CONTAINER SOILS - WATER MOVEMENT and RETENTION XXII
Hello! Houzz's new format has presented some challenges, but it
looks like there has been some changes that will allow me to repost the thread
in its entirety. Hopefully you'll view that as a GOOD thing. I need to
thank all the growers and good people who have 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 was to offer soil-related information with
the potential to help you increase the reward you get in return for your
efforts. What might I do to increase the value of this offering?
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. Vitality is something
you have much control over. It is the visual signals we get from the parts of
the plant we CAN see that allow us to take measure of the condition of the
roots, their vitality. Soil choice, combined with watering habits, have a very
significant impact on root health. As you read, keep in mind that good root
health and root function is an essential PREREQUISITE to a healthy plant.
You 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
help 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 us 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.
I started this
thread about 10 years ago, in March of '05. So far, it has reached the maximum
number of posts GW allows to a single thread twenty times, which is much more
attention than I ever imagined it would garner. 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 stem so often from the subject
are in themselves enough to make me hope the subject continues to pique
interest, and the exchanges provide helpful information. Most of the motivation
for posting 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. I'll provide links to some of the more
recent of the previous dozen threads and more than 3,000 posts at the end of
what I have written - just in case you have interest in reviewing them. 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 the soil. It will follow the information.
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'
soil, but also points to the fact that not all growers look at container soils
from the same perspective, which is why growers so often disagree on what makes
a 'good' soil. I hope you find it thought provoking:
IS SOIL 'X' A GOOD SOIL?
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 soil X a quality or suitable
How do we determine if soil A or soil B is a good soil? and before we do
that, we'd 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 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 soil 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 good soil from the
individual grower's perspective; but let's change our focus from the pointless
to the possible.
We're only interested in the comparative degrees of 'good' and
'better' here. It would be presumptive to label any soil "best".
'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 soil 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 soil 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 soil should be, at a minimum, that you
can water that soil properly. That means, that at any time during the growth
cycle, you can 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 or metabolism due to (take your pick) too much water
or too little air in the root zone.
I think it's very reasonable to withhold the comparative basic
descriptor, 'GOOD', from soils that can't be watered properly without
compromising root function, or worse, suffering one of the fungaluglies that
cause root rot. I also think anyone wishing to make the case from the plant's
perspective that a soil that can't be watered to beyond saturation w/o
compromising root health can be called 'good', is fighting on the UP side logic
So I contend that 'GOOD' SOILS ARE SOILS WE CAN WATER CORRECTLY;
THAT IS, WE CAN FLUSH THE SOIL WHEN WE WATER WITHOUT CONCERN FOR COMPROMISING
ROOT HEALTH/ FUNCTION/ METABOLISM. If you ask yourself, "Can I water
correctly if I use this soil?" and the answer is 'NO' .... it's not a good
soil .... for the reasons stated above.
Can you water correctly using most of the bagged soils readily
available? 'NO', I don't think I need to point to a conclusion.
What about 'BETTER'? Can we determine what might make a better
soil? Yes, we can. If we start with a soil that meets the minimum standard of
'good', and improve either the physical and/or chemical properties of that soil,
or make it last longer, 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 that any soil that reduces excess water retention, increases aeration,
ensures increased potential for optimal root health, and lasts longer than
soils that only meet some one's individual and arbitrary standard of 'good', is
a 'better' soil.
All the plants we grow, unless grown from seed, have the genetic
potential to be beautiful specimens. It's easy to say, and easy to see the
absolute truth in the idea that if you give a plant everything it wants it will
flourish and grow; after all, plants are programmed to grow just that way. Our
growing skills are defined by our ability to give plants what they want. 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 soil discussion, let's see what the plant wants from its
soil. The plant 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, and 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 soil that is
evenly damp, never wet or soggy. Giving a plant what it wants, to flourish and
grow, doesn't include a soil that is half saturated for a week before aeration
returns to the entire soil mass, even if you only water in small sips. Plants
might do 'ok' in some soils, but to actually flourish, like they are
genetically programmed to do, they would need to be unencumbered by wet, soggy
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 clearing out those influences that stand
in the way of the plant reaching its genetic potential. Even if we are able to
make every other factor that influences plant growth/ vitality absolutely
perfect, it could not make up for a substandard soil. For a plant to grow to
its genetic potential, every factor has to be perfect, including the soil. Of
course, we'll never manage to get to that point, but the good news is that as
we get closer and closer, our plants get better and better; and hopefully,
we'll get more from our growing experience.
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 soil has a future full of potential, where
a $10 plant in a 2-bit soil has only a future filled with limitations.' ~
CONTAINER SOILS - WATER MOVEMENT and RETENTION
As container gardeners, our first priority should be to ensure the
soils we use are adequately aerated for the life of the planting, or in the
case of perennial material (trees, shrubs, garden perennials), from repot to
repot. Soil aeration/drainage is the most important consideration in any
container planting. Soils are the foundation that 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 soils or primary components with particles larger
than peat/compost/coir. 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 also hits pretty hard against the futility in using a
drainage layer of coarse materials in attempt to improve drainage. It just
doesn't work. All it does is reduce the total volume of soil available for root
colonization. A wick can be employed to remove water from the saturated layer
of soil at the container bottom, but a drainage layer is not effective. A wick
can be made to work in reverse of the self-watering pots widely being discussed
on this forum now.
CONSIDER THIS IF YOU WILL:
Container soils are all about structure, and particle size plays
the primary role in determining whether a soil is suited or unsuited to the
application. Soil 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 that are
the 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 soils are so limiting in their affect.
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 and retention of water in container soil(s).
There are two forces that cause water to move through soil - one
is gravity, the other capillary action. Gravity needs little explanation, but
for this writing I would like to note: Gravitational flow potential (GFP) is
greater for water at the top of the container than it is for water at the
bottom. I'll return to that later.
Capillarity is a function of the natural forces of adhesion and
cohesion. Adhesion is water's tendency to stick to solid objects like soil
particles and the sides of the pot. 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;
cohesion is what makes water form drops. Capillary action is 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 GFP equals the capillary attraction of
the fibers in the paper.
There will be a naturally occurring "perched water
table" (PWT) in containers when soil particulate size is under about .100
(just under 1/8) inch. Perched water is water that occupies a layer of soil at
the bottom of containers or above coarse drainage layers that tends to remain
saturated & will not drain from the portion of the pot it occupies. It can
evaporate or be used by the plant, but physical forces will not allow it to
drain. It is there because the capillary pull of the soil at some point will
surpass the GFP; therefore, the water does not drain, it is said to be
'perched'. The smaller the size of the particles in a soil, 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 soil on top of a coarse drainage layer, where it will not drain.
Imagine that we have five cylinders of varying heights, shapes, and
diameters, each with drain holes. If we fill them all with the same soil mix,
then saturate the soil, the PWT will be exactly the same height in each container.
This saturated area of the container is where roots initially seldom penetrate
& where root problems frequently begin due to a lack of aeration and the
production of noxious gasses. Water and nutrient uptake are also compromised by
lack of air in the 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 soil that supports
perched water, tall growing containers will always have a higher percentage of
unsaturated soil than squat containers when using the same soil mix. The
reason: The level of the PWT will be the same in each container, with the
taller container providing more usable, air holding soil above the PWT. From
this, we could make a good case that taller containers are easier to grow in.