Book excerpt: This book provides the most up-to-date knowledge on water in soils and applications for the best use of our water resources. It first addresses the influence of soils on water quality, which is linked to rock weathering, soil formation, acidity and waterlogging. Here, the constituents of soils – such as clay minerals and iron oxides – play a major role. These modifications also have an impact on biogeochemical processes at the global scale, including the carbon cycle and the composition of the atmosphere. Secondly, this book discusses soil salinity, alkalinity and sodification in climates spanning from Mediterranean to arid. Here, water quality results from the concentration of solutes by evaporation and the transpiration of plants. The proper management of irrigation both protects soils against acidification and ensures sustainable agroecological development, while improper management leads to soil degradation and groundwater overexploitation. Lastly, the book describes how excess transfer of phosphorus in lakes results from a cascade of liberation and immobilization in the structure of the surrounding landscape. This leads to a general integrative method to limit eutrophication and restore the quality of water bodies.