Plants lose water by evaporation through their leaves to the atmosphere. The loss is made good by water flowing from the soil into the roots, and thence within the xylem to the leaves. The factors that provide resistances to the flow of water from soil to root are set by the nature of the soil, the proliferation of roots within it, and the contact between the soil and the root. Root system architecture and the proliferation of lateral roots are covered in the following chapter, section 4.1.
Water removed by transpiration results in drier soil around roots compared with bulk soil. This provides the gradient in suction necessary for water flow towards roots to continue. As soil dries near the root surface, water flows radially from bulk soil to replenish it. When the bulk soil becomes drier, greater suctions are necessary to sustain the flow. The next section describes the capacity of different types of soils to hold water when wet, and release it to plants as they dry. It explains the terms field capacity and permanent wilting point. Following sections quantify the uptake of water by roots, and describe a major barrier to water uptake – the soil:root interface.