Ask a Question forum: Does a pot hole need to have access to air?

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Virginia (Zone 7a)
Rez
Aug 3, 2016 11:21 AM CST
Some (plastic) pant pots have their holes on the side but some other have it below so when I put them on a tray the hole doesn't get any air. I've made a couple of pots from buckets I got from the dollar shop and burned a hole on their bottom so again they don't really get ventilation from the hole. Is this alright or should I place the pots in such a way that they do get such ventilation? Thanks.
springfield MO area (Zone 6a)
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Frillylily
Aug 3, 2016 11:28 AM CST
I wonder this myself. I had a large plastic tub (probably 30 inches tall and across) and I cracked the bottom of it w my shovel. then I filled it with rocks and then soil and planted a clem vine in it. It is sitting directly on the ground with nothing lifting it up off the soil, my clem has been looking kind of ratty lately and I blamed on the 100F temps. But we have had TONS of rain this past couple months and now am wondering if the pot isn't draining as it should be? I guess in the back of my mind, I am thinking I should have put it up on some 2x4s or something to lift it up and allow it to drain. Afraid having it sit right on the soil kind of sealed it so that it won't drain. Now I don't know what to do because it is heavy and there are plants around it now and the vine has been in it for a year now. It did bloom really beautifully this past spring-before all the rain though. I think if you are starting out-haven't got them filled and planted already, I'd probably raise them up a tad, I think it surely would help them to drain better. Wish I had did mine. D'Oh!
Name: Daisy
Reno, Nv (Zone 6b)
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DaisyI
Aug 3, 2016 11:41 AM CST
Its not exposure to air your pot holes need but they do clog with dirt easily if the pots are sitting directly on the ground. For pots whose bottoms will be sitting directly on the soil, lift them a little. I use old tiles left from a house project and put them under the edges of the pot.
Name: Jay
Nederland, Texas (Zone 9a)
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Horntoad
Aug 3, 2016 11:42 AM CST
If the holes are on the bottom then it probably should be lifted on boards, pot feet or whatever allows a space between the hole and surface. Not for ventilation but for drainage.
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Name: Sue
Ontario, Canada (Zone 4a)
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sooby
Aug 3, 2016 11:43 AM CST
I don't think it is usually a problem, I have pots with only holes on the bottom and they drain OK. I think the water draining through perhaps brings in air from the top. Frillylilly if your pot isn't draining excess water well it could be because of the rocks in the bottom. Having coarse material at the base of a pot causes the potting mix above it to retain more water (perched water table). Having the same texture throughout the pot drains better.
springfield MO area (Zone 6a)
I was one of the first 300 contributors to the plant database! Plant Identifier
Frillylily
Aug 3, 2016 3:37 PM CST
sooby said:I don't think it is usually a problem, I have pots with only holes on the bottom and they drain OK. I think the water draining through perhaps brings in air from the top. Frillylilly if your pot isn't draining excess water well it could be because of the rocks in the bottom. Having coarse material at the base of a pot causes the potting mix above it to retain more water (perched water table). Having the same texture throughout the pot drains better.


Can you explain this some more? I don't understand? I put the rocks in because I thought it would allow for better drainage Confused
Name: Sue
Ontario, Canada (Zone 4a)
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sooby
Aug 3, 2016 4:17 PM CST
When you put fine material over coarse material in a pot, the fine material will not drain into the coarse material until the fine material on top is so wet that it cannot hold any more water. The water does not easily cross the interface between the two different textured materials. Yes it is counter-intuitive so I tested it myself with potting mix over polystyrene packing peanuts and it was absolutely true, the mix in that pot held more water than the same sized pot with all potting mix. This has been known for a very long time yet so many articles still recommend using gravel or other coarse material at the bottom of a pot for drainage. There are some old pictures on the web that show what happens when water is poured into a container of fine medium over coarse, but I'm not sure if the link will work, let's try:

