NATURE OF SODIC SOIL, GYPSUM REQUIREMENT, EFFECT ON PLANT GROWTH AND MANAGEMENT
Soils containing sodium as a significant proportion of their total exchangeable cations. Sodic soils tend to have poor drainage due to poor sloi structure.
i. Sodic soils - Soils containing sodium salts capable of alkaline hydrolysis, mainly Na2CO3, these soils have also been termed as ‘Alkali’ in older literature.
ii.Saline soils - Soils containing sufficient neutral soluble salts to adversely affect the growth of most crop plants. The soluble salts are chiefly sodium chloride and sodium sulphate. But saline soils also contain appreciable quantities of chlorides and sulphates of calcium and magnesium.
Nature of Sodic Soils
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 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.
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.
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.
Sodic soils and plant growth
Plant growth is adversely affected in sodic soils due to one or more of the following factors:
i. Excess exchangeable sodium in sodic soils has a marked influence on the physical soil properties. As the proportion of exchangeable sodium increases, the soil tends to become more dispersed which results in the breakdown of soil aggregates and lowers the permeability of the soil to air and water (Figure 20). Dispersion also results in the formation of dense, impermeable surface crusts that hinder the emergence of seedlings.
ii. A second effect of excess exchangeable sodium on plant growth is through its effect on soil pH. Although high pH of sodic soils has no direct adverse effect on plant growth per se, it frequently results in lowering the availability of some essential plant nutrients. For example, the concentration of the elements calcium and magnesium in the soil solution is reduced as the pH increases (Table 22) due to formation of relatively insoluble calcium and magnesium carbonates by reaction with soluble carbonate of sodium, etc. and results in their deficiency for plant growth. Similarly, the solubility in soils and availability to plants of several other essential nutrient elements, e.g. P, Fe, Mn and Zn, are likely to be affected as will be discussed in a later section.
iii. Accumulation of certain elements in plants at toxic levels may result in plant injury or reduced growth and even death (specific ion effects). Elements more commonly toxic in sodic soils include sodium, molybdenum and boron.
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.
EXCHANGEABLE SODIUM PERCENTAGE (ESP) AND SODICITY HAZARD
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
Reclamation and management
II. Organic manures
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.
A. Kind of amendments
Chemical amendments for sodic soil reclamation can be broadly grouped into three categories:
a. Soluble calcium salts, e.g. gypsum, calcium chloride.
b. Acids or acid forming substances, e.g. sulphuric acid, iron sulphate, aluminium sulphate, lime-sulphur, sulphur, pyrite, etc.
c. Calcium salts of low solubility, e.g. ground limestone.
The suitability of one or another amendment for sodic soil reclamation will largely depend on the nature of the soil and cost considerations. Ground limestone, CaCO3, is an effective amendment only in soils having pH below about 7.0 because its solubility rapidly decreases as the soil pH increases (Table 22). It is apparent that the effectiveness of limestone as an amendment is markedly decreased at pH values above 7.0. Some soils that contain excess exchangeable sodium also contain appreciable quantities of exchangeable hydrogen and therefore have an acidic reaction, e.g. degraded sodic soils
However, lime is not an effective amendment for most sodic soils as their pH is always high. In fact, sodic soils contain measurable to appreciable quantities of sodium carbonate which imparts to these soils a high pH, always more than 8.2 when measured on a saturated soil paste, and up to 10.8 or so when appreciable quantities of free sodium carbonate are present. In such soils only amendments comprising soluble calcium salts or acids or acid-forming substances are beneficial. The following chemical equations illustrate the manner in which some of the amendments react in these soils.
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:
Na2CO3 + CaSO4 Û CaSO3 + Na2SO4 (leachable)
Ø 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:
Na2CO3 + CaCl2 Û CaCO3 + 2 NaCl (leachable)
Ø Sulphuric acid
Sulphuric acid is chemically H2SO4. It is an oily corrosive liquid and is usually about 95 percent pure. Upon application to soils containing calcium carbonate it immediately reacts to form calcium sulphate and thus provides soluble calcium indirectly. Chemical reactions involved are:
Na2CO3 + H2SO4 Û CO2 + H2O + Na2SO4 (leachable)
CaCO3 + H2SO4 Û CaSO4 + H2O + CO2
Ø Iron sulphate and aluminium sulphate (alum)
Chemically these com-pounds are FeSO4.7H2O and Al2(SO4)3.18H2O respectively. Both these solid granular materials usually have a nigh degree of purity and are soluble in water. When applied to soils, these compounds dissolve in soil water and hydrolyse to form sulphuric acid, which in turn supplies soluble calcium through its reaction with lime present in sodic soils. Chemical reactions involved are:
FeSO4 + 2H2O Û H2SO4 + Fe (OH)2
Ø 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:
2 S + 3 O2 ® 2 SO3 (microbiological oxidation)
Pyrite (FeS2) is another material that has been suggested as a possible amendment for sodic soil reclamation. Reactions leading to oxidation of pyrite are complex and appear to consist of chemical as well as biological processes.EQUIVALENT QUANTITIES OF SOME COMMON AMENDMENTS FOR SODIC SOIL RECLAMATION
Relative quantity 1/
| Gypsum (CaSO4 2H2O) || |
| Calcium chloride (CaCl2 2 H2O) || |
| Sulphuric acid (H2SO4) || |
| Iron sulphate (FeSO4.7 H2O) || |
| Aluminium sulphate (Al2 (SO4)3.18 H2O) || |
| Sulphur (S) || |
| Pyrite (FeS2) - 30% sulphur || |
| Calcium polysulphide (CaS5) - 24% sulphur || |
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.
B. Quantity of amendment
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
= 86 x 10-5 kg/kg soil
= 86 x 10-5 x 2.24 x 106 kg/ha
= 1926 kg or 1.96 t/ha
If it is desired to replace greater quantities of adsorbed sodium, the quantity of gypsum can be accordingly increased.
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.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
II. 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.
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)
| Control || |
| FYM 25 t/ha || |
| FYM 50 t/ha || |
| Gypsum 11 t/ha || |
| Gypsum 11 t/ha + FYM 25 t/ha || |
| Gypsum 11 t/ha + FYM 50 t/ha || |
2/ Zinc was applied as zinc sulphate at 45 kg/ha before planting rice.