Container Soils - Water Movement and Retention 8/26/21

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Hello! This post has 3 pages. Houzz's format won't allow me to post the thread in it's entirety, so the thread will continue in the 2 text boxes subsequent to my OP. It's necessary to repost the thread from time to time because after a given number of replies, part 2 and/or 3 get archived and are difficult to find. I'll try to be better about reposting as soon as the number of posts reaches that point.

I want to thank all the growers and good people who have participated and been so supportive of the thread over the years. Their contributions are one of the main reasons viewers continue to find interest in it, so thank you very much! If you find value in the information I have set down in this post and feel there is anything pertaining to the topic that should be added or explored in more detail, please contribute your suggestions. My goal is to offer information that allows you to avoid an ongoing battle with your medium for control of your plants' vitality, information that will help you increase the reward you get in return for your efforts.

I started this thread about 15 years ago, in March of '05. So far, it has reached the maximum number of posts GW allowed to a single thread twenty two times, which is much more attention than I ever imagined it would garner. Currently, there have been just over 6,200 posts to the singular topic. I have reposted it in no small part because it has been great fun, and a wonderful catalyst in the forging of new friendships and in increasing my list of acquaintances with similar growing interests. The forum and email exchanges that so often stem from the subject matter are in themselves enough to make me hope the subject continues to pique interest, and the exchanges provide helpful information. Most of the motivation to post this thread another time comes from the reinforcement of hundreds of participants over the years that strongly suggests the information provided in good-spirited collective exchange has made a significant difference in the quality of their growing experience. Thank you for taking the time to examine this topic - I hope that any/all who read it take at least something interesting and helpful from it. I know it's long, and grows a little longer each time it's reposted. My hope is that you find it worth the read, and the time you invest results in a significantly improved growing experience. Since there are many questions about soils appropriate for use in containers, I'll post basic mix recipes later, in case any would like to try them. Also, you will find I use the terms medium/ media and soil/ soils interchangeably.

As you eye the length of this post, one thing you might ask yourself is, "Why the interest and all this talk about soils and water retention?" In all honesty, soils obscure the reason we talk about them - they hide the roots and the roots' state of vitality. Vitality is not the same as vigor. Vigor is a genetic factor, something the plant is endowed with because of how it was programmed by Mother Nature. It is also something we have no sway over. Vitality, on the other hand, is dynamic and variable, essentially a measure of how well a plant is/ has been able to deal with the cultural hand it has been dealt by the grower. Vitality is something you have much, perhaps nearly total control over. The visual signals we get from the parts of the plant we CAN see allow us to take measure of the condition of the roots and the plant's overall vitality. Soil choice and watering habits, have a very significant impact on root health. As you read, keep in mind that good root health and root function are an essential PREREQUISITE to a healthy plant. We cannot expect to grow healthy plants w/o a healthy root system - it is impossible; which brings us full circle to why we discuss soils.

Poor root health is responsible for a very high percentage of the ills that befall plants, and the reasons people flock to the forums seeking remediation for widely varying issues. Poor root health means a reduction in vitality, which leaves the plant looking shabby while compromising its ability to defend itself against insects and diseases. So, let's talk about some things we can implement that should go a very long way toward providing you with the ability to consistently keep the root systems of your plants happy.

Before we get started, I'd like to mention that I wrote a reply and posted it to a thread some time ago, and I think it is well worth considering. It not only sets a minimum standard for what constitutes a 'GOOD' container medium, but also points to the fact that not all growers look at container media from the same perspective, which is why growers so often disagree on what makes a medium 'good, bad, or somewhere in between. I hope you find it thought provoking:


I think any discussion on this topic must largely center around the word "GOOD", and we can broaden the term 'good' so it also includes 'quality' or 'suitable', as in "Is X a quality or suitable medium?" How do we determine if medium A or medium B is a good one? And before we do that, we had better decide if we are going to look at it from the plant's perspective or from the grower's perspective, because often there is a considerable amount of conflict to be found in the overlap - so much so that one can often be mutually exclusive of the other.

We can imagine that grower A might not be happy or satisfied unless knows he is squeezing every bit of potential from his plants, and grower Z might not be happy or content unless he can water his plants before leaving on a 2-week jaunt and still have a weeks worth of not-having-to-water when (s)he returns. Everyone else is somewhere between A and Z; with B, D, F, H, J, L, N, P, R, T, V, X, and Y either unaware of how much difference medium choice can make, or, they understand but don't care.

