Is the Soil Water Potential at Permanent Wilt Really 15 Bars?.

The idea of a universal water potential at permanent wilting point  has become widely accepted, and is generally thought to be -1.5 MPa (15 bars), but research establishing this value was done prior to the development of accurate water potential meters.  The present experiment uses an accurate measurement method to investigate whether a universal permanent wilting point does in fact exist and establishes its approximate value using a Decagon Devices WP4 Dew Point Potentiameter.     Dwarf sunflowers in four soil textures were sealed in pots and monitored until permanent wilting point was reached.  The water potential and water content of the soil was then measured.  Results show that, although a permanent wilting point does exist for sunflower, its value is around -2.5 MPa (25 bars), not -1.5 MPa (15 bars) as expected.  This information is useful in plant growth and irrigation studies.