I geology, a PWT is the water that occupies a mass of soil comprised of small particles (usually clay or silty clay) situated above a dissimilar soil and will not drain because the substrata is of large particles (coarse sand/gravel). The capillary attraction of the soil holding the PWT is greater than the gravitational flow potential of the water in the upper strata plus any capillary 'pull' of the substrata, so water 'perches (like a bird) in the upper strata.

In container media, perched water is the water that occupies the lower parts of a medium's mass and will not drain because it is held too tightly for the gravitational flow potential to overcome the capillary attraction of the medium. Illustration: if you saturate a sponge and hold it in the horizontal position until it stops draining, then hold it by a corner, a considerable amount of perched water will exit the sponge because of the increase in GFP when you change its orientation. The height of a PWT is directly related to particle size with soils comprised primarily of of small particles (peat, coir, compost, sand, topsoil, or other particles of similar size) holding greater volumes of perched water than soils comprised of larger particulates like pine bark, and various other materials coarser than those previously listed.

The airless conditions associated with perched water compromise root function and metabolism, and provide a favorable environment for the various root rot fungi that thrive in anoxic conditions.

Al

Thank you. I can see that PWT can be avoided by using the proper mix of soil components to avoid root rot.

..... exactly true. IMO, a lack of understanding about what causes a PWT and how to deal with it is the most serious impediment to proficiency for a very large % of practicing container gardeners.

Al