1. Soil as a Part of the Soil–Plant–Atmosphere Continuum (SPAC)

Soil is a part of the Soil–Plant–Atmosphere Continuum (SPAC); water and energy are transported mostly through all three parts of the SPAC system, but in the case of bare soil or the water table, the SPAC system is reduced to the Soil–Atmosphere Continuum (SAC) or to the Water-Table–Atmosphere Continuum (WAC). This chapter contains basic information about all three subsystems of SPAC. Conceptual approaches to the soil-water and energy transport are defined, as well as the representative elementary volume of soil (REV) needed to measure soil characteristics. The basic characteristics and properties of water (water vapour), soil, plant canopy and atmosphere are presented and their role in water and energy transport in the SPAC system is discussed. Soil water is always a low concentrated solution, but its physical properties are not changed significantly, and therefore it can be assumed to be clean water. The solute concentration is considered to evaluate the increased risk of soil salinization. The role of carbon dioxide in the SPAC system and its role in climate change are discussed.