Dealing with Water-Retentive Soils

11 years ago

Dealing with Water-Retentive Soilscolor>size>

A good friend recently asked me if putting a brick in the bottom of a container interferes with drainage? After reading the question, it occurred to me that there are aspects to the question that Ive discussed very little here at GW. It also occurred to me that I could use her question to help those who grow in heavy (water-retentive) soils. IÂm going to define those soils, but this isnÂt about disparaging soil types - itÂs about helping you try to squeeze the most plant vitality (and the water) out of them. Heavy soils are based on fine ingredients. If the soil contains more than 30-40% of any combination of peat, coir, compost, or other fine ingredients like builders sand or topsoil, it will retain appreciable amounts of 'perched water' and remain soggy after itÂs saturated - and this is about dealing with soggy soils.

Perched water is water that remains in the soil after the soil stops draining. If you wet a sponge & hold it by a corner until it stops draining, the water that is forced out of the sponge when you squeeze it is perched water. From the plantÂs perspective, perched water is unhealthy because it occupies air spaces that are needed for normal root function and metabolism. The gasses produced under anoxic (airless) conditions (CO2, sulfurous compounds, methane) are also an issue. The main issue though, is that roots deprived of sufficient oxygen begin to die within hours. You donÂt actually see this, but the finest, most important roots die first. The plant then has to spend stored energy or current photosynthetic (food production) to regenerate lost roots - an expensive energy outlay that would otherwise have been spent on blooms, fruit, branch extension, increasing biomass, systems maintenance Â.. Perhaps the plant would have stored the energy for a winterÂs rest and the spring flush of growth instead of expending it on root regeneration.

You can see that perched water, from the plantÂs perspective, is not a good thing. From our own perspective, we think itÂs rather convenient when we only need to water our plantings every 4-5 days, but because we canÂt see it, there is a sacrifice in potential growth/vitality for our convenience - like driving on low tires reduces fuel economy. How we choose to resolve this issue is of no concern to me - we all arrange our priorities & few of us are willing to water plants every hour to squeeze the last wee bit of vitality from them. Growing is about compromise in more cases than not. There is no judgment passed here on soil choice.

If you donÂt agree that perched water is generally a bad thing in containers, thereÂs no need to read on. If youÂre still interested, IÂll lay a little groundwork here before I outline some things remedial you can do to combat excess water retention. Almost all out-of-the-bag soils retain a considerable amount of perched water after they have been saturated. Each individual soil formulation will retain a specific height of perched water unique to THAT soil. No matter what the shape or size of the container - height, width, round, square ÂÂ the height of the PWT (perched water table) will be the same. You can fill a 1" diameter pipe with a particular soil or a 55 gallon S-shaped drum with the same soil, and both will have exactly the same PWT height.

LetÂs do some imagining for the purpose of illustration. Most peat or compost based soils retain in excess of 3 inches of perched water, so lets imagine a soil that retains 3 inches of perched water. Also, imagine a funnel that is 10 inches between the exit hole & the mouth and is filled with soil. Because we are imagining, the mouth is enclosed & has a drain hole in it. In your minds eye, picture the funnel filled with a soil that holds 3 inches of perched water, and the soil is saturated. If the funnel is placed so the large opening, the mouth, is down, you can see the largest possible volume of soil possible when using this container is saturated, the first 3 inches; but, turn the funnel over and what happens? We KNOW that the PWT level is constant at 3 inches, but there is a very large difference in the volume of soil in the lower 3 inches of the funnel after it is placed small end down. This means there is only a small fraction of the volume of perched water in the small-end-down application vs. the large-end-down. When you tip the funnel so the small end is down, all but a small fraction of the perched water runs out the bottom hole as the large water column seeks its 3 inch level in the small volume of soil. In a way, you have employed gravity to help you push the extra water out of the soil.

You havenÂt affected the DRAINAGE characteristics of the soil or its level of aeration, but you HAVE affected the o/a water retention of the container. This allows air to return to the soil much faster and greatly reduces any issues associated with excess water retention. OK - we can see that tapered containers will hold a reduced VOLUME of perched water, even when drainage characteristics, aeration, and the actual height of the PWT remain unchanged, but we donÂt and wonÂt all grow in funnels, so lets see how we can apply this information PRACTICALLY to other containers.

Drainage layers donÂt work. The soil rests on top of drainage layers, then the water ÂperchesÂ in the soil above - just as it would if the soil was resting on the container bottom. Drainage layers simply raises the LOCATION where the PWT resides. But what if we put a brick or several bricks on the bottom of the container? LetÂs look at that idea, using the soil with the 3inch PWT again. LetÂs say the brick is 4x8x3 inches tall, and the container is a rectangle 10x12x12 inches high. The volume of soil occupied by perched water is going to be 10x12x3, or 360 cubic inches. If we add the brick to the bottom of the container so the height of the brick is 3 inches, it reduces the volume of soil that can hold perched water, so for every brick you add (4x8x3=96) you reduce the volume of soil that can hold perched water by 96 cubic inches. If you add 3 bricks, the volume of soil that holds perched water would be 360-288, or only 72 cubic inches, so you have reduced the amount of perched water in the container by 80% Â.. quite a feat for a brick.

