4. SODIC SOILS AND THEIR MANAGEMENT

The chief characteristic of sodic soils from the agricultural stand point is that they contain sufficient exchangeable sodium to adversely affect the growth of most crop plants. For the purpose of definition, sodic soils are those which have an exchangeable sodium percentage (ESP) of more than 15. Excess exchangeable sodium has an adverse effect on the physical and nutritional properties of the soil, with consequent reduction in crop growth, significantly or entirely. The soils lack appreciable quantities of neutral soluble salts but contain measurable to appreciable quantities of salts capable of alkaline hydrolysis, e.g. sodium carbonate. The electrical conductivity of saturation soil extracts are, therefore, likely to be variable but are often less than 4 dS/m at 25 °C. The pH of saturated soil pastes is 8.2 or more and in extreme cases may be above 10.5. Dispersed and dissolved organic matter present in the soil solution of highly sodic soils may be deposited on the soil surface by evaporation causing a dark surface which is why these soils have also been termed as black sodic soils.

Under field conditions after an irrigation or rainfall, sodic soils typically have convex surfaces. The soil a few centimetres below the surface may be saturated with water while at the same time the surface is dry and hard. Upon dehydration cracks 1-2 cm across and several centimetres deep form and close when wetted. The cracks, generally, appear at the same place on the surface each time the soil dries unless it has been disturbed mechanically. The physico-chemical characteristics of two soil profiles are presented in Table 20.

The principal cause of alkaline reaction of soils is the hydrolysis of either the exchangeable cations or of such salts as CaCO₃, MgCO₃, Na₃CO₃, etc. Hydrolysis of the exchangeable cations takes place according to the following reactions

In this reaction H⁺ is inactivated by exchange adsorption in place of Na⁺. The displaced Na does not combine with, or inactivate OH^- ions which results in an increase in the OH^- ion concentration and increased soil pH. The extent to which exchangeable cations hydrolyse depends on their ability to compete with H⁺ ions for exchange sites. Ions such as Na⁺ are unable to compete as strongly as the more tightly held ions such as Ca²⁺ and Mg²⁺. For this reason exchangeable Na⁺² and K⁺² are hydrolysed to a much greater extent and produce a higher pH than do exchangeable Ca²⁺ or Mg²⁺. Hydrolysis of exchangeable Ca²⁺ and Mg²⁺ ions, in fact, is so limited that it results in a soil having only by a mildly alkaline reaction. Hydrolysis of compounds like CaCO₃, and MgCO₃, takes place according to the reaction:

In this reaction H⁺ from water is inactivated through combination with carbonate to form weakly ionized carbonic acid. Hydroxyl ions are not inactivated through combination with Ca²⁺ resulting in an alkaline solution. The hydrolysis of CaCO₃ and of MgCO₃, is limited due to their low solubilities and therefore they tend to produce a pH in soils no higher than about 8.0 to 8.2. Soils containing measurable quantities of Na₂CO₃, have a pH of more than 8.2; the pH increases with increasing amounts of Na₂CO₃, and may be as high as 10.0 to 10.5. This is due to the higher solubility of Na₂CO₃ and therefore the greater potential for hydrolysis. According to Cruz-Romero and Coleman (1975) exchangeable sodium and CaCO₃ react in low CO₂ - low neutral salt environments to produce high pH and appreciable concentrations of Na₂CO₃. Since the soils of arid and semi-arid regions nearly always contain some calcium carbonate, a build up in the exchangeable sodium in the absence of an appreciable quantity of neutral soluble salts will always result in high pH; the exact value depending on the concentration of Na₂CO₃, formed or the level of ESP.

Table 20a CHARACTERISTICS OF A SODIC SOIL, KARNAL, INDIA (Bhargava - Personal communication)

Depth cm	pH₂^1/	dS/m	clay (<2m) %	CEC me/100g	Saturation extract composition
					Na⁺	(Ca+Mg)⁺⁺	K⁺	CO₃^-	HCO₃^-	Cl^-	SO₄^-	ESP
					me/l
0-5	10.3	8.2	22	10.2	85	0.4	0.2	30	37	13	6	97
5-24	10.3	8.0	29	12.8	84	0.4	0.1	27	41	15	6	94
24-56	9.8	1.9	33	14.8	18	0.6	0.1	2.5	9	6	3	90
56-85	9.8	1.4	31	14.6	14	0.5	0.1	2.2	8	3	2	85
85-118	9.6	1.0	27	11.2	10	0.8	0.1	2.2	5	3	1	68
118-140	9.2	0.9	23	9.8	9	1.0	0.1	1.5	5	3	1	39

Depth cm	pH	EC dS/m	Composition of 1:2 extract
			Na⁺	(Ca + Mg)⁺⁺	K⁺	CO₃^-	HCO₃^-	Cl^-	SO₄^-
			me/100 g soil
0-3	10.0	17.5	48	tr	2.6	6.0	35	2.7	2.6
3-6	9.7	12.5	37	tr	2.0	5.2	26	2.4	3.3
10-15	8.8	5.9	13	1.3	0.9	0.6	8	1.0	4.8
16-26	8.3.	5.3	10	1.1	0.7	0.1	6	1.5	4.0
26-36	7.6	2.9	5	0.8	0.3	0	3	1.1	1.6
42-51	7.4	2.6	3	1.5	0.4	0	2	0.6	2.4
55-66	7.6	3.6	3	4.6	0.5	0	2	0.2	5.6

Figure 17 Relationship between the pH of saturated soil paste, and exchangeable sodium percentage (ESP) (Abrol et al., 1980)

Abrol et al. (1980) and Bhargava and Abrol (1978) showed a relationship between the pH of the saturated soil paste and the exchangeable sodium percentage of the soils studied by them (Figure 17). Since pH can be relatively easily determined, these workers suggested that pH could be used as an approximate measure of the exchangeable sodium percentage. Several other workers (Agarwal and Yadav, 1956; Chang and Dregne, 1955; Kovda, 1965) also reported that pH and ESP are correlated. Gupta et al. (1981) pointed out that for soils containing sodium-carbonate type of salts the exchangeable sodium ratio (ESR) and pH were quantitatively related and that the-relationship governing their dependence can be derived from Na⁺ - (Ca + Mg)²⁺ exchange equilibria in soils. Based on these considerations and on published information, Abrol et al. (1980) suggested an approximate relationship between pH of saturated soil paste and ESP (Table 21) which can be used for inferring the approximate ESP of soil from pH measurements.

pH of saturated soil paste	Approximate ESP
8.0 - 8.2	5 - 15
8.2 - 8.4	15 - 30
8.4 - 8.6	30 - 50
8.6 - 8.8	50 - 70
8.8	70

The relationship between soil pH and ESP of the kind shown in Figure 17 (Table 21) exists only for specific kinds of sodic soils, that is, soil having measurable to appreciable quantities of salts capable of alkaline hydrolysis and having a saturated soil paste pH above 8.0. Such a relationship does not exist for saline soils,, i.e. soils dominated by neutral soluble salts, the pH of which is normally less than 8.0. Calcareous soils, even at very low ESP values, have a pH mainly determined by the ambient CO₂, partial pressure (about 8.2 in contact with a standard atmosphere; about 1 unit lower under a high CO₂ concentration as in some water-saturated soils, and higher where CO₂, pressures are lower). As this relationship is not a universal one and may only be applied for specific and similar conditions, it is not advisable to use pH as a general index of sodicity.

For the purposes of definition, US Salinity Laboratory researchers (Richards 1954) had suggested a saturated soil paste pH of 8.5 or more for characterizing soils as ‘alkali’. In later publications however, the US scientists preferred the term ‘sodic’ to ‘alkali’ and in the definition of sodic soil a reference to soil pH was omitted. As already discussed, there is a relationship between pH and soil sodicity for soils containing calcium carbonate as do most soils of semi-arid regions. Studies (Gupta et al., 1982, 1983) have also shown that pH strongly influences the soil physico-chemical behaviour as distinct from the effect of exchangeable sodium on soil properties. For this reason these workers suggest that pH should be an integral part of the definition of sodic soils.

An ESP of 15 is generally recognized as a limit above which the soils are characterized as sodic (alkali) (Richards, 1954). This limit, though tentative, has been increasingly found useful because many soils show a sharp deterioration in physical properties around or above this ESP (Abrol et al., 1978; Acharya and Abrol, 1978; Varallyay, 1977; Gardner et al., 1959), although for some soils a lower ESP (6) has been suggested as a limiting value (Northcote and Skene, 1972). A survey of published data (Abrol et al., 1980) showed that for sodic soils, most often an ESP of 15 to 20 is associated with a saturation paste pH of 8.2. For diagnostic purposes therefore it was suggested that a saturation paste pH of 8.2 will be more realistic than the value of 8.5 which is nearly always associated with higher values of ESP.

