A Simple, Gravimetric Method to Quantify Inorganic Carbon in Calcareous Soils

Abstract

Core Ideas A carbonate meter that measures inorganic C in calcareous soils was assembled. A technique was developed to determine inorganic C in calcareous soils using the meter. The carbonate meter is simple and can be easily assembled in any soil testing lab. The carbonate meter gravimetric method is easy, with good precision and accuracy. The carbonate meter method is more efficient than the pressure transducer method. Total C (TC) in calcareous soils has two components: inorganic C (IC) and organic C (OC). The IC must be measured and subtracted from the TC to obtain OC. Our objective was to develop a simple gravimetric technique to quantify IC. This technique, named the carbonate meter (CM), can be assembled in any soil testing laboratory with a 6‐mL glass vial, a no. 1 rubber stopper with one hole, a 6.5‐cm glass tube with 0.5‐ and 0.3‐cm outside and inside diameters, respectively, and a 50‐mL Erlenmeyer flask. Precision and recovery was tested with 0.728 and 2.57% IC standard calcareous soils and pure calcite and dolomite, using 0.2000 ± 0.0200 g of finely ground samples and 4.0 mL of fresh 4 mol L−1 HCl–3% FeCl2⋅ 4H2O. Twelve US calcareous soils with 0.11 to 8.52% IC were used to validate our CM procedure. Inorganic C recoveries after 22 to 24 h for two standard calcareous soils, calcite, and dolomite were 100.55, 99.73, 100.45, and 99.58%, respectively. The detection limit (DL) and the lower limit of quantitation (LLOQ) for calcite were 0.31 and 1.0 g IC kg−1, respectively. The DL and the LLOQ for dolomite were 0.42 and 1.4 g IC kg−1, respectively. Mean IC recovery with a modified pressure calcimeter (MPC) in reference standards was 56.42%, while the CM recovery was 100.84%. The MPC loses accuracy in calcareous soils with >5.11% IC, whereas the CM had good recoveries up to 8.40% IC. The CM does not require pre‐analysis, it is easy, non‐tedious, and inexpensive, with good precision and accuracy.