Dealing with Water-Retentive Soils
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 Iíve 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. Iím going to define those soils, but this isnít about disparaging soil types - itís 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 itís 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 plantís 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 donít 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 Ö.. Perhaps the plant would have stored the energy for a winterís rest and the spring flush of growth instead of expending it on root regeneration.
You can see that perched water, from the plantís perspective, is not a good thing. From our own perspective, we think itís rather convenient when we only need to water our plantings every 4-5 days, but because we canít 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 donít agree that perched water is generally a bad thing in containers, thereís no need to read on. If youíre still interested, Iíll 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 ÖÖ 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.
Letís 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 havenít 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 donít and wonít all grow in funnels, so lets see how we can apply this information PRACTICALLY to other containers.
Drainage layers donít work. The soil rests on top of drainage layers, then the water Ďperchesí 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? Letís look at that idea, using the soil with the 3inch PWT again. Letís 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% Ö.. 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 doesnít 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 donít want to trade one stress for another.
How else might we Ďtrickí the water in the container into leaving? Letís think about the following in 2 dimensions, because itís 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, Iíve often talked about wicks, so Iíll 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 ífoolí 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 doesnít mean itís a good thing to use water retentive soils, simply because you have tricks to help you deal with them. For years, Iíve been using highly aerated soils and biting the Ďwater more oftení bullet because Iíve 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 wonít 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.
Dealing with Water-Retentive Soils
Dealing with Water-Retentive Soils
AL - your writing skills are excellent - I had no trouble visualizing the containers and the perched water table (a new term to me). I found "aha's" at several places and found myself really understanding why I had to water all my containers of fig trees every day. Since I have the time and since my dog gets a chance to catch the frisbee when the water timer clicks off, it is an enjoyable part of my day. Tomorrow, I will go look for the largest funnels I can find (L.O.L.)
Thank you so much for being willing to tutor us all.
What do you recommend for improving container-soil drainage, if "builder's sand" is too fine? It seems to me that half the reason to plant in containers rather than in the ground is to have affordable control over the soil.
I've been killing off seedlings in insert cells and Dixie Cups for years by ignorantly using "rich, moist" potting mix. I knew they were too damp, but tryin g to get it to dry out by just not watering is like waiting for a long stretch of sunny weather in the Pacific NorthWet's winter. (Ten minutes of sun in mid-winter is enough to stop traffic and we talk about it for days. maybe that's an exaggeration.)
I finally started mixing in 1/4 or more sand to get it to drain better. At first I added fine play sand and some coir to commercial potting mixes. Maybe a higher % of seedlings are surviving now, but I can still hardly ever water them.
Next, for raised bed soil, I switched to adding "Multi-Purpose" or "medium-coarse" sand which I guess is in the 1/4 mm to 1 mm range. Here, I'm trying to lighten heavy clay with aged manure and sand and peat and coir. And a fall/winter cover crop. And coffee grounds.
I've seen "rose sand" which looks much the same as "Multi-Purpose" sand, plus a small fraction of very coarse grains over 2 mm.
Should I be shopping for crushed rock?
What I really think I want is 5-10 cubic yards of very sandy sub-soil, delivered and far below cost. I'll screen it myself.
Wandering aorund this forum, I quickly found this thread, which answers my question:
For pots, avoid sand and commercial potting mixes. Use pine bark fines, sphagnum peat and perlite in pots.
That seems expensive for raised beds that started life as 80% or more clay. Would you advocate adding rubbed and screened pine bark "mulch" (around 1/8" to 1/4" particles? Coarser?) to heavy, closed fine clay soils in raised beds? Maybe "it depends" on what else is in the soil to start with.
(Given that bark is bound to break down in a few years, I suspect I still should be adding some kind of coarse mineral fraction to raised beds.)
I'm happy watering every day or three when there's no rain, but for 2/3rds or 3/4s of the year, the drainage ditch stays busy from all the rain.
To ensure good drainage in container soils, you need to START with large particles. You can't add large particles (like perlite) to fine particles (peat/compost/coir) and expect them to increase drainage r lower the height of the PWT. It just doesn't work. What perlite does is take up space that would otherwise be occupied by water-retentive material, so all it does is reduce the VOLUME of water in the PWT of soils. It really doesn't improve drainage or aeration. I use the illustration of perlite and pudding. How much perlite do you need to add to pudding to get it to drain well & improve aeration? Maybe 60-70% perlite and 30% pudding is a start, but then you're really adding the pudding to perlite because perlite is the largest fraction. It works the same way with fine-particled soils. If you start with bark, and add enough peat or compost until you have the desired water retention, you're in much better shape from the perspective of drainage/aeration.
Try a mix of screened perlite and screened Turface for your seedlings. Practically speaking, it's sterile, and seedlings insist on good drainage and lots of aeration to grow well.