4.2 Measuring sodicity/alkali status of soils

4.2.1 pH measurement

pH measurement is a significant diagnosis of salt-affected soils but dependence of pH value upon the soil-water ratio of the suspension in which it is measured is frequently ignored in the reports and pH data are given with no indication of the dilution factor used. Interpretation of such data is difficult, even impossible, pH data in Table 21 were measured on a saturated soil paste and Figure 18 gives the relationship between pH of saturated soil paste and pH of 1:2 soil-water suspension. It is seen that pH of 1:2 soil-water suspension is greater than the pH of saturated soil paste by about 1 unit. Thus for characterizing soils as sodic, if the pH is measured in 1:2 soil-water suspension, the limiting pH value will be about 9.0 instead of 8.2 as suggested above.

Figure 18 Relationship between pH of saturation paste, pHs and pH of 1:2 soil-water suspension, pH₂ for soil of varying ESP²

Highly saline soils have a pH of saturated soil paste around neutral because that is the pH of the neutral salts constituting most of the solutes in the soil solution.

The influence of soil-water ratio and salinity on pH is illustrated in Figure 19 (Dregne, 1976). With a high soil-water ratio as represented by the saturated paste, the pH varied from 7.1 in the highly saline soil to 8.0 in the non saline soil. Diluting the system to give a soil-water ratio of 1:5 resulted in a pH of 7.8 for highly saline and 8.7 for the non saline soil. The difference between pH of the saturated paste and 1:2 or 1:5 soil-water suspension tends to be greater in sodic than in non-sodic or saline soils. This observation has led to the suggestion that a difference of about 1 pH unit between the two readings indicates that the soil contains more than 15 percent exchangeable sodium. This leads to the conclusion that when properly measured, pH can be used as a criterion for distinguishing sodic from normal and saline soils. Opinions vary as to the proper method of making pH readings but it is desirable to select a definite procedure and follow it closely, so that the readings will be consistent and have maximum diagnostic value. The method used should be described accurately so as to aid others in the interpretation of results.

Figure 19 Soil pH as affected by soil-water ratio and the salinity status (Dregne, 1976)

4.2.2 Evaluating ESP

Every soil has a definite capacity to adsorb the positively charged constituents of dissolved salts, such as calcium, magnesium, potassium, sodium, etc. This is termed the cation exchange capacity. The various adsorbed cations can be exchanged one for another and the extent of exchange depends upon their relative concentrations in the soil solution, the valency and size of the cation involved, nature and amounts of other cations present in the solution or on the exchange complex, etc. Exchangeable sodium percentage (ESP) is, accordingly, the amount of adsorbed sodium on the soil exchange complex expressed in percent of the cation exchange capacity in milliequivalents per 100 g of soil. Thus,

Experimental details for measuring ESP can be found in several publications (Richards. 1954; FAO, 1970; Hesse 1971). Measured values of cation exchange capacity and exchangeable cations can vary considerably depending on the method of determination adopted. Only the data determined by the same method can be compared, which implies that during survey and monitoring, the same laboratory methods must be adopted.

4.2.3 SAR as an index of sodicity hazards

Experimental determination of exchangeable sodium percentage is tedious, time consuming and subject to errors. Incomplete removal of index salt solution during the washing step of CEC determinations can lead to high CEC values and therefore low ESP estimates. Similarly, hydrolysis of exchangeable cations during the removal of the index salt solution, fixing of ammonium ions from the index or replacement solution by the soil minerals and the dissolution of calcium carbonate or gypsum in the index or replacing solutions can all lead to low values of cation exchange capacity and therefore to high ESP estimates. Problems of CEC and ESP determinations are also encountered in soils of high pH containing zeolite minerals. These minerals, e.g. analcime, contain replaceable monovalent cations in their lattice which are readily replaced by monovalent cations used as the index or replacement cation resulting in unusually high values of ESP (Gupta et al., 1984). To overcome some of these difficulties several workers prefer to obtain an estimate of the exchangeable sodium percentage from an analysis of the saturated soil extract. Workers at the US Salinity Laboratory (Richards 1954) proposed that the sodium adsorption ratio (SAR) of the soil solution adequately defines the soil sodicity problem and is quantitatively related to the exchangeable sodium percentage of the soils. Sodium adsorption ratio, SAR, is defined by the equation:

Although some studies have shown that grouping calcium and magnesium in the above equation is not strictly valid, there appears only little loss of accuracy when this is done. Further, in many laboratories of the world calcium plus magnesium in the soil extracts and waters are estimated in a single determination - thus it is convenient to group these two elements together for the calculation of SAR. The calcium plus magnesium concentration is divided by two because most ion exchange equations express concentrations as mol/litre or mmol/litre rather than mmol (+)/1. The exchangeable sodium status of soils can be predicted fairly well from the SAR of the saturated soil extract since the two are related by the expression:

where the exchangeable ion concentrations are in cmol (+)/kg (the subscript ex indicates exchangeable), and K_G is the exchange constant called Gapon’s constant. Several studies have shown that there is a linear relationship between SAR of the soil solution and ESR up to an ESP of about 50 so that SAR of the soil solution can be used as a fair measure of the exchangeable sodium status of soils. For a better estimate of exchangeable sodium, the value of constant K_G needs to be determined experimentally for each major group of soils. The value of K_G obtained by salinity laboratory workers (Bower, 1959) for a group of soils from the Western United States has been widely used. Up to an SAR of the saturation extract of about 30 the ESP values are roughly similar to SAR, but above this limit, they diverge and the full expression above must be used.

4.3 Sodic soils and plant growth

Plant growth is adversely affected in sodic soils due to one or more of the following factors:

Figure 20 Schematic diagram showing the relative hydraulic conductivity of a soil as affected by increasing ESP

pH	Solubility of CaCO₃ me/l
6.21	19.3
6.50	14.4
7.12	7.1
7.85	2.7
8.60	1.1
9.20	0.8
10.12	0.4

Under field conditions plant growth is adversely affected due to a combination of two or more of the above factors, depending on the level of exchangeable sodium, nature of the crops and the overall level of management. Table 23 gives the approximate extent of hazard in relation to ESP and crops.

Approx. ESP	Sodicity hazard	Remarks
< 15	None to slight	The adverse effect of exchangeable sodium on the growth and yield of crops in various classes occurs according to the relative crop tolerance to excess sodicity. Whereas the growth and yield of only sensitive crops are affected at ESP levels below 15, only extremely tolerant native grasses grow at ESP above 70 to 80.
15 - 30	Light to moderate
30 - 50	Moderate to high
50 - 70	High to very high
> 70	Extremely high

4.4 Reclamation and management

4.4.1 Amendments

Basically, reclamation or improvement of sodic soils requires the removal of part or most of the exchangeable sodium and its replacement by the more favourable calcium ions in the root zone. This can be accomplished in many ways, the best dictated by local conditions, available resources and the kind of crops to be grown on the reclaimed soils. If the cultivator can spend very little for reclamation and the amendments are expensive or not available, and he is willing to wait many years before he can get good crop yields, soil can still be reclaimed but at a slow rate by long-continued irrigated cropping, ideally including a rice crop and sodic tolerant crops in the cropping sequence, along with the incorporation of organic residues and/or farmyard manure. For reasonably quick results cropping must be preceded by the application of chemical soil amendments followed by leaching for removal of salts derived from the reaction of the amendment with the sodic soil.

Soil amendments are materials, such as gypsum or calcium chloride, that directly supply soluble calcium for the replacement of exchangeable sodium, or other substances, such as sulphuric acid and sulphur, that indirectly through chemical or biological action, make the relatively insoluble calcium carbonate commonly found in sodic soils, available for replacement of sodium. Organic matter (i.e. straw, farm and green manures), decomposition and plant root action also help dissolve the calcium compounds found in most soils, thus promoting reclamation but this is relatively a slow process. The kind and quantity of a chemical amendment to be used for replacement of exchangeable sodium in the soils depend on the soil characteristics including the extent of soil deterioration, desired level of soil improvement including crops intended to be grown and economic considerations.

Chemical amendments for sodic soil reclamation can be broadly grouped into three categories:

Gypsum Gypsum is chemically CaSO₄.2H₂O and is a white mineral that occurs extensively in natural deposits. It must be ground before it is applied to the soil. Gypsum is soluble in water to the extent of about one-fourth of 1 percent and is, therefore, a direct source of soluble calcium. Gypsum reacts with both the Na₂CO₃, and the adsorbed sodium as follows:

Calcium chloride Calcium chloride is chemically CaCl₂ 2H₂O. It is a highly soluble salt which supplies soluble calcium directly. Its reactions in sodic soil are similar to those of gypsum:

Sulphur (S) Sulphur is a yellow powder ranging in purity from 50 percent to more than 99 percent. It is not soluble in water and does not supply calcium directly for replacement of adsorbed sodium. When applied for sodic soil reclamation, sulphur has to undergo oxidation to form sulphuric acid which in turn reacts with lime present in the soil to form soluble calcium in the form of calcium sulphate:

Others In some localities cheap acidic industrial wastes may be available which can be profitably used for sodic soil improvement. Pressmud, a waste product from sugar factories, is one such material commonly used for soil improvement. Pressmud contains either lime or some gypsum depending on whether the sugar factory is adopting carbonation or a sulphitation process for the clarification of juice. It also contains variable quantities of organic matter.

