Salt-Affected Soil Soil 206 Soil Ecosystem Lab Objectives: After completing this laboratory the student should be able to: 1. Define three classifications of salt-affected soils. 2. Perform calculations to classify a salt-affected soil and determine the reclamation procedures. 3. Explain the reclamation procedures for salt-affected soils. 4. Determine the of assigned soil. 5. Set up a percolation tube, leach with assigned reagent and describe the chemical reactions that occur during the leaching processes. Introduction Salt-affected soils develop in low-rainfall regions where lack of leaching results in a high concentration of basic cations. When drainage of these soils is restricted and surface evaporation exceeds precipitation, soluble salts accumulate in the surface horizon. These types of soils are said to be "saltaffected" and are classified as saline, sodic, or saline-sodic, refer to Table 1 for the characteristics of these soils. Many salt-affected soils develop because changes in the local water balance, usually brought about by human activities, increase the input of salt-bearing water more than they increase the output of drainage water. Increased evaporation, waterlogging, and rising water tables usually result. It is worth remembering the irony that salts usually become a problem when too much water is supplied, not too little. During the past three decades, low-income countries in the dry regions of the world have greatly expanded the area of their land under irrigation in order to produce the food needed by their rapidly growing human populations. Consequently, the proportion of arable land that is irrigated has increased dramatically. Initially, the expanded irrigation stimulated phenomenal increases in food-crop production. Many of these irrigation projects failed to address the need for drainage of excess irrigation water. The result has been the acceleration of salinization, the accumulation of neutral soluble salts in the surface horizons, and salts have accumulated to levels that are already adversely affecting crop production. The diagnosis of salt-affected soils requires several chemical tests. Management is specific for each type of salt-affected soil. Salt-Affected Soils Structure is a highly desirable soil property: it greatly influences water movement (infiltration and erosion), heat transfer (seedling emergence), aeration (gas exchange), and porosity (root penetration and water holding capacity). In the presence of calcium, soils are typically flocculated, a condition where colloidal particles are bound together. The flocculated soil particles are in turn bound by organic and inorganic materials to form soil aggregates. These aggregates form the basis of soil structure and are referred to as granular, platy, blocky and prism-like. Spring 2006 1
Consider for a moment a suspension of clay particles. Clay particles are negatively charged as the result of isomorphous substitution or broken edges so they naturally repel each other (like charges repel). However, the presence of adsorbed cations tends to mask this negative charge (repulsion) to varying degrees, depending on the type of cation. Small multi-charged cations (i.e., Al +3, Ca +2, Mg +2 ) are strongly adsorbed by colloids (unlike charges attract), greatly reducing the negative charge. If the negative charge of the colloid is sufficiently reduced, flocculation will occur as represented shown in Figure 1B. If, on the other hand, the colloids are saturated with large weakly charged cations (i.e. Na +, K + ), dispersion will occur as represented in Figure 1A. Sodium, a weakly charged cation that encourages dispersion and calcium, a multi-charged cation that encourages flocculation, provide an example of the intricate relationship between soil chemistry and the physical condition of a soil. The importance of the physical condition and the influence of sodium, creates the situation where knowing the concentration of this cation in a soil is essential. Figure 1: Soil Particles in a Dispersed and Flocculated Condition A B The exact mechanisms of flocculation and dispersion are quite complex and best explained using more advanced chemistry principles. The main point to remember