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Perched Water Table In Container

In a nut shell Perched Water Table (PWT) is the characteristic of a given soil to hold water and resist drainage. It is due to the combined effect of Cohesion, adhesion and capillary action.

PWT is independent of pot shape (height and with). Here is a picture to demonstrate the principle.

In the above picture various size of pots are filled with the same medium, and watered to saturation. After a while letting to drain they all will have up to equal height of water remaining and not draining.

This effect is dramatic in a short pot in which most seedlings are grown. If the media has a high PWT the seedlings' roots will be sitting in water most of the time. It does not matter how many and how big the drain holes are. As a result of oxygen deprivation root rotting can reult as known as damping off.

You can do an experiment with your potting mix and find out it PWT, using a clear solo cup, as shown in this picture.

Sey

Comments (10)

  • Jean
    7 years ago
    last modified: 7 years ago

    A principle well-known by soil scientists but seldom by gardeners.

    As it turns out, we should add more info to this to be even more meaningful to gardeners:

    When gardeners "add coarse stuff in the bottom of the pot to improve drainage," they simply move that perched water table higher in the container. Thereby limiting the available rooting space.

  • Seysonn_ 8a-NC/HZ-7
    Original Author
    7 years ago

    Thanks Jean

    That is exactly right. Adding coarse stuff at the bottom of container just raising the bottom of container and thus PWT. But If, for example, just add the coarse stuff next to the wall and leaving the center then PWT will be unchanged.

    But most importan action would be to choose a medium with low PWT.


    Sey

  • Seysonn_ 8a-NC/HZ-7
    Original Author
    7 years ago
    last modified: 7 years ago

    Medium and PWT

    Most of soil less potting mixes sold commercially are peat moss based. Peat moss has a high PWT. Now then they also add vermiculite to it, to make the matter worse.(Moisture control ?)

    Adding perlite can help, provided it is of sufficient amount, percentage wise.

    Alternative:

    Pine bar fines ( particle sizes from 1/8" to 3/8"), when mixed with peat moss can reduce PWT. But percentage wise bark to peat ration has to be high (3 to 1 ) .

    Alternative Moisture Retentive:

    The material called floor dry is baked DE. It won't fall apart and won't compromise drainage , yet will retain moisture. I have tested it myself. One unit weight of FD will absorb one unit weight of water. It will keep the medium moist but not soggy wet.

    In container planting, especially growing tomatoes, balanced moisture level is a challenge. When it goes from soggy to dry it may cause BER among other things. I think a blend of peat moss, pine bark fine and DE (floor Dry) can offer a solution.

    Sey

  • lucillle
    7 years ago
    last modified: 7 years ago

    If I'm understanding what he said correctly, according to Tapla adding some perlite to peat is like adding bb's to pudding, it doesn't help lighten the mix or change the perch. Unless, of course, you are saying 5 parts perlite to one part peat.

    Of course, everyone has the right to grow stuff in whatever medium they wish.

  • lucillle
    7 years ago
    last modified: 7 years ago
  • Seysonn_ 8a-NC/HZ-7
    Original Author
    7 years ago

    Al.

    Thanks for coming by and commenting.

    .

    Sey



  • Lin Gee
    3 years ago

    I'd like to ask Al - tapla about wicking tubs and his use of pine bark mulch, turface and crushed marble will this media work for such a system. will i get enough capillary to wick water from the bottom reservoir held in 6 inch drain pipe with a drain hole at 5 inches to allow for air. the soil in filled in around the pipes and goes the the top of the container. what is his evaluation of such a system?

  • coolbythecoast
    3 years ago

    A commercial compost company owner once told me that you don't want all fines, you want bits of pine bark or equivalent that naturally end up somewhat vertical, that way the water wicks down the outside of the bark where it manages to find a piece of bark that is lower, etc and finally out the bottom of the pot.