Your job though, is to be sure that what you add to the bottom of the container to reduce the volume of soil that can hold perched water doesnÂt create stress later on when the planting has matured. Be sure the container has a large enough volume of soil to produce plants free from the stress of excessive root constriction. You donÂt want to trade one stress for another.

How else might we ÂtrickÂ the water in the container into leaving? LetÂs think about the following in 2 dimensions, because itÂs easier to visualize. If you look at the side view of a cylindrical or rectangular container, you see a rectangle, so imagine a cylinder or rectangle 10 inches wide or 10 inches in diameter and 8" deep. Both side views are rectangles. Now, draw a horizontal line 3 inches above the bottom to represent the level of the PWT. Remember, this line will always remain horizontal and 3 inches above the bottom. Now tip the container at a 45 degree angle and notice what happens. The profile is now a triangle with an apex pointing downward and the base is of course the line of the PWT 3 inches above the bottom. Can you see there is a much lower volume of soil in the bottom 3 inches of the triangle than in the bottom 3 inches of the rectangle? The PWT line is level at 3 inches above the apex, so by simply tilting your containers after you water, you can trick a large fraction of the unwanted perched water to exit the container. Sometimes it helps to have a drain hole on the bottom outside edge of the pot, but not always. Only when the location of the hole is above the natural level of the PWT when the pot has been tilted does it affect how much additional water might have been removed.

On the forums, IÂve often talked about wicks, so IÂll just touch on them lightly. If you push a wick through the drain hole and allow it to dangle several inches below the bottom of your container immediately after watering, the wick will ÂfoolÂ the perched water into behaving as though the container was deeper than it actually is. The water will move down the wick, seeking the bottom of the container and will then be pushed off the end of the wick by the additional water moving down behind it.

A variation of the wick, is the pot-in-pot technique, in which you place/nest one container inside another container with several inches of the same soil in the bottom and fill in around the sides. Leaving the drain hole of the top container open allows an unobstructed soil bridge between containers. Water will move downward through the soil bridge from the top container into the bottom container seeking its natural level; so all of the perched water the soil is capable of holding ends up in the bottom container, leaving you with much better aeration in your growing container.

The immediately above example employs the soil in the lower container as a wick, but you can achieve the same results by partially burying containers in the yard or garden, essentially employing the earth as a giant wick. These techniques change the physical dynamics of water movement and retention from the way water normally behaves in containers to the way water behaves in the earth. Essentially, you have turned your containers into mini raised beds, from the perspective of hydrology.

What I shared doesnÂt mean itÂs a good thing to use water retentive soils, simply because you have tricks to help you deal with them. For years, IÂve been using highly aerated soils and biting the Âwater more oftenÂ bullet because IÂve seen the considerable difference these durable and highly aerated soils make when it comes to plant growth and vitality. Many others have come to the same realization and are freely sharing their thoughts and encouragement all across the forums, so I wonÂt go into detail about soils here.

It should also be noted that roots are the heart of the plant, and it is impossible to maximize the health and vitality of above-ground parts without first maximizing the health and vitality of roots. Healthy roots also reduce the incidence of disease and insect predation by keeping metabolisms and vitality high so the plant can maximize the production of bio-compounds essential to defense.

The soil/medium is the foundation of every conventional container planting, and plantings are not unlike buildings in that you cannot build much on a weak foundation. A good soil is much easier to grow in, and offers a much wider margin for error for growers across the board, no matter their level of experience. But regardless of what soils you choose, I hope the outline here provides you with some useful strategies if you DO find yourself having to deal with a heavy soil.

Featured Answer

tapla
Original Author

7 years ago

last modified: 7 years ago

First, thanks for the very kind words. ..... and 6-8" here in MI is a light dusting. ;-) My friend from VA is wringing her hands over the 8-12 they're expecting, too.

1) If you're using lime and the medium is moist, 2 weeks would be great, but I often plant next day. Peppers and tomatoes are sort of predisposed to BER, so giving some time for the reactive phase to take place can be helpful, even though BER is usually more of a physiological issue than an actual soil deficiency of Ca.

2) I mix ingredients on a tarp. I pit down the bark and moisten it, then I put down the peat, lime, and any additives, followed by the perlite, which I also moisten. Don't bother with wetting the peat because it'll be hydrophobic at the mixing stage. Then I mix everything with a garden rake, careful not to wreck the tarp. It's easy to avoid poking holes in it or tearing it. I then tug the corners so the mix is in the center and go from there. Within an hour the tendency of the peat to be hydrophobic will be "broken". Was that an answer to your question?

