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ILLINOIS NRES 201 - Lab 9 Soil pH & Liming

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Introductory Soils Lab 9 Soil pH & Liming NRES 201 LABORATORY 9. SOIL pH AND LIME RECOMMENDATIONS Soil pH is one of the most interesting and informative soil properties. Soil pH is a measure of the hydrogen ion concentration in the soil solution. Theoretically, soil pH is the negative logarithm of the hydrogen ion concentration in the soil solution. pH = –log (H+) In practice, uncertainties in measuring pH associated with dilution of the soil solution and colloidal effects result in soil pH being an approximate value. Soil pH is an indicator of soil weathering. Soil pH values reflect the mineral content of the parent material, the length of time and severity of weathering, and especially the leaching of basic materials from the soil profile. Factors such as the type of vegetation, annual rainfall, and drainage as well as the activities of man also influence soil pH. The availabilities of iron, copper, phosphorus, zinc, and other nutrients, as well as the toxicities of various substances, are controlled in large part by soil pH. Some potentially toxic substances in soils, such as aluminum (Al3+) and lead (Pb2+), have little effect on plant growth under alkaline conditions, but are a serious concern when the same concentrations occur in acid soils. Many nutrients, notably phosphorus, show their greatest availability in slightly acid to neutral soils, with markedly lower availability with increases or decreases in soil pH. Soil pH is also an indicator of serious soil problems. Soil pH values above 8.5 are indicative of sodic soils, whereas pH values below 4 suggest the oxidation of reduced sulfur compounds. 9.1 ROLE OF WATER. Water is one of the most important species in aqueous systems such as soils, not only because it is an excellent solvent but also because of its role in acid-base reactions. Water will autohydrolyze into a hydronium (H3O+) or as more commonly written a hydrogen (H+) ion and a hydroxyl (OH–) ion: 2H2O ↔ H3O+ + OH- Kw = 10-14 at 25°C or H2O ↔ H+ + OH-, where Kw is the equilibrium constant for the autohydrolysis reaction and is given by: Kw = (H+)(OH-) = 10-14. The equilibrium constant expression for water has special significance for aqueous Page 1 of 17Introductory Soils Lab 9 Soil pH & Liming NRES 201 systems. This expression in its logarithmic form not only establishes the pH scale, and hence the definition of acidic, basic, and neutral solutions, but also illustrates the interdependence of H+ and OH– concentrations. In logarithmic form: log (H+) + log (OH-) = –14. In terms of negative logarithms, p = –log and pH = –log (H+): pH + pOH = 14. These equations illustrate that in aqueous solutions the concentrations of the H+ and OH– ions cannot be varied independently. When one species is increased there must be a corresponding decrease in the concentration of the other, such that the product of their concentrations is a constant (Kw). Moreover, it must be remembered that pH is a negative logarithmic scale. Hence, a change in pH from 5 to 4 is a tenfold increase in H+ ion concentration, and a pH change from 4 to 8 is a ten thousand fold decrease in hydrogen ion concentration. 9.2 MECHANISMS THAT CONTROL SOIL pH. Table 9-1 Mechanisms that control soil pH. Soil pH range Major mechanism(s) operating 2 to 4 Oxidation of pyrite and other reduced sulfur minerals. Dissolution of soil minerals. 4 to 5.5 Exchangeable Al3+ and its associated hydroxy ions. Exchangeable H+. 5.5 to 6.8 Exchangeable H+. Weak acid groups associated with soil minerals and humic substances. Dissolved CO2gas and aqueous species. 6.8 to 7.2 Weak acid groups on soil organic matter and humic materials. 7.2 to 8.5 Dissolution of solid divalent carbonates (CaCO3s). 8.5 to 10.5 Exchangeable Na+ under low salt conditions. Dissolution of Na2CO3s. Note: For the pH range of 4 to 6.8, soil pH is controlled by the percent of the cation exchange complex occupied by acidic cations (H+ and Al3+) Oxidation of reduced sulfur compounds. When soils or deposits that contain reduced forms of sulfur such as iron sulfide are Page 2 of 17Introductory Soils Lab 9 Soil pH & Liming NRES 201 exposed to aerobic environments, conditions are created that favor the oxidation of the reduced sulfur compounds. Associated with the oxidation of the reduced sulfur minerals is the production of H+ ions and their release to the soil solution: 2FeS2pyrite + 8.5O2gas + 2H2O ↔ α-Fe2O3hematite + 4H+ + 4SO42-. The reduced sulfur compounds are oxidized by chemoautotrophic bacteria such as Thiobacillus. The chemical energy released by these oxidations is used by the bacteria to drive their metabolic processes. The initial process of sulfur oxidation in neutral soils is carried out by bacteria, but once the soil has been acidified to pH values around 3.5 or less the oxidation of the sulfide ions can be coupled with the reduction of ferric iron and the process becomes chemical (abiotic) in nature: Fe3+ + S2- + 4H2O ↔ Fe2+ + SO42- + 8H+. The source of the acidity in these soils is the oxidation of reduced sulfur. The total amount of acidity that can potentially be produced is determined by the quantity of reduced sulfur minerals in the soil. The rate at which the acidity is produced is governed by the rates of the biological and chemical mechanisms. If the pH values of these soils are determined and compared to the amount of acid being produced, it is apparent that some buffering mechanism in the soil is operating and reacting with the H+ and preventing the establishment of extremely low pH values. In acid sulfate soils, it has been shown that more than 98% of the acid released during pyrite oxidation reacts with soil minerals and is neutralized, 1 to 2% reacts with dissolved alkalinity (HCO3–), and less than 1% remains in the soil solution as free acid: AI(OH)3gibbsite + 2H+ ↔ Al(OH)2+ + 2H2O. The equation showing gibbsite reacting with a H+ ion is an example of how a soil mineral can neutralize the H+ ions produced by sulfur oxidation. Soil minerals in addition to gibbsite can also be dissolved in response to elevated H+ levels and contribute to overall soil buffering. Exchangeable aluminum. The trivalent aluminum ion (Al3+) is the cation of a weak base and as such has the potential to hydrolyze water, producing hydrogen ions. The combination of the aluminum ion, its hydrolysis species, and the insoluble weak base Al(OH)3amorphous represents


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