I said all that to illustrate the large measure of futility in trying to establish any sort of standard as to what makes a medium 'good' from the individual grower's perspective; but let's change our focus from the pointless to the possible. We're really only interested in the comparative degrees of 'good' and 'better' here. It would be presumptive to label any medium "best", though 'best I've found' or 'best I've used' CAN sometimes be useful for comparative purposes, but that's a very subjective judgment. Let's tackle 'good', then move on to 'better', and finally see what we can do about qualifying these descriptors so they can apply to all growers.

I would like to think that everyone would prefer to use a medium that can be described as 'good' from the plant's perspective. How do we determine what a plant wants? Surprisingly, we can use %s established by truly scientific studies that are widely accepted in the greenhouse and nursery trades to determine if a medium is good or not good - from the plant's perspective, that is. Rather than use confusing numbers that mean nothing to the hobby grower, I can suggest that our standard for a good should be, at a minimum, that you can water properly. This means at any time during the growth cycle you should be able to water your plantings to beyond the point of saturation (so excess water is draining from the pot) without the fear of root rot or compromised root function due to (take your pick) an excessive volume of water or too little air in the root zone.

I think it is very reasonable to withhold the comparative adjective, 'GOOD', from media that can't be watered properly without compromising root function, or worse, suffering disease from one of the fungaluglies that cause root rot. I also think anyone wishing to make a case from the plant's perspective that a medium which can't be watered to beyond saturation w/o compromising root health can be called 'good', is fighting uphill against logic.

So I contend that 'GOOD' soils are those we can water correctly; that is to say, we can flush the medium when we water without concern we would be compromising root health/function due to long term retention of excess water. If you ask yourself, "Can I water correctly if I use this medium?" and the answer is 'NO' .... it's not a good choice .... for the reasons stated above. Can you water correctly using most of the bagged bagged potting media readily available? 'NO', not without work-arounds in place, and we can talk about those.

What about 'BETTER'? Can we determine what might make a better medium? Yes, we can. If we start with a medium which meets the minimum standard of 'good', and improve either the physical or chemical properties of it, or make it last longer structurally, then we have 'better'. Even if we cannot agree on how low we wish to set the bar for what constitutes 'good', we should be able to agree any medium that reduces excess water retention and increases aeration ensures better potential for optimal root health than media which only meet some one's individual and arbitrary standard of 'good'.

All the plants we grow, unless grown from seed, have the genetic potential to become beautiful specimens. It's easy to see the absolute truth in the idea that if you give a plant everything it needs to prosper it will flourish. The better we are at it, the better our plants will grow. But we all know it's not that easy. Lifetimes are spent in careful study, trying to determine just exactly what it is that plants want and need to make them grow best.

Since this is a discussion, let's examine what the plant wants from its soil. It wants a soil in which we have endeavored to provide in available form, all the essential nutrients, in the ratio in at which the plant uses them, at a concentration high enough to prevent deficiencies yet low enough to make it easy to take up water (and the nutrients dissolved in the water). First and foremost, though, the plant wants a container medium that is evenly damp, never wet or soggy. Giving a plant what it wants to flourish and grow doesn't include a medium half saturated for a week before aeration returns to the entire mass, even if we only water in small sips. Plants might do 'ok' in some water-retentive soils if the grower is highly skilled, but in order to actually flourish as plants are genetically programmed to do, they would need to be unencumbered by media which do not remain wet/ soggy for extended periods.

What defines our proficiency as growers is our ability to identify and reduce the effects of limiting factors, or by eliminating those limiting factors entirely; in other words, by addressing those influences inhibiting the plant from reaching its genetic potential. Even if we are able to provide every other factor that influences plant growth/ vitality perfectly, it could not make up for a substandard medium. For a plant to realize its full genetic potential, every factor has to be perfect, including the medium. Of course, we'll never manage to get to that point, but the good news is, as we get closer and closer, our plants get better and better; and almost certainly, we'll find our growing experience more rewarding.

In my travels, I've discovered it almost always ends up being that one little factor that we willingly or unwittingly overlooked that limits us in our abilities, and our plants in their potential. Mother nature always sides with the hidden flaw.

Food for thought:

A 2-bit plant in a $10 has a future full of potential, where a $10 plant in a 2-bit has only a future filled with limitations. ~ Al


This is where we get to the meat of the subject. As container gardeners, our first priority should be to ensure the media we use provide adequate aeration for the intended life of the planting, or in the case of perennial material (trees, shrubs, garden perennials), from repot to repot. Aeration/drainage is arguably the most important consideration in any container planting. The soil is the foundation all container plantings are built on, and aeration is the very cornerstone of that foundation. Since aeration and drainage are inversely linked to soil particle size, it makes good sense to try to find and use (or make, in the case of 'media/ soils') soils or primary components with particles larger than peat/ compost/ coir/sand/ top or other traditionally 'fine' ingredients.. Durability and stability of soil components so they contribute to the retention of soil structure for extended periods is also extremely important. Pine and some other types of conifer bark fit the bill nicely, but I'll talk more about various components later.

