There are three principal problems that can arise from the quality of irrigation water delivered to the agricultural fields: 1. Salinity Hazard 2. Sodicity (alkali) Hazard 3. Bicarbonate Hazard. Soils may be affected only by salinity or by a combination of both salinity and sodium.
Essay # 1. Salinity Hazard:
A salinity hazard 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. 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.
Salinity is determined (evaluated) by measuring the ability of water to conduct an electrical current. Salinity is expressed in two different ways, either as electrical conductivity (EC) or total dissolved salts (TDS). There are several units commonly used to express EC: deciSeimens per meter (dS m-1), Seimens per meter (S m-1), microSeimens per centimeter (wS cm-1), millimhos per centimeter (mmhos cm-1), or micromhos per centimeter (µmhos cm-1).
The relationship between these units is:
1 dS m-1 = 0.1 S m-1 = 1000 µS cm-1
= 1 mmhos cm-1 = 1000 µmhos cm-1
Total dissolved salts are expressed in parts per million (ppm) or milligrams per liter (mg H) and are, generally, not measured directly, but calculated from the EC measurement.
1 milligram l-1 (mg l-1) = 1 part per million (ppm)
EC (mmhos cm-1 or dS m-1) x 640 = TDS (mg l-1 or ppm)
The ratio of total dissolved salt to EC of various salt solutions ranges from 550 to 700 ppm per dS nm-1. The most common salt in saline water, sodium chloride, has a TDS of 640 ppm at an EC of 1.0 dS m-1. Most laboratories use this relationship to calculate TDS from EC, but some multiply by other factors (Table 12.3).
The EC values can also be used for inferring the salt concentration expressed in meq l-1 and such relations for mixed soil solutions having EC up to 5 dS m-1 is represented by:
Total ions (meq l-1) = EC (dS m-1) × 10
Most surface irrigation water, whose source is rivers, has a total salinity of less than about 0.5 to 0.6 dS m-1. Groundwater in the semiarid and arid regions has, generally, higher salinity and may vary from less than one dS m-1 to more than 12 to 15 dS m-1. Sea water is highly saline with average total soluble salts content of about 35 g l-1 corresponding to an electrical conductivity of about 50 dS m-1.
The higher the total salinity of 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 utilisation of high salinity water for irrigation. Similarly, factors that promote leaching of salts from the root zone through periodic leaching favour the utilisation of high salinity water for irrigation. Classification of saline water proposed by FAO 1992 is given in Table 12.4.
Apart from salt concentration (EC), sodium adsorption ratio (SAR) and anionic properties. (Cl: SO4 ratios) in saline water also influence salt accumulation and salt balance in the soil.
Mg:Ca Ratio:
Increased proportion of Mg relative to Ca increases the sodification of soils. Mg:Ca ratios more than 3 are critical for optimum growth of plants. Since Mg:Ca ratios in most saline waters varies from 1 to 6, it would be worthwhile to work out critical Mg:Ca ratios for saline waters in relation to soils and crops.
Cl:SO4 Ratio:
Sulphate salts of Ca, Mg and Na are much less soluble than their chloride salts. Irrigation with such waters leads to less salt accumulation in the soil. Sodification of soils is more for SO4 than for Cl salts when the salty soils are not subjected to leaching.
The standard set up by the United States Soil Salinity Laboratory (USSSL) is shown in Table 12.5.
Adverse effects of saline water can be summarised as indicated below:
1. Salt accumulation and reduced availability of water to plants
2. Excessive salt accumulation in the soil leads to early wilting and the effects are almost similar to drought
3. Visual symptoms are stunted growth with smaller, thicker and dark green leaves compared to plants with normal water.
Mostly, cations like Na+, Ca2+, Mg2+, and anions like Cl–, SO42-, MCO32- and CO32- are the major salt constituents contained in saline water. Plant growth is adversely affected with saline irrigation, primarily, through the effect of excessive salts on osmotic pressures of the soil solution, through excessive concentration and absorption of individual ions. Na+, CI–, B etc. may-prove toxic to plants and/ or retard uptake of other nutrients.