https://www.google.ca/search?q=%22perched+water+table%22&cli...
Name: Daisy
Reno, Nv (Zone 6b)
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DaisyI
Aug 3, 2016 4:21 PM CST
I agree It would be a better use of gravel to put it under the pot.
Name: Carol Roberts
Huntington Beach, CA (Zone 10b)
Sunset 24
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CarolHB
Aug 3, 2016 4:36 PM CST
I've stopped using anything in the bottom of the pots - only potting mix and the potting mix does not fall out, nor do I usually have drainage problems. I have some pots over 20" that didn't seem to have enough holes in the bottom so I've drilled holes as close to the bottom as I can get but in the sides. This way I know they're draining because I can see it. I put all my containers on gravel, pavers, elevate with a couple of bricks, etc., and never let them touch actual ground - no grass no dirt - mainly because of slugs and other undesireables, but also the drainage problem. My actual ground is all concrete-like clay which doesn't absorb anything so all my gardening is done in containers.
Can't complain too loud about how the ball bounces when I'm the one who dropped it.
Name: Carol
Santa Ana,Ca. (Zone 10b)
Sunset zone 22
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ctcarol
Aug 3, 2016 4:52 PM CST
I also put my pots on pavers or something, but it's to keep the roots from growing into the ground beneath them.
Name: Carol Roberts
Huntington Beach, CA (Zone 10b)
Sunset 24
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CarolHB
Aug 3, 2016 5:00 PM CST
Oh, yeah - I forgot about that one. Roots can be a problem, especially if they're tap roots and you don't know if you want that plant to be in that spot forever. Ya wanna ask me how I know? D'Oh!
Can't complain too loud about how the ball bounces when I'm the one who dropped it.
Name: Rick Corey
Everett WA 98204 (Zone 8a)
Sunset Zone 5. Koppen Csb. Eco 2f
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RickCorey
Aug 3, 2016 5:15 PM CST
If the mix is coarse enough to have some open air channels despite the mix being saturated with water, air can diffuse pretty well down the whole depth of the root zone (the height of the pot). That's how natural soil works: the rest of the earth acts as a wick and lets the topsoil drain down to its "field capacity". Capillary force can wick water up from the water table into a "capillary fringe" whose depth depends on the fineness of the soil at that depth.

I think that any container is mostly "capillary fringe" or "perched layer" until it is dry.

I usually THINK I partly understand "perched water" right when I'm reading those articles, but then nothing sticks to my brain afterwards. So I must not UNDERSTAND it. But for some reason, it's only a problem in pots, not in the field. ???

Anyway, if the mix in the pot is so fine that water stays perched or held by capillary force to FILL most of the small pores in the pot, oxygen can;t diffuse through the liquid as it would through air. Gasses diffuse 10,000 times faster through air than through water.

So if your potting mix is coarse enough, the holes in the pot only need to let water out. The potting mix will then let water in from the top.

If your potting mix is fine and dense, with few pores in the soil larger than 75 microns, those pores will fill with water each time you water, and seal air OUT until the plant drinks enough to lower the perched and saturated layers below the parts of the roots you didn't want to drown.

However, as a plan, that's like a fraternity that decides to cram themselves into a phone booth and then have someone fill it with beer - they want to see if they can drink the beer fast enough that only a few of them drown.

Better to plan for your potted plants, or potted frat brothers in a phone booth, to have enough breathing holes that water and beer can escape freely, and then air can enter freely.

I always drill some holes in the sides of any tall pot, "to let some air in near the bottom". And to let the water out, for sure!
[Last edited by RickCorey - Aug 3, 2016 6:18 PM (+)]
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Name: Rick Corey
Everett WA 98204 (Zone 8a)
Sunset Zone 5. Koppen Csb. Eco 2f
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RickCorey
Aug 3, 2016 5:15 PM CST
https://en.wikipedia.org/wiki/Pore_space_in_soil


- Macropores are over 75 microns in diameter. These are big enough that water in them does not experience much capillary force, and can drain out freely due to the force of gravity. Air will enter these pores even when the soil is at “field capacity” and allow rapid diffusion of oxygen, even if plant roots don’t “suck them empty”.

- Mesopores are 30 – 75 microns in diameter. Water in these pores won’t just flow down and out because of gravity, but plant root hairs CAN extract water from them. These fill with water at field capacity, and then plants extract the water over the next few days or weeks.

I think that water in this size of pore is somewhat mobile, up-down and sideways, in response to capillary and other forces. This is where capillarity and “other forces” are approximately equal. Those articles start to have a lot of math at this point!

- Micropores are 5-30 microns in diameter. Capillary forces LOCK that water in place so it doesn’t flow sideway, up or down, but apparently plant root hairs CAN suck at least some water out of them. This is where the plants and soil capillarity play tug-of-war.

https://en.wikipedia.org/wiki/Pore_space_in_soil#Micropore

Name: Ken Ramsey
Starkville, MS (Zone 8a)
[url=www.tropicalplantsandmore.com]
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drdawg
Aug 3, 2016 5:15 PM CST
Not just roots, how about earthworms. They love to get into my pots and do so even though my pots generally are on slated wood platforms that raise the pots 2" off the ground. I cannot tell you how many times I have un-potted a fiddle leaf or banana leaf ficus to prepare for shipping, and low and behold, there is a large earthworm happily living there. I am careful to look for them after the episode with the lady in New York City. Of course, if I could have it my way, I would love to have my potted plants as homes for earthworms.
drdawg (Ken Ramsey) - Tropical Plants & More
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Name: Sue
Ontario, Canada (Zone 4a)
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sooby
Aug 3, 2016 5:47 PM CST
"But for some reason, it's only a problem in pots, not in the field. ???"

