Organic growing media (peat, sawdust, coir, compost) is being used more and more in the production of greenhouse tomatoes. Research continues to develop new substrates containing a sawdust/sphagnum peat mix and to find an appropriate irrigation strategy. These media, however, have a tendency to accumulate salts. The development of an efficient irrigation strategy requires an understanding of the dynamics of salt leaching during irrigation. A priori, these substrates contain a mobile and an immobile water phase, the result of the presence of dead plant structures (hydrocysts and vessels). Experiments were conducted under steady-state unsaturated conditions to evaluate the transport of NaCl during leaching, and to quantify the immobile water content and the transfer coefficient between the mobile and the immobile water phases. Results indicate that the proportion of immobile water is important in these media and that the exchange between the mobile and immobile water is extremely slow, in agreement with conclusions obtained in gas transfer studies. This indicates a poor leaching efficiency with such substrates and a need for continuous monitoring of salinity when growing tomatoes at high salt levels.