Reduced availability at high salinity leads to water deficits for plants and plant growth gets inhibited when soil solution concentration reaches a critical value, often referred to as threshold salinity (ECt). Under field situations, the first reaction to use of saline water is reduction in germination, but the detrimental effects include reduced initial growth resulting in smaller plants.
Management Practices for Efficient Use of Saline Water:
Soluble salt concentration of most river water is less than 1.0 g 1_1 and usually between 0.2 and 0.4 g l-1. Under conditions of good drainage, this water can be used practically for all crops continuously. If the irrigation water has a concentration of about 0.5 to 1.0 g l-1, it may contain sodium, then requiring special measures.
On the other hand irrigation practices in Asia and some other countries shows the use of irrigation water with concentration of 3 to 8 g H. This is only possible as a result of the following conditions :
i. Water is used to sandy soils with high permeability, perfect leaching and natural drainage
ii. Many of these saline waters contain considerable amounts of gypsum which precipitate in the soil, thus limiting the harmful effect of saline soil solution
ii. Relatively salt tolerant crops are grown.
Increase of salts in the root zone of soil depends on the salt content of irrigation water, number of irrigations and salts raised by capillarity from groundwater. On the other hand, increased use of irrigation water decrease the salt content if the leaching water is removed by drainage. General process of salt accumulation in irrigated soils by saline water is shown in Fig. 12.4. Every watering without leaching increases the salinity of irrigated soil.
If 1.000 m3 of water containing 1 g H salt is put into the field, 1.000 kg ha-1 of salts will be added to the soil, with a concentration of 10 g H, 10.000 kg ha-1 will be added. Consequently, after 20 waterings, the total salinity of irrigated soil will increase unto 20 or 200 t ha-1 and the average salinity of soil will increase to 0.4 to 1.0 per cent (FAO/UNESCO 1973). Proper management practices can mitigate the adverse effect of poor quality irrigation water, when it is inevitable to use such water for irrigating the crops.
Management practices for optimum crop production with poor quality irrigation water must aim at preventing build up of salinity, sodicity and toxic ions in the root zone to levels that limit the productivity of soils, control salt balance in salt-water system as well as minimise the damage effects of salinity on crop growth.
There can be very wide variations in the permissible limits of salinity levels of water for irrigation. For this reason any rigid generalisations 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:
i. Selection of salt tolerant crops, varieties and crop sequences
ii. Improved agronomic practices
Residue management
Seed rate and spacing
Method of sowing
Soil fertility management.
iii. Irrigation water management
Presouring irrigation
More frequent irrigation
Change surface irrigation method Use of extra water for leaching
onjunctive use of fresh and saline waters.
iv. Chemical amendments
Gypsum Pyrites
1. Selection of Salt Tolerant Crops, Varieties and Crop Sequences:
There is 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 utilising saline water resources for crop production.
Mass and Hoffman (1977) and Mass (1984) have produced salt tolerance tables (Table 12.14). Table shows crop tolerance and yield potential of selected crops as influenced by irrigation water quality (EC). Salt tolerance data are used in the calculation of leaching requirement. A full yield potential should be obtainable for nearly all crops when using a water which has a salinity less than 0.7 dS m-1.
From the long term experiments at CSSRI, Kamal it was observed that pearmillet-wheat, pearmillet-barley, pearmillet-mustard, sorghum (fodder)-wheat and sorghum (fodder)—mustard sequences were more remunerative on saline soils.
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.
