Time domain reflectance (TDR) gives reliable estimates of the water content of structure-less soil. The effect of soil structure on the performance of TDR has received relatively little attention in comparison with the development of new calibration models and also the use of TDR for simultaneously measuring water content and the electrical conductivity of the pore water. In this paper we report on the effect of aggregate size on the reliability of water content determined from TDR measurements. The experiments that we report are relevant to loose seed-beds. We show that as aggregate size increases TDR progressively under- estimates the volumetric water content when a standard calibration function is used. We suggest a simple rule of thumb to avoid large errors in the TDR estimated water content.