Soil specific surface area can be a strong predictor of organic carbon (SOC) contents in soils. Specific surface area of soils can be estimated reliably and cost-effectively from water adsorption by air-dry soil samples, but SOC can also adsorb water. For estimating the mineral component of specific surface area, it is, therefore, necessary to exclude water-adsorption by SOC. Here, we refer to ‘apparent specific surface area’ for measurements that include water adsorption by both mineral soil and SOC. We used a mathematical approach to estimate water adsorption by SOC so that this component can be subtracted from measurements of apparent specific surface area.

We used a data set of apparent specific surface area and soil carbon at seven depths from 50 soil cores collected from a research farm in the Manawatu region in New Zealand. Both apparent specific surface area and SOC content decreased with soil depth with very high correlation (r² = 0.98). We estimated the SOC contribution to apparent specific surface area from the slope of the relationship between changes in apparent specific surface area and SOC content. For our soils, the SOC contribution to apparent specific surface area was estimated as 0.43 ± 0.02 m² mgC^-1. This parameter allows apparent specific surface area measurements to be corrected for the water adsorption by SOC to calculate the functionally relevant mineral specific surface area.