2. Improved Agronomic Practices:
Adequate stand establishment of crop on saline soils is a challenging task. An ideal package of cultural practices besides soil fertility and irrigation water management can ensure adequate crop stand for economic yields.
i. Residue Management:
The common saying “salt loves bare soils” refers to the fact that exposed soils have higher evaporation rates than those covered by residues. Residues left on the soil surface reduce evaporation. Thus, less salt will accumulate and rainfall will be more effective in providing for leaching.
ii. Seed Rate and Spacing:
Seedling mortality and poor tillering are common on saline soils. Relatively higher seed rate than normal can counteract these effects. Around 25 per cent higher seed rate appears reasonable for saline soils. In the case of lowland rice, number of seedlings per hill has to be increased (4-6), besides transplanting aged seedlings.
Crops differ considerably in their ability to tolerate salinity/sodicity. The values of salinity for obtaining specific crop yields were computed by Mannas and Gupta (1990) as per response equation: RY = 100 – S (ECe – ECt), where, ECt is threshold salinity. Oilseed crops requiring less water can tolerate higher levels of ECiw, whereas most pulse crops are very sensitive to salts.
Thus, for successful irrigation with saline water in a specific zone, selection of crops should be such as to suit salinity of water, as it may not be possible to change the quality of irrigation water. High water requiring crops like sugarcane and rice should be avoided with brackish waters as these aggravate the salinity problems.
Tolerant crops (Table 12.15) and varieties appear to be the most practicable way of crop production with poor quality irrigation water.
Barley, sugarbeet and rape are tolerant to salinity. Rice, maize, fingermillet, sorghum, pigeonpea, groundnut, sunflower etc. are semitolerant. Wheat, oats, sugarcane, cotton etc. are semitolerant to sodic soils. There is wide variation in inherent salt tolerance of crop varieties.
Usually there is a negative correlation between tolerance and their yield potential. Varieties like Damodar in rice and Kharchia in wheat are tolerant to salinity but have low yield potential. Varieties showing stable yield under saline conditions should be preferred.
iii. Methods of Sowing:
Failure to achieve satisfactory germination and thus the required plant population is the major factor limiting crop production with saline waters. In furrow irrigation, salts accumulate in the center of the ridge between furrows and on the top of the ridge (Fig 12.5). If the seed is placed on the side of the ridge or at the bottom of the ridge, the problem of salinity can be minimised.
Sloping beds either on one side or on both the sides with seeds just above the water line ensure optimum crop stand. Transplanting leads to better crop establishment in fingermillet and pearlmillet. Closer spacing is better than wide spacing. Wherever possible, a heavy presowmg irrigation with nonsaline water should be given to leach out the salts out of seeding zone. Sowing on northeastern side of ridges can also reduce salt accumulation near the seed zone.
iv. Soil Fertility Management:
Nitrogen requirement of crops is higher in saline soils than in normal soils. High concentration of salts (KCl or K2SO4) inhibits nitrification leading to NH4-N accumulation. Nitrogen fertilisers should be applied in split doses to reduce the losses through volatilisation and denitrification.
Wheat, barley, mustard, pearmillet and cotton significantly responds to higher doses (around 25 per cent higher dose over recommended dose to crops grown on normal soils) of fertilisers. Required dose of P and K along with first dose of N should be applied at or before sowing. Remaining N dose should be applied in two splits at first and second irrigations (25 and 45 DAS).
3. Irrigation Water Management:
On saline soils crop evapotranspiration needs are modified by the presence of salts. Availability of water to plants decreases with increasing salinity. On-farm water management technology has to be optimised for minimising the adverse effects of saline water for efficient water use by the crops.
i. Pre-Sowing/Plant Irrigation:
Salts often accumulate near the soil surface during fallow period, particularly when water tables are high or when off-season rainfall is below normal. Under these conditions, seed germination and seedling growth can be seriously reduced unless the soil is leached before planting.
ii. More Frequent Irrigation:
The adverse effects of the high salinity of irrigation water on the crops can be minimised 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 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.
iii. Changing Surface Irrigation Method:
Surface irrigation methods, such as flood, basin, furrow and border are usually not sufficiently flexible to permit changes in frequency of irrigation or depth of water applied per irrigation. With furrow irrigation, it may not be possible to reduce the depth of water applied below 2 to 3 cm.