Raised bed soils are a horse of a different color. Raised beds are like growing in the garden, but conventional container growing is more like hydroponics than growing in the garden. Most often, I find myself helping those who are trying diligently to bring their gardening practices to container culture and have met with difficulties. Raised bed soil composition isn't as demanding as container media. Sand mixed with 30-40% organic matter will work well in a garden, but is often death in a container because although sand usually drains quite well, it's so fine that organic particles like peat & compost clog the macro-pores and increase water retention/reduce aeration. When I think of 'sand' for containers, I'm thinking of a material of uniform size like crushed granite/silica/pumice in particles of 1/16" or larger.
The soil in my raised beds:
I think the soil in my raised beds started as something like equal parts of builder's sand, Turface, partially composted pine bark, and reed/sedge (Michigan) peat. I was probably a little heavy on the bark, and if you need additional water retention, some vermiculite would be an ok addition. The soil you see above has excellent tilth and friability - you can actually probably SEE it. It's full of soil life, though I did need to supplement the N supply for the first year due to some N immobilization.
FWIW, I would just lay down newspaper or cardboard & build the beds on top of it w/o turning the clay over ...... unless you have the ditch working for you to drain any bathtubs you're creating as your effort or soil life loosens the soil beneath the beds.
>> To ensure good drainage in container soils, you need to START with large particles. You can't add large particles (like perlite) to fine particles (peat/compost/coir) and expect them to increase drainage
I understand. I need to think of those as "soiless mixes" though that may not be the right term.
>> Try a mix of screened perlite and screened Turface for your seedlings. Practically speaking, it's sterile, and seedlings insist on good drainage and lots of aeration to grow well.
Wow, that seems coarse for teeny-tiny seedlings, but I'll try it out. No Jiffy Mix? I recently started using a layer of vermiculite to keep the babies more nearly sterile, but this seems a very different approach: coarse. By 'screened', do you mean 1/16" to 1/8" (1.6 mm to 3.2 mm)? Even for seedlings?
(Not disputing, just want to be sure I'm getting it.)
>> Raised bed soils are a horse of a different color. ... Sand mixed with 30-40% organic matter will work well in a garden, but is often death in a container ... When I think of 'sand' for containers, I'm thinking of a material of uniform size like crushed granite/silica/pumice in particles of 1/16" or larger.
I think of that as VERY coarse sand or crushed whatever. Pumice or lava rock or tufa would be nice!
Unfortunately for my budget, I don't hear you saying "clay" anywhere in your raised bed mix. I have plenty of clay for free! For example, the hill that I removed from the first few beds while amending what I didn't remove.
I've been trying to get it to drain at least semi-sufficiently.
But I am now guessing this advice for containers also applies to some degree to raised beds:
>> To ensure good drainage in container soils, you need to START with large particles.
Sigh. Total soil removal and replacement. First, multiply my budget by ten, then invent the anti-gravity wheelbarrow. But it makes sense. I may have to change my goal to "drain as well as I can afford".
I splurged on 4 cubic yards of topsoil once (it seemed much better draining in the vendor's pile than it was in my bed - probably becuase a pile drains better than a bathtub!) Maybe I used it inefficiently making my best bed too deep, but what seemed so large in the truck seemed so small in my yard! I don't think he shorted me, it's just that a yard is much bigger than a truck, no matter how small the yard or large the truck. Anyway, his prices have gone way up and that won't happen again soon.
I was hoping for some magic formula like "these organic components plus those mineral miracles make crumbs stable enough to create drainage between the crumbs". Maybe chopped rock wool.
>> FWIW, I would just lay down newspaper or cardboard & build the beds on top of it w/o turning the clay over ... unless you have the ditch working for you to drain any bathtubs you're creating as your effort or soil life loosens the soil beneath the beds.
Exactly - for now each bed has usable soil only down to the clay level. Where I excavated clay below grade, the ditches are even further below grade. I WISH the clay were good enoguh to support weeds, necessitating cardboard! What a luxury that would be! At least the clay supports firm, solid, permanent ditches with walls like concrete!
Planning for the future (and being lazy), I shape the floor of each new bed to slope towards one corner. I drain from that lowest corner down eventually to the main ditch.
I try to keep the mini-ditch leading away from the lowest corner several inches lower than the corner of the floor, and much lower than most of the floor. I hope this encourages worms, frost, elluviation and miracles to convert that unredeemed bottom clay to something that roots can get into (some year in the future).
After my initial "DUHH!! Rain plus gravity plus pure clay equals stagnant mud!!" discovery, most of one year was devoted to The Great Ditch, which runs the length of my yard down a rather shallow slope. It needs yet more work, but works well enough from Point X downhill to Point Y. When those two points are remedied, two more parts of the yard will have growing zones considerably deeper.
That is to say, it will be possible to excavate the bathtub drains deeper, or make new beds deeper than the old beds.
(Despite it being a small yard, I can't afford or carry enough soil amendments or soil replacements to go purely UP, so I also feel a need to go down. Over time.)
