Аннотация:This chapter studies critical components of a physics-based approach to define several characteristic points on the water retention curve. The idea is to find segments of the water retention curve at which the forces causing water retention have distinctly different physical natures and wet soil has distinctly different rheological behavior. The boundaries among these segments define the key soil water contents. Voronin defined four key soil water contents: maximum adsorbed water content, maximum molecular water content, maximum capillary-sorptive water content, and maximum capillary water content. These are all defined on a mass basis. The chapter presents methods to measure all four key water contents. In a study discussed in the chapter, there were simple relationships between the key water contents and soil matrix potentials that had been held for soils of different origin and composition. The chapter presents four regression equations that have been found. The practical advantage of the key water content concept consists of the opportunity to use traditional measurements of field capacity, liquid limit, and plastic limit and to couple those values with values of soil matrix potential using these four regression equations. Such a pedotransfer approach relates easy-to-measure field values to water retention curves.