3) My 5-1-1 mix last year would just NOT dry out even using a wick (perched water?) and numerous peppers and tomatoed passed away. Any way to remedy this? Now that you have a year of experience, you can pay close attention to how fine the bark is. That's what dictates how much peat to add, or how much extra perlite if the bark is very fine (for the 5:1:1). Raising this issue is a good example of why understanding the concept is more important than following a recipe, although that statement is more true for the 5:1:1 than the gritty, if we allow for substitutions. ..... and don't water so much! ;-)

4) I usually don't fertilize transplants for about 2 weeks after the fact. There is a lot of support for the observation that roots colonize the soil mass much faster if the plant is slightly under-nourished for a period after transplanting. I think though, that this is less important when you're using a soil that doesn't set you worrying about soil staying soggy for so long it's sure to be an issue. Still, I always wait. That opportunity does go right out the window, though, when/if we add a starter charge of fertilizer when we mix the soil.

Now might be a good time to remind readers that when you're making your own soils and want to add a slow release or controlled release starter charge to your mix, don't do it when you MAKE it, do it when you actually USE it. CRFs (like Osmocote/Dynamite) and slow release fertilizers (the granular stuff, but not the stuff you mix w/water) will continue to release or dissolve as long as the soil is moist, so fertilizer burn is a real possibility if you add the fertilizer when you MAKE the soil.

When temps are between 65-85, I usually fertilize weekly with 9-3-6 @ 4-5 tsp/2.5 gallons of water, so about 1.5-2 tsp per gallon. When it's below 55*, I don't fertilize (to guard against ammonium toxicity), and if it's above 85* I don't fertilize, or do so sparingly. A higher salt content in the soil solution makes it harder for the plant to absorb and move water when it needs to be most efficient at that chore.

Lots of growers use something like a 3:1:1 for SWCs, but again - "ideal" depends on bark size.

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Comments (80)

jojosplants
11 years ago
last modified: 7 years ago
O.K.
What's up? GW glitch or what? I know I saw more posts here earlier, and was planning on catching up on some reading. Where did they go?

JoJo
sissysimone
11 years ago
last modified: 7 years ago
Will someone please tell me what is going on ? i have been enjoying what Al and others have to say until for some reason the things people were saying were just thrown away.Why should I come here when good information is erased?what is going on around here, i know there are a lot of writings missing?
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tapla
Original Author
11 years ago
last modified: 7 years ago
Sorry, guys. I had written a note to GW admin asking them to review this thread because of the nature of some posts that have since been deleted, which is why it appears that some of the info I had posted had been deleted along with it. I don't think they deleted my posts because they found anything offensive in them, I was careful to ensure there wasn't; it's just that they were replies to posts no longer here, so not much has actually been lost ...... same thing on the 'Pine Needles Thread'.

I'm working on another thread that goes into more detail about limiting factors and how they affect plant growth. Actually, having to explain why perfecting one or all of the factors that potentially limit growth can't make up for the one factor that does limit growth was a good thing. It gave me an idea for another thread that I'm hoping will garner some interest and start another fruitful discussion. I hope you'll watch for it.

Thanks for being so attentive! ;o)

Al
engineeredgarden
11 years ago
last modified: 7 years ago
Al, thanks for supplying us readers with some great information. Gosh, i've learned so much from you about growing in containers, and just wanted to let you know that it is greatly appreciated. My plants appreciate it, too!

EG
tapla
Original Author
11 years ago
last modified: 7 years ago
You're welcome. I often speak at various gatherings - mostly gardening or garden-related clubs, MG groups and such. Often, at the end of the talk when thank yous are being expressed, I ask the group to take the time to teach others what they've learned and what they know. So many times, the thought that I would have arrived at wherever it is I am in my journey to being able to grow plants well so much sooner, had someone been available to share some of the things I'm able to share with you guys.

I realize that some people can't be helped, or don't want my help. You can't push someone up a ladder unless they want to climb, but the rewards that come in the form of personal satisfaction from helping people like you, and so many others, seem like a natural extension of the rewards I get from nurturing plants. Nurturing plants/nurturing plant people - it doesn't matter much - the rewards are really very similar ....... and either way, it's enjoyable. People make better friends, though. ;o)

Thank you, EG.

Al
happy_fl_gardener; 9a, near DeLand
11 years ago
last modified: 7 years ago
Al - I have several Earthboxes. What type of planting medium would you use and would you still use the 2 cups of granular fertilizer and plastic "shower cap" covering that is recommended? I had been using a peat potting mix with 8-10-10 or 16-4-8, depending what I was growing.

What do you suggest for the planting medium and fertilizer? Thanks so much for you help.

Christine
tapla
Original Author
11 years ago
last modified: 7 years ago
Hi, Christine. I don't use SWCs, so would defer to others with more practical experience, but from the discussions I've had, it seems like modifying the 5:1:1 mix to something like 5:3:1, bark:peat:perlite has worked well in the past, but the best ratio will depend on the size of the particles of the largest fraction of the soil, that being the bark.

I would find a thread specifically directed at the discussion of SWCs and ask about the +/-s of using fertilizer strips vs CRFs vs fertigating (watering + fertilizing) with solubles from the top or bottom.

I'm guessing the 'shower cap' is suggested to help keep water vapor from evaporating as it nears the surface of the soil, thus keeping the soil more evenly moist and providing a favorable environment for the important fine roots in a larger % of the soil, and to facilitate dissolution of the fertilizer in the strip. I'm envisioning it as also serving to slow the upward creep in pH usually associated with the accumulation of (bi)carbonates in the media. The more water that evaporates from the surface, the more carbonates are left behind. These accumulating compounds drive alkalinity/pH upward; and since we're not watering from above, there is no way to flush them from the soil.

