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Perched Water table


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Whats up guys!! i already hear you all asking... WTF is a perched water table, and can i Drink it?

this is some very important information regarding pots and mediums that i feel should be available here on the forum.

Well, basically, the perched table is the saturation point, where the capillary action in the soil is canceled out by the force of gravity. Every type of growing media has a different perched table. Capillary action will pull water up from a certain point, and below that point, gravity keeps the water from moving up. The size of the container does not affect the height at which the perched table occurs. See the figure below.

enter image description here

 

If we fill five cylinders of varying heights and diameters with the same soil mix and provide each cylinder with a drainage hole, the PWT will be exactly the same height in each container. This is the area of the pot where roots seldom penetrate & where root problems begin due to a lack of aeration. From this we can draw the conclusion that tall growing containers are a superior choice over squat containers when using the same soil mix. The reason: the level of the PWT will be the same in each container, with the taller container providing more usable, air holding soil above the PWT.

gravel-in-pots-perched-water-table

This information is very important for us cannabis growers as finding a happy medium is always tricky.

If the same potting medium is used, irrespective of the size or shape of the pot, the perched water table always stays the same height because it is determined by the wicking ability of the potting medium, since gravity doesn’t change.

The way to increase drainage of the perched water table is to add materials throughout all of the potting medium to increasing the air spaces in the mix and reduce capillary action.

Perlite and vermiculte are materials which are used as soil amendments, and both are minerals that are made more porous by expanding them with heat, much like popcorn. Because they have large air spaces within them, they are used to increase the drainage and aeration in potting mixes. Perlite mainly increases drainage, while vermiculite will also hold some moisture and help retain nutrients too. Mixing either of these amendment materials right though a potting mix will increase aeration, improve drainage and reduce the height of the perched water table.

The lower the perched water table, the more frequently you will need to water, the higher the perched water table, the riskier it becomes for the plants roots which are navigating below the PWT.

another way to lower the PWT if you already have plants in your medium, is to raise the pot off the floor onto a grid of sorts so that the floor doesnt stop the drainage of water as easily if it were flat on the floor. 

 

EDIT - Sources

https://gardening.stackexchange.com/questions/13774/what-is-a-perched-water-table#:~:text=There is%2C in every pot,not allow it to drain.

https://deepgreenpermaculture.com/2019/09/06/should-you-put-gravel-or-rocks-at-the-bottom-of-plant-pots-for-drainage/

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So basically, it is easier to drown a plant in a smaller / shorter pot compared to a bigger / taller pot, even when both have had water added to them until run off?

 

Any idea how this differs between coco and soil and the mixes with added perlite etc?

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3 hours ago, PsyCLown said:

So basically, it is easier to drown a plant in a smaller / shorter pot compared to a bigger / taller pot, even when both have had water added to them until run off?

 

Any idea how this differs between coco and soil and the mixes with added perlite etc?

Basically, the finer the substrate, the easier it is for the capillary action to do its thing. 

So soil would have a range of the tiniest to the largest particles... But mostly small and sand.... So without enough perlite or large bits in the soil it would likely have the highest PWT by a long shot. 

Coco has mostly small to medium sized particles with tiny particles mostly rinsed out. This makes the coco very good to hold not only water, but allow oxygen to permeate the substrate. Plain coco would have a PWT below that of soil, often times below that of soil with amended perlite too... 

Coco with perlite would be the go to choice over vermiculite as the vermiculite can hold a lot of water which may raise the PWT instead of lowering it. 

Straight up... I have no idea what the ideal PWT is... 1cm...5cm... No cooking clue... But experts recommend a 20-30% perlite in the mix is ideal... Perhaps they have taken this into consideration 

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