for this lab is flocculation and dispersion reactions are the result of cation exchange. Cation exchange is the interaction between a cation in solution and another cation on the surface of any surface-active material, such as clay or organic matter. In this lab, we will observe the cation exchange that occurs between sodium and calcium on clay mineral surfaces. Salt-affected soils are classified using electrical conductivity, exchangeable sodium percentage and ph measurements. Electrical conductivity () is the capacity of the soil to conduct or transmit electrical current and will be measured in deci-siemens per meter (ds/m). This soil property is determined largely by the concentration of soluble salts. A concentration sufficient to interfere with plant growth is generally defined as that which produces an electrical conductivity in the saturation extract () greater than 4 ds/m. The exchangeable sodium percentage (ESP) identifies the degree to which the exchange complex is saturated with sodium. A concentration above 15% is considered to be sodic and is toxic to many plants and can be extremely detrimental to soil structure. Refer to Example 1 for the calculation to determine the ESP of a soil. Spring 2006 2
Example 1: ESP Calculation ESP = Exchangable Sodium, cmol c / kg soil * 100 Soil C cmol c / kg soil 6.9 cmol c Sodium/ kg * 100 = 23% ESP 30 cmol c / kg soil The ph is the negative logarithm of the activity or concentration of the hydrogen ion. The ph of a saturated soil paste is measured using an electrode referenced to a standard. The relationship of these three soil properties and their use as an indicator of salt-affected soils are summarized in Table 1. Table 1: Classification of Salt-affected Soils (ds/m) ESP (%) ph Saline >4 <15 <8.5 Sodic <4 >15 >8.5 Saline-sodic >4 >15 <8.5 Reclamation Three general methods are employed to improve the productivity of salt-affected soils. The exact method used depends on the nature of the salt problem. Eradication techniques, used for saline soils, involve improving soil drainage followed by soil flooding. The objective of eradication is to lower the soluble salt content in the root zone. Conversion techniques are used for saline-sodic and sodic soils. Conversion involves improving soil drainage and the incorporation of gypsum prior to leaching. Gypsum applications replace the exchangeable sodium with calcium in order to promote flocculation. Example 2 reviews the steps necessary to classify a salt-affected soil. Example 2: Classification of Salt-affected Soil ph C Ex-Na+ ESP Soil (ds/ m) (cmol c / kg) (cmol c / kg) (%) <4 6.5 ---No Additional Info. Needed to Classify--- A Not Salt-affected, Reclamation not required. >4 7 35 5 7 B Saline Soil, Reclamation Needed, Eradication Technique <4 9 30 6.9 23 C Sodic Soil, Reclamation Needed, Conversion Technique Example 3 summarizes how to calculate the gypsum requirement for the sodic soil in Example 2, Soil B. Replacing exchangeable sodium with calcium allows the soluble sodium to be leached with no adverse affect on soil structure. Leaching saline-sodic and sodic soils before adding gypsum can actually intensify the sodium problem by dispersing the remaining aggregates. This allows the clay colloids to migrate through the profile until the pores clog, reducing the porosity and permeability of the soil. This may permanently impair the hydraulic conductivity of the soil making it unfit for use in biomass production. Spring 2006 3
Example 3: Gypsum Calculation for Conversion Reclamation Technique The exchangeable sodium content of the soil that is to be replaced determines the gypsum (CaSO 4 ) requirement. Exchangeable sodium can be calculated from the C of the soil and the percentage of sodium that is on the exchange complex. Once the cmol c of exchangeable sodium is known, the amount of CaSO 4 needed to effectively replace it on the soil colloid can be calculated. Calculate the kilograms of CaSO 4 per hectare-15 cm required to replace 60% of the exchangeable sodium in a soil with a C = 30 cmol c / kg and an ESP of 23%. First calculate the percent exchangeable sodium to be replaced and then the total C of the soil that is to be replaced in cmol c. 23% * 60% = 