Best luck!

Al
jodik_gw
11 years ago
last modified: 7 years ago
I'm eager to learn the scientific reasoning behind the idea that light can trump fine, soggy soils and excessive watering. This is something I've never heard of, nor would I think it prudent to keep a declining plant in a detrimental medium, or continue to over-water, when it's too easy to just re-pot using a better medium and stop over-watering.

The right amount of light is essential to good plant growth... this is an elementary piece of gardening knowledge, and it goes without saying, but how adding more light will cure a soil issue and over-watering is something I don't understand. Please expound, will you, Jane? I'm very eager to learn exactly how light will trump my poor soils and heavy watering hand.

And a note to tapla... thank you so much for your continued diligence in sharing prudent and factual information. I'm not certain if I've mentioned it, but my lavender flowered Dendrobium grown in mid-light, in my rendition of your medium mix, has very healthy roots and is about to bloom for the first time since I rescued it from a clearance cart at Lowe's! Thank you, for all you do! :-)
DMForcier
9 years ago
last modified: 7 years ago
Interesting article. I had the same "what's the point of the brick" question as emgardener, but you answered that well.

Now I have one that I don't see answered, though the double-pot techniques kind of approaches it.

...

Why not use a very deep pot? Put the PWT down below where the roots would naturally want to go?
greentiger87
9 years ago
last modified: 7 years ago
To preface, I'm not at all agreeing with jane_ny's comment, but I think the gist of what she's saying is important. Sorry if this has been rehashed before.

One of the well accepted limitations of Liebig's Law of the Minimum is that each factor must be as close to independent as possible. When factors interact, the model becomes less predictive. When looking at the big picture though, Liebig's Law is of course still extremely useful.

In the case of increased light, and what I think is the implied increase in temperature, these factors interact with the "excess water" factor.

For example, suppose I have two plants in a water retentive medium. I put one in an area where it gets some afternoon shade (Plant A) I put the other in the middle of the lawn where it will get the full heat of the sun all day (Plant B) Identical plants in a coarse, well drained medium are also put in these situations (Plant C and D). All plants are watered exactly the same amount, once a day.

Plant A and B are both limited by water-retentive soil. Plant B will be forced to transpire to maintain leaf temperature. It can keep its stomata open because it has a large supply of water available in its pot, which maintains leaf water potential. This reduces forced photorespiration caused by the closing of stomata to prevent water loss (triggered by leaf water potential). Light in excess of photosynthetic capacity can still be used up by photorespiration to prevent even more harmful light-triggered reactions. As the water is used up on a daily basis very quickly, the problems of an anoxic perched water table are lessened.

Plant A feels the full effects of a perched water table, and has decreased growth as a result.

Plant C has the ideal conditions for this particular plant - bright light in the morning for optimum photosynthesis, with a soil medium that is both evenly moist and has plenty of porosity for air. It flourishes.

Plant D has the same great soil, but oppressive sunlight and heat in the afternoon. It cannot maintain open stomata and cool itself because its soil medium doesn't retain enough water. It suffers from decreased growth due to the water stress and excessive light and leaf temperatures.

So in this case, excess light that might be a limiting factor alone interacts with excess water-retention to decrease (not eliminate) each of their negative effects.

Does more light "trump" overwatering? I'm not really sure what that means. Light in excess of what would normally be "ideal" may reduce the effects of overwatering. If for some reason you insist on using a water retentive soil, putting your plant in a place where the soil be relatively dry by the end of the day is one way to decrease the overall stress on the plant. It's much better of course, to use a good medium and the appropriate amount of light.

I'm not presuming that this argument is news to Al or anyone else here, but I really needed to write it out for myself. Liebig's law is one of the most useful tools of biology, but it's not a dogma.. it has to be fine tuned when you approach the details and nuances of such a complicated system.

Feel free to now tear this apart, you won't hurt my feelings. Just explain why.

PS: I know I'm conflating light and heat in some places, but unless we're talking about grow lights, they do usually come together.
TheMasterGardener1
9 years ago
last modified: 7 years ago
These topics are the best!!

A soil that holds water- When watered for a day or so the plant is in too much water, for a few days it is in too little water to "dry" properly.

With the 511 it could hold water and air in the right ratios!
greentiger87
9 years ago
last modified: 7 years ago
Oh wow. I didn't realize this thread was so old!
dsws
9 years ago
last modified: 7 years ago
After reading the OP, I don't see why a coarser / more-porous layer would be so different from a wick.

It seems to me as though the best configuration would be a continuous gradation from very coarse material at the bottom (that has air space even when it's just been soaked), to slightly finer material on top of it (that has air space when just soaked, but only because the stuff underneath it provides a little help to wick the water downward), and so on to peat-based medium everywhere above three inches (or whatever the saturation height is for the medium at hand).