The choice of an amendment at any place will depend upon its relative effectiveness as judged from improvement of soil properties and crop growth and the relative costs involved. The time required for an amendment to react in the soil and effectively replace adsorbed sodium is also a consideration in the choice of an amendment. Because of its high solubility in water, calcium chloride is the most readily available source of soluble calcium but it has rarely been used for reclamation on an extensive scale because of its high cost. Similarly iron and aluminium sulphates are usually too costly and have not been used for any large-scale improvement of sodic soils in the past.

Because amendments like sulphur and pyrite must first be oxidized to sulphuric acid by soil microorganisms before they are available for reaction, the amendments are relatively slow acting. Being cheapest and most abundantly available, gypsum is the most widely used amendment. Sulphuric acid has also been used extensively in some parts of the world, particularly in western United States and parts of USSR. Several studies have attempted to evaluate the effectiveness of various amendments under varying soil and climatic conditions. Overstreet et al. (1951) compared gypsum, sulphur and sulphuric acid for reclaiming sodic soil of the Fresno series in the USA. When applied in chemically equivalent quantities, the response in terms of the yield of irrigated pasture was markedly higher in the sulphuric acid treated plots than in the gypsum and sulphur plots over a two year period following treatment. The yields in sulphur plots were never significantly higher than those of the check plots indicating that sulphur did not oxidize sufficiently to improve the soil effectively.

Verma and Abrol (1980 a,b) compared the effect of chemically equivalent quantities of gypsum and pyrite at. 5 application rates on soil properties and yield of rice and wheat in a highly sodic soil. The pyrite used had 31 percent total sulphur and the gypsum 85 percent CaSO₄ 2 H₂O. Results (Table 24) showed that pyrite was only about one-fourth as effective as gypsum. This is apparently due to lack of complete oxidation of pyrite once incorporated in sodic soils of high pH. Starkey (1966) pointed out that the best pH ranges for the activity of some sulphur oxidizing microorganisms, e.g. T. thiooxidans and T. ferrooxidans were in the range 2.0 to 3.0 and 2.2 to 4.7 respectively. Since the pH of sodic soils is usually very high, it is doubtful if the oxidation of sulphur/pyrite will proceed sufficiently. In order to be as effective as soluble calcium compounds, all the sulphur must undergo oxidation to form sulphuric acid. In experiments by Verma and Abrol (1980 a,b) the soil improvement in pyrite plots did not approach the improvement obtained in gypsum treated plots even three years after the amendments were applied. These results tend to show that the efficiency of sulphur compounds that must oxidize to produce sulphuric acid before they can replace adsorbed sodium is likely to be low in sodic soils due to their high pH.

Table 24 EFFECT OF GYPSUM AND EQUIVALENT QUANTITIES OF PYRITE ON SOIL PROPERTIES AND CROP YIELD IN A HIGHLY SODIC SOIL (Verma and Abrol, 1980 a,b)

Treatment		Soil properties			Crop yield t/ha
		Infiltration rate mm/day	ESP		Rice	Wheat
		Infiltration rate mm/day	0-15 cm	15-30 cm	Rice	Wheat
Control		3.34	76.5	92.4	3.85	0.19
Gypsum t/ha	7.1	6.82	33.4	75.1	6.71	1.46
	14.2	8.58	32.4	79.2	6.85	3.14
	21.3	12.25	19.2	59.5	7.43	3.60
	28.4	12.42	13.6	56.5	7.24	4.22
Pyrite t/ha	3.6	3.38	64.1	90.2	5.71	0.15
	7.2	4.05	52.3	86.4	6.04	0.54
	10.8	5.62	44.1	80.2	6.71	1.35
	14.4	5.65	38.8	80.3	6.91	1.35

In another field study on a highly sodic soil Milap Chand et al. (1977) compared the performance of several amendments on the yield of barley grown in a highly sodic soil. Their data (Table 25) show that gypsum, sulphuric acid and aluminium sulphate were nearly equally effective in improving the yield of barley. As expected, farmyard manure or pressmud (C) from the sugar factories adopting the carbonation process had little beneficial effect and pressmud (S) from factories adopting the sulphitation process increased the yield since it contained about 9.3 percent calcium sulphate and about 36 percent organic matter.

Table 25 EFFECT OF SEVERAL AMENDMENTS ON THE YIELD OF BARLEY GROWN IN HIGHLY SODIC SOIL (Milap Chand et al. 1977)

Amendment	Barley yield t/ha
Control	0.02
Gypsum	3.10
Sulphuric acid	3.20
Aluminium sulphate	3.12
Ferrous sulphate	2.62
Farmyard manure	0.29
Pressmud (C)	0.10
Pressmud (S)	0.61

Table 26 EFFECT OF GYPSUM AND EQUIVALENT QUANTITIES OF SULPHURIC ACID ON CROP YIELDS t/ha (Yadav, 1973)

Treatment	1969-70		1970-71		1971-72
Treatment	Rice	Wheat	Rice	Wheat	Rice	Wheat
Control	1.51	0.72	2.12	1.71	3.56	1.46
Gypsum	3.67	1.92	4.36	2.61	5.13	2.74
Sulphuric acid	4.56	2.56	4.68	3.10	5.53	3.05

Some workers have postulated that sulphuric acid is more effective because calcium sulphate formed in situ as a result of its reaction with soil calcium carbonate is more effective in neutralizing free sodium carbonate in soils and in replacing the adsorbed sodium. Miyamoto et al. (1975) presented equations for predicting the changes in exchangeable sodium and dissolved salts results from known applications of sulphuric acid. Prather et al. (1978) suggested that combining either CaCl₂ or H₂SO₄ with CaSO₄ as an amendment can appreciably shorten the time of reclamation and improve water efficiency as compared to CaSO₄ alone under certain soil conditions.

Large-scale use of sulphuric acid for improving sodic soils presents handling and application difficulties. However, in many parts of the USA sulphuric acid is being increasingly used for improving calcareous sodic soils because its cost compared to gypsum is not prohibitive and its use can be economic as larger and quicker returns from the land, in the form of crop yields, will more than pay for the higher initial cost. Similarly in USSR, a large-scale soil reclamation programme is being undertaken in the Araraat Valley, in the Armenian Republic, by application of 1 percent sulphuric acid along with heavy leaching (Petrosian, 1977). However, gypsum is by far the most commonly used amendment because of its abundant availability and low cost. The quantities of different amendments that will cause equivalent improvement by replacing adsorbed sodium are given in Table 27, relative to 1 ton gypsum. The figures in Table 27 assume 100 percent oxidation of materials like sulphur or pyrite in order to be as effective as soluble calcium compounds. Optimum conditions for the oxidation of these compounds are not fully understood but available information indicates that oxidation is unlikely to be sufficient in soils of high pH to warrant their large-scale use for reclamation.

Table 27 EQUIVALENT QUANTITIES OF SOME COMMON AMENDMENTS FOR SODIC SOIL RECLAMATION

Amendment	Relative quantity ^1/
Gypsum (CaSO₄ 2H₂O)	1.00
Calcium chloride (CaCl₂ 2 H₂O)	0.85
Sulphuric acid (H₂SO₄)	0.57
Iron sulphate (FeSO₄.7 H₂O)	1.62
Aluminium sulphate (Al₂ (SO₄)₃.18 H₂O)	1.29
Sulphur (S)	0.19
Pyrite (FeS₂) - 30% sulphur	0.63
Calcium polysulphide (CaS₅) - 24% sulphur	0.77

¹/ These quantities are based on 100 percent pure materials. If the material is not 100 percent pure necessary correction must be made. Thus if gypsum is only 80 percent pure the quantity to be added will be

tons instead of 1.00 ton.

The quantity of an amendment necessary to reclaim sodic soil depends on the total quantity of sodium that must be replaced. This, in turn, depends on such factors as the soil texture and mineralogical make up of the clay, extent of soil deterioration as measured by exchangeable sodium percentage (ESP) and the crops intended to be grown. The relative tolerance of a crop to exchangeable sodium and its normal rooting depth will largely determine the soil depth up to which excess adsorbed sodium must be replaced for satisfactory crop growth. If a quantitative exchange of applied soluble calcium for adsorbed sodium is assumed, replacement of each mole of adsorbed sodium per 100 g soil will require half a mole of soluble calcium. The quantity of pure gypsum required to supply half a cmole of calcium per kg soil for the upper 15 cm soil depth will be

In many laboratories the quantity of gypsum required for reclaiming sodic soil is determined by the gypsum requirement (GR) test suggested by Schoonover (1952). The test is performed by mixing a small soil sample (5 g) with a relatively large volume of saturated gypsum solution and measuring the calcium lost from the solution after reaction with soil. Sodium salts in an sodic soil are so diluted by this treatment that nearly complete displacement of exchangeable sodium by calcium from the gypsum solution occurs. The decrease in calcium from the solution when expressed on the basis of tons of CaSO₄.2H₂O per 30 cm of soil is the gypsum requirement of the soil.

