the user will probably experience soil or cropping problems. With water quality values between these guides, a gradually increasing restriction on use is likely to be experienced as the water quality deteriorates. Specific conditions that may warrant a modification in the suggested values include the leaching fraction, the conditions of drainage, method of irrigation, the climate including rainfall, physical soil conditions, tolerance of crops shown to salinity and the chemical soil characteristics.

Several workers (Pla and Dappo, 1976; Massoumi, 1976) have proposed classifications in respect of water quality to suit local soil and environmental conditions. Bhumbla and Abrol (1972) suggested guidelines (Table 44) for Indian conditions where invariably a rainfall of 300 to 400 mm or more is received in the monsoon season which leaches down the salts accumulated in the preceding cropping season.


Potential Irrigation Problem


Degree of Restriction on Use


Slight to Moderate


Salinity (affects crop water availability)2


Ecw (or)


< 0.7

0.7 - 3.0

> 3.0



< 450

450 - 2000

> 2000

Infiltration (affects infiltration rate of water into the soil. Evaluate using ECW and SAR together)3



= 0 - 3 and ECW


> 0.7

0.7 - 0.2

< 0.2

= 3 - 6 w


> 1.2

1.2 - 0.3

< 0.3

= 6 - 12


> 1.9

1.9 - 0.5

< 0.5

= 12 - 20


> 2.9

2.9 - 1.3

< 1.3

= 20 - 40


> 5.0

5.0 - 2.9

< 2.9

Specific Ion Toxicity (affects sensitive crops)


Sodium (Na)4


surface irrigation


< 3

3 - 9

> 9

sprinkler irrigation


< 3

> 3

Chloride (Cl)4


surface irrigation


< 4

4 - 10

> 10

sprinkler irrigation


< 3

> 3

Boron (B)5


< 0.7

0.7 - 3.0

> 3.0

Trace Elements (see Table 21)

Miscellaneous Effects (affects susceptible crops)


Nitrogen (NO3 - N)6


< 5

5 - 30

> 30

Bicarbonate (HCO3) (overhead sprinkling only)


< 1.5

1.5 - 8.5

> 8.5


Normal Range 6.5 - 8.4


Adapted from University of California Committee of Consultants 1974.


ECW means electrical conductivity, a measure of the water salinity, reported in deci Siemens per metre at 25°C (dS/m) formerly millimhos per centimetre (mmho/cm).

Both are equivalent. TDS means total dissolved solids, reported in milligrams per litre (mg/l).


SAR means sodium adsorption ratio. SAR is sometimes reported by the symbol RNa. At a given SAR, infiltration rate increases as water salinity increases.


For surface irrigation, most tree crops and woody plants are sensitive to sodium and chloride, while most annual crops are not sensitive. For chloride tolerance of selected fruit crops, see Table 18. With overhead sprinkler irrigation and low humidity (< 30 percent), sodium and chloride may be absorbed through the leaves of sensitive crops.


For boron tolerances, see Table 45.


NO3 -N means nitrate nitrogen reported in terms of elemental nitrogen (NH4 -N and Organic-N should be included when wastewater is being tested).



Crops to be grown

Upper permissible limit of EC of water for safe use for irrigation, dS/m

Deep black soils and alluvial soils having a clay content of more than 30 percent. Soils that are fairly to moderately well drained.





Heavy textured soils having a clay content of 20-30%. Soils that are well drained internally and have a good surface drainage system.





Medium textured soils having a clay content of 10-20%. Soils that are very well drained internally and have a good surface drainage system.





Light textured soils having a clay content of less than 10%. Soils that have excellent internal and surface drainage.





Qualifying remarks:


A monsoon rainfall of 300 to 400 mm is common for most areas having a groundwater quality problem. This rainfall periodically leaches out salts accumulated in the root zone during the previous season.


In the above proposed limits of water quality it is presumed that the groundwater table at no time of the year is within 1.5 metres from the surface. If the water table does come up within the root zone the above limits need to be reduced to half the above values.


If the soils have impeded internal drainage either on account of presence of hard pans, unusually high amounts of clay or other morphologic reasons, for advisory purposes, the limit of water quality should again be reduced to half.


If the waters contain soluble sodium percentage more than 70, gypsum should be added to soil occasionally.


If supplemental canal irrigation is available, water of higher electrical conductivity could be used in periods of water shortage.

