19D1 Field test for soil carbonates with dilute HCl and effervescence class assessment

This qualitative field test (Soil Survey Division Staff 1993) uses 1.0 M HCl to provide a rapid means of confirming the presence of soil carbonates and their ‘effervescence class’. As such, the test provides a relative index of the amount of carbonates in the soil matrix. The test relies on the visible effervescence of CO₂ when dilute HCl reacts with carbonate minerals in the soil. The rate and extent of effervescence are affected by the chemical and physical nature of the carbonates present, including particle size and mineralogy, together with other variables such as temperature, water content and the strength of the HCl. The chemical reaction described in Method 19B1 applies.

Reagents

1 M Hydrochloric Acid

Dilute 100 mL of 10 M HCl, mix and make volume to 1.0 L with deionised water.

Procedure

Carefully obtain a representative sub-sample of the soil matrix and crush or sieve to a particle size of <2 mm (field moist sample). Place a sufficient amount of the sub-sample into a spot-plate depression, add 1–2 drops of 1.0 M HCl, observe the initial reaction and wait around 2 min before assessing the final extent of effervescence and assigning the appropriate effervescence class, guided by Table 19.4 and report for each soil segment and location.

Report the CaCO₃ effervescence class and the corresponding ≈CaCO₃-equivalent (%) on a field-moist basis.

Table 19.4. Effervescence classes used to describe the entire soil matrix using 1.0 M HCl. Ratings and approximate CaCO₃ equivalents (%) are adapted from Kalra and Maynard (1991).