That's for the configuration it settles to: you would want some extra height of coarse material to allow for some finer stuff from each layer falling or being washed into the gaps in the next layer down. But any further coarseness of the upper layers just reduces your water-holding capacity without providing any additional aeration.
tapla
Original Author
9 years ago
last modified: 7 years ago
If you wish to construct that type of layering system, it can be made to be effective.

Al
dsws
9 years ago
last modified: 7 years ago
Thank you.

Btw, I may sound from time to time as though I'm disagreeing just to be contrary. What I'm actually trying to do is bash my half-baked notions and half-remembered understandings against others' practical knowledge, to see what breaks. I expect it to be my arguments that fall apart most of the time, and I appreciate the many well-informed explanations I've seen in your posts on this forum.

Edit: And I edit compulsively. I don't have any excuse for that. It's just habit.
tapla
Original Author
9 years ago
last modified: 7 years ago
Thanks for mentioning that. I had noted how many of my old threads you were bumping, and was hoping the reason was one that would end up turning into a learning opportunity for all of us. It looks like you have most of the pieces assembled, and will be able to make quick work of what might be missing, so good for you!

Al
jodik_gw
9 years ago
last modified: 7 years ago
Another great in depth article, Al... thank you!
dsws
9 years ago
last modified: 7 years ago
Part of what I've been thinking as I read about water in soil is going back and forth between thinking of the water as sitting on the bottom or hanging from the top. It does both.

An image that may help make this idea clear is a slinky.

When you water thoroughly, you put in the slinky, stacked all the way from bottom to top. Then the slinky slithers out the bottom until it's hanging from the top all the way to the hole in the bottom. As the slinky goes out the hole, it vanishes. When there are only a few turns of slinky sitting on the bottom, their weight is no longer enough to overcome friction and push the slinky out the hole. Those few turns are sitting on the bottom; the rest of the slinky is hanging from the top.

Changing the medium is like putting a fluid around the slinky for buoyancy. If the fluid is so dense that the slinky actually floats, that corresponds to a solid lump of clay. There's no air space, no matter how tall the container is. Gritty mix is like air, so that only one or two turns of slinky sit on the bottom.

One way this analogy is a little off is that slinkies can't flow sideways, but water can. To imagine what happens with a brick or a wick or a funnel, you have to imagine turns of slinky being able to jump sideways when the tension on the next slinky pulls harder than the tension on the slinky they're in.
sunrisemadness
7 years ago
Tapla

What a great resource you are here on GW/houzz. Need your advice on prepating 5-1-1 for peppers and tomatoes. 1) How long in advance should the medium be prepared before planting? 2) How much water should you add to the mix before it is drained and ready for planting and does it need to be "saturated" to be hydrated? 3) My 5-1-1 mix last year would just NOT dry out even using a wick (perched water?) and numerous peppers and tomatoed passed away. Any way to remedy this? 4) When should the transplant be watered with Foliage Pro 9-3-6 initially and then ongoing and at what rate to sustain tomatoes and peppers?

Thanks again for your valuable education on soils. I am still experimenting with container growing and advice on a mix for SWC would be appreciated.

Happy Growing to all from the Piedmont of NC. Predicting 6" to 8" of snow tonight! Where is Spring?
halocline
7 years ago
last modified: 7 years ago

Hi Al, We're getting a nice wet & heavy snow here too.

My question is, if I am going to pre-mix my 5:1:1 & store it for a month or 2, does it need to be stored dry?

If so, (& maybe this is common sense) would I need to moisten it (to avoid hydrophobia) at the time of use?

Thanks,Rob.
Nil13 usda:10a sunset:21 LA,CA (Mount Wash.)
7 years ago
I wouldn't store it WET but it doesn't have to be super dry either. Just don't mix in the CRF before you use it. An overnight soak will hydrate it prior to use if it is dry.
tapla
Original Author
7 years ago
Rob - I agree you shouldn't store it wet. Wet soil often sees a marked lowering of pH as organic acids form during anaerobic composting, but damp is ok unless the soil is compacted to the point you're getting the anaerobic composting. The pH of pine bark varies quite a bit, and the reason is usually related to how it's stored. Bark that's windrowed and not turned occasionally would usually be expected to come in at a much lower pH than bark that wasn't stacked to the heavens and turned often.

If you're going to store the soil you make for a long time, it's better to store it dry, but if it's only a couple of months & you can store it so it's no more than damp and not compacted, you should be fine.

Tip: If you have a completely dry 5:1:1 mix & want to moisten it so you can plant in it, measure out how much soil you'll need, then put half the volume in a tub, add water and stir it up - so it ends up sort of muddy. Then, add the rest of the dry stuff to the muddy stuff and mix. Within 10-20 minutes, all of the soil will be damp/moist, and the tendency toward hydrophobia will have been eliminated.

Al
halocline
7 years ago
Thanks Nil, Al that's exactly what I needed to know.

