Specific Heat of Soil Solids and Its Influence On Soil Water Content Measurements From the Heat Pulse Method.

Soil volumetric water content (q) measured with the heat pulse method is influenced by the specific heat capacity of soil solids (c_s), yet there are inconsistent reports about c_s values. In this study, we determined soil c_s with differential scanning calorimetry on nine soils, and partitioned c_s into specific heats of soil minerals (c_m), organic matter (c_o), and tightly bound water (c_tbw). Soil c_m was measured for organic matter-removed samples dried at 200^oC, and c_s values were measured for original samples dried at 105^oC. For the nine soils, c_m averaged 0.736 MJ Mg^-1 K^-1 (from 0.717 to 0.759 MJ Mg^-1 K^-1), and c_s averaged 0.750 MJ Mg^-1 K^-1 (from 0.718 to 0.772 MJ Mg^-1 K^-1). Tightly bound water showed negligible influence on c_s values. Soil c_m was influenced mainly by soil specific surface area and varied linearly with temperature from 10 to 55^oC. The heat pulse measurements were conducted on Soils 6, 8 and 9. When the widely accepted c_s value of 0.725 MJ Mg^-1 K^-1 was used, the heat pulse method overestimated q by 0.054 m³ m^-3 on average. The error was reduced significantly (0.042 m³ m^-3) when c_s values of 0.761 MJ Mg^-1 K^-1 (Soil 6), 0.745 MJ Mg^-1 K^-1 (Soil 8) and 0.781 MJ Mg^-1 K^-1 (Soil 9) were used.