Application of time-domain reflectometry (TDR) for measuring water content was tested for different horticultural substrates. TDR calibration curves were determined for a mix of composted agricultural wastes (grape marc and separated cattle manure), red tuff (scoria, granulated volcanic ash), and a mix of the two. Results were expressed as volumetric water content as a function of the ratio of apparent to actual length of the TDR probe. Calibration equations obtained by regression analysis of measured data were compared with Ledieu's equation, which is widely used in soils. The latter largely underestimated the water content of the compost, probably due to the presence of bound water. The smaller but still large differences observed for tuff were attributed to water in occluded pores. Results were well described by linear equations, each with an almost identical slope but with different intercepts. For most practical purposes, a single measurement may be enough to determine the particular intercept for the specific substrate being tested.