Reclamation Procedures

The term "salty" generally refers to a soil that contains sufficient salts to impair its productivity.

There are several methods of soil analysis to determine the salt content. Most of them, however, are insufficient for characterizing salinity status because the influence of the salts on the crop growth depends on many factors. These include the composition and properties of the different soil layers and horizons, seasonal variations of the capillary zone and the depth of groundwater, soil temperature and its seasonal fluctuations, and the stage of development of crops. Moreover, salinization is a dynamic process, changing with time and varying over short distances in the field and along the soil profile.

The system developed by the U.S. Salinity Laboratory is currently the most commonly used for the classification of salty soils [24]. The diagnostic parameters in this system follow:

• The electrical conductivity of the saturated soil extract, ECe (dS/m), is a temperature-sensitive measurement and therefore is standardized at 25°C.

• The exchangeable sodium percentage (ESP) can be written as

where adj RNa (adjusted sodium adsorption ratio) is defined as [24]

in which Na and Mg are the concentrations (in me/L) of sodium and magnesium in the soil extract and Cax is a modified value of the calcium concentration, which takes into account the effects of carbon dioxide (CO2), bicarbonate (HCO3), and salinity of the soil water (ECTO) upon the calcium originally present. Values for Cax are given in Table 4.5.

With reference to the reclamation procedures, the salty soils have been separated into the following three groups [25]: saline, saline-sodic, and sodic.

Saline Soils

Saline soils are those for which the conductivity of the saturated soil extract ECe > 4 dS/m at 25°C and ESP < 15. Ordinarily, the pH is less than 8.5.

The chemical characteristics of saline soils are the following:

• Sodium seldom comprises more than half of the soluble cations and hence is not adsorbed to any significant extent.

• The amounts of calcium and magnesium may vary considerably.

• Soluble and exchangeable potassium are ordinarily minor constituents, but sometimes they may appear as major constituents.

• The main anions are chloride, sulfate, and nitrate.

Because of the presence of excess salts and the absence of significant amounts of exchangeable sodium, saline soils generally are flocculated and therefore have permeabilities similar to those of nonsaline soils.

Table 4.5. Modified calcium concentration Cax (meq/L) as function of electrical conductivity of soil water and of ratio of bicarbonate to calcium

. IT a. Electrical Conductivity of Applied Water, EC„, (dS/m)

