Determination of Inorganic, Organic and Total Carbon in Calcareous Soils.

Total carbon (TC) in calcareous soils has two components, inorganic carbon (IC) and organic carbon (OC).  Organic carbon, affects many important physical, chemical, and microbiological soil properties. One indirect procedure to obtain OC in calcareous soils is measuring the IC and subtracting this value from the TC.  Another approach is eliminating the IC as CO₂ through a decomposition reaction and quantifying the OC in the dry soil residue. Both approaches can overestimate or underestimate OC, if IC determination or IC elimination is not done properly. Our objective was to develop a technique to quantify IC and OC. The IC determination is done gravimetrically using the Carbonate-Meter.  The Carbonate-Meter can be prepared in any soil testing laboratory with the following components: one 50-ml Erlenmeyer flask, one 6-ml glass vial, and one number-1 rubber stopper with one hole.  The IC as carbon dioxide from a check soil containing 8.22 % of IC was analyzed with the Carbonate-Meter using 0.200 g of fine soil and 4.0 ml of 0.8 M HCl.  Results after 6 hrs of shaking at 150 oscillations per minute showed 0.60 % precision and 96.36 % of recovery. The OC is determined in the soil residue with the TruSpec Leco Instrument. The Erlenmeyer flask, containing the OC and the acidic solution is transferred with 35 ml of distilled water to a 50-ml centrifuge tube, placed in a reciprocating shaker for 10 min. at 180 rpm, and the soil residue is separated by centrifugation at 6,000 rpm for 10 min. The clean soil is transferred quantitatively with a transfer pipette into a 5cm x 5cm tin foil and dried overnight at 50⁰C. The OC from a calcareous check soil containing 1.97 % OC was analyzed with this technique. Results showed 1.44 % precision and a recovery of 106.80 %. The procedure is easy to follow in the laboratory, and has good precision and accuracy.