Introductory Soils Lab 8 Soil pH & Liming NRES 201 Page 1 of 11 LABORATORY 8. 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. 8.1 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 aqueousIntroductory Soils Lab 8 Soil pH & Liming NRES 201 Page 2 of 11 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. 8.2 MECHANISMS THAT CONTROL SOIL pH. Table 8-1 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+)Introductory Soils Lab 8 Soil pH & Liming NRES 201 Page 3 of 11 8.3 MEASUREMENT OF SOIL pH. Soil pH as used in this text refers to the negative logarithm of the hydrogen ion concentration (H+) in the soil solution. Predictions about the chemistry of a soil constituent are often based on the pH of the soil solution. In actual practice these predictions are based on measured pH values, which may or may not be reliable estimates of the actual pH of the soil solution. McLean (1982) identified the following factors that may influence the accuracy of soil pH measurements: 1. The nature and type of inorganic and organic constituents that contribute to soil acidity. 2. The soil to solution ratio used in measuring pH. 3. The salt content of the diluting solution used to achieve the desired soil to solution ratio. 4. The CO2gas content of the soil and solution. 5. Errors associated with standardization of the equipment used to measure pH. The measurement of soil pH generally requires the addition of water or a salt solution. The most common soil to solution ratio is 1:1, although 1:2 and 1:10 dilutions are also employed. As might be expected, the addition of water or a salt solution can change the H+ concentration from that of the original soil solution. For example, weak acid groups associated with soil organic matter as well as soil minerals may either associate or dissociate upon dilution. The salt content of the diluting solution can result in the release of H+ ions from the exchange complex. Depending upon the nature and type of colloids and weak acid groups present, the effect of dilution may differ from one soil to another, making consistent correction difficult if not impossible. The use of 0.01 M CaCl2 has been recommended to minimize the effects of dilution, since this concentration of salt is generally considered to represent the salinity of a “normal” soil solution in the field. Dilution of the soil solution may also result in differences in the type and amount of dissolved gases. Gases such as CO2gas are present in the soil in higher concentrations than in the bulk atmosphere. Hence, dilution and stirring as used in the actual measurement of soil pH can promote the loss of such gases and lead to differences between the measured and actual pH of the soil. Soil pH measurements are normally made using either electrometric or colorimetric techniques. Colorimetric techniques are based on structural changes in