Calcium (meq/L3) 0.1 0.2 0.3 0.5 0.7 1.0 1.5 2.0 3.0 4.0 6.0 8.0

0.05 13.20 13.61 13.92 14.40 14.79 15.26 15.91 16.43 17.28 17.97 19.07 19.94

0.10 8.31 8.57 8.77 9.07 9.31 9.62 10.02 10.35 10.89 11.32 12.01 12.56

0.15 6.34 6.54 6.69 6.92 7.11 7.34 7.65 7.90 8.31 8.64 9.17 9.58

0.20 5.24 5.40 5.52 5.71 5.87 6.06 6.31 6.52 6.86 7.13 7.57 7.91

0.25 4.51 4.65 4.76 4.92 5.06 5.22 5.44 5.62 5.91 6.15 6.52 6.82

0.30 4.00 4.12 4.21 4.36 4.48 4.62 4.82 4.98 5.24 5.44 5.77 6.04

0.35 3.61 3.72 3.80 3.94 4.04 4.17 4.35 4.49 4.72 4.91 5.21 5.45

0.40 3.30 3.40 3.48 3.60 3.70 3.82 3.98 4.11 4.32 4.49 4.77 4.98

0.45 3.05 3.14 3.22 3.33 3.42 3.53 3.68 3.80 4.00 4.15 4.41 4.61

0.50 2.84 2.93 3.00 3.10 3.19 3.29 3.43 3.54 3.72 3.87 4.11 4.30

0.75 2.17 2.24 2.29 2.37 2.43 2.51 2.62 2.70 2.84 2.95 3.14 3.28

1.00 1.79 1.85 1.89 1.96 2.01 2.09 2.16 2.23 2.35 2.44 2.59 2.71

1.25 1.54 1.59 1.63 1.68 1.73 1.78 1.86 1.92 2.02 2.10 2.23 2.33

1.50 1.37 1.41 1.44 1.49 1.53 1.58 1.65 1.70 1.79 1.86 1.97 2.07

1.75 1.23 1.27 1.30 1.35 1.38 1.43 1.49 1.54 1.62 1.68 1.78 1.86

2.00 1.13 1.16 1.19 1.23 1.26 1.31 1.36 1.40 1.48 1.54 1.63 1.70

2.25 1.04 1.08 1.10 1.14 1.17 1.21 1.26 1.30 1.37 1.42 1.51 1.58

2.50 0.97 1.00 1.02 1.06 1.09 1.12 1.17 1.21 1.27 1.32 1.40 1.47

3.00 0.85 0.89 0.91 0.94 0.96 1.00 1.04 1.07 1.13 1.17 1.24 1.30

3.50 0.78 0.80 0.82 0.85 0.87 0.90 0.94 0.97 1.02 1.06 1.12 1.17

4.00 0.71 0.73 0.75 0.78 0.80 0.82 0.86 0.88 0.93 0.97 1.03 1.07

4.50 0.66 0.68 0.69 0.72 0.74 0.76 0.79 0.82 0.86 0.90 0.95 0.99

5.00 0.61 0.63 0.65 0.67 0.69 0.71 0.74 0.76 0.80 0.83 0.88 0.93

7.00 0.49 0.50 0.52 0.53 0.55 0.57 0.59 0.61 0.64 0.67 0.71 0.74

10.00 0.39 0.40 0.41 0.42 0.43 0.45 0.47 0.48 0.51 0.53 0.56 0.58

20.00 0.24 0.25 0.26 0.26 0.27 0.28 0.29 0.30 0.32 0.33 0.35 0.37

30.00 0.18 0.19 0.20 0.20 0.21 0.21 0.22 0.23 0.24 0.25 0.27 0.28

Reclamation techniques normally depend on suitable irrigation water, appropriate means of water application, and good drainage. Large quantities of water with low salt content are needed to leach salts from the soil profile. Surface basins and sprinkle irrigation are usually appropriate because these apply water on the entire soil surface. Salttolerant vegetation can help to reclaim saline soils, especially those with fine textures. Plant roots help to keep the soil permeable and the top growth prevents erosion. Good drainage is required to remove the leaching water fast enough and to keep groundwater levels from rising. Sometimes leaching is done intermittently both to save water and to give more time for drainage.

Reclamation requires that land be well managed afterward. The water table has to be kept low enough to keep from making the soil saline again. Suitable amounts of irrigation water have to be applied to let drainage remove the salt brought in by the irrigation water.

Saline-Sodic Soils

The term "saline-sodic" is applied to soils for which ECe > 4 dS/m at 25°C and ESP >15. Normally, the pH is seldom higher than 8.5. These soils form as a result of the combined processes of salinization and sodification.

As the concentration of salts in the soil solution decreases, some of the exchangeable sodium hydrolyzes and forms sodium hydroxide. This salt may change to sodium carbonate on reaction with carbon dioxide absorbed from the atmosphere. Upon leaching, the soil may become strongly alkaline, and after the consequent dispersion of the fine particles, unfavorable for the entry and movement of water and for tillage. Although the return of the soluble salts may help particles to flocculate, the management of saline-sodic soils continues to be a problem until the excess salts' exchangeable sodium is removed from the root zone.

When gypsum is added to saline-sodic soils and such soils are leached, the excess salts are removed and calcium replaces the exchangeable sodium. The reclamation of saline-sodic soils requires amendments—normally gypsum, sulfur, or calcium chloride—to replace sodium. Amendments have to be applied prior to the leaching process.

The amendment requirement (AR) can be calculated from the following formula [4]:

where CEC is the cation exchange capacity in meq/100 g of soil; ESPin and ESPfn are, respectively, the initial and final exchangeable sodium percentages (before and after the reclamation); and AR is the amendment requirement in meq/100 g of soil.

CEC usually is measured by saturating the soil complex with one type of cation (e.g., Na+), then replacing this cation and measuring the displaced quantity in the soil water extract (standardized at pH = 7.0). If, for example, ESPin = 30, ESPfn = 10, and CEC = 24, then AR = 4.8 meq of amendment/100 g of soil.

When the amendment is gypsum [1 meq of gypsum/100 g of soil equals 860 ppm (parts per million) of gypsum], the amount of gypsum required for 1 ha to depth of 20 cm (roughly 3.106 kg) will be 12,384 kg.

This calculation is based on 100% replacement of sodium by calcium. Because of the presence in some saline-sodic soils of free soda, the actual efficiency is lower. Thus, it is recommended that the amount of applied gypsum be increased in accordance with equivalents of free sodium carbonate and bicarbonate.

Reclamation with gypsum—sometimes added to the irrigation water—is usually less expensive than with other amendments, but the amount required is larger and the process is slow. Sulfur also is cheap and widely used, but also has it own limitations. Soil bacteria must first oxidize the sulfur to sulfuric acid. The hydrogen ion of the acid then must react with the soil lime to release calcium, which, in turn, exchanges for the soil-adsorbed sodium. Only then is the sodium ready for leaching and the whole process may take months or years.

Sodic Soils

Sodic soils are those for which the percentage of exchangeable sodium is greater than 15 and the conductivity of the saturated soil extract is less than 4 dS/m at 25°C. The pH readings usually range between 8.5 and 10. These soils frequently occur in small irregular areas of arid and semiarid regions, which often are referred to as "slick spots."

The removal of excess salts due to leaching tends to increase the rate of hydrolysis of the exchangeable sodium and can lead to a rise in soil pH. Moreover, evaporation may deposit organic matter present in the soil solution on the ground surface causing the formation of a dark crust.

These soils are characterized by the presence of sodium carbonates, sodium hydrogen-carbonates, and other sodium salts in the soil solution. These salts are very harmful, particularly if the soil does not contain gypsum. Physical properties are affected by the presence of exchangeable sodium in the soil. As ESP increases, the soil tends to become more dispersed. The pH may increase, sometimes becoming as high as 10. At these high pH values and in the presence of carbonate ions, calcium and magnesium are precipitated, resulting in an increase in the concentration of sodium in the soil solution.

Sodic soils are difficult to reclaim because their permeability is too low for water to carry amendments to the soil colloids. Mechanical mixing and deep plowing may help because they get the amendment directly into the soil and open up some passages for water percolation.

Few plants will grow in sodic soils until some reclamation has been achieved. To this end, even weeds should be encouraged because their roots open channels and crevices that improve soil permeability.

In the initial stages of sodic soil reclamation, the application of salty water is much better than pure water because salts help to flocculate the soil colloids and increase the soil permeability, often by one or two orders of magnitude. In this regard, water containing calcium is especially helpful because this ion replaces exchangeable sodium.

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