I believe it can occur in nature but it isn't always a problem in growing plants, it can be used to advantage. USGA golf greens (and I think other sports turf) are deliberately constructed that way for example.

Section 6.3.7 below shows the container experiment better than the one I found above, perhaps.

http://personal.psu.edu/asm4/water/drain.html
Name: Bob
Vernon N.J. (Zone 6a)
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NJBob
Aug 3, 2016 6:58 PM CST
Ken I put a piece of screen over the holes in the bottom of pots before filling, ants and earwigs more the problem here.
Name: Rick Corey
Everett WA 98204 (Zone 8a)
Sunset Zone 5. Koppen Csb. Eco 2f
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RickCorey
Aug 3, 2016 7:12 PM CST
For some reason I have trouble making sense of their diagrams or photos - they seem very blurry and I didn't figure out how they show "where the water is". But I agree with the text.

As long as I just "take it on faith" that the boundary where a fine-textured layer sits on top of a coarse-textured layer is practically an IMPERVIOUS layer until the surface layers are "saturated" and water HAS TO come out the bottom when more comes in the top, I can agree with whatever articles ay about perched payers.

But why is that?

I understand that capillarity is stronger than gravity in soil. If there is no capillary connection between (say) a layer of peat moss sitting on top of gravel, and the gravel under it, the only force acting to pull water out of the peat is "mere gravity" and the capillary attraction of close-packed peat fibers trumps gravity by a large amount.

Again, WHY is there no capillary connection?

Today my best answer is: "capillarity comes from small things spaced closely together. With gravel, the ONLY capillary sites are where one grain touches another grain - like 3-4 POINTS of contact per grain of gravel". Be generous and say there are 1-2 grains of gravel in every cubic centimeter. Phooey, that's only 6-8 POINTS of contact per cc!

A cubic cm of peat must have hundreds if not thousands of fine fibers, each touching the other for their entire length.

In this way of looking at it, water in the peat layer is pulled HARD into thousands of times as many capillary crevices in each cc of peat, as in gravel. Maybe tens or hundreds of thousands times more capillary area. And in peat, every capillary layer is in intimate contact with other capillary layers, and water can be pulled from one to the next, continuously through the whole mass of peat.

And what happens to a brave water molecule that manages to escape from the peat into the gravel? It would just sit on whatever gravel grain it reached, finding no capillary path to move along. Then the water would stop moving into the gravel.

Or, one in ten or one in a hundred brave water molecules might reach a grain of gravel right NEAR a point where that grain touches one other grain! Big whoop, that one lucky water molecule might transfer to the NEXT grain of gravel by capillary attraction. It would stop there.

So whether or not there is any "connection" from the peat layer into the gravel layer, there just plain ain't no CAPILLARY PATHS in gravel!

The only way water can move through gravel or even INTO gravel is to become SATURATED enough in the upper layers that bulk amounts of water are FORCED out of the peat's capillary embrace and into the gaps between grains of gravel.

Where gravity is strong enough to make it flow downhill (between grains, due to gravity - not through capillary paths due to strong capillary forces.

Now that the water has escaped from capillary forces and is IN the gravel layer, there are almost no capillary sites except for the few points of actual contact between one grain and its neighbor. Those sites might hold a tiny amount of capillary water, but they are not connected, so there is no capillary flow.

Only when there is enough water in the gravel to saturate all the capillary sites can water move through gravel. The "free", excess water has no capillary forces, so it can just drop like a rock under the (weak) force of gravity.

I don't know if that's an accurate way of looking at it, but it lets me get to sleep at night!

If it sounds weird that capillarity is stronger than gravity, consider a capillary tube of glass. The ID of that tube is WAY larger than 30-75 microns! Despite that, the tiny "collar" where the water meets the glass lets the water feel the capillary attraction of the glass. That force is enough to lift ALL the water in that tube several mm, against gravity! The tiny capillary fringe around the circumference of the tube, plus surface tension, HAULS that heavy water up MULTIPLE millimeters, against the force of gravity! Like a flea hauling a dog on a leash. To moe the dog at all, it has to be one STRONG flea.

Now imagine the same effect in the soil, running through micropores and mesopores only a few tens of microns in diameter (a couple hundredths of a millimeter - say, room for several bacteria to wander in without being very crowded). Down there, it isn't 30 microns of capillary fringe hauling a huge mm-sized column of water, it's just 30 microns of capillary film hauling on 30 microns of water! Of course it's much stronger than gravity.
[Last edited by RickCorey - Aug 4, 2016 11:56 AM (+)]
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Name: Ken Ramsey
Starkville, MS (Zone 8a)
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drdawg
Aug 3, 2016 8:11 PM CST
Bob, I would need a lot of screens, a really lot of screens. I have approximately 1000 potted tropical plants, but probably only 300-400 are actually on the ground (or actually just off the ground). Still a lot of screening.
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