Method 58c Determination of pH in soils with soluble salts

See the discussion under 'pH extractants' in Chapter 4. Reagents.

• Calcium chloride, 1.0 M - completely dissolve 14.7 g CaCl2.2H2O in water and make up to 100 ml.

• Calcium chloride, 0.01 M - completely dissolve 1.47 g CaCl2.2H2O in water and make up to 1 l.

Procedure. Proceed as in Method 5.8a., replacing water with a solution of 0.01 M CaCl2. Alternatively, add 5 drops (0.25 ml) of 1 M CaCl2 to the suspension following the pH determination in Method 5.8a.

Calculation. The difference in pH between water and salt solution extracts is known as the salt effect, and given the symbol A pH. Thus,

A pH = soil pH in CaCl2 solution - soil pH in water and A pH values are positive for soils with a net positive charge, and negative for soils with a net negative charge, with magnitude proportional to charge.

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