Gravitational water: This is the water which is susceptible to the forces of gravity. It exists after significant rainfall, and after substantial irrigation. This is the water which fills all the pore-space, and leaves no room for oxygen and gaseous exchange. In “light” soils, this tends to drain away quickly. In heavy soils, this can take time.
Capillary water: This is the water which is held with the force of SURFACE TENSION by the soil particles, and is resistent to the forces of gravity. This is the water which is present after the gravitational water has drained away, leaving spaces free for gaseous exchange. When the soil is holding it’s MAXIMUM capillary water (after the gravitational water has drained), this is called FIELD CAPACITY. At this point, the plant is able to take up water easily, and has the oxygen that it needs in the root zone.
Hygroscopic water: This is the water which is held so tightly (by surface tension) to the soil particles that the plant roots can’t take it up. It’s there…….but it’s unavailable. At this stage there’s generally sufficient oxygen, but there just isn’t enough available water. The plant wilts, and will eventually die if it doesn’t get water. When the plant wilts and is unable to recover, this is called the PERMANENT WILTING POINT.