By Suren Mishra, Ph.D. Tetra Technologies, Inc. 25025 I-45 North The Woodlands, TX 77380 AMMENDMENT OF SALT AFFECTED SOIL INTRODUCTION In general, soil is composed of sand, silt, humus and clays. Sand and silt are siliceous, and are coarse in texture. They are relatively inactive from ionic exchange point of view. On the other hand, humus and clays, which contribute to the fines, are highly surface active particles. They are negatively charged in the aqueous environment. Consequently, they readily attract and hold positively charged ionic species and repel negatively charged ionic species. There are great variations in the amounts of the various elements that are present in the soil and plants. Typical concentrations of essential nutrients in soil that are essential for plants and vegetation are given in Table 1. As can be noted, sodium is not included in this list. In fact, the presence of Na + is particularly detrimental, as it has toxic effect on plants and vegetation. The primary source of Na + in soil is from its salt content. As Na + being monovalent is only weakly adsorbed on the soil particles, it is inefficient in neutralizing their negative charge sites. It, consequently, tends to disintegrate the soil structure and reduces the soil permeability and tilth. This, in turn, adversely affects the seed germination and the overall plant growth. Table 1 Typical Concentrations of Essential Nutrients in Mineral Soils, Annual Plant Uptake, and Ratio of Content in 10 cm Layer of Soil to Uptake (after Bohn et al., 1985) Nutrient Soil Content Annual Plant Soil Content, ( wt%) Uptake (kg/ha) 10 cm layer, to annual uptake Calcium 1 50 260 Potassium 1 30 430 Nitrogen 0.1 30 50 Phosphorus 0.08 7 150 Magnesium 0.6 4 2,000 Sulfur 0.05 2 320 Iron 4.0 0.5 100,000 Manganese 0.08 0.4 3,000 Zinc 0.005 0.3 2,000 Copper 0.002 0.1 1,000 Chlorine 0.01 0.06 200 Boron 0.001 0.03 400 Molybdenum 0.0003 0.003 1,000
Problem of salt, the source of Na +, is prevalent in the arid and semi-arid regions, where runoff water collected in certain areas of land evaporates and the salts in the water accumulate. In these areas, water also moves upward from artesian sources and shallow water tables. Through evaporation, salts are deposited and contribute to the various stages of contamination that are known as saline, saline-sodic, or sodic soils (to be defined later). Salt in the soil can reduce the crop growth and yield in two ways: by osmotic influence, and by specific ion toxicity. The osmotic effects are the processes, which impede the plant growth. Normally, the concentration of solutes in the plant root cells is higher than that in the soil moisture. This allows moisture to move freely into the plant root. However, when the salt concentration in the soil increases, the difference in concentration between the soil moisture and the root cells diminishes. This makes moisture less available to the roots. To prevent salts in the soil moisture from reducing moisture availability to the plant, the plant cells must adjust osmotically. That is, they must either accumulate salts or synthesize organic compounds such as sugars and organic acids. These processes use energy that could otherwise be used for crop growth. The result is smaller plant that may appear healthy in all other aspects. Some plants adjust more efficiently, giving them greater salinity tolerance. Specific ions in this case are Na + and Cl -. While Na + is toxic to the plant and vegetation, plants have greater degree of tolerance for Cl -. In fact, as discussed earlier, Cl - is considered essential micronutrient to most of the crops. Table 2 contains estimates of the maximum allowable chloride concentrations of saturated soil extract for various fruit crop cultivars and root stocks. To overcome the problem of these specific ions, it is important that they are displaced from the soil structure. Table 2 Chloride Tolerance Limits of Some Fruit Crop Cultivars and Rootstocks (after Hanson et al, 1993) Crops Cl - (mg/l) Rootstocks Avocado 178-266 Grapefruit 888 Oranges 355-533 Grapes 1,065-1,420 Cultivars Berries 178-355 Grapes 355-710
ION EXCHANGE IN SOIL Ion exchange occurs in soils at the mineral surfaces, organic matters, and roots. Cationic exchange is generally considered to be more important, since the anion exchange capacity of most soil is comparatively much smaller than the cationic exchange capacity. Anionic exchange capacity (AEC) of a soil increases with ph; and it is much greater in soils containing kaolinitic clays and those which contain oxides of Fe and Al than those soils that contain montmorillonitic clays. Anions such as Cl - and NO 3 - may be adsorbed into the soil system, although not to the extent of H 2 PO 4 - and SO 4 2- (Borrow, 1985). The order of adsorption strength is as follows: H 2 PO 4 - > SO 4 2- > NO 3 - = Cl - In fact, in most soils, H 2 PO 4 - is the primary anionic