Rob
Seysonn_ 8a-NC/HZ-7
7 years ago
last modified: 7 years ago
Thanks Al

Your brick/funnel example reinforce an idea that I always though that putting some chunky gravel or small stone at the bottom of pot can facilitate drainage. I am not talking about sand or finer gravel. This was without ever coming to know the PWT .
By your funnel analogy the chunky gravel would form irregular funnels that are filled with the given soil. So then the PWT would be mostly restricted to the gravel layer filled with the medium which has much less volume. So in conclusion the gravel has helped the drainage by forming funnels and reducing the amount of soil in the PWT height.
But whenever I tried to defend this, I was told that no matter what, the PWT will exist on top of the gravel layer.

Seysonn.
tapla thanked Seysonn_ 8a-NC/HZ-7
Nil13 usda:10a sunset:21 LA,CA (Mount Wash.)
7 years ago
That would work if you thouroughly mixed the gravel and medium first. If you just layer it, there will be an interface zone and it will take time for it to progress all the way through the gravel layer. In the mean time you have a raised PWT.
tapla thanked Nil13 usda:10a sunset:21 LA,CA (Mount Wash.)
tapla
Original Author
7 years ago
All it would take for it to work is a single uninterrupted soil column from the main body of soil to the bottom of the container. Let's say you had an 8X8 square pot and are using a soil that supports 3" of perched water. If you put an 8x8x3 brick in the bottom, there will be a 3" PWT above the brick; but, if there was a hole all the way through the brick that was filled with soil, the PWT above the brick would disappear. The only PWT the soil would support would be contained within whatever size hole the brick had in it. If the soil supported 4" of perched water, there would be 1" of perched water above the bricks upper surface.

Al
sunrisemadness
7 years ago
Talpa or any other member please chime in

A couple of questions. I think I am finally gaining an understanding of soil porosity and water retention. How do you determine the PWT (level) of a soil mix for a given size container (i.e. 5 gallon bucket, 8 inch black nursery pot, 20 inch ceramic glazed pot with drainage hole on the bottom, 15 x 20 "grow bag"? Is using a wick dryness the best method for determining when to water in non-SIP containers.

One last question. Assuming non screened pine bark fines with about 20% particles less than 1/8 inch, what ratio of Perlite and ProMixMZ would you use to make an adequate SIP container mix that had a soil collum of 3 inches x 3 inches under an aerated platform (drainage holes at 2 1/2 inches in a 5 gallon bucket? Not trying to put anyone on the spot but just attempting to make a mix that may work.
tapla
Original Author
7 years ago
A soil that supports perched water will support the same perched water table (PWT) height in any size or shape container. You can actually SEE the ht of a PWT if you fill a use a clear plastic cup with a drain hole. Cover the drain hole (usually with tape) and completely saturate the soil. After the soil is saturated, allow it to drain. You'll see a defined line and will note the soil below the line is completely saturated and the soil above the line contains air. That tells you the height of the PWT that particular soil will support in any container.

Using a wick or employing the earth as a wick changes how the water behaves in relation to the ht of the PWT. For instance, if your grow bag is resting on soil, or your container is partially buried - the PWT will probably disappear entirely.

Is using a wick dryness the best method for determining when to water in non-SIP containers? I'm a little confused. The soil IS the wick in SIPs. I must be missing something obvious that would clear things up if I understood what it is I'm missing. That's called a line of logic that eats its own tail. ;-)

For your last question, in order to take a SWAG, I'd need to know how far above the water line the main soil mass is when the reservoir is full.

Al
sunrisemadness
7 years ago
Man am I getting an education in container gardening! Sorry for the confusion on the wick dryness question. Let me better explain my experiment: I have an 8 inch black plastic nursery container (actually several with different combinations of 5-1-1) filled with 5-1-1 (the mix I made) and put a 5 inch wick in one of the drain holes. The wick is in the bottom of the pot and extends out about 3 inches. The container is elevated 4 inches so it will drain and in a large plastic container. The mix was damp when put in the container and water drained into the plastic container and the wick was saturated. Three days later the wick was dry and water in the container had evaporated. I again watered the container, the water drained and the wick was wet again. So, does this mean that I will need to water any plant in the container every three days or will the 5-1-1 retain enough moisture to sustain the plant for say a week or so? This is very interesting trying to get the ratio of the mix correct so as not to destroy any more plants!
sunrisemadness
7 years ago
Talpa

The last last question : Assuming non screened pine bark fines with about 20% particles less than 1/8 inch, what ratio of Perlite and ProMixMZ would you use to make an adequate SIP container mix that had a soil collum of 3 inches x 3 inches under an aerated platform (drainage holes at 2 1/2 inches in a 5 gallon bucket? Not trying to put anyone on the spot but just attempting to make a mix that may work.