6.3 Management practices for efficient use of high salinity water

6.3.1 More frequent irrigation
6.3.2 Selection of salt tolerant crops and varieties
6.3.3 Use of extra water for leaching
6.3.4 Conjunctive use of fresh and saline waters
6.3.5 Cultural practices

It would thus seem that there can be very wide variations in the permissible limits of salinity levels of water for irrigation. For this reason any rigid generalizations may prove disadvantageous for field level workers and there is need to develop guidelines for each major area having similar soil, climatic and agricultural conditions. More important however is our ability to use a water of a particular salinity level under a given set of conditions. Management practices can often be modified to obtain a more favourable distribution of salts in the profile and therefore better crop yields, water quality remaining the same. Management practices that can help to overcome a high salinity problem of the irrigation water are discussed below. Desalinization of water to remove soluble salts has often been referred to as a technical possibility but at the present stage of available technologies it is doubtful if this method can have any large-scale application in the utilization of saline water for irrigation of most agricultural crops, at least in the near future.

6.3.1 More frequent irrigation

The adverse effects of the high salinity of irrigation water on the crops can be minimized by irrigating them frequently. More frequent irrigations maintain higher soil water contents in the upper parts of the root zone while reducing the concentration of soluble salts. Both these factors result in reduced effect of high salts on the availability of water to plants and therefore promote better crop growth. The sprinkler method of irrigation is generally more amenable to increased frequency of water applications. In surface irrigation methods however, more frequent irrigations almost invariably result in an appreciable increase in water use.

6.3.2 Selection of salt tolerant crops and varieties

As indicated in previous sections, there is a wide range in the relative tolerance of agricultural crops to soil salinity. Proper choice of crops can result in good returns even when using high salinity water, whereas use of such water for growing a relatively salt-sensitive crop may be questionable. Similarly, selection and breeding of salt-resistant crop varieties offer tremendous possibilities of utilizing saline water resources for crop production. Some workers have suggested induction of salt tolerance by soaking seeds for a certain period in salt solutions as a method for obtaining increased yields in saline water irrigated soils, while others suggest that growing seeds obtained from parents that have been irrigated with saline water helps in obtaining higher crop yields. These suggestions, however, have not been tested extensively on a field scale.

6.3.3 Use of extra water for leaching

To prevent excessive salt accumulation in the soil, it is necessary to remove salts periodically by application of water in excess of the consumptive use. The excess water applied will remove salts from the root zone provided the soil has adequate internal drainage. This concept (Richards, 1954) is quantified in the term ‘leaching requirement’ often referred to by the abbreviation, LR. By definition, leaching requirement (LR) is the fraction of total water applied that must drain below the root zone to restrict salinity to a specified level according to the level of tolerance of the crop.

where D is the depth of water,

and dw and iw refer respectively to the drainage and irrigation water. Assuming strict salt balance conditions in the soil-water system:

Diw x Ciw = Ddw x Cdw where C refers to the concentration of salts.

This would imply that the excess amount of irrigation water of a known EC that must be applied is determined by the maximum permissible EC of the drainage water specified for a particular crop. The values of ECdw represent the maximum salinity tolerated by the species grown under particular conditions. The leaching requirement for a particular crop may be illustrated by use of salt tolerance data (Figure 9). For barley, where a value of ECdw = 8 dS/m can be tolerated, leaching requirement = ECiw/8. Thus for irrigation water with conductivities of 1, 2 and 4 dS/m respectively, the leaching requirement will be 12, 25 and 50 percent.

In actual irrigation practice, the applicability of the leaching requirement concept has had some limitations. In the normal surface irrigation methods there are invariably 10 to 20 percent or more losses due to deep percolation of water beyond the root zone in most light and medium textured soils and this takes care of the leaching requirements for salinity control. In heavy textured soils and in soils having expanding type clay minerals applying 15 to 20 percent more water is often difficult during the crop season due to poor permeability and consequent aeration problems. Leaching accomplished periodically through seasonal rainfall may also result in adequate salt removal from the root zone.

Application of excess water, above that needed for meeting the evapotranspirational needs, though useful for salinity control, puts a high demand on the water resources on the one hand and increases the salt load of the drainage water on the other. Studies by Bernstein and Francois (1973) have shown that reducing the leaching fraction has only a small effect on the salinity of the upper root zone since this area is adequately leached during each irrigation. As a result of these and other studies (Rhoades et al., 1973), it is now suggested that the leaching fraction can be reduced from the values suggested by earlier methods and adequate crop yields can still be obtained.

It therefore appears that controlling the interval between irrigations is the most important management practice for obtaining higher yields with high salinity water and this could be achieved by the sprinkler, drip or the surface irrigation methods.