species that adsorb. Some acidic soils may adsorb significant quantities of SO 4 2-. In most cases, the anionic adsorption by soil particles can be attributed more to specific or chemisorsption than simple Coulombic adsorption due to counter ionic interactions. The major exchangeable cations in soil are Al 3+, Ca 2+, Mg 2+, K +, Na + and H +. Other exchangeable cations, which are minor components of soil but have nutrient values, are Cu 2+, Fe 3+, Zn 2+, Mn 2+ and NH 4 +. Among these, Ca 2+, Mg 2+, K + and Na + are the dominant exchangeable cations in most soils. Their abundance generally parallels the energy of the adsorption sequence, with Ca 2+ most abundant and Na + or K + are the least abundant. Valance is the major factor affecting the likelihood that a cation can be adsorbed. A trivalent Al 3+ is more likely to adsorb than a divalent Ca 2+, which is more strongly adsorbed than a monovalent Na +. However, for cations of similar valance, smaller cations with greater charge density per unit volume adsorb more strongly. The diameter of these ions includes the water molecules that are adsorbed onto the them, forming a hydrated layer. It is the hydrated diameter of the cations that are involved in the adsorption at the particle surfaces. For this reason, Ca 2+ with hydrated ionic radius of 6A adsorbs with a much stronger energy than Mg 2+ with hydrated radius of 8A. Similarly, NH 4 + or K + with their respective hydrated radii of 2.5A and 3A adsorbs with much stronger energy than Na + with hydrated ion radius of 4A. The general order of selectivity or replaceability of some important exchangeable hydrated cations (hydrated radius of each ion is given in parenthis) is as follows: Al 3+ (9A) > Ca 2+ (6A) > Mg 2+ (8A) > NH 4 + (2.6A) > K + (3A) > Na + (4A)
Among the above exchangeable hydrated cations, Al 3+ though has the largest hydrated radius but due to its higher valence can adsorb most strongly. However, at ph values higher than 4.0, there is drastic reduction in concentration of free Al 3+ ions in the soil environment. At the soil ph > 5.0 at which any vegetation could sustain, aluminum electrolyte can not be used for soil amendment. Consequently, calcium is the most effective ions that could be used for soil amendment. Cationic Exchange Capacity (CEC): The cationic adsorption capacity of a soil is expressed in terms of its Cationic Exchange Capacity (CEC), which represents the total quantity of negative charge available to attract positively charged ions in solution. It is expressed in terms of milliequivalent of negative charge per 100 g of oven dried soil (meq/100g). The CEC also represents the total meq/100 g of cations held on the negative charge sites of the soil particles. The CEC of a soil is strongly influenced by the nature and amount of mineral and organic compounds present. Soils containing high clay and organic matters have higher cationic exchange capacity than sandy soils, which are low in organic matter content. The CEC of soil with different textures are given in Table 3. Table 3 CEC Values of Different Soil Textures Soil Textures CEC (meq/100 g) Sands (light colored) 3 5 Sands (dark colored) 10 20 Loams 10 15 Silt Loams 15 25 Clay and Clay Loams 20 50 Organic Soils 50 100 It is evident that a soil with high clay content or organic matters has high CEC value, and will tend to adsorb a larger amount of cationic species. However, the capacity for the adsorption of cations will also depend on the base saturation of the soil, which is defined as the percentage of total CEC occupied by basic cations, e.g. Ca 2+, Mg 2+, K + and Na +. The percentage base saturation is expressed as follows: % Base Saturation = (total bases/cec) * 100 (1) where total bases and CEC are in terms of meq/100 g. As the presence of Na + in the soil structure is critical to its fertility, it is important that they are replaced by more desirable cations. There is a direct relationship between the Exchangeable Sodium Percentage (ESP) of the soil and its CEC value. This can be expressed as follows:
ESP = (Na Ex / CEC) * 100 (2) Where Na Ex and CEC are in terms of meq/100 g ESP can also be estimated by the following empirical relationship based on the Sodium Adsorption Ratio (SAR) of the soil: ESP = 100 (-0.0126 + 0.01475 SAR) / [1 + (-0.0126 + 0.01475 SAR)] (3) SAR is calculated as follows: SAR = Na / [ (Ca + Mg) / 2 ] 0.5 (4) Where the concentrations of Na, Ca and Mg are expressed in meq/l In equation (4) K + is not included, as in most soil its presence is in a minute quantity and can be considered negligible for the calculation of SAR value. It should be noted that the degree of dispersion of a soil depends on its mineralogical composition, in particular on its clay content. The clay component of the soil can be classified into two broad categories: kaolinite and montmorillonite. A soil with high kaolinitic clays and with lower cationic