Leaching techniques for saline wastes composts used as growing media in organic agriculture: assessment and modelling

The purpose of this work was to examine solute release by the effect of leaching of a saline compost with two main objectives: (1) to identify the most efficient method for this purpose, in order to minimize the environmental impact of this process in terms of water consumption and (2) to study the composition of the leachates to manage them properly and avoid possible contamination. A laboratory method involving column leaching with distilled water (CL) and two field methods involving saturation leaching (SL) and drip leaching (DL) were compared to this end. In order to more accurately assess nutrient release and compare the three leaching techniques, the cumulative amounts of ions leached were processed by using an exponential growth model. All target ions fitted properly, and so did the curve for the ions as a whole. Salts were removed mainly by effect of the leaching of major ions in the substrate (Na(+), Cl(-), inorganic N, SO4 (2-) and K(+)). SL and CL proved similarly efficient and reduced the salt content of the substrate to an electrical conductivity below 2 dS m(-1) in the saturation extract, which is the optimum level for nursery crops. By contrast, the DL method provided poor results: salt contents were reduced to an electrical conductivity of only 8 dS m(-1) in the saturation extract, so the resulting substrate can only be useful to grow highly salt-tolerant crops.