Many sodic soils contain, in addition to excessive quantities of exchangeable sodium, appreciable amounts of soluble sodium carbonate. In such cases the gypsum requirement test evaluates the amount of calcium required to replace the exchangeable sodium plus that required to neutralize all the soluble sodium carbonate in the soil. Some workers (Hausenbuiller, 1978) maintain that sufficient amendment must be added to react with both soluble sodium carbonate and exchangeable sodium to achieve complete reclamation. However studies by Abrol and Dahiya (1974) showed that, when gypsum was surface applied and leached, only a small fraction of the soluble carbonates reacted with applied calcium and that a major fraction of the soluble carbonates leached without reacting with applied gypsum. Under field conditions one irrigation prior to application of an amendment would further ensure leaching of soluble carbonates, eliminating the need of additional quantities of gypsum for neutralizing the free sodium carbonate.

For the above reasons, a modification in the method of determining the gypsum requirement of soils has been proposed (Abrol et al., 1975). In the modified procedure, the soil is washed free of soluble carbonates with alcohol before proceeding with the gypsum requirement test. The modified procedure gives a more realistic estimate of the gypsum needs of sodic soils containing varying amounts of soluble carbonate.

It has been earlier pointed out a relationship between soil pH and the exchangeable sodium percentage for some indian soils. Such a relationship was established for sodic soils of the Indo-Gangetic plains in India (Figure 17), and based on this a graphical relationship between pH of 1:2 soil-water suspension and the gypsum requirement of the surface 15 cm depth was established. This is presented in Figure 21. Since pH can be determined easily and since it is measured on 1:2 soil-water suspension in most Indian laboratories. Figure 21 has been found very useful in predicting the approximate gypsum requirements of some indian sodic soils. Similar relationships for groups of like soils may be investigated for estimating the amendment needs of soils.

Figure 21 Relationship between pH of 1:2 soil-water suspension and the gypsum requirements of sodic soils of the Indo-Gangetic plains. Light, medium and heavy refer to soils with a clay content of approximately 10, 15 and 20 percent, respectively. A cation exchange capacity of 10 cmol (+)/kg soil is common for most medium textured soils

Amendments like gypsum are normally applied broadcast and then incorporated with the soil by disking or ploughing. Elgabaly (1971) reported that gypsum mixed with the surface 15 cm was more effective in the removal of exchangeable sodium than gypsum applied on the soil surface. Khosla et al. (1973) found that mixing limited quantities of gypsum in shallower depths was more beneficial than mixing with deeper depths (Table 28). Mixing gypsum in deeper depths resulted in its dilution resulting in lesser ESP decrease throughout the depth. Also when gypsum is mixed to greater soil depths there is greater likelihood that a fraction of gypsum will be used in neutralizing soluble carbonates in the entire 30 cm soil depth at the expense of exchangeable sodium replacement at the shallower soil depth. This will decrease the seed germination rate and consequently the yield (Table 28), when gypsum at 50 percent of the laboratory estimated gypsum requirement of the soil was surface applied, only 1.7% of the soluble carbonates were precipitated compared to 80.8% when gypsum was mixed in the entire soil. This, in turn, resulted in increased exchangeable sodium replacement and therefore higher hydraulic conductivity in the surface application treatment (Table 29). When the problem of exchangeable sodium is only mild, gypsum applied in dissolved form was found more beneficial for the establishment of pasture in comparison to soil application treatments (Davidson and Quirk, 1961).

Table 28 EFFECT OF DEPTH OF MIXING 13 TON GYPSUM PER HECTARE ON THE GRAIN YIELD OF BARLEY, RICE AND WHEAT GROWN IN SUCCESSION IN A HIGHLY SODIC SOIL (Khosla et al., 1973)

Depth of ^1/mixing cm	pH ^2/	ESP ^3/	Grain yield t/ha
Depth of ^1/mixing cm	pH ^2/	ESP ^3/	Barley	Rice	Wheat
10	9.3	25	2.6	7.0	3.3
20	9.5	37	2.5	6.1	3.2
30	9.9	75	0.5	5.6	2.0

Amendment	Application rate as % of GR ^1/	Soluble carbonate precipitated		Hydraulic conductivity cm/day		Exch. Ca me/100 g
Amendment	Application rate as % of GR ^1/	S ^2/	M ^2/	S	M	S	M
Gypsum	50	1.7	80.8	1.41	0.08	6.3	3.4
(CaSO₄ 2 H₂O) Calcium chloride	100	5.5	100.0	9.09	4.65	6.0	6.5
(CaCl₂ 2 H₂O)	100	50.8	100.0	8.70	0.59	6.5	5.6

Because of hazards in handling, the application of sulphuric acid is difficult under ordinary field conditions. However, special equipment is now available in some countries which sprays the concentrated acid on the soil surface. Amendments are sometimes applied in the irrigation water and for this special equipment is also available (Follett et al. 1981).

At mine sites, gypsum is obtained in the form of lumps which require grinding before application in sodic soil reclamation. The fineness to which gypsum must be ground is a matter of economic consideration. Very fine grinding entails higher cost although, based on physico-chemical considerations (Aylmore et al., 1971), it is often maintained that the finer the gypsum particles, the more effective they are likely to be for the reclamation of sodic soils. El Gibaly (1960) carried out laboratory studies to evaluate the relative effectiveness of gypsum passed through different mesh sieves and observed no significant difference in the total sodium removal when a sodic soil was leached with water after mixing it with gypsum passed through 100, 150 and 200 mesh sieves, although the total removal of sodium in these treatments was higher than that with the treatment in which the gypsum passed through a 60 mesh sieve. Studies of Chawla and Abrol (1982) with a highly sodic soil containing free sodium carbonate showed that treatment of soil with very finely ground gypsum resulted in high initial hydraulic conductivity which decreased sharply with time (Figure 22). On the other hand, treatment with gypsum passed through 2 mm mesh and having a range of particle size distribution helped in maintaining permeability at higher level and for a longer period. Their results showed that higher solubility of finer particles caused them to react with free sodium carbonate, inactivating the soluble calcium due to formation of insoluble calcium carbonate.

Figure 22 Hydraulic conductivity of a highly sodic soil as affected by gypsum fineness (Chawla and Abrol. 1982)

These studies suggested that gypsum passed through 2 mm sieve and with a wide particle size distribution was likely to be more efficient for the reclamation of sodic soils having appreciable quantities of sodium carbonate.

Limited solubility of gypsum in water (0.25 percent at 25°C) is sometimes cited as a major drawback of this amendment when rapid reclamation is desired. Similarly, the quantities of water required for sodic soil reclamation are generally calculated on the basis of gypsum solubility in free water. For example, if a sodic soil requires an application of 12.5 tons gypsum per hectare, the quantity of water required to dissolve this quantity will be above 50 cm depth. That this is not likely to be the case in practice was shown by Abrol et al. (1979) and Oster and Frenkel (1980). When gypsum is mixed in a highly sodic soil its solubility increases several fold because of the preference of exchange sites for divalent calcium ions compared to sodium ions. The higher the degree of sodium saturation the greater will be the dissolution of gypsum mixed in soil. Hira et al. (1980 a,b) observed that under 14 cm water were required to dissolve gypsum applied at 12.4 tonnes per hectare and leach the reaction products from the surface 15 cm soil in a highly sodic soil (ESP 94.0). This quantity is only about one-fourth the quantity of water calculated from considerations of solubility in water alone.

In recent years successful efforts have been made to predict the reclamation of sodic soils following application of amendments and leaching with water under controlled conditions (Dutt et al., 1972; Oster and Frenkel, 1980), and computer models have been developed. Testing of these under field conditions and under conditions of group crops will greatly assist in planning large-scale reclamation.

4.4.2 Organic manures

Organic manures have long been known to facilitate the reclamation of sodic soils (Yadav and Agarwal, 1961; Kanwar et al., 1965). The mechanisms involved and the precise reasons for observed responses are not always clear. Dargan et al. (1976) studied the effect of gypsum and farmyard manure singly and in combinations on the yield of berseem and a subsequent rice crop in a highly sodic soil. A strong interacting effect of gypsum and FYM on the yield of berseem appears, at least in part, due to the supply of micronutrients such as Zn, as observed by responses to the subsequent rice crop (Table 30). Puttaswamygowda and Pratt (1973) attributed the beneficial effect of straw incorporated in an sodic soil under submerged conditions to (i) the decomposition of organic matter, evolution of CO₂ and certain organic acids; (ii) lowering of pH and the release of cations by solubilization of CaCO₃ and other soil minerals thereby increasing the EC; and (iii) replacement of exchangeable Na by Ca and Mg and thereby lowering the ESP. Submerged anaerobic conditions were optimum for these processes according to these workers. Similar observations were made by Swarup (1981). In recent studies Gupta et al. (1984) studied the effect of organic materials on the dispersion behaviour of soils and inferred that at high ESP, the role of organic matter in improving soil physical properties was somewhat questionable. However when applied in conjunction with inorganic amendments or when applied in soils of mild sodicity, organic materials have always proved beneficial and therefore their use in the reclamation of sodic soils occupies an important place.