What I will write will also hit heavily against the futility in using a “drainage layer” of coarse materials in attempt to improve drainage. It just doesn't work. All drainage layers do is reduce the total volume of medium available for root colonization. A wick can be employed to remove water from the saturated layer of medium at the container bottom, but a drainage layer is not effective. A wick can be made to work in reverse of 'self-watering' pots.


If we agree we should avoid ingredients that are phytotoxic (toxic to plants) our primary consideration when building/buying a container medium should be the medium's structure and its structural stability. Size of the particles that make up a medium, the ratio at which they are combined, and the long term durability (resistance to breaking down into smaller and smaller particles) plays a primary role in determining whether a medium is suited or unsuited to the application. The medium fills only a few needs in container culture. Among them are: Anchorage - a place for roots to extend, securing the plant and preventing it from toppling. Nutrient Retention - it must retain a nutrient supply in available form sufficient to sustain plant systems. Gas Exchange - it must be amply porous to allow air to move through the root system and gasses, which are a by-product of decomposition, to escape. Water - it must retain water enough in liquid and/or vapor form to sustain plants between waterings. Air - it must contain a volume of air sufficient to ensure that root function/metabolism/growth is not impaired. This is extremely important and the primary reason that heavy, water-retentive media are so limiting. Most plants can be grown without soil as long as we can provide air, nutrients, and water, (witness hydroponics). Here, I will concentrate primarily on the movement/ retention of water in container media.

There are two forces that cause water to remain in or move through soils – one is gravity, the other capillary action. Gravity needs little explanation because all things are pulled toward the center of the earth. For this writing I would like to note: Capillarity is a function of the natural forces of adhesion and cohesion. Adhesion is water's tendency to stick to solid objects like particles and the sides of the pot; it is the reason water can stick to the inside of your vertical shower door. Cohesion is the tendency for water to stick to itself. Cohesion is why we often find water in droplet form – because cohesion is at times stronger than adhesion; in other words, water's bond to itself can be stronger than the bond to the object it might be in contact with. Capillary action is adhesion + cohesion, forces in evidence when we dip a paper towel in water. The water will soak into the towel and rise several inches above the surface of the water. It will not drain back into the source, and it will stop rising when the force of gravity is equal to the capillary attraction of the fibers in the paper.

There will be a naturally occurring "perched water table" (PWT), also referred to as a 'perched water column' in containers when soil particle size is under about .100 (a little smaller than 1/8) inch. It is created whenever the capillary pull of the medium (remember how the water climbed up the paper towel and defied gravity) surpasses the Gravitational Flow Potential GFP (weight of the water column in the pot); therefore, the water does not drain, it is said to be 'perched'. The smaller the size of the particles in a medium, the more surface area there is for water to 'stick to', so the greater the capillarity and the greater the height of the PWT. Perched water can be tightly held in heavy (comprised of small particles) soils where it perches (think of a bird on a perch) just above the container bottom where it will not drain; or, it can perch in a layer of heavy medium on top of a coarse drainage layer, where again it will not drain. It is water that resists gravity whenever the force of capillarity is stronger (when particle size is too small) and will not allow the PW to drain from the portion of the pot it occupies. It can evaporate or be used by the plant, but ambient physical forces will not allow it to drain naturally.

Imagine we have five cylinders of varying heights, shapes, and diameters, each with drain holes. If we fill them all with the same medium mixture, then saturate the medium, the PWT will be exactly the same height in all the containers.

This saturated area of the container is where roots initially seldom penetrate and where root problems frequently begin due to a lack of aeration and the production of noxious gasses which cannot escape waterlogged media. Water and nutrient uptake are also compromised by lack of air in the rhizosphere (root zone). Keeping in mind the fact that the PWT height is dependent on soil particle size and has nothing to do with height or shape of the container, we can draw the conclusion that: If using a medium that supports perched water, taller growing containers will always have a higher percentage of unsaturated than squat containers when using the same medium. The reason: The level of the PWT will be the same in each container, with the taller container providing more usable, air holding above the PWT. From this, we could make a good case that taller containers are easier to grow in.

~ continued in the text box below ~

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