As a result, irrigating more frequently might improve water availability to the crop but might also waste water. Converting to surge flow irrigation may be the solution for many furrow systems. Otherwise, sprinkler or drip irrigation system may be required.
iv. Sub-Surface Drainage:
Very saline, shallow water tables occur in many areas. Shallow water tables complicate salinity management since water may actually move upward into the root zone, carrying with it dissolved salts.
Water is then extracted by crops and evaporation, leaving behind the salts. Shallow water tables also contribute to the salinity problem by restricting the downward leaching of salts through soil profile. Installation of subsurface drainage system is the only solution for this situation.
v. 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 consumptive use. Excess water applied will remove salts from the root zone provided the soil has adequate internal drainage. This concept is quantified in the term leaching requirement.
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 × Ciw = Ddw × Cdw
where, C refers to the concentration of salts.
Therefore,
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. For barley, where a value of ECdw = 8 dS m-1 can be tolerated, leaching requirement = ECiw/8. Thus, for irrigation water with conductivities of 1, 2 and 4 dSm-1 respectively, the leaching requirement will be 12, 25 and 50 per cent.
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 per cent 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 per cent 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 evapotranspirational needs, though useful for salinity control, puts high demand on the water resources on the one hand and increases salt load of drainage water on the other. Studies 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, 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.
Therefore it 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.
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 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.
Salinity of a high salinity water source should improve proportionally to the mixing ratio with low salinity water. For example, a water source with an EC of 3 dS m-1 mixed equally with a source with an EC of 1.0 dS m-1 should result in a blend with a salinity of approximately 2.0 dS m-1. A chemical analysis of the blend should be performed to confirm this.
Salinity of the mixture can be calculated with the equation:
Mixing of irrigation sources can occur in irrigation ponds or within the irrigation system itself. When mixing water sources in irrigation ponds, the non saline water should be added immediately prior to being used so as to reduce evaporative losses. Evaporation of surface water is not only an inefficient use of water; it also increases the salinity of the water remaining in the pond. Alternatively, good quality water could be used for irrigation at the more critical stages of growth (stand establishment, germination) and the saline water at the stages where the crop has relatively more tolerance.
Desalinisation 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 utilisation of saline water for irrigation of most agricultural crops, at least in the near future.
4. Chemical Amendments:
In sodic soils (sodium affected soils), sodium ions have become attached to and adsorbed onto the soil particles. This causes a breakdown in soil structure and results in soil sealing or cementing, making it difficult for water to infiltrate. Chemical amendments are used to facilitate the displacement of these sodium ions.
Amendments are composed of sulphur in its elemental form or related compounds such as sulfuric acid and gypsum. Gypsum also contains calcium which is an important element in correcting these conditions. Some chemical amendments render the natural calcium in the soil more soluble. As a result, calcium replaces the adsorbed sodium which helps to restore the infiltration capacity of the soil.
Polymers are also beginning to be used for treating sodic soils. It is important to note that use of amendments does not eliminate the need for leaching. Excess water must still be applied to leach out the displaced sodium. Chemical amendments are only effective on sodium affected soils. Amendments are ineffective for saline soil conditions and often will increase the existing salinity problem. Table 12.16 lists the most common amendments.
Essay # 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.
One of the greatest hazards associated with the use of brackish irrigation water is the possible increase of exchangeable sodium percentage (ESP) of soil’s clay fraction. Due to its effect on the soil and plant, sodium is considered to be one of the major factors governing water quality. Several methods were proposed for expressing the sodium hazard. Previously, water quality was defined on the basis of its sodium percentage alone.
The soluble sodium percentage (SSP) may be calculated by the formula:
Increase in SAR depends on the ratio of sodium ion concentration to the square root of the mean concentration of the divalent cations (Ca and Mg) present in the solution.