14% * 30 cmol c = 4.2 cmol c 1 kg soil 1 kg soil Next, calculate the kilograms CaSO 4 for each kilogram of soil: 4.2 cmol c * 1 mol c * 1mol CaSO 4 * 136g CaSO 4 * 1kg = 2.8x10-3 kg 1 kg soil 100cmol c 2mol c 1 mol CaSO 4 1000g 1 kg soil Finally, calculate the kilograms CaSO 4 for each hectare-15 cm of soil: 2.8x10-3 kg CaSO 4 * 2x10 6 kg soil = 5600 kg CaSO 4 1 kg soil 1ha-15cm 1ha-15c Gypsum is added to the saline soil to replace the Na + ion from the exchange complex with the Ca +2 ion. When gypsum is added the following reaction takes place: 2Na X + CaSO 4 Ca X + Na 2 SO 4 Sodium sulfate is a soluble salt that can be easily leached from the soil. Calcium has been exchange for the sodium on the exchange complex so flocculation should be encouraged. Now when the leaching occurs, the pores will not be clogged with the dispersed clay-size fraction and the hydraulic conductivity will not be impacted. However, tons of gypsum per hectare can be required, which Example 3 indicates, so this is not a task without effort. Control methods for salt-affected soils include controlling evaporation, proper irrigation scheduling, application of elemental sulfur or sulfuric acid, and the use of salt-tolerant crops. See chapter 9 in Gardiner and Miller for more information on the management and reclamation of salt-affected soils. Spring 2006 4
Procedure for Assigned Soils 1. From each of your assigned soils, weigh out 25 grams of well ground soil into a 100 beaker. 2. Add 25 of distilled water. Stir. Let stand for 1 hour, minimum. 3. Filter using a vacuum filter flask. 4. Pour leachate into a centrifuge tube. Measure and record the and the ph of each soil. 5. Clean-up all lab equipment. Data Sheet for Soil Salinity Classification Soil ph Soil Salinity Classification Soil Reclamation Procedure 1. Construct a percolation tube as illustrated in the diagram. You will be assigned a solution for leaching. Enter your solution on the data sheet. 2. Leach the soil column with the FIRST assigned solution under constant-head (refer to diagram) conditions for 10 minutes. Before continuing make sure your flow rate is within 5% of your competing group. This is critical to verify the affects of our reclamation procedures. 3. Collect the leachate in centrifuge tubes. One tube for each minute. This must be collected under constant flow. 4. Measure and record the amount of solution collected each minute and measure and record the for each sample. Measure and record the ph of the 10 th collection. 5. At the end of the first leaching period, let the tube drain and stand for 5 minutes. 6. SLOWLY refill the percolation tube with the SOND assigned solution leaving the tube open until the aggregates are completely submerged. Filling the tube slowly will avoid air entrapment. Do not jiggle or tap the tube as this will create a conductivity artifact. 7. Leach the soil column with the SOND assigned solution under constant-head conditions for 10 minutes. Collect the leachate in centrifuge tubes. One tube for each minute. This must be collected under constant flow. 8. Measure and record the amount of solution and the for each interval as you did in step 4. Measure and record the ph of the 10 th sample.!!!!make sure you use the SAME and ph meter that you used in Step 4!!!! 9. Compute the cumulative solution collected for the total length of each leaching period. 10. Fill in the blank graphs with the and the Q. Spring 2006 5
Maintain Constant Head of Reagent One Piece of Crumpled Kimwipe Saturated Soil Aggregates (NaCl or CaCl 2 ) ½ Cotton Ball Maximum Q = 15 min -1 Data Sheet Treatment 1 min 2 min 3 min 4 min 5 min 6 min 7 min 8 min 9 min 10 min collected Total Leachate Collected ph of #10 treatment leachate Soil Salinity Classification after first treatment Spring 2006 6
Reclamation Treatment collected 1 min 2 min 3 min 4 min 5 min 6 min 7 min 8 min 9 min 10 min Total Leachate Collected ph of #10 reclamation leachate Soil Salinity Classification after Reclamation Treatment Treatment Graph 80 40 72 36 64 32 56 28 48 24 40 20 32 16 24 12 16 8 8 4 min 1 2 3 4 5 6 7 8 9 10 Reclamation Treatment Graph 80 40 72 36 64 32 56 28 48 24 40 20 32 16 24 12 16 8 8 4 min 1 2 3 4 5 6 7 8 9 10 Spring 2006 7