Electromagnetic based soil water content sensors respond to changes in bulk density, salinity and organic matter content. A field calibration of a multisensor capacitance soil water content sensor was conducted for a weathered clay loam soil and silica sand under field and laboratory conditions, respectively. The main objectives of this study were to: 1)quantify the effect of organic matter levels on the major soil physical properties, e.g. bulk density, and total porosity; 2) evaluate the effect of OM level on the calibration of the tested soil water monitor sensors under field and laboratory conditions. Four levels of OM (0, 2, 4 and 6 % per volume) were added to either a clay loam soil in the field or to a quart sand in the laboratory. After drying, there was an average 3% reduction of the volume of the clay loam soil as a result of shrinking. There was a significant statistical effect of OM level on total porosity. Porosity increased as a function of OM level; the highest increase was for the clay loam with an average increase of 20% of the total porosity as compared with the control treatment. Different calibration equations were established for the different OM levels and media.