The ratio is called sodium adsorption ratio (SAR), defined as:
The higher the ratio, the greater the hazard that the soil will be sodified. The ionic concentrations are expressed in meq l-1. As indicated in Fig. 12.2, an empirical relation has been drawn up between SAR and ESP.
The classification of water according to the SAR is also related to the water’s electrical conductivity (and therefore its salt concentration). Four groups are indicated: low, medium, high and very high electrical conductivity. For EC = 100 micromhos cm-1, the dividing points are at the SAR values, 6, 10 and 18.
As per USSSL, sodium hazard is evaluated as indicated below:
The effects of high SAR on irrigation water infiltration are dependent on the electrical conductivity of water. For a given SAR, lower the EC poorer the infiltration properties, higher the EC better the infiltration. For example, irrigation water with SAR =15 has poor infiltration properties if the EC = 0.5 dS m-1 but good infiltration properties if the EC = 2.0 dS m-1. Fig 12.3 may be referred for the relationship between SAR and EC.
A good rule of thumb, based on this figure, is that if the SAR is more than 10 times greater than the EC, then poorer water infiltration is likely to occur.
Harmful effects of sodic water can be summarised as indicated below:
1. Destruction of soil structure leading to puddled condition of fine textured soils when wet and hard when dry
2. Poor seedling emergence due to soil crust formation
3. Increased pH reduces the availability of N, Zn and Fe
4. Decrease in the availability of Ca and Mg and increase in Na toxicity
5. Toxicity of B, Mo and Se due to their excessive solubility.
Management of Sodicity Hazard:
As in the case of irrigation water with a salinity hazard, appropriate management practices can often help in more efficient use of water with a high sodicity hazard. These practices include: 1. Application of Amendments 2. Mixing with an Alternate Source of Water 3. More Frequent Irrigation 4. Growing Crops with Low Water Requirements and Others.
1. 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 neutralise 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. It has been recommended that for RSC values up to 2.0 mmol (+) l-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 1.0 mmol (+) l-1 works out to 850 kg ha-m-1 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.
The amount of agricultural grade gypsum (70-80% purity) for neutralisation of each me l-1 of RSC is 90 kg ha-1 per irrigation of 7.5 cm depth. Gypsum requirement can thus be estimated by the RSC to be neutralised and the quantity of water required for irrigation during the growing season or on yearly basis. For example, if the RSC of well-water is 10 me l-1 and the depth of irrigation is 5.0 cm, the gypsum requirement would be 4.5 t ha-1: (gypsum in kg ha-1) × 10 (RSC to be neutralised) × 5 (number of irrigations to the crop).
It is easier to apply gypsum to soil than to apply through irrigation water. Gypsum may be broadcast on a previously levelled field and mixed with soil to around 10 cm depth with cultivator or disk. Ideal time of application is after the harvest of rabi crops around May- June. Otherwise, it should be applied with the good monsoon rain.
The other way of reclaiming sodic water is to pass it through a specially designed gypsum bed/gypsum chamber (2.0 × 1.5 × 1.0 m). Gypsum chamber is a brick-cement- concrete chamber. The chamber is connected to a water fall box on one side and to a water channel on the other side. Sodic water flowing from below dissolves gypsum placed in chamber and reclaims it.
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 neutralises 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.
2. 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.
3. More Frequent Irrigation:
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.
4. 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.
5. Crops and Cropping Systems:
Growing crops, tolerant of excess exchangeable sodium and poor soil physical conditions will help to obtain better returns than if sensitive crops are grown. Table 12.17 gives relative tolerance of crops to sodicity or alkalinity.
Rice enjoys a favourable place in any cropping sequence for adoption on sodic soils. Rice based cropping systems such as Rice-wheat-dhaincha, Rice-berseem, Rice-mustard and Rice-wheat are more remunerative on sodic soils.