Table 30 EFFECT OF APPLICATION OF FARMYARD MANURE (FYM) AND GYPSUM ON THE YIELD OF BERSEEM AND OF SUBSEQUENT APPLICATION OF ZINC ON THE YIELD OF RICE IN A HIGHLY SODIC SOIL ^1/(Dargan et al., 1976)

Treatment	Berseem yield	Rice
Treatment	t/ha	No Zinc	Zinc ^2/
Control	0.15	5.4	7.2
FYM 25 t/ha	0.83	6.6	7.8
FYM 50 t/ha	1.74	7.7	8.5
Gypsum 11 t/ha	9.49	6.7	8.9
Gypsum 11 t/ha + FYM 25 t/ha	29.48	7.7	9.2
Gypsum 11 t/ha + FYM 50 t/ha	31.89	8.4	8.9

4.5 Crops in sodic soils

Proper choice of crops during reclamation of sodic soils is important. Growing crops tolerant to excess exchangeable sodium can ensure reasonable returns during the initial phases of reclamation or when the crops are grown with irrigation water having a sodicity hazard. Abrol and Bhumbla (1979) reported results of long-term field studies to evaluate the effect of exchangeable sodium on the performance of several field crops. Under field conditions, varying levels of exchangeable sodium were achieved by applying different quantities of gypsum to a highly sodic soil. In these studies gypsum was applied only once initially. Data on actual crop yields as a result of application of different levels of gypsum are presented in Table 31, and Figure 23 (a and b) depicts the relationship between exchangeable sodium percentage and the yield of selected crops. These data bring out that there are wide variations in the tolerance of crops to sodic conditions: rice and dhaincha appear to be tolerant, wheat and bajra are only moderately tolerant and legume crops like mash and lentil are relatively sensitive to excess exchangeable sodium (Table 31). Relative tolerance of rice and wheat are clearly brought out in Figure 24 which shows that at an ESP of about 50 the yield of rice was virtually unaffected, while the wheat crop almost failed at this high ESP (Plates 6 and 7). Based on these and other studies (Chhabra et al., 1979; Singh et al., 1979, 1980, 1981) crops are listed in Table 32 according to their relative tolerance to exchangeable sodium. It has been observed that, generally, crops that are able to withstand excess moisture conditions resulting in short-term oxygen deficiencies are also more tolerant of sodic conditions because the excess exchangeable sodium adversely affects crop growth chiefly through its adverse effect on soil physical properties.

Figure 23 Relationship between exchangeable sodium percentage (ESP) and the yield of selected crops (Abrol and Bhumbla, 1979) (A)

Figure 23 Relationship between exchangeable sodium percentage (ESP) and the yield of selected crops (Abrol and Bhumbla, 1979) (B)

Figure 24 Relative tolerance of rice and wheat crops to exchangeable sodium percentage

Table 32 RELATIVE TOLERANCE OF SELECTED CROPS AND GRASSES TO EXCHANGEABLE SODIUM ^1/(Abrol, 1982)

Tolerant		Semi-tolerant			Sensitive
Karnal grass		Wheat			Cowpeas
	Diplachne fusca			Triticum aestivum		Vigna sinensis
Rhodes grass		Barley			Gram
	Chloris gayana			Hordeum vulgare		Cicer arietinum
Para grass		Oats			Groundnut
	Brachiaria mutica		Avena sativa			Arachis hypogaea
Bermuda grass		Raya			Lentil
	Cynodon dactylon		Brassica juncea			Lens esculenta
Rice		Senji			Mash
	Oryza sativa			Melilotus parviflora		Phaseolus mungo
Dhaincha		Bajra			Maize
	Sesbania aculeata			Pennisetum typhoides		Zea mays
		Cotton			Cotton, at germination
				Gossypium hirsutum		Gossypium hirsutum
Sugarbeet		Berseem			Mung
	Beta vulgaris			Trifolium alexandrinum		Phaseolus aurus
		Sugarcane			Peas
				Saccharum officinarum		Pisum sativum

Tolerance, to ESP and range at which affected	Crops	Growth response under field conditions
Extremely sensitive (ESP = 2-10)	Deciduous fruits	Sodium toxity symptoms even at low ESP values.
	Nuts
	Citrus (Citrus spp.)
	Avocado (Persea americana Mill.)
Sensitive (ESP - 10-20)	Beans (Phaseolus vulgaris L.)	Stunted growth at these ESP values even though the physical condition of the soil may be good.
Moderately tolerant (ESP - 20-40)	Clover (Trifolium spp.)	Stunted growth due to both nutritional factors and adverse soil conditions.
	Oats (Avena saliva L.)
	Tall fescue (Festuca arundinacea Schreb.)
	Rice (Oryza saliva L.)
	Dallisgrass (Paspalum dilatum Poir.)
Tolerant (ESP - 40-60)	Wheat (Triticum aestivum L.)	Stunted growth usully due to adverse physical conditions of soil.
	Cotton (Gossypium hirsutum L.)
	Alfalfa (Medicago sativa L.)
	Barley (Hordeum vulgare L.)
	Tomatoes (Lycopersicon esculentum Mill.)
	Beet, garden (Beta vulgaris L.)
Most tolerant (ESP more than 60)	Crested and Fairway wheatgrass (Agropyron spp.)	Stunted growth usually due to adverse physical conditions of soil.
	Tall wheatgrass (Agropyron elongatum Host Beau.)
	Rhodes grass (Chloris gayana Kunth)

Based upon earlier studies (Bernstein and Pearson, 1956; Pearson and Bernstein, 1958), Pearson (1960) listed crops according to their tolerance to exchangeable sodium (Table 33). A comparison with the listing in Table 32 brings out wide variations in the observed tolerance to exchangeable sodium of some crops. For example, Pearson (1960) rated rice only as a moderately tolerant crop, while crops like wheat, cotton, barley, tomatoes, etc. were rated as tolerant. However, the research results upon which Pearson (1960) based his crop tolerance ratings were obtained with soils whose structure had been stabilized with substances like Krilium or other soil conditioners. The applicability of these results under field conditions, therefore, is questionable. This is because under field conditions crop growth in sodic soils is adversely affected, largely due to the effect of excess exchangeable sodium on the soil physical conditions and it is difficult to visualize conditions in which the soils will have good physical conditions when excess sodium occupies exchange sites. Results of experiments on which crop tolerance ratings in Table 31 are based were largely conducted under field conditions and therefore the tolerance limits are based on the overall effects of exchangeable sodium on the physical and nutritional properties of soils.

4.5.1 Rice as a reclamative crop

The high tolerance of rice to exchangeable sodium arises chiefly because of its ability to withstand, and in fact its need for, a layer of water on the field throughout the growing season. Also, the high pH of sodic soils is reduced under continuous flooding. Thus, Ponnamperuma and his colleagues observed pH to decrease from 8.8 to 7 twelve weeks after flooding. This was ascribed to evolution of large quantities of carbon dioxide from bacterial action and its accumulation because of restricted diffusion of gases in flooded soils (Ponnamperuma, 1965; Ponnamperuma et al. 1966).

The low permeability of sodic soils is a further advantage to rice because losses of water due to deep percolation are restricted, although in most cases they are sufficient to leach soluble salts resulting from the exchange of sodium present in the root zone. These factors make rice an ideal crop during the reclamation of sodic soils and it can enhance the reclamation process considerably. Apart from being tolerant to high sodicity, growing rice results in continuous soil improvement through reduction in soil sodicity.

McNeal et al. (1966) experimenting with sodic soils from Pakistan concluded that rice cultivation indirectly facilitates the removal of exchangeable sodium by increasing the cross-sectional area of conducting pores, resulting in increased permeability. Those workers questioned, however, the rice crop’s appreciable direct effect resulting from the dissolving action of carbon dioxide on the release of calcium from calcium carbonate.

The effect of growing rice on changes in soil sodicity in a field study (Abrol and Bhumbla, 1979) are presented in Table 34. These and other controlled laboratory studies (Chhabra and Abrol. 1977) showed that rice growth resulted in high cumulative removal of exchangeable sodium caused by mobilization of native insoluble calcium carbonate as a result of increased hydrolysis, and CO., liberated by plant roots. Long-term field studies (unpublished) showed that when rice was included in the crop sequence, there was a gradual reduction in sodicity so that in a period of about 10 years, the upper 100 to 125 cm of soil were nearly free of sodicity problems whereas the entire profile was initially highly sodic. Growing rice has an additional advantage that up to 12 to 15 cm of rain water can be stored in the rice fields and this reduces the drainage needs of the sodic soil areas on the one hand and encourages groundwater recharge on the other (Narayana, 1980). In conclusion, its relatively shallow and superficial root system, its high sodicity tolerance and reclamative action together with the need and possibility of storing a large fraction of the rain water makes rice an ideal crop during reclamation of sodic soils.