6.3.4 Conjunctive use of fresh and saline waters

There are situations where good quality water is available for irrigation but not in adequate quantities to meet the evapotranspirational needs of crops. Under these conditions, the strategies for obtaining maximum crop production could include mixing of high salinity water with good quality water to obtain irrigation water of medium salinity for use throughout the cropping season. Alternatively, good quality water could be used for irrigation at the more critical stages of growth, e.g. germination, and the saline water at the stages where the crop has relatively more tolerance. Further research is needed to define the best options considering the tolerance of crops at different growth stages, critical stages of growth vis-a-vis soil salinity, etc.

6.3.5 Cultural practices

Cultural practices can often be modified to reduce the hazard of high salts in the irrigation water. Similarly a modification in the method of irrigation can result in improved use of water for some crops. These aspects have been discussed earlier.

6.4 The sodicity (alkali) problem

6.4.1 Role of magnesium
6.4.2 Management practices for efficient use of water with sodicity hazard

Prolonged use of certain irrigation water results in reduced crop yields due to deterioration in the soil physical properties. The adverse effect of irrigation water quality on soil physical properties is associated with the accumulation of sodium ion on the soil exchange complex which imparts instability to the soil aggregates and whose disruption followed by dispersion of clay particles results in clogging of soil pores. Sodium adsorption ratio (SAR) of the irrigation water defined by the equation below,

with concentration of ions in millimol (+)/1, has, for a long time, been suggested as a measure of the sodicity/alkali hazard of irrigation water (Richards, 1954). For SAR values greater than 6 to 9, the irrigation water could be expected to cause permeability problems in soils which contain swelling type clay minerals.

Eaton (1950) suggested that Residual Sodium Carbonate (RSC) defined by the formula:

RSC = (CO3 - + HCO3-) - (Ca++ + Mg++),
is a good index of the sodicity hazard of an irrigation water. The anions HCO3- and CO3 - in the irrigation water tend to precipitate calcium and magnesium ions in the soil resulting in an increase in the proportion of the sodium ions. For this reason, RSC was considered to be indicative of the sodicity hazard of water. Wilcox (1958) concluded that water with more than 2.5 mmol (+)/1 of RSC is not suitable for irrigation. Water containing 1.25 to 2.5 mmol (+)/1 was considered marginal and that with less than 1.25 mmol (+)/1 probably safe.

Bower and Maasland (1963) proposed a modification in the old SAR procedure to include changes in soil water composition that are expected to result due to dissolution/precipitation of lime in the soil upon irrigation. This approach used the Langelier saturation index to estimate carbonate precipitation as a function of CaCO3 saturation of the soil solution. The index as applied to soils is, SI = (8.4 - pH) where 8.4 is the approximate pH of a non-sodic soil in equilibrium with CaCO3 and pH = (pK2 - pKC) + p (Ca + Mg) + p (CO3 + HCO3) where K2 and KC are the second dissociation constant of H2CO3 and the solubility constant of CaCO3 respectively and (Ca + Mg) and (CO3 + HCO3) are the molar concentrations of the respective ions; p refers to the negative logarithm of the various variables.

Ayers and Westcot (1976) calculated adjusted SAR using the semi-quantitative equation:

adj. SAR = SAR [1 + (8.4 - pHC)]
It was held that the adj. SAR would more correctly predict the sodicity hazard of an irrigation water than either the SAR or the RSC concept. However, in their revision and updating of Irrigation and Drainage Paper No. 29 (1985, Rev. 1), they state that the procedure is no longer recommended.

Excess exchangeable sodium causes the stable soil aggregates to disperse and impart poor air/water permeability only in the absence of excess electrolytes. In nature generally, as the salinity of the waters increases, the SAR also increases. Thus most irrigation water that has a high salinity hazard also has a high SAR but such water does not have a sodicity (alkali) hazard. Thus it is extremely doubtful if SAR or adj. SAR alone could predict the sodicity hazard of an irrigation water. On the other hand, when appreciable quantities of residual sodium carbonate (RSC) are present, the total salinity of water is often low to medium and rarely more than 2 dS/m. Under conditions of low to medium total salinity, water having high residual sodium carbonate (RSC) can have an appreciable sodicity hazard. The concept of residual sodium carbonate appears to relate better to the sodicity problem in the field. However, further research will lead to better predictive approaches for judging the suitability of water for irrigation.

6.4.1 Role of magnesium

In recent years research efforts have been made to define precisely the relative role of magnesium ions vis-a-vis monovalent sodium and divalent calcium ions in influencing soil properties and therefore in developing appropriate modifications in the criteria for quality rating of water with a high proportion of magnesium ions. Amongst the earlier efforts was the work of Antipov-Karataev and Manaeva (1958) and of Arany (1956). These authors believed that the role of magnesium was different to that of calcium because of their varying affinities for adsorption on the exchange complex. Others (Quirk and Schofield, 1955; El-Swaify et al., 1970) had shown similarity between the effects of magnesium and calcium.