(I'm also reading your marathon "Container Soil" threads, and wondeirng if coarse conifer bark mulch would help the raised beds. That or shredded wood mulch was always on my list for top-dressing and eventual incorporation, but buying it anew every few years seems more expensive than extra-coarse sand. I had better look at prices again: the per-volume matters more than the per-pound price.)
Seedling and cuttings LOVE a barely damp, highly aerated medium. These types of media, especially when sterile, discourage damping off diseases and promote rapid growth. Fertilize about the time the first true leaves are appearing.
I think you're mixing the discussion between containers and raised beds, so I'm confused in places. Making your own container soils (the 5:1:1 mix) is much less expensive than buying commercially made soils. If you buy the bark in 2-3 cu ft bags and the perlite in 3-4 cu ft bags, it should cost about half as much as bagged soils. RB soils can be 60% good topsoil or sand + organic matter. Some clay mixed in won't hurt, but try to make sure the clumps aren't too large. Soil life will break them down in a year or so.
Pine bark is a very good mulch. It breaks down slowly and enriches the soil. I compost, but produce less than a yard per year. Even so, all my display gardens have superb soil, resultant from the enrichment they receive from a 2" layer of fine pine bark every other year.
True, there are at least three mutually exclusive topics:
- raised beds outside, starting from 90% heavy clay
- starting seedlings in little cells indoors in trays
- containers (big, small, outdoors, indoors)
>> screened perlite and screened Turface for your seedlings.
I agree that cost isn't a big issue for seedlings in inserts in trays. They need cubic inches of mix, not cubic yards.
I was moaning about the thought of totally replacing (buying) soil for my raised beds. But I'm thinking now that would be necessary only if I try to acheive desirable fast-draining soil. I will instead think in terms of not-so-slow-draining but affordable soil for the raised beds. Like, try to get to medium-heavy loam.
-- Changing topic to starting seedlings in small cells in trays --
>> I don't often start seeds, but for the few that I do, I use Turface, straight from the bag. Since I always screen my components for bonsai soil, I always have plenty of Turface fines on hand & I cover the seeds with that. It's really hard to beat the jiffy sterile seed starting blends of peat & vermiculite though. They work well, even if they do collapse quickly.
>> Try a mix of screened perlite and screened Turface for your seedlings. Practically speaking, it's sterile, and seedlings insist on good drainage and lots of aeration to grow well.
I'm still surprised that tiny seedlings with tiny roots would be happy with coarse particles, straight from the bag or even screened, if this still refers to sizes no smaller than 1/16" (1.6 mm) and averaging just under 1/8" (3 mm)? What do you use for a screen?
Do you have to water seedlings in this more often than once per day, once the humidity cover comes off?
P.S. Thanks for your patience and the many hours spent answering questions here.
* "I'm still surprised that tiny seedlings with tiny roots would be happy with coarse particles, straight from the bag or even screened, if this still refers to sizes no smaller than 1/16" (1.6 mm) and averaging just under 1/8" (3 mm)? What do you use for a screen?"
Seedlings/many cuttings will grow like crazzy in a jar or plastic bag, resting on top of nothing more than a damp paper towel - no soil at all - just high humidity. I wouldn't complain if the particles were 1/16 - 3/16, but the PWT disappears as particle size approaches 1/10", so 3/32 would be the ideal lower limit.
See what I made for screens below.
* "Do you have to water seedlings in this more often than once per day, once the humidity cover comes off?"
No - even materials with no internal porosity (crushed granite, perlite) usually hold enough moisture to co several days w/o watering. It depends on things like air movement, temperature, r-humidity levels, size of the seedling(s) in the container, what the container is made from (gas permeable?) ..... You can always mix ingredients - like Turface & granite or perlite to get the water retention you want.
Thanks, I think my questions around "size matters" are answered. Even for seedlings, drainage is not excessive with a 0.1" (2.5 mm). VERY coarse sand indeed, if I were using sand. I guess that would be considered "medium" perlite.
I'm still re-arranging my long-standing assumptions and beliefs, which is a slow process. I always liked Jiffy mix, and fine or medium sand seemed to help it some.
Those are very nice screens! The one in front looks like 4 mesh (called 1/4", but 0.225 openings). The one on the far right looks much finer.
For containers and seedlings, I have been screening commercial potting soil or potting mix with Ĺ" or ľ" hardware cloth, but I've been keeping what passes through and throwing away sticks and gravel. I understand now why my seedlings don't drain!
For screening things to add to raised beds, I tie galvanized hardware cloth over steel industrial shelving for support. The steel shelving all by itself is good for breaking big clay clods and removing big rocks - I throw shovels-full against steeply sloping shelving, and most of what goes through is worth finer screening.
For my beds, I always screen at least down to "rapidly passes Ĺ" cloth". However, if much of that is coarse, I screen again with ľ" cloth and use the back of a steel rake to encourage gravel and the smaller clay balls to roll off rather than pass through. Small clay balls may break up over time, but if I can exclude anything higher in clay than average, I'm happier.