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Container Gardening

Q. Re Perched Water Table Depth

18 years ago

This may be a silly question, but do all pots have the same perched water depth? Or, does it depend of how large the pot, and how deep the potting soil? I suppose it also depends of the porosity of the planting medium...the more air that it can pull in behind it as it drains, the more will drain out. Does any of that make any sense?

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Comments (5)

tapla (mid-Michigan, USDA z5b-6a)
18 years ago
The level (height) of the PWT has nothing to do with pot size or shape and everything to do with soil particle size and shape.

Mix a batch of soil. Now, fill ten pots of varying proportions with soil from the same batch. Make sure you have one that is three feet deep & only 6 inches in diameter and one that is a foot deep and 3 feet in diameter. Mix in a cone shaped one & a pot-bellied one - a rectangular one & an octagonal one if that suits you. Line 'em all up. If you pick a pot at random & measure the height of the PWT & tell me what it is, I can tell you immediately how high the PWT will be in each of the other pots. It will be exactly the same height.

Practically speaking, the smaller and less uniform the size of soil particulates, the higher the PWT will be. Using a wick to drain the PWT makes the water occupying the PWT react as though the pot was deeper. Gravity pulls water down the wick because it is actually an extension of the soil & causes the water to behave essentially as if the pot bottom does not exist.

Al
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terryisthinking
18 years ago
I believe you, but
That is just weird.
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If the mix drains well enough there will be very little perched water. If there is a little perched water and the rest of the medium is dry, I would probably have to water. If you made the 511 correctly there is probably no perched water, or very little anyway. If you have shallow roots that's probably why the top is drying out faster. That's the beauty of the 511.......almost no chance of overwatering. When in doubt water.
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I am seeing this thread for the first time, and while I did not read the whole thing I am wondering if the original question was really addressed. The original poster is talking about situations where the root zone is far away from the perched water table. Instead of addressing that specific issue, somehow this thread got directed to the question of whether perched water table matters at all (clearly it does). Let's consider a specific example. Say I have an oil vase that is four feet high. I fill that will 511. Then I plant shallow rooted plants in that oil vase, so those roots are about 3.5 feet away from the bottom of the pot. In this post, a user makes the point that you want the root zone to have less than 5 kPa of pressure/tension in order to be able to take water out of the soil and nourish the roots. I do not understand this issue well, but using the data in that post (i.e., 1 kPa = 3.94 inches for a particular soil mix), that seems to imply that this soil mix would get to 5 kPa at about 20 inches of height from the top of the perched water table. You would get to 5 kPa at: 3.94 in/kPa * 5 kPa = 19.7 in ~= 20 in. That might imply that you want to keep the bottom of your root zone within 20 inches of the top of the PWT, otherwise the roots might have problems getting the best extraction of water out of the soil, in the days after watering. So back to the original question that was posted, it seems to me that there might be value in looking at the root behavior of what you are planting, and the size of the pot, and you might want to keep the bottom of your roots within an appropriate distance from the top of the PWT, in order to give the roots an easier time in extracting water out of the soil. That's way more complexity than most of us are willing to deal with, and I am sure there is a lot of science and practical understanding that I do not have on this issue. But maybe it is not a good idea to put a shallow rooted plant into a very deep container.
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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. Al
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A PWT is called 'perched' because while there is a hole in the container, surface tension keeps the water 'perched' above the hole. If there were no media in the pot, all the water would just drain out the hole instead of 'perching' above it.
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Millie_36
Original Author
18 years ago
Well duh! I should have known that....kind of guessed, but wasn't sure. I could see how the bigger particle size would allow more air space and break the water tension. Pot depth will only matter to the particular plant and how much root room it needs.

What had me straining the gray matter on this is that another forum member and I have been playing with using composted pine bark and composted cotton burrs as rooting medium under mist. I believe from other posts that she has a timer and I do not. So far I haven't rotted a rose cutting, but I do think I probably need to sift out the "fines" for insurance. The saving factor for me is that I am using the 3/4 to 1 gallon per hour mist (fog) nozzles, and have them 2' above the cuttings and spraying upward so that I get even more drift.

Al, One more question or opinion. Will making cuts up the sides of the pots do anything to break water tension? I made large holes in cottage cheese cartons and then took them to a band saw to cut from the drain hole up about 3 inches into the sides of the pot.
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tapla (mid-Michigan, USDA z5b-6a)
18 years ago
If the soil is holding too much water, there would be some benefit in the way of evaporation from the additional soil that is exposed to air at the saw cuts or holes in container sides, but drainage would be exactly the same with or without the cuts. Aside from the evaporative factor, there is no advantage in having 100 drain holes in container bottoms over a single hole. Both arrangements are equally effective at draining water.

If the mix is highly aerated, there would probably be some additional gas exchange or air moving through the soil as a result of the extra holes/saw cuts, but again - drainage would remain the same.

Al
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Millie_36
Original Author
18 years ago
Ok, thanks for the advice. So far, so good with this experiment. I kind of like the idea of eliminating one pot-up step.
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