Perched Water Tables

12 years ago

With all my searching on perched water tables, I'm still unsure of a few things...

1) How can you identify a pwt in a container?

2) I know the pwt is said to disappear as the particle size reaches 1/8", so does that mean I could screen my turface with a 1/16" screen, then mix it with my growers sized gran-i-grit and have a mixture with no pwt? When screening turface over 1/8" screen I lose a lot of turface, so I'm hoping I can sick with screening over my regular 1/16" window screen and that the addition of grit will eliminate the pwt.

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Comments (5)

nallred91
Original Author
12 years ago
{{gwi:53888}}

This first mix is a mix of pea gravel and turface. I did screen the turface, but I'm not sure how I can check to see if there is a pwt in the bottom of the pot?

{{gwi:53890}}

This mix is pea gravel, perlite, and bark. The particle size for these are pretty big, so I'm assuming no pwt. The plants have been happy and growing in this mix since late summer, so I assume there is no pwt. But I would still like to know for sure. Either way, I plan on repotting these two plants into probably a mix of turface and grit in the spring.

{{gwi:53892}}

Here's another example of the turface/pea gravel. although here the pea gravel was a little dirty... But does the presence of the pea gravel here help to eliminate whatever pwt the turface may be causing?

{{gwi:53894}}

And here's the granite I plan on mixing with turface for all of my future pottings. by the looks of it, i don't think screening it with a 1/16" screen would do much.

any advice would be great. Hopefully I'll figure out a simple way to get a turface/grit/perlite mix that doesn't hold a pwt.
tapla (mid-Michigan, USDA z5b-6a)
12 years ago
If almost all of your soil particulates are larger than 1/10", you don't have a PWT. You can also see if your soil has a pwt by filling a plastic glass with your soil then melting or drilling a hole in the bottom and allowing the free water to drain. You should be able to identify the ht of any PWT by observing any delineation between soil that's saturated near the bottom and soil that ISN'T saturated nearer the top. You'll be able to tell by observing the difference between air-filled pores & water filled-pores.

Al
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Edit, I took a closer look and really most of the leaves on the top part of the plant look to have this dirty appearance. I understand that aphids can have this effect but I don't know how it would have all-of-a-sudden gotten a massive aphid infestation.
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You could repot and mix in chicken grit or sand. Chicken grit is a very, very fine gravel. You can also buy some NAPA floor dry, (NOT item 8822, you do not want diatomaceous earth). This is fired clay, so it wont break down, it holds moisture but not too much. I have used this stuff as 100% of my soil in many potted plants. On thing I have found with my bamboos, is that they love bark in the soil. So rather than gravel at the bottom of the pot I put pine bark mulch. It drains and once the roots get down that far, they root to the bark. This post was edited by botanicalbill on Wed, Mar 26, 14 at 19:57
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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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Appreciate the additional comments folks! Gardengal: Thank you for the clarification on PWT, I think I probably overestimated how much 5-1-1 would reduce the PWT. In time I will get deeper pots, but for now I think I will just regularly rock the pot after watering. I think these pots are borderline too big for these trees right now, which is maybe why water is staying in them longer than I expect. The trees are only starting their second season with me, they are not big trees. Ken: I probably shouldn't have tagged this in the houseplants forum because they are outdoors. I did so because I saw much of the 5-1-1 and PWT chatter seemed to happen in that forum, and I thought the concepts would apply whether indoor or outdoor. You are correct that there are many variables here and I'm less concerned about solving this particular instance of YLD because I think on the whole my trees are healthy. I was mostly wondering if my drainage hole situation could be causing a higher PWT or if I just need to adapt a little to the properties of my mix. Based on posts above, I am going with the latter. Silica: Yes, both trees are covered in flower buds and/or new leaves. Only a few buds have opened so far. This is my first season of active flowering with both trees, so I'm not sure what to expect in terms of YLD related to flowering. It's not hard for me to imagine that reduced fertilizing over the winter left them under prepared for this new growth. We also got several days of rain (rare for Phoenix) that may have contributed to them getting a little water logged. I assumed 5-1-1 mix, they could take it. I also haven't added any vinegar to my hard water, so there could be pH issues as well. 😕
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nallred91
Original Author
12 years ago
Ah thank you, I should have thought about that... that seems like a perfect way to check...
Ruby Chang
12 years ago
Thank you for providing this easy experiment. I used this mechanism to check as you stated above. However...I think I have significant perch water b/c:

1) when I drill hole in bottom to allow free water to drain...most drains out...so that is "free water to drain"..but once it stops draining, I tilted the pot...even more water will drain (that means there is perched water right?)

2) I can't tell very well the delineation between the saturated and very saturated soil...so I also inserted a wick to see if more water will drain if I poke a tooth pick in...my soil did not have extra water that drain due to the tooth pick insert...but did drain more (regardless of inserting tooth pick insert or not)...

Another question: how much perched water is significant trouble if I am using a regular 4in pot?

Ruby
Ruby Chang
12 years ago
...ok...another dumb question...to use the experiment above...how big of the container must it be? I know from reading the forum that the PWT height will always be the same no matter what size of the container it is (Al's example with the funnel)...But I don't think I am understanding the concepts very well...

There must be at least 3in of soil in the container if the PWT height is 3inches right? ...so how much soil should we have in how large of a container? I am confused...I am going to reread that thread again...maybe I will understand a little better?

Perched Water Tables

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nallred91
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tapla (mid-Michigan, USDA z5b-6a)

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Perched Water Tables

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nallred91Original Author

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

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nallred91Original Author

Ruby Chang

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nallred91
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nallred91
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