Table 34 EFFECT OF GROWING RICE ON THE CHANGES IN SOIL ESP (ABROL AND BHUMBLA, 1979)

Treatment ^1/	Exchangeable sodium percentage (ESP)
gypsum	Before rice (April 1971)		After rice (November 1971)
t/ha	0 - 15 cm	15 - 30 cm	0 - 15 cm	15 - 30 cm
0	87.0	94.0	50.0	63.0
7.5	67.0	87.0	29.5	63.0
15.0	33.0	83.0	25.5	56.0
22.5	15.5	64.5	16.0	32.5
30.0	14.0	57.0	12.5	36.0

4.5.2 Grasses

Grasses are, in general, more tolerant of sodic conditions than most field crops. Field and greenhouse studies have shown that Karnal grass (Diplachne fusca), Rhodes grass (Chloris gayana). Para grass (Brachiaria mutica) and Bermuda grass (Cynodon dactylon) are highly tolerant of sodic conditions and can be successfully grown in sodic soils (Ashok Kumar and Abrol 1979, 1983). Karnal grass grows extremely well in soils of very high ESP (80 to 90) even when no amendment is applied. Yield of five grasses in response to three levels of gypsum application in a highly sodic soil (ESP is 90) relative to their respective yields under normal, non-sodic soil conditions (taken as 100) are shown in Figure 25. Karnal grass gave high yield even in the control plots (no gypsum) indicating its high tolerance of sodic conditions (Plate 8). Rhodes grass yielded next highest. Karnal grass and para grass are also highly tolerant to ponded water conditions, typically obtained in sodic soil areas during the rainy season and even after each irrigation. In fact the yield of these two grasses increased with submergence up to 8 days following each irrigation in greenhouse studies (Figure 26) and this factor makes these grasses extremely suitable for sodic soil conditions. When grasses are grown there is a continuous decrease in soil sodicity with time and an improvement in soil physical properties due to the biological action of grass roots. Thus growing tolerant grasses will not only provide much needed forage but also improve the soils resulting in increased absorption of rain water, reduced runoff and soil losses due to erosion. Figure 27 depicts the relative tolerance to exchangeable sodium of a few selected grasses.

Figure 26 Effect of periodic submergence (2, 4 and 8 days) on the relative yield of selected grasses (Ashok Kumar and Abrol, 1983)

4.5.3 Trees

The recent emphasis on the concentration of and need for additional sources of energy has demanded that a sizeable fraction of available land resources be diverted to forestry. Since there is keen competition for good land for producing food crops, there is a greater possibility for utilizing relatively marginal lands for forestry. Sodic soils constitute one such group. Earlier attempts to grow trees in highly sodic soils were largely a failure. Field studies by Yadav et al. (1975), however showed that species like Eucalyptus hybrid, Prosopis juliflora and Acacia nilotica could be grown in highly sodic soils if the seedlings were planted in pits 90 cm deep and 90 cm diameter after the pit soil had been amended with gypsum and manure. More recently, Sandhu and Abrol (1981) demonstrated that if the tree seedlings were planted in auger holes 15 cm diameter and 150 cm deep, filled with a mixture of original soil, 2 kg gypsum and 7 to 8 kg manure, seedlings made excellent growth and there was 100 percent survival. In this technique a favourable environment is created for root growth and penetration; the roots nearly bypass the sodicity and problems of hard subsurface soil layers and proliferate in the zone of continuous moisture availability (Plate 9). Using this technique a large number of auger holes can be made mechanically with a tractor-operated auger (Plate 10). Research to find techniques suited to particular soil, climatic and prevailing socio-economic conditions and a search for better-suited tree species will provide the necessary stimulus for organizing the much needed forestry programmes in such marginal lands.

4.6 Factors influencing tolerance of crops to exchangeable sodium

As in the case of high salinity, tolerance of crops to excess exchangeable sodium is not a fixed property of a species. Several factors influence the tolerance; some of the important ones are given below.

4.6.1 Growth stage

Tolerance of crops changes with the stage of growth. Preliminary experiments showed that the sodicity tolerance of rice increased with age in the initial growth stages and it was found beneficial to transplant somewhat older rice seedlings, 35 to 40 days of age, in sodic soils instead of the usually recommended 30 day old seedlings. It is also, possibly, for this reason that the workers at the US Salinity Laboratory (Pearson, 1960) categorized rice as only a semi-tolerant crop for sodic conditions because in their studies the crop was raised from seeds and not from transplanted seedlings. Many legumes germinate well in sodic soils but their subsequent growth is arrested due to low tolerance. In many small seeded crops germination failures are largely responsible for poor or uneven crop stands. Cotton, a crop considered tolerant of saline conditions, is only moderately tolerant of sodic conditions and relatively sensitive to sodicity at the germination stage. Unlike soluble salts in saline soils, considerable variations in the exchangeable sodium status of soils are not likely to occur during a crop season, although as mentioned in a previous section, ESP tends to decrease during rice growth. For this reason overall crop responses to a specified sodicity are generally more meaningful.

4.6.2 Environmental factors

The relative tolerance of crops to soil sodicity is strongly influenced by the prevailing atmospheric evaporative demand. Rice crops grown under submerged conditions tolerate higher sodicity levels in wet years when the rainfall is well distributed and atmospheric humidity is higher during the crop season than in the dry years when the atmospheric evaporative demand is high. This was attributed to the accumulation of sodium in toxic quantities when the evapotranspiration was high as is seen from data in Table 35 (Singh et al. 1979).

For crops other than rice there is a strong interaction between exchangeable sodium level of the soil and water supply to plants on one hand and the evaporative demand on the other. Acharya et al. (1979) studied the drying pattern of two soils with exchangeable sodium percentages of 4 and 38 in the surface 0-15 cm layer in the field in winter and summer months when the average evaporative demands were 2.5 and 12.5 mm/day, respectively. Their results (Figure 28, a and b) showed that in soils of high ESP, compared to low ESP soils, the supply of water to meet evaporation from the soil surface was limited much more in periods of high evaporation demand than in the periods of low evaporative demand. These studies show that the adverse effect of high ESP on plant growth is likely to be accentuated under conditions of high evaporative demands, e.g. summer months, and this would make water management for crops more demanding.

Figure 28a Soil water contents of the profiles initially (0) and in day 1 to 10 under high evaporative demand for ESP 38 (i) and 4 (ii) soils (Acharya et al., 1979)

Figure 28b Soil water contents of the profiles initially (0) and in days 2 to 27 under low evaporative demand for ESP 38 (i) and 4(ii) soils

Table 35 SODIUM CONTENT OF 30 DAY OLD RICE PLANTS IN A WET AND DRY YEAR AS AFFECTED BY GYPSUM APPLICATION IN A HIGHLY SODIC SOIL (Singh et al., 1979)

Gypsum applied t/ha	Na Content (%)
Gypsum applied t/ha	Wet year	Dry year
0	1.57	2.26
2.5	1.35	2.15
5.0	1.08	2.05
10.0	0.79	1.83

4.6.3 Crop varieties

Even within the same crop there are large variations between crop varieties in their tolerance to sodic soil conditions. Although there have been several studies aimed at identifying genotypes and breeding new crop varieties tolerant of salinity conditions, there appears only limited effort in this direction with regard to sodic soils. Mishra and Bhattacharya (1980) compared the performance of a few tall indica genotypes known for their tolerance to salinity and a high yielding semi-dwarf rice variety IR 8 in soils of varying sodicities in a pot culture experiment. Varieties CSR 1, CSR 2 and CSR 3 though low yielders in normal soils of low ESP levels tended to yield more than the variety IR 8 at very high ESP levels. Some of the observed trends are shown in Figure 29. Since the absolute yield of a crop will be a major consideration for most farmers, it is seen from Figure 29 that over a large sodicity range the improved high yielding varieties would perform better than the relatively tolerant native ones.

Figure 29 Relative (i) and absolute (ii) yields of tolerant native (B) and high yielding dwarf (A) rice varieties in sodic conditions (i)

Figure 29 Relative (i) and absolute (ii) yields of tolerant native (B) and high yielding dwarf (A) rice varieties in sodic conditions (ii)

4.7 Nutrient requirements of crops

High levels of exchangeable sodium and accompanying high pH of sodic soils affect the transformations and availability of several essential plant nutrients (Figure 30). For this reason, optimum crop production in sodic soils calls for special fertilizer management practices compared to soils unaffected by sodicity. Our knowledge of the nutrient relations of crops in sodic soils is limited and generalizations can be made only with caution.