exchange capacity may require as high as 40% Na saturation in its cationic exchangeable structure. When compared against a soil with high montmorillonitic clays with a high cationic exchange capacity that may require as low as only 15% Na saturation before the soil is dispersed. However, in general, soils with low clay content are subjected to less of problems as they have a low cationic exchange capacity and are also more water and air permeable. EFFECT OF SOIL ELECTROCHEMISTRY ON VEGETATION Soil contaminated with sodium can be classified into three groups: saline, sodic and saline/ sodic. Saline Soils have saturated Extract Conductivity (EC se ) > 4 mmhos (or ms)/cm, ph < 8.5, and ESP < 15 %. The concentration of soluble salts in such soil is sufficient to interfere with vegetation or plant growth. However, when adequate drainage is established, the excessive soluble salts may be removed by leaching and the soil again becomes normal. Saline soils are often recognized by the presence of white crusts of salts on the surface. Sodic Soils occur when ESP > 15 %, EC se < 4 mmhos/cm, and ph > 8.5. In the sodic soil, the excess of sodium disperses the colloidal fraction and its presence creates nutritional disorder in most plants. In contrast to saline soil, when salts are leached out, the exchangeable sodium hydrolyses and the ph increases, which results in a sodic soil. In this case, the EC se value is lower that the saline soil as the free salt is leached out.
A combination Saline/ Sodic Soil has high salt concentration with EC se > 4 mmhos/cm and high ESP of > 15 %. However, it has ph < 8.5. Although the return of the soluble salts may lower the ph, the management of this soil continues to be a problem until the excess salts and exchangeable sodium are removed from the plant/ vegetation root zone and a favorable physical condition of the soil is restored. The high EC se value of soil due to high salt concentration increases the osmotic pressure of the soil solution. As the plant or vegetation roots in contact with this soil solution do not possess the ability to overcome the osmotic pressure of the soil, they die from the lack of water (Heywood and Wadleigh, 1949). A statistical correlation between osmotic pressure and EC se does exist. This means that easily measurable EC se allows estimation of the potential damage for crops from increased osmotic pressure of the soil solution. The uptake of salts from contaminated soil disrupts nutrient uptake and utilization. Consequently, excess salt in the soil solution reduces the availability of essential plant nutrients, resulting in drop in plant yields and in many cases total devastation. There is no one threshold salinity level of all plants or vegetation. Generally, vegetables are more sensitive to salts than grains and grasses. A general crop tolerance levels to salt has been developed by the US Salinity Laboratory (1954) as given in Table 4, which indicates no effect on the plant yields at EC se lower than 2 mmhos/cm. Table 4 General Crop Response as a Function of EC se EC se (mmhos/cm) Effect on Crop Yield 0 2 None 2 4 Slight 4 8 Many crops affected 8 16 Only tolerant crops yield well > 16 Only very tolerant crops yield well At higher EC se value than 16 mmhos/cm, either there is drastic reduction in plant yield or there is a complete devastation. Sodicity or ESP of the soil also influences the crop yield, as shown in Table 5. Table 5 Reduction in Crop Yield as a Function of Soil ESP Type of Soil ESP Average % Reduction in Crop Yield Slightly Sodic 7 15 20 40 Moderate Sodic 15 20 40 60 Very Sodic 20 30 60 80 Extremely Sodic > 30 > 80
TREATMENT OF SOIL Soil that has been exposed to salt, either through natural source as discussed earlier or through intentional means as is the case when brines from mining or oil & gas industrial sources as disposed of as the waste material, goes through different phases. The soil freshly exposed to the brine solution is at the saline state, when majority of salt is still in the free state. With time, when the soil is frequently irrigated naturally or artificially, the free salt solution drains to the lower level of the soil cross section. However, due to ion exchange phenomenon, the cationic Na + species are adsorbed into the soil structure. Cl - species of the salt, on the other hand, do not adsorb at the like charge surface of the soil structure. Substantial portion of Cl - species drains to a lower level and remain free to be drained to even lower level of the cross section, if additional irrigation is applied. However, a significant portion of Cl - species can be mechanically entrapped in the soil structure. This mechanical entrapment is further compounded by the fact that due to the adsorption of Na + soil becomes dispersed. The soil at this stage, as defined earlier, is considered sodic. The ESP value of such soil will determine the degree of its