6. 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, organic matter applications are especially recommended if irrigation water has a sodicity hazard. Inclusion of green manure crops in cropping systems also limits sodicity buildup.
7. Deep Tillage/Sub-Soiling:
With the development of sodicity on the surface soil, clay particles in sodic water irrigated soil become prone to dispersion and displacement and thus the possibility of formation of dense subsoil layer (plow sole) increases. Such soils become very hard and dense on drying. Both these factors retard root proliferation leading to poor crop yield. Deep ploughing is a short term measure to overcome the physical hardness in such soils.
Essay # 3. Toxicity Hazards:
i. Bicarbonate Hazard:
The bicarbonate anion is important in irrigation water as regards precipitation of calcium and to a lesser degree, also of magnesium in the soil. This brings about a change in the SSP in the irrigation water and, therefore, an increase of the sodium hazard. The term residual sodium carbonate (RSC) has been introduced.
RSC (meq l-1) = (CO3— + HCO3) – (Ca++ + Mg++)
Alkali hazard is classified as indicated below:
Normal water (RSC traces)
Low alkali water (RSC < 2.5 meq l-1)
Medium alkali water (RSC 2.5 to 5.0 meq l-1)
High alkali water (RSC 5.0 to 10.0 l-1)
Very high alkali water (RSC > 10.0 meq l-1)
Irrigation with sodic water containing high Na relative to Ca and Mg and high carbonates (CO3 and HCO3) leads to increase in alkalinity and sodium saturation in soils. This increase in ESP adversely affects soil physical properties including water infiltration and soil aeration. On drying, the soil becomes very hard (flocculation) and on wetting, the soil particles get dispersed (deflocculation) and clog the pores that affect root respiration and development.
Modified SAR (adj SAR) Index:
Permeability problem is related to CO3 and HCO3 content in irrigation water. This is not considered in the SAR procedure. When soils get dried between irrigation, part of CO3 and HCO3, precipitate as Ca-Mg CO3 thus removing Ca and Mg from soil water and increasing the relative proportion of Na which would increase the sodium hazard. Wilcox et at (1953) suggested RSC values on which water’s suitability could be judged. Refinement of RSC and SAR procedures have been attempted as adj SAR
where, pHc = calculated pH.
New adj SAR index:
Adj SAR procedure to evaluate the permeability problem over predicted the sodium hazard. A new adj SAR method is derived which adjusts the calcium concentration of the irrigation water to the expected equilibrium value and includes the effects of carbon dioxide (CO2), bicarbonate (HCO3) and of salinity (EC) upon the calcium originally present in the applied water but now a part of the soil water. The new adj SAR is termed widely as adj RNa.
where, Cax is modified calcium concentration value on me l-1 expected to remain in near surface soil water following irrigation with water of given HCO3/Ca ratio and EC available from the standard tables.
ii. Ions, Trace Elements and Other Problems:
A number of other substances may be found in irrigation water and can cause toxic reactions in plants (Table 12.6). After sodium, chloride and boron are of most concerned.
TABLE 12.6: Recommended limits for constituents in reclaimed water for irrigation
Crops grown on soils having an imbalance of calcium and magnesium may also exhibit toxic symptoms. Sulphate salts affect sensitive crops by limiting the uptake of calcium and increasing the adsorption of sodium and potassium, resulting in a disturbance in the cationic balance within the plant.
The bicarbonate ion in soil solution harms mineral nutrition of the plant through its effects on the uptake and metabolism of nutrients. High concentrations of potassium may introduce a magnesium deficiency and iron chlorosis. An imbalance of magnesium and potassium may be toxic, but the effects of both can be reduced by high calcium levels.
iii. Water Quality Effects on Plants and Crop Yield:
Potential yield reduction due to water salinity levels is given in Table 12.14. Generally, forage crops are the most resistant to salinity, followed by field crops, vegetable crops and fruit crops which are, generally, the most sensitive.