Nitrogen Owing to their low organic matter content, sodic soils are generally deficient in available nitrogen. Further, excess sodium on the soil exchange complex imparts structural instability to the soil giving these soils characteristic, poor physical properties. The infiltration rate of the soils is low and the soils have restricted internal drainage. For this reason the surface soil layers remain nearly saturated for prolonged periods following irrigation or rain resulting in temporary anaerobic conditions. Similarly, between two irrigation cycles, water movement to plant roots from subsurface soil layers is restricted causing the surface soil layers to dry too soon. Thus the surface soil layers experience extremes of the water regime during the crop growth period. Patrick and Wyatt (1964) reviewing the literature on elemental nitrogen losses from soil concluded that losses were likely to be highest under alternate aerobic and anaerobic conditions, a situation exactly met within sodic soils. There is, however, a need to evaluate experimentally the extent of such losses under field conditions. Considerable losses of N in ammonia form due to volatilization are likely to occur in sodic soils due to their high pH (Rao and Batra, 1983). High pH of sodic soils and poor soil physical conditions are also likely to adversely affect the transformations and availability of applied nitrogenous fertilizers. Studies by Nitant and Bhumbla (1974) brought out that with increasing soil pH and sodicity the time required for nearly complete hydrolysis of urea increased from about 3 days in a soil of pH 8.6 (1:2 soil-water suspension) to about 7 days for soil of pH 9.8. Reduced hydrolysis in soils of high sodicity was attributed to the possible effect of high pH on the activity of the enzyme urease or the direct effect of carbonate ions on the formation of ammonium carbonate.

In view of the above factors, crop yields in sodic soils are adversely affected unless additional nitrogen is applied to compensate for losses due to denitrification, volatilization, etc. Dutch work with potatoes (Van Hoorn, 1958) showed that under conditions of poor soil structure, twice as much nitrogen was needed as when under conditions of good soil structure. In a number of field trials at the Central Soil Salinity Research Institute, Karnal, India (Annual Reports 1970 to 1980), responses of rice and wheat grown in sodic soils were studied to levels of applied nitrogen. These studies showed that crops grown in sodic soils generally responded to higher levels of N application compared to crops grown in non-sodic soils but otherwise similar soil and climatic conditions. Based on these results it is generally recommended that crops grown in sodic soils be fertilized at 25 percent excess over the rates recommended for normal soils.

Obrejanu and Sandu (1971) studied the effect of gypsum and nitrogen levels on the yield of sugarbeet in solonetzic soil. Their data (Table 36) show that yields were best with high gypsum and N applications, even though responses to N application were slightly higher without gypsum (and relative responses much higher).

Table 36 SUGARBEET YIELD (t/ha) AS AFFECTED BY LEVELS OF GYPSUM AND NITROGEN IN A SOLONETZIC SOIL (Obrejanu and Sandu, 1971)

Nitrogen level kg/ha	Gypsum level t/ha
Nitrogen level kg/ha	0	8	16
0	21 (100) ^1/	27 (100)	60 (100)
60	31 (147)	35 (129)	65 (108)
120	38 (181)	51 (189)	77 (128)
180	52 (247)	56 (207)	88 (138)

Phosphorus The general trend of phosphorus availability in relation to pH and degree of sodium saturation was shown (Figure 32) by Pratt and Thorne (1947) based on measurements made in clay suspensions. Chhabra et al. (1980) analysed a large number of soil samples from barren sodic soils and reported that these soils generally contained high amounts of extractable phosphorus and that there was a positive correlation between soluble P status and the electrical conductivity of the soil. Presence of sodium carbonate in these soils resulted in the formation of soluble sodium phosphates and hence a positive correlation between electrical conductivity and soluble P status. However, when a soil contains significant amounts of sodium carbonate (and also soluble P) most of the soil calcium is in the calcium carbonate form and not available to the plants resulting in complete crop failures. When an amendment, say gypsum, is applied to improve sodic soils, the soluble sodium-phosphates are converted to less soluble Ca-phosphates. Chhabra and Abrol (1981) observed that crops grown in freshly reclaimed sodic soils did not respond to applied P fertilizers for 4-5 years because of their high available P status. These studies have clearly shown that proper evaluation of the fertilizer needs of crops grown in sodic soils could considerably reduce the cost of crop production in these soils.

Figure 31 Effect of exchangeable sodium (ESP) on the relative dry matter yield as affected by increased N fertilization (Abrol, 1968)

Figure 32 Solubility of phosphate in water from suspensions of bentonite clay of varying levels of sodium saturation (Pratt and Thorne, 1948)

Potassium Several studies have shown that increasing soil sodicity resulted in reduced uptake of potassium by most crops (Singh et al., 1979, 1980, 1981) although the opposite was true for some other crops (Chhabra et al., 1979; Martin and Bingham, 1954).

The significance of reduced uptake of potassium with increasing ESP (Table 37) in K fertilization needs of crops has been investigated in detail. Lack of response to applied K in sodic soils observed in some studies at Karnal was attributed to the presence in the soil of K-bearing minerals which could supply sufficient K to meet the crop requirements (Pal and Mondal, 1980).

Table 37 EFFECT OF EXCHANGEABLE SODIUM ON THE K CONTENT OF SOME PLANTS (Singh et al., 1979, 1980, 1981; Chhabra et al., 1979)

ESP	K % in 30 day old plants
ESP	Safflower	Linseed	Cowpeas	Raya	Sunflower
7.6	3.06	1.66	2.04	3.94	2.24
12.5	2.53	1.56	1.96	3.49	2.46
16.6	1.95	1.40	1.92	3.38	2.63
23.0	1.58	1.23	1.92	2.87	3.02
44.2	1.25	0.95	1.89	2.12	2.64

Calcium Increasing soil sodicity nearly always results in an increased uptake of sodium and decreased uptake of calcium by plants (Table 38). However, as can be seen from the data, with an increase in ESP the increase in sodium concentration of plants is usually much larger compared to the decrease in the calcium concentration. For this reason the plants often accumulate sodium in toxic quantities before the calcium becomes limiting for plant growth. However, when the exchangeable sodium levels are very high, calcium is often the first limiting nutrient, for example when the soils contain appreciable quantities of free sodium carbonate and the soil pH is high such that application of amendments is absolutely necessary.

Table 38 EFFECT OF EXCHANGEABLE SODIUM ON THE SODIUM AND CALCIUM CONTENTS OF SOME 30 DAY OLD PLANTS (Singh et al., 1979, 1980, 1981; Chhabra et al., 1979)

ESP	Safflower		Linseed		Cowpeas		Raya		Sunflower
ESP	Na	Ca	Na	Ca	Na	Ca	Na	Ca	Na	Ca
7.6	1.01	1.36	1.48	0.46	0.16	2.35	0.50	2.98	0.09	2.78
12.5	1.42	1.22	1.53	0.46	0.24	2.33	0.73	2.91	0.10	2.87
16.6	1.85	1.18	1.76	0.44	0.25	2.24	1.00	2.80	0.26	2.68
23.0	2.28	0.88	2.10	0.34	0.32	2.05	1.31	2.35	0.41	2.62
44.2	2.81	0.63	2.40	0.27	0.66	1.72	3.02	1.84	0.52	2.25

Table 39 EFFECT OF LEVELS OF ZINC ON THE YIELD AND COMPOSITION OF RICE GROWN IN AN AMENDED SODIC SOIL (Singh et al., 1982)

ZnSO₄ kg/ha	Yield		Zn content ug/g
ZnSO₄ kg/ha	Dry matter g/4 plants	Grain t/ha	At tillering	Grain	Straw
0	12.99	5.06	13.3	10.9	15.8
10	16.23	6.01	25.1	12.3	20.4
20	16.11	6.04	29.3	12.3	22.3
40	17.44	6.00	34.0	12.6	25.1

Micronutrients High pH, low organic matter content and presence of calcium carbonate strongly modify the availability of micronutrients to plants grown in sodic soils. Zinc deficiency has been widely reported for crops grown in sodic soils (Plate 11) and is accentuated when an amendment is applied to a Zn-deficient sodic soil (Singh et al. 1982). Several field studies have shown significant increase in crop yields due to application of zinc. Field studies by Singh et al. (1982) (Table 39) showed that application of 10 kg ZnSO₄/ha was sufficient to mitigate the deficiency of Zn in rice grown in an amended, highly sodic soil.

Next to zinc, iron is often the limiting micronutrient in sodic soils due to high pH and presence of calcium carbonate. Addition of iron salts to correct the deficiency was generally not useful unless it was accompanied by changes in the oxidation status of the soil brought about by prolonged submergence and addition of organic matter (Katyal and Sharma, 1980). Swarup (1980) showed a marked increase in the extractable Fe and Mn status of a sodic soil upon submergence up to 60 days; the increase was more when organic materials like rice husk or farmyard manure were incorporated in the soil.

