This paper develops a theory of tritium deposition, which is then combined with the experimental results, depicting the temperature and moisture dependence of the HT oxidation rate. The model describes the atmosphere-to-soil HT deposition process in terms of the atmospheric transport in the surface boundary layer, the transport in soil and the oxidation process by microbial activity. This model is favorably validated with the field data from the Canadian tritium release experiment. Based on this fundamental but sophisticated model, a practical model of HT deposition velocity is developed as a simple function of the soil temperature and the soil moisture. This model drastically reduces the uncertainty in the deposition velocity from a few orders of magnitude to a factor of 2.