sodicity. As discussed earlier, with subsequent exposure to salt, the soil may reach a complex state of sodic/ saline type. It is important that the fates of Na + and Cl - species in the soil structure are clearly understood. The general chemical environment of most of natural soils is such that Cl - species do not adsorb at the particles. They remain free. However, Na + species do adsorb at the predominantly negatively charged surface of the soil particles. Once the soil that is dispersed due to the adsorption of Na + species can be destabilized through amendment, its permeability and, therefore, drainage property improves. This results in the removal of Cl - species that are mechanically entrapped in the soil structure to further lower level. If adequate amount of soil amendment and irrigation are applied, both Na + species that liberated due to ion exchange phenomenon and the mechanically liberated Cl - species can be removed from the soil section to a desired level. Soil at the root zone, thus, can be restored to the state of fertility. Amendment of salt affected soil through cationic exchange is postulated in Figure 1. Due to the adsorption of Na+ species the soil particles, particularly the clay fractions, are dispersed. The parallel plate like arrangement of the dispersed clay particles impedes the movement of moisture in the soil structure. As shown in Figure 1(A), the Na + while is adsorbed into the clay structure its counter ion Cl - remains free but entrapped in the dispersed clay system. At this stage, both Na + and Cl - in the soil would be measured to be high. Once cationic additives are added to the soil for amendment, the replacement of the adsorbed Na + species in the clay structure with more preferred cations would liberate Na + from the system. The adsorption of the preferred cations aggregates otherwise
dispersed particles. This results in better drainage of moisture in the soil. With adequate irrigation, respectively chemically and mechanically liberated Na + and Cl - from the system will drain to a lower level, as shown in 1(B). Removal of these species from the plant or vegetation root zone restores the fertility of soil. At this stage, if Na + and Cl - were measured at the root zone, significant reduction in their concentrations would be noted. Na + Na + Ca 2+ Ca 2+ Ca 2+ Cl - Na + Cl - Cl - Na + Cl - Na + A: Sodium Affected Dispersed Soil B: Ca 2+ Amended Open Soil Structure Figure 1: Soil Amendment Postulate As discussed earlier, calcium remains to be a preferred cationic species for the amendment of salt affected soil. There are a number of calcium compounds that are available for use. However, their effectiveness is based on the amount of calcium that could be available for cationic exchange in the soil structure. The availability of calcium in these compounds depends on their water solubility. Limestone, lime, gypsum, and dolomite though are frequently used for soil amendment, yet their effectiveness is grossly limited by their poor water solubility. Consequently, the amount of these materials that is required to achieve the required degree of amendment can be restrictive. In many areas, where there are low rainfalls and artificial irrigation is the only source of water, use of these materials for soil amendment is grossly ineffective. Calcium chloride, due to its high water solubility is a preferred additive for soil amendment. The effectiveness of these additives can be compared in terms of their solubility products that determines their water solubility, as shown in Table 6. Table 6 Solubility Products of Some Soil Amendment Additives with Calcium Source (at 25 o C) Amendment Solubility Water Solubility Additives Product (K sp ) (g/100g) Limestone, CaCO 3 4.95 x 10-9 0.0007 Lime, Ca(OH) 2 7.88 x 10-6 0.147 Gypsum, CaSO 4 1.2 x 10-6 0.0149 Dolomite*, CaMg(CO 3 ) 2 5.26 x 10-6 0.0423 Calcium Chloride**, CaCl 2 8,883.66 82.0 *Calculated based on the K sp of CaCO 3 and MgCO 3 ** Calculated using the solubility data on CaCl 2
CALCIUM CHLORIDE TREATMENT OF SOIL As can be noted in Table 6, among the relatively insoluble materials, lime is most soluble. However, its use is restricted to acidic soil as its use in the alkaline or moderately acidic soil would render the soil undesirably alkaline. Although, gypsum is more forgiving, its solubility being low limits its effectiveness in soil amendment. When compared against these additives, the solubility of calcium chloride is tremendously higher and, therefore, is preferred in soil amendment. Following are some field test examples of using calcium chloride based additive in the treatment of soil that had been exposed to salt brine. In a West Texas location at Lamesa, the sites were located in cotton and peanut fields. These areas were exposed to underground pipeline leaks that in certain areas killed the existing crops. The salt damage ranged from 1 ft to 