PWT info can be found here:
Learning new things is good.
-----
"Perched water is water that defies gravity and remains in the soil after the soil stops draining. If you wet a sponge & hold it by a corner until it stops draining, the water that is forced out of the sponge when you squeeze it is perched water. From the plant's perspective, perched water is unhealthy because it occupies air spaces that are needed for normal root function and metabolism. Poor exchange of the gasses produced under anoxic (airless) conditions (CO2, sulfurous compounds, methane) are also an issue. The main issue though, is that roots deprived of sufficient oxygen begin to die within hours. You do not actually see this, but the finest, almost microscopic, and most important roots die first. The plant then has to spend stored energy or current photosynthate (food/ carbohydrate) to regenerate lost roots - a very expensive energy outlay that would otherwise have been spent on maintenance of the currently living mass, blooms, fruit, or normal growth. Perhaps the plant would have stored the energy for a winter's rest and the spring flush of growth instead of expending it on root regeneration necessitated by soil saturation"