Boron and molybdenum are not likely to be limiting elements for plant nutrition in sodic soils. In fact, they are often likely to be present in the toxic range. Kanwar and Singh (1961) observed a positive correlation between water soluble boron and the pH and EC of soils. In a laboratory study Gupta and Chandra (1972) observed a marked reduction in the water soluble boron content of a highly sodic soil upon addition of gypsum. At high pH and sodicity, boron is present as highly soluble sodium metaborate which upon addition of gypsum is converted to relatively insoluble calcium metaborate. Reduced uptake of boron by grasses (Table 40) with decreasing ESP due to gypsum application was also reported by Ashok Kumar and Abrol (1982). As with B, solubility of Mo increases with pH (Pasricha and Randhawa, 1971) and for this reason forage grown on sodic soils is likely to accumulate Mo in excessive quantities, which may prove toxic to the animals feeding on them. Chhabra et al. (1980) and Gupta et al. (1982) studied the effect of sodicity on the solubility of fluoride, an element important from the animal nutrition viewpoint. Water extractable fluoride increased with increasing sodicity and pH (Figure 33), the latter having a more important role in determining the behaviour of fluoride in soils. It was further shown that the F content of plants increased with increasing ESP and decreased with application of P fertilizer (Singh et al. 1979, 1980).

It is seen from the above that the optimum management of nutrients is extremely important for crop production in sodic soils. As our knowledge of the physico-chemical behaviour of nutrients in these soils advances we will be able to manage them better.

Table 40 AVERAGE BORON CONTENT (ppm) OF POUR GRASSES AS INFLUENCED BY LEVEL OF GYPSUM APPLICATION IN A HIGHLY SODIC SOIL (Ashok Kumar and Abrol, 1982)

Grass	Gypsum level (t/ha)
Grass	0	3.12	6.25	9.37	12.50
Bermuda grass	42.8	39.4	35.7	33.2	29.7
Para grass	39.4	37.6	34.7	33.8	30.3
Setaria grass	41.3	35.7	33.8	31.9	29.2
Hybrid Napier	51.3	48.8	46.3	41.9	41.3

Figure 33 Effect of ESP on water extractable fluoride in soil (Chhabra et al., 1980)

Figure 33 Effect of pH on water extractable fluoride in soil (Chhabra et al., 1980)

4.8 Water management

4.8.1 Drainage

The drainage needs of an area and the best methods to meet them are likely to vary with soil and climatic characteristics and the prevailing socio-economic conditions. Drainage of sodic soils presents special problems.

Narayana et al. (1977) summarized the results of studies at the Central Soil Salinity Research Institute, India, to evaluate the drainage needs of representative sodic soil areas. The average annual rainfall of the area is about 700 mm and nearly 90 percent of this is received during the months of July to September. The average annual open pan evaporation is approximately 1 900 mm thus giving a net annual water deficit of about 1 200 mm. Generally this water deficit is experienced in all months except during July and August. Also Narayana (1979) analysed the maximum storm rainfall and the period of dry spells for different return periods for the region. Their data (Table 41) showed that for a five year return period, the maximum two day rainfall was 200 mm, and the length of dry spell 34 days. Such a situation, therefore, calls for a greater emphasis on the conservation of rainfall not only from a drainage point of view, but also to meet the water requirements of crops during the dry spells of the growing season. For such a situation a rainwater management procedure consisting of a three tier system with the following features was recommended (Narayana, 1979).

Table 41 MAXIMUM STORM RAINFALL AND DRY SPELLS OF DIFFERENT RETURN PERIODS IN KARNAL REGION

Similar efforts have been made for the improvement of salt-affected soils in China in recent years. According to You and Wang (1983), in regions where the groundwater is fresh or only slightly saline, well irrigation can simultaneously lower the groundwater level, leach the salts downward from the soil, alleviate the harmful effects of flooding and promote the salt leaching effect by natural precipitation. In regions where the groundwater is highly mineralized, as is the case in areas dominated by saline soils, extraction and drainage of the highly mineralized groundwater by wells can control the groundwater level if the drain water can be removed, e.g., to the sea. Irrigation with fresh water diverted from rivers can then accelerate salt leaching from soil and prevent secondary salinization. However, well drainage cannot replace ditch drainage and, generally, a combination of well drainage and ditch drainage should be adopted to improve the salt-affected soils. Figure 34 schematically describes the effect of pumped well irrigation and drainage on the control of the water table, salinization/sodication, control of drought, etc.

4.8.2 Irrigation

The depth of water to be applied at each irrigation and the time interval between successive irrigations are important considerations in irrigation scheduling. Applying water too frequently or too infrequently can result in reduced crop yields or reduced water use efficiency. The optimum depth of irrigation and the interval between two irrigations are determined by such factors as the atmospheric evaporativity, proliferation and depth of root penetration, capacity of the soil to store and transmit water and the nature of plant responses to soil moisture stress. For crops other than rice, irrigation management presents major difficulties in obtaining optimum crop yields in sodic soils. Excess exchangeable sodium profoundly influences soil water behaviour and therefore the irrigation management needs of these soils.

Figure 35 gives the post-infiltration moisture content distribution with depth in a sodic and a normal soil. It is seen that the moisture content in the sodic soil is about 5 percent less by weight (or nearly 7 to 8 percent by volume) compared to the normal soil. These differences were attributed to restricted rate of water entry at the soil surface in the case of sodic soil, indicating that the total soil water storage capacity is effectively reduced because of restricted entry from the surface. Similarly it was shown that the available water capacity defined by classical concepts (moisture content in the range of field capacity, i.e. 0.1 bar to wilting point, i.e. 15 bars suction), is significantly reduced with increasing exchangeable sodium (Abrol and Acharya, 1975). Thus the storage of water, as also the ability of soils to retain water in an available form, is reduced due to their poor structural status.

Figure 35 Post infiltration moisture distribution in a sodic soil and a normal soil (Abrol and Acharya, 1975)

Following infiltration, the supply of water for meeting the evapotranspirational needs of crops must occur through water movement under unsaturated conditions from lower soil layers. High ESP of subsoil layers could drastically reduce the soil hydraulic conductivity (Acharya and Abrol, 1978) and therefore the ability of soil to transmit water to the growing roots. It was shown by Acharya et al. (1979) that with increasing ESP of the surface soil, the water uptake rate was reduced and the contribution of the lower sodic soil layers was low to practically none when the ESP was high (Table 42). Apart from water transmission to the roots, the root penetration is extremely restricted in sodic soil layers as was shown by Abrol and Acharya (1975).

Table 42 PERCENTAGE OF TOTAL WATER EXTRACTION FROM DIFFERENT DEPTH INCREMENTS IN RELATION TO EXCHANGEABLE Na OF THE SURFACE SOIL (Acharya et al. 1979)

Exchangeable Na % (0-15 cm)	4	11	16	23	38
Rz ^1/ cm/day	0.233	0.241	0.200	0.180	0.141
Depth (cm)	Percent of R_z
0-15	46.4	44.0	67.5	75.0	78.0
15-30	30.0	31.1	28.0	25.0	22.0
30-45	16.3	18.7	4.5	0.0	0.0
45-60	6.4	6.2	0.0	0.0	0.0

Figure 36 Drying pattern of surface soil layers under field conditions in the first 4 days after irrigation, soil of ESP 4

Figure 36 Drying pattern of surface soil layers under field conditions in the first 4 days after irrigation, soil of ESP 38

From these considerations it would appear that for best results in sodic soils, the depth of irrigation should be governed by the infiltration characteristics of the soil rather than the conventional criteria of the soil storage capacity. Quantities of water in excess of what can be absorbed in a few hours could result in an oxygen imbalance in the root zone adversely affecting crop growth. The interval between two irrigations is determined largely by such factors as the atmospheric evaporative demand, proliferation and depth of root penetration and capacity of the soil to store and transmit water. In sodic soils, limited root penetration, lowered capacity to store water in an available form and poor transmission characteristics require that the irrigations are applied more frequently than would be required for crops grown under normal soil conditions. The optimum frequency will depend chiefly on the stage of soil deterioration. For the same reasons Yadav and Girdhar (1973) found sprinkler irrigation better than surface water application because of the ease of applying light and frequent irrigations through this method.

4.8.3 Cultural practices

Figure 37a Effect of thickness of rice husk mulch, 0, 3, 5 and 8 cm (D0, D1, D2 and D3 respectively) on salt distribution in a highly sodic soil. EC was measured in 1:2 soil-water suspension (Dhankar and Abrol, 1973)

Figure 38 Effect of continuous cropping with rice-wheat rotation on soil sodicity over a ten-year period (No Gypsum)

Figure 38 Effect of continuous cropping with rice-wheat rotation on soil sodicity over a ten-year period (Gypsum)

Duration of event	Return period in years
Duration of event	1.01	2.33	5	10	25	100
Maximum 1-day rainfall mm	41	120	152	183	221	282
Maximum 2-day rainfall mm	51	155	201	238	285	355
Maximum 4-day rainfall mm	67	179	228	268	318	394
Maximum dry spell in the rainy season (days)	15	28	34	39	45	54