The chloroform‐fumigation extraction (CFE) method is widely used to estimate soil microbial biomass. The relationship between extraction efficiency and the extractant‐to‐soil ratio has not been fully investigated in chloroform fumigated and non‐fumigated soils. We analyzed the extraction efficiency of 0.5 M K 2 SO 4 in five soils at six extractant‐to‐moist soil ratios (3:1, 5:1, 7.5:1, 10:1, 12.5:1, and 15:1). A positive relationship between extractant‐to‐soil ratio and the amount of extracted dissolved organic carbon (DOC) was observed for both non‐fumigated and fumigated samples. The extractant‐to‐dry soil ratio required for 90% extraction averaged 12.0 for non‐fumigated samples and 8.2 for fumigated samples. A minor, yet statistically significant, positive relationship between extractant‐to‐dry soil ratio and chloroform‐labile carbon was observed in four of five soils, with a 5.6 average extractant‐to‐dry soil ratio required for 90% extraction. For analyses where DOC extracted from non‐fumigated samples is a critical variable, an extractant‐to‐soil ratio greater than 5:1 should be considered.