2 ft depth. At the time of soil amendment with the calcium chloride based additive, tilling or disking was not possible due to the living crops surrounding the affected areas. Water from the existing irrigation system was used for the dilution and application of the additive. Rainwater was the only source of post treatment irrigation of the treated soil. Test results are summarized in Table 7. Table 7 Remediation of Salt Contaminated Sites at Lamesa, TX (after Mishra et al., 1999) Location General Na (mg/kg) % Na Soil Type 4/2/96 8/2/96 Reduction 1 Clayey Loam 1660 173 89.5 2 Clayey Loam 750 218 70.9 3 Clayey Loam 1770 860 51.4 4 Clayey Loam 530 220 58.4 5 Clayey Loam 3370 68 97.9 As can be noted in Table 7, at these locations, in amount four months after soil was treated the sodium content of the soil was reduced by 58 to 97%. Crops were completely restored by the following growing season. Test work in a grassland area in Houston, Texas, where salt damage occurred from pipeline leaks. The sites were treated with calcium chloride based additive. For the dilution and application of the additive water was brought in from an outside source. However, post treatment irrigation was primarily from the natural rainfalls. No tilling or disking was possible due to thick brush and vegetation surrounding these areas. As shown in Table 7, in about five weeks after the site was treated with the calcium chloride based additive, sodium levels at these locations were reduced by 81-85%. Grass seeds were
applied after one week of the site treatment. Total vegetation of the site was restored in approximately four months. Table 8 Remediation of a Salt Contaminated Site in Houston, Texas (after Mishra et al., 1999) Location General Na (mg/kg) % Na Soil Type 10/1/96 11/7/96 Reduction 1 Clayey Loam 842 120 85.7 2 Clayey Loam 1190 218 81.7 The high rate of soil restoration by calcium chloride can be demonstration by the following example. In this case, the salt spillage occurred due to a storage tank into a pastureland. As the site was a pastureland, tilling was possible at the time of additive applications. Similar to other sites discussed above, the soil of this site was also classified as clayey loam. In this case, following the application of chemical, the site was immediately irrigated with water. To evaluate the quick action of the calcium chloride amendment, in this case, a set of samples was collected after only three hours of treatment. These samples were taken from the top 6-in. of the surface and were analyzed for sodium content and for SAR and ESP values. Test results are summarized in Table 9, which indicate that within a short period of three hours there were significant reduction in the sodium content, SAR and ESP values of the treated soil. This behavior suggests rapid rate of interaction between the cationic calcium chloride additive and the soil system. Table 9 Remediation of a Salt Contaminated Site at Kilgore, Texas (after Mishra et al., 1999) Sample Na (mg/kg) SAR ESP% Before After %Red. Before After %Red. Before After %Red 1 5626 1950 65 44 14.5 67 75.5 4.6 94 2 3993 1516 62 27.5 2.6 91 41.3 8.7 79 CONCLUDING REMARKS Problems associated with salt affected soil are primarily attributed to the Na + and not to the Cl -. Adsorption of Na + in the soil structure, mainly the clays, while disperses and drastically impeded the moisture permeability of the soil, it is also toxic to the plants and vegetation. Most effective method of amending such soil is through the replacement of undesirable Na + with another cations that functions as nutrient to the plants and vegetation. Ca 2+ is the most effective cations for this role. As water is the workhorse for the cationic exchange process in the soil, solubility of the calcium-containing chemical is critical to its effectiveness. Calcium chloride being highly water-soluble is the most effective chemical additive that is used for the amendment of the salt affected soil.
REFERENCES: Bohn, H.L., B.L. McNeal and G.A. O Connor, 1985. Soil Chemistry, John Wiley & Sons, New York Grattan, Stephen R., 1993. Sodium and Chloride Toxicity in Plants, Agricultural Salinity and Drainage: A Handbook for Water Managers, (eds: Hanson, B., Stephen R. Grattan and Allan Fulton) University of California Irrigation Program, University of California, Davis, pp. 11-13. Haywood, H.E. and C.H. Waldeigh, 1949. Plant Growth on Saline and Alkali Soils, Advance in Agron., Agron. Soc. of America, Madison, WI, Volume 1, pp. 1-38. Mishra, Surendra K., William J. Evans and John I. Lacik, 1999. Treatment of Salt Affected Soil in the Oil Field, Proceedings of SPE/EPA 1999 Exploration and Production Environmental Conference, 103 March 1999, Austin, Texas, pp. 145-153. U.S. Salinity Laboratory Staff, 1954. Diagnosis and Improvement of Saline and Alkali Soil, Agric. Handbook No. 60, USDA, US Government Printing Office, Washington DC.