1. An enriching agent for the fertilization of soils, characterized in that it consists of a mixture of lignite of low bitumen content that has been submitted to a treatment endowing it with a high content of humic acids, and of natural or synthetic polymer that retains water and is non-biodegradable or only slightly biodegradable, and in that the respective proportions in the dry state of the different constituents of this enriching agent vary between 95% and 50% by weight of the treated lignite and between 5% and 50% by weight of the water-retainer.
