Perched Water Table In Container

8 years ago

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

Featured Answer

tapla (mid-Michigan, USDA z5b-6a)

8 years ago

Imagine a jar full of pudding or fine sand. In your mind's eye, start
adding perlite to either. Does 1 perlite particle make a difference in
drainage or aeration, the properties you would be seeking to improve
with the addition of perlite? Of course not. 100 pieces? No. 1,000? No.
There is a level at which perlite starts to make a notable difference in
the properties mentioned (aeration/drainage). It's called the threshold
level and it's established at the point when there is no longer enough
sand or pudding to fill all the air spaces between the perlite
particles. Even in the mind's eye you can see that it takes a very large
fraction of coarse material and a small fraction of fine material to
make a notable difference in drainage/aeration in most cases. That's why
you can't effectively amend soils based on fine particles by adding
even a pretty healthy measure of pine bark or perlite.If you end up
having 75% pine bark and the rest of the 25% fraction comprised of equal
measures of peat or potting soil + perlite, you're not amending the
peat or potting soil with bark - you're amending the fraction that's
larger by far with the ingredients that in combination make up the
lesser volume of 25%

If you start with a gallon of Miracle-Gro
potting soil and add 1 gallon of pine bark and/or perlite to it, have
we created notably better aeration or drainage? No. Even at 2 gallons of
bark and perlite there is still enough potting soil to fill all the
large air spaces between the coarse material - so little gain. At 3
gallons of bark and/or perlite, when there is not enough fine material
in the gallon of potting soil to fill the spaces between the large
particles, we start to make notable headway. This is why you can use 3
ingredients that when mixed together CAN make an extremely productive,
root-friendly medium, and mix them in such a way that the end product is
almost unusable.

Soggy mediums are particularly problematic because they make Ca unavailable, not to mention the fact that the
low
Oxygen levels accompanying soil saturation also cause iron and manganese to become electron acceptors, which leads to their precipitation, and even
though these elements might be present in soils, the plant’s
inability to assimilate them.

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Comments (10)

Jean
8 years ago
last modified: 8 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
8 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
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Seysonn_ 8a-NC/HZ-7
Original Author
8 years ago
last modified: 8 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
8 years ago
last modified: 8 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.
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lucillle
8 years ago
last modified: 8 years ago
Here is more info. I really like the pudding example best, though.

http://oregonstate.edu/dept/nursery-weeds/feature_articles/physical_properties/physical_properties.html
Seysonn_ 8a-NC/HZ-7
Original Author
8 years ago
Al.

Thanks for coming by and commenting.

.

Sey
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tapla (mid-Michigan, USDA z5b-6a)
8 years ago
Lin Gee
4 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?
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coolbythecoast
4 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.
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