Standard Test Methods for pH of Soils
5.1 The pH of the soil is a useful variable in determining the solubility of soil minerals, the mobility of ions in the soil, and assessing the viability of the soil-plant environment.
5.2 pH measurements are made in both test water and a calcium chloride solution because the calcium displaces some of the exchangeable aluminum. The low ionic strength counters the dilution effect on the exchange equilibrium by setting the salt concentration of the solution closer to that expected in the soil solution. The pH values obtained in the solution of calcium chloride are slightly lower than those measured in water due to the release of more aluminum ions which then hydrolyses. Therefore, both measurements are needed to fully define the character of the soil's pH.
5.3 For the purpose of these test methods, the test specimens are sieved through a 2.00 mm (No. 10) sieve. Measurements on soils or soil fractions having particle sizes larger than 2.0 mm by these test methods may be invalid. If soil or soil fractions with particles larger than 2.0 mm are used, it must be stated in the report since the results may be significantly different.
Note 2: The quality of the result produced by this standard is dependent on the competence of the personnel performing it, and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing/sampling/inspection/etc. Users of this standard are cautioned that compliance with Practice D3740 does not in itself assure reliable results. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.
1.1 These test methods cover the measurement of the pH of soils that will pass the 2.00 mm (No. 10) sieve. Such measurements are used in the agricultural, environmental, geotechnical, and natural resources fields. This measurement determines the degree of acidity or alkalinity in soil materials suspended in water and a 0.01 M calcium chloride solution. Measurements in both liquids are necessary to fully define the soil's pH. This variable is useful in determining the solubility of soil minerals and the mobility of ions in the soil and assessing the viability of the soil-plant environment. A more detailed discussion of the usefulness of this parameter is given in Refs (1-6)2.
1.2 Two methods for measuring the pH of soils are provided. The method to be used shall be specified by the requesting authority. When no method is specified, Method A shall be used. The pH is determined in test water and a calcium chloride solution for both methods.
1.2.1 Method A—The pH is measured using a potentiometer having a pH sensitive electrode system. This method can be used for any application and must be used when the application warrants a higher level of resolution.
1.2.2 Method B—The pH is measured using pH sensitive paper. This method can be used for any application, however, because paper typically has a lower resolution, it provides an approximate estimate of the pH of the soil and should not be used when the application requires a higher level of resolution (Note 1).
Note 1: For example, paper with a sensitivity to the nearest 1 pH unit placed into a buffer solution of 4 should indicate a pH of 4, however, it would not indicate if the pH is 4.449 or 3.449.
1.3 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.4 All measured and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D6026.
1.4.1 The procedures used to specify how data are collected/recorded and calculated in the standard are regarded as the industry standard. In addition, they are representative of the significant digits that generally should be retained. The procedures used do not consider material variation, purpose for obtaining the data, special purpose studies, or any considerations for the user’s objectives; and it is common practice to increase or reduce significant digits of reported data to be commensurate with these considerations. It is beyond the scope of these test methods to consider significant digits used in analysis methods for engineering data.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
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