Estimating residual water content in air-dried soil from organic carbon and clay content

Measurement of soil organic carbon (SOC) or other elements in air-dried soil samples leads to underestimation of mass fraction per unit dry soil (concentration) unless the residual water (RW) content is accounted for. However, RW measurements are time-consuming and costly and thus often neglected. The resulting bias can lead to dramatic erroneous results, especially when two differently treated datasets are compared. We therefore, derived pedotransfer functions to estimate residual water content in air-dried soil samples from: (i) SOC content, (ii) clay content and (iii) both variables together. The uncertainty of prediction decreased in the order (i)–(iii), with root mean squared deviation (RMSD) values of 0.64, 0.46 and 0.34% of RW content, respectively. These functions represent a potential step towards more harmonized and transparent SOC determination.