Water availability is a key factor affecting the development of soil and ecosystem type. The availability of soil water is controlled by the rates at which water is replenished from the surrounding environment and at which it is removed by vegetation and drainage. Water is replenished in soils from precipitation, groundwater and to a small degree through condensation of atmospheric water. In addition, trees are capable of moving water within the soil profile from areas of high to areas of low water potential. The concept of water potential is discussed below. Most soils in northern forests characteristically have a flow of water driven by gravity. The water retained in the soil after free draining is known as the field capacity, which is determined by a number of soil properties, both chemical (adsorption) and physical (aggregate size). Excess water in soils results in filling of the pore volume, resulting in waterlogging or gleying of soils, with consequences for soil organisms and the development of root systems.

Introduction and background

Water-binding forces in soils

Water retention in soils is controlled by a number of factors. The binding of water in soils is controlled by both the interaction of water with charged surfaces of the soil solid phase, and the interaction between the water molecules themselves. The soil solid phase is made up from minerals of differing sizes such as clays and sand, and soil organic matter (Chapter 2).