Estimation of Mobile/Immobile Phase in Organic Growing Media: Leaching Efficiency.

The production of greenhouse tomato is more and more performed in organic growing media (peat, sawdust, coir, compost). Research is ongoing to develop new substrates containing a sawdust/sphagnum peat mix and an appropriate irrigation strategy. These media, however, have the tendency to accumulate salts and the development of an efficient irrigation strategy requires an understanding of the dynamics of salt leaching during the irrigation. A priori, these substrates contain a mobile and an immobile water phase, results of the presence of dead plant structures (hydrocysts and vessels). Experiments were conducted in steady-state unsaturated conditions to evaluate the transport of NaCl during leaching and to quantify the immobile water and the transfer coefficient between the mobile and the immobile water phase. The results show that the proportion of immobile water is very 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 study. This therefore indicates a poor leaching efficiency with such substrates and a need for continuous monitoring of salinity when growing tomato at high salt levels.