Main soil mass will be about 1 inch above the full line in the 5 gallon bucket (meaning there will be about 2 inches of water in the bottom of the bucket where the soil wick rests. Hope this is what you need to answer. Thanks
tapla
Original Author
7 years ago
I have an 8 inch black plastic nursery container (actually several with different combinations of 5-1-1) filled with 5-1-1 (the mix I made) and put a 5 inch wick in one of the drain holes. The wick is in the bottom of the pot and extends out about 3 inches. The container is elevated 4 inches so it will drain and in a large plastic container. The mix was damp when put in the container and water drained into the plastic container and the wick was saturated. Three days later the wick was dry and water in the container had evaporated. I again watered the container, the water drained and the wick was wet again. So, does this mean that I will need to water any plant in the container every three days or will the 5-1-1 retain enough moisture to sustain the plant for say a week or so? This is very interesting trying to get the ratio of the mix correct so as not to destroy any more plants! The 5:1:1 mix usually does support SOME perched water, but no where near as much as media based on all fine particulates with a little perlite added. What makes water move down the wick is the weight of the water column above, and the taller the water column is, the more downward pressure (gravitational flow potential) it exerts. When the downward pressure of the water column is no longer heavy enough (tall enough) to overcome the combined strength of capillarity (adhesion [of water to soil particles] and cohesion [of water to itself), the water stops draining out of the drain. Adding a wick increases the ht of the column. Easiest way to think of it is, it "tricks" the water in the pot into "thinking" the pot is deeper than it really is, so the water starts to move downward "looking" for the bottom of the pot and gets pushed off the bottom of the wick by the water moving down from above. The water stops dripping off the wick when the downward pressure of the water column is no longer heavy enough (tall enough) to overcome the combined strength of capillarity - same reason it stops flowing out of the drain holes.

So, a dry wick doesn't mean a dry soil. It just means there is no longer a water column heavy enough to keep pushing water down the wick. If you're using the wick as a gauge to test the dryness of the soil, feel the wick right where it enters the soil or where it goes through whatever you have covering the drain to keep soil from escaping.

Your SIP set-up seems about the same as most, and many favor 3:2:1, PBFs: peat: perlite, but as in most cases, the size of the bark and how much very fine material it contains will ultimately determine how much peat and perlite are appropriate.

I hope that covers it, and that you'll have an extraordinary growing season. I had calls from 3 garden friends today asking about nothing in particular. I think they just wanted to be reassured they weren't crazy in their thinking we actually might get a spring somewhere down the road.

Al
Cheryl
4 years ago
New to the forum and recently interested in the science behind my plants. I appreciated the shared knowledge and usefulness of your information. It makes sense. Certainly, it is a lot to think about but I do like to think and this has pushed me to understand some of the physics of hydrology. - thanks!
tapla
Original Author
4 years ago
You're welcome. The time you spend learning about what makes plants work, soil science, fertilizing, watering efficiently ....... is going to be way, way more productive than the time you spend on mundane plant-related chores. Learning by getting bit on the butt by our mistakes isn't nearly as enlightening as learning to avoid the mistakes entirely, so keep at it. Growing well isn't difficult at all, as long as you make sure you have a very few basics covered.

Al
Cheryl
4 years ago
Well said. ; - } I always loved science, want to know why for everything, the more I learn the more I crave explore especially when the benefits are so rewarding.
tapla thanked Cheryl
halocline
4 years ago
Cheryl - When I found the GardenWeb; I was researching Bonsai soils, and although I was a pretty seasoned gardener, Al saved me about 10 years of research in about 2 years. Not to mention all the forward progress I've made; w/o having to search out everything I've learned from different sources.

Happy growing,

Rob
tapla thanked halocline
Cheryl
4 years ago
Ahhh, soooooo pretty! I am loving this site. Everyone has a good sense about themselves and experience beyond me. My plants are becoming so important to our home. I can understand them better and am enjoying the interaction of their responses. I just finished the evening repotting my micro mini African violets for wick watering. Next , as soon as it warms a bit there are plans to flush the salts from my plants as I want them to be as healthy and hav the full amount of uptake as possible for their nutrition. I love the science and that there are understandable facts behind the do’s. It opens up a whole world I didn’t even know existed.
tapla thanked Cheryl
illsstep
4 years ago
Al,

If methods exist to reliably drain the PWT in water-retentive soils, what benefits are to be had from using a highly porous soil other than convenience?

You have mentioned gas exchange in the root zone is facilitated by more frequent watering. I would imagine that even with the PWT wicked away, a water-retentive soil will need to be watered less often than a highly porous one - which means fewer opportunities to facilitate gas exchange in the root zone. Is this likely to affect vitality in any noticeable way? As in, will a container grown plant grow less as a result of a less frequent watering schedule (due to less gas exchange at the root zone), or would this be an insignificant contributor to growth rates of container grown plants?
tapla
Original Author
4 years ago
Methods exist to reliably drain fluid from inside the pericardium, but that shouldn't be looked at as license to walk about with an effused heart sac. Taking steps to remove excess water from our containerized plantings is assuredly a step in the right direction, but it doesn't fix soil compaction or ensure the high levels of air porosity that plants respond so well to.

I think that, if we're able to eliminate excess water retention by employing some simple science to do the work, it's a worthwhile half measure that helps our plants regain some, but not all of the potential lost to soils with aeration/ compaction/ water retention issues. Well aerated soils provide a pathway by which harmful soil gasses can escape, and allows a good supply of oxygen to fuel root metabolic processes, not to mention the fact that they reduce the collective adhesive attraction component of the forces that combine to create capillarity. The greater the reduction of capillary attraction in soils allows, the more free water gravity alone is able to force from the pot. Another way of looking at it is, reducing the amount of excess water a compacted/poorly aerated soil can hold still leaves you with a compacted and poorly aerated soil and the inherent issues (other than excess water retention) that accompany it.