Table 12.7 lists the chloride tolerance of agricultural crops. Boron is a major concern in some areas. While a necessary nutrient, high boron levels cause plant toxicity and concentrations should not exceed those given in Table 12.7.
TABLE 12.7: Chloride tolerance Level of crops in order to tolerance1
Many crops have little tolerance for salinity during seed germination, but significant tolerance during later growth stages. Some crops such as barley, wheat and corn are known to be more sensitive to salinity during the early growth period than during germination and later growth periods. Sugarbeet and safflower are relatively more sensitive during germination, while the tolerance of soybeans may increase or decrease during different growth periods depending on the variety.
Management of Toxicity Problem:
1. Toxic Constituents of Major Concern:
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 center 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.
Several Ca containing soil amendments are used to replace Na in sodic soils in conjunction with leaching to remove salts from the root zone. The amendments counteract Na by providing Ca either directly (contain Ca) or indirectly (provide acid to dissolve calcium carbonate present in the soil).
Calcium arising from the soil amendments reacts with soil Na to displace it from the cation exchange sites on clay and organic matter particles. The released Na can then be leached out of the soil profile as sodium sulfate. The most commonly used amendments for the treatment of sodic soils are gypsum and elemental sulphur.
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 liter in the irrigation water may produce toxicity symptoms in citrus leaves; 1 mg l-1 may reduce the yields of citrus and certain stone fruits and 4 mg l-1 is harmful to many crops.
Recent revision of data on boron tolerance of crops is given below:
Very sensitive (< 0.5 mg l-1): Lemon, blackberry.
Sensitive (0.5 to 0.75 mg l-1): Avocado, grapefruit, citrus, peach, cherry, plum, grape, walnut, cowpea, onion.
Sensitive (0.75 to 1.0): Garlic, sweet potato, wheat, barley, sunflower, bean, sesame, groundnut.
Moderately sensitive (1.0 to 2.0 mg I-1): Pea, carrot, radish, potato, cucumber.
Moderately tolerant (2.0 to 4.0 mg l-1): Cabbage, turnip, oats, maize, tobacco, mustard, muskmelon, pumpkin.
Tolerant (4.0 to 6.0 mg l-1): Sorghum, tomato, alfalfa, sugarbeet, palm, broadbean.
Very tolerant (6.0 to 15.0 mg I-1): Cotton, asparagus.
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. Recommended maximum concentration of trace elements in irrigation water is given in Table 12.18.
2. Field Practices to Eliminate or Reduce the Hazard:
Field practices that can eliminate or reduce the hazard due to presence of toxic elements include irrigating the crops more frequently as discussed under conjunctive use of fresh and saline waters. 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 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 fertilisers in optimum doses to obtain otherwise vigorously growing plants etc. are some of the other practices that will help to overcome toxicity problems.
Water Quality Guidelines for Irrigation:
It is evident from the above that apart from its composition, determination of suitability of specific water requires specification of conditions (soil, climate, crop etc.) of its use and the irrigation and other management practices followed.
Therefore, the following broad guidelines, for assessing suitability of irrigation waters, have been suggested from time to time for average use conditions:
1. Growing relatively tolerant crops and varieties
2. Sowing on northeastern side of ridges
3. Using around 20 per cent higher seed rate and quick postsowing irrigation (within 2-3 days) for better germination
4. Use of gypsum for saline water having SAR > 20 and/or Mg: Ca> 3 and rich in silica
5. Fallow during rainy season when SAR > 20 and higher saline waters are used in low-rainfall areas
6. Additional phosphorus application when CI: SO4 ratio is > 2
7. Canal water, preferably, at early growth stages including presowing irrigation for conjunctive use with saline water
8. When ECiw < ECe (0-45 cm soil depth at harvest of rabi crops), saline water irrigation just before onset of monsoon.