Emerson and Chi (1977) observed that the ESP required to produce a given decrease in the flow rate was reduced when magnesium was the complementary cation rather than calcium. Similarly Yadav and Girdhar (1981) reported increased dispersion of clay particles and reduced hydraulic conductivity when the Mg/Ca ratio was increased at a given SAR and electrolyte concentration of the leaching water. Studies by Rowell and Shainberg (1979) and Alperovitch et al. (1981) similarly have shown that magnesium has a specific effect on clay dispersion and loss of hydraulic conductivity of non-calcareous soils, but the calcareous soils which have higher mineral dissolution rates were less affected and did not show the specific effect of magnesium on either clay dispersion or hydraulic conductivity losses. A better understanding of the role of magnesium in relation to other ions will provide more sound criteria for judging the suitability of different water for irrigation under different soil and agroclimatic conditions.

6.4.2 Management practices for efficient use of water with sodicity hazard

As in the case of irrigation water with a salinity hazard, appropriate management practices can often help in better and more efficient use of water with a high sodicity hazard. These practices include:

i. Application of amendments Since accumulation of the sodium ion on the exchange complex is mainly responsible for poor soil physical properties, irrigation water having a sodicity hazard could be improved by increasing the soluble calcium status of the water, thereby decreasing the proportion of sodium to the divalent cations and therefore its adsorption on the soil exchange complex. Applied soluble calcium salts will also neutralize the bicarbonate and carbonate ions thereby reducing the sodicity hazard of the water. The quantity of an amendment that must be applied, the mode and frequency of application etc., are some of the practical questions. Bhumbla and Abrol (1972) recommended that for RSC values up to 2 mmol (+)/1 there was no need to apply an amendment. For higher RSC values, the required amounts of amendment should be calculated and the recommendations made accordingly. Thus the gypsum needed to decrease RSC by 1 mmol (+)/1 works out to 850 kg per hectare metre of water. Gypsum can be either incorporated in the soil or lumps of gypsum can be suitably placed in the water channel to dissolve gradually.

Sulphuric acid has also been used to amend water quality and can be applied directly to the soil or in the irrigation water. It rapidly neutralizes the sodic constituents of water or reacts with lime in the soil to produce soluble calcium. On an equivalent basis, however, the effect is nearly the same as that of gypsum. Being corrosive, handling of sulphuric acid presents problems which must be overcome through proper application techniques.

ii. Mixing with an alternate source of water If an alternate source of irrigation water is available, mixing the two sources may be helpful in obtaining water which is acceptable for irrigation considering its sodicity hazard. Detailed chemical analysis and the quantities in which the water is available from the two sources can help in deciding the proportions in which they need to be mixed.

iii. Irrigating more frequently Irrigating frequently with small quantities of water is an effective way to manage water with a sodicity hazard. Reduced permeability of the soils restricts water supply to the roots. Also applying large amounts at a time can result in surface stagnation which affects most crops adversely. Frequent irrigations could also reduce the precipitation of calcium by reaction with bicarbonates in water by keeping the soils wet. Using sprinkler irrigation with the ability to supply controlled amounts of water at a time should be considered where feasible.

iv. Growing crops with low water requirements When the irrigation water tends to create a sodicity problem, it is advisable to use small quantities of water, waters with significant quantities of residual sodium carbonate (RSC) will cause a continuous increase in the exchangeable sodium status of soils and therefore the need to limit water use. Unlike saline water, where application over and above the evapotranspiration requirements is recommended, extra application of water with a sodicity hazard will further aggravate the problem. If feasible, growing crops and irrigating during periods of high evapotranspiration demands should be avoided.

v. Growing tolerant crops Growing crops tolerant of excess exchangeable sodium and poor soil physical conditions will help obtain better returns than if sensitive crops are grown.

vi. Organic matter applications Heavy dressings of organic manures, regular incorporation of crop residues, application of such organic materials as rice hulls, sawdust, sugar factory wastes, etc., have all been found useful in maintaining and improving soil physical properties and in counteracting the adverse effect of high levels of exchangeable sodium. Wherever feasible therefore, organic matter applications are especially recommended if irrigation water has a sodicity hazard.