Al
illsstep
4 years ago
last modified: 4 years ago
Al -

Here is another question, if you don't mind.

If a grower is stuck with a water-retentive soil for some reason (not the right time of year to repot, etc.), would it be beneficial to water more often than moisture levels deem necessary, to help facilitate gas exchange in the root zone (and of course take action to drain the PWT each time)? This is assuming the act of watering helps push old soil gasses out and draw new air in, even in dense soil mixes.
halocline
4 years ago
You don't have to over water every time, but it is a good idea to flush the soil from time to time.

Rob
tapla thanked halocline
illsstep
4 years ago
last modified: 4 years ago
Thank you.

My question doesn't relate to avoiding salt buildup in the soil, though.

I am asking about ways to facilitate gas exchange at the root zone in soils that lack proper aeration for this to happen passively. Since the act of watering displaces soil gasses, pushing out what is already there and drawing in fresh oxygen behind the water, I was curious if watering on a more frequent schedule than necessary would help mitigate the lack of passive gas exchange. Of course, you would have to manually drain the PWT each watering.

There seem to be two benefits of a highly aerated container soil, from what I have read and understood so far, and they are closely related:

1. Reduces or eliminates areas that allow no gas exchange (the dreaded PWT - waterlogged areas with insufficient oxygen available for roots to respire)

2. Reduces or eliminates areas that allow insufficient gas exchange (compacted areas where the available oxygen is quickly used up and the other gasses, byproducts of root respiration, end up trapped instead of being able to diffuse out to allow new oxygen in).

Now, 1. can be addressed, in part or completely, by the methods described in this thread. Ways to manually drain the soil's PWT when the soil is too dense for it to drain away passively.

Are there similar strategies for dealing with 2. manually when a soil is too dense for reasonable gas exchange to occur passively? Will a more frequent watering schedule act as a manual gas exchange?
tapla thanked illsstep
tapla
Original Author
4 years ago
There ARE mechanical methods, like turning on a vacuum cleaner and putting the hose end over the drainage hole, but that would be something akin to misting your plants to raise RH surrounding the plant. Its effect works for only a very short time.

I think it's a far superior arrangement when healthy drainage and gas exchange are the result of a medium designed so this happens passively. That approach allows the grower to repurpose the time and effort that would otherwise be spent battling the soil for control of a plant's general wellbeing on other aspects of husbandry or whatever else seems appropriate. The grower might even spend time at a garden forum site helping others learn how to get more from the growing experience instead of in far-off growing areas defending plants against the ravages of inappropriate soils.
Al
illsstep
4 years ago
Fair enough. Thank you :)
tapla thanked illsstep
Cheryl
4 years ago
Hahahahahaha - love it
tapla thanked Cheryl
WMS Lee
2 years ago
What about using newspaper or paper towels to wick away water? Will that drain too much water from the pot?
tapla thanked WMS Lee
tapla
Original Author
2 years ago
It depends on your methodology. E.G. putting them in the bottom of the pot would be a decidedly bad idea. They are used most effectively outside the pot by setting a depotted plant's root mass directly on the paper where their capillarity will pull all excess water from the root/soil mass, after which the plant can go back in the pot. There are, however, other ways of ensuring little to no excess (perched) water can or will remain in the soil after a complete saturation of the entire soil column in the pot.

If you want them to drain a pot w/o depotting the plant, either the soil column must be in direct contact with the paper or there must be a bridge of material that readily supports conduction of water flow. Example - a pot with a recessed volume won't drain into papers because the soil column is broken. If there's a wick made of rayon or other absorbent and conductive material that forms a bridge between the soil in the pot and the paper (because the wick IS in contact with the paper), the root/soil mass will drain as completely as it would if the plant was depotted and it's root/soil mass was resting on the paper, though not as quickly.

Al
tropicofcancer (6b SW-PA)
2 years ago
Good to see this thread back on top.
WMS Lee: You cannot drain away too much water with passive methods - only the saturated portion which is good. The only way take away more water is to use something active like a vacuum pump.
tapla thanked tropicofcancer (6b SW-PA)
WMS Lee
2 years ago
Al, TOC, thanks for taking the time to respond.

I'm just setting the pot directly on top of the paper towels, and there is contact with the soil through the drain holes. I use other methods to remove the perched water prior to this (such as tilting the container and moving the container down then quickly upwards), yet there seems to be quite a lot of water still being wicked away by the paper towels. Wasn't sure if the paper towels may be too absorbent and wicking away too much water, but TOC has cleared that up.
tapla thanked WMS Lee
tropicofcancer (6b SW-PA)
2 years ago
If your pots has holes at the edge of the bottom of pot then tilting/shaking can get rid of the excess water fairly easily. Otherwise there is still a bit of excess water left behind. Imagine the tilted pot and water level tilts with the pot. Near the edge there is no exit point and so a little is left behind. Usually that little bit should not be a problem. But it is there. The paper/towel/wick essentially can get rid of that bit too.

Dealing with Water-Retentive Soils

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