soil water, that water held in the pore spaces of the soil within the potential rooting zone for vines. When all the pore space is filled with water, the soil is said to be saturated (this defines its water-holding capacity). Unless the soil is a very heavy clay (see soil texture), it will normally drain within 48 hours to a state called its field capacity (FC), which corresponds to a soil water suction of about 10 kiloPascals (kPa) (see water stress). Ideally, 10–15% of the pore space should be filled with air at the FC. With soil evaporation and, most importantly, extraction of water by the vines, the soil water content decreases until a point is reached when the vine can no longer extract water fast enough to prevent its wilting during the day, and non-recovery at night. This condition is called the permanent wilting point (PWP). The amount of water held between FC and PWP, per metre depth of soil, defines the available water capacity (AWC). The amount of plant available water (PAW) in a soil profile is the product of the AWC and soil depth. Soils that do not drain to an FC remain waterlogged, in which case a lack of oxygen quickly kills the vine roots.