6.5 The toxicity problem

Apart from the salinity or the sodicity hazard, the constituents of much irrigation water may cause toxicity problems when taken up by the plants in excess amounts. The toxic constituents of major concern are sodium, chloride and boron. Fruit trees, vines and woody ornamentals are especially sensitive to sodium and chloride ions. Most annual crops are not so sensitive but may be affected by higher concentrations. Sodium and chloride ions are freely taken up by the plants and become concentrated as water is lost through transpiration. Toxicity results when the concentration of these elements exceeds the tolerance limits of the plants. ‘Leaf burn’ scorch, and dead tissue along the outside edges of leaves are typical symptoms of sodium toxicity which first occur in the oldest leaves, usually appearing as a burn or drying of tissue at the outer edges of the leaf. As the severity increases, the drying progresses towards the leaf centre until the entire tissue is dead. Injury due to chloride toxicity however, typically, starts at the extreme leaf tip of older leaves and progresses from the tip back as the severity increases.

A slight excess of boron in the irrigation water or in the soil solution can cause toxicity to a variety of crops. Boron is taken up by the crop and is accumulated. For example, as little as 0.6 mg elemental boron per litre in the irrigation water may produce toxicity symptoms in citrus leaves; 1 mg/l may reduce the yields of citrus and certain stone fruits and 4 mg/l is harmful to many crops. Table 45 presents the recent revision of the data on boron tolerance of agricultural crops (Maas, 1984).

Other constituents of some irrigation water, such as lithium, selenium, molybdenum, fluoride and chromium may have deleterious effects on plants or animals even at very low concentration; however their occurrence in irrigation water has only very occasionally been reported.

6.6 Management practices

Field practices that can eliminate or reduce the hazard due to presence of toxic elements include irrigating the crops more frequently. Frequent irrigations reduce the effective concentration of toxic constituents and therefore their adverse effect. Occasional application of excess water to leach the salts will further reduce the amounts of toxic elements in the root zone. Accumulation of sodium in plant parts can usually be reduced by maintaining a favourable concentration of calcium ions in the soil solution. Adequate quantities of calcium in the irrigation water and soil solution prevent excessive uptake of sodium by plants. Application of amendments, such as soluble calcium salts or sulphuric acid, can therefore greatly reduce the toxicity hazard due to excess sodium. Blending of water supplies, planting less sensitive crops, improving drainage conditions through profile modification, use of fertilizers in optimum doses to obtain otherwise vigorously growing plants etc. are some of the other practices that will help overcome toxicity problems.


Very Sensitive (<0.5 mg/l)


Citrus limon


Rubus spp.

Sensitive (0.5 - 0.75 mg/l)


Persea americana


Citrus X paradisi


Citrus sinensis


Prunus armeniaca


Prunus persica


Prunus avium


Prunus domestica


Diospyros kaki

Fig, kadota

Ficus carica


Vitis vinifera


Juglans regia


Carya illinoiensis


Vigna unguiculata


Allium cepa

Sensitive (0.75 - 1 mg/l)


Allium sativum

Sweet potato

Ipomoea batatas


Triticum eastivum


Hordeum vulgare


Helianthus annuus

Bean, mung

Vigna radiata


Sesamum indicum


Lupinus hartwegii


Fragaria spp.

Artichoke, Jerusalem

Helianthus tuberosus

Bean, kidney

Phaseolus vulgaris

Bean, lima

Phaseolus lunatus


Arachis hypogaea

American elm

Ulmus americana


Pyrus communis


Malus sylvestris mill

Moderately Sensitive (1-2 mg/l)

Pepper, red

Capsicum annuum


Pisum saliva


Daucus carota


Raphanus sativus


Solanum tuberosum


Cucumis sativus

Moderately Tolerant (2-4 mg/l)


Lactuca saliva


Brassica oleracea capitata


Apium graveolens


Brassica rapa

Bluegrass, Kentucky

Poa pratensis


Avena sativa


Zea mays


Cynara scolymus


Nicotiana tabacum


Brassica juncea

Clover, sweet

Melilotus indica


Cucurbita pepo


Cucumis melo

Ragged-robin rose

Rosa spp.


Olea europaea


Zinia elegans Jacq.


Cucurbita spp.

Tolerant (4-6 mg/l)


Sorghum bicolor


Lycopersicon lycopersicum


Medicago saliva

Vetch, purple

Vicia benghalensis


Petroselinum crispum

Beet, red

Beta vulgaris


Beta vulgaris


Tamarix aphylla


Phoenix canariensis

Date palm

P. dactylifera


Beta vulgaris

Garden beet

Beta vulgaris


Gladiolus spp.


Vicia feba

Very Tolerant (6-15 mg/l)


Gossypium hirsutum


Asparagus officinalis

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