The importance of microbial biomass of low concentration, regular additions of soluble organic substrate to carbon-limited soils, such as might occur with throughfall, has been studied. The conclusions depended upon the method used to measure C concentrations. Using a TOCsin automated analyser, the high level treatment enhanced biomass C, whereas the lower level treatment had small and variable effects, but using dichromate oxidation led to the conclusion that the low level treatment had a negative effect, and even the high level treatment had no beneficial effect. A further evaluation showed that the TOCsin procedure gave low recoveries of biomass C for bacterial monoculture suspension, an effect attributed to transport-related phenomena, and could not cope with continuous SO₄^2- inputs. However, use of Cl^- to extract C before and after fumigation altered the amount of C obtained. Therefore dichromate oxidation was used subsequently. The changes in microbial biomass, ATP concentration and respired CO₂ for glucose-amended soil have been measured over 9 days, and the dynamic responses shown to differ. The large changes in ATP-to-biomass C ratio show that ATP should not be used as a surrogate for biomass C determination after fresh substrate addition. Subsequently, ¹⁴C-labelled pea plant residues have been used to study differences in response of the three determinants to incorporation of water-soluble (WS), water-insoluble (WI), and unwashed (UW) plant residue materials. For plant materials too, ATP-to-biomass C ratios changed in response to substrate additions, increasing substantially (as for glucose) for WS additions, but decreasing after W1 residue incorporation. In the shorter term, higher biomass C was favoured more by incorporation of WI than of WS plant residues.