Container Soils: Water Movement and Retention II

How to Grow in Container, Soil mixes, Fertilizers Etc

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Container Soils: Water Movement and Retention II

Post by KAMasud » August 3rd, 2012, 10:49 pm

Second series of "tapla lectures" 2007.

As container gardeners, our first priority should be to insure 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. Soil is the foundation that all container plantings are built on, and aeration is the 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. That components retain their structure for extended periods is also extremely important. Pine and some other types of conifer bark fit the bill nicely and I’ll talk more about them later.

The following also hits pretty hard against the futility of using a drainage layer in an attempt to improve drainage. It just doesn't work. All it does is reduce the amount soil available for root colonization. A wick will remove water from the saturated layer of soil at the container bottom. It works in reverse of the self-watering pots widely being discussed on this forum now.

Since there are many questions about soils appropriate for use in containers, I'll post my basic mix later, in case any would like to try it. It will follow the Water Movement info.

Water Movement and Water Retention in Container Soils

Consider this if you will:

Soil need fill only a few needs in plant culture. Anchorage - A place for roots to extend, securing the plant and preventing it from toppling. Nutrient Sink - It must retain sufficient nutrients in available form to sustain plant systems. Gas Exchange - It must be sufficiently porous to allow air to the root system and by-product gasses to escape. And finally, Water - It must retain water enough in liquid and/or vapor form to sustain plants between waterings. Most plants could be grown without soil as long as we can provide air, nutrients, and water, (witness hydroponics). Here, I will concentrate primarily on the movement of water in 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, water’s bond to itself can be stronger than the bond to the object it might be in contact with; in this condition it forms a drop. 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. 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 .125 (1/8) inch.. This is water that occupies a layer of soil that is always 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 "perched". The smaller the size of the particles in a soil, the greater the height of the PWT.

If we fill five cylinders of varying heights and diameters with the same soil mix and provide each cylinder with a drainage hole, the PWT will be exactly the same height in each container. This saturated area of the pot is where roots seldom penetrate & where root problems frequently begin due to a lack of aeration. 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 soil dependent and has nothing to do with height or shape of the container, we can draw the conclusion that: 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. Physiology dictates that plants must have oxygen at the root zone in order to maintain normal root function.

A given volume of large soil particles has less overall surface area when compared to the same volume of small particles and therefore less overall adhesive attraction to water. So, in soils with large particles, GFP more readily overcomes capillary attraction. They drain better. We all know this, but the reason, often unclear, is that the height of the PWT is lower in coarse soils than in fine soils. The key to good drainage is size and uniformity of soil particles. Mixing large particles with small is often very ineffective because the smaller particles fit between the large, increasing surface area which increases the capillary attraction and thus the water holding potential.

When we add a coarse drainage layer under our soil, it does not improve drainage. It does though, conserve on the volume of soil required to fill a pot and it makes the pot lighter. When we employ this exercise in an attempt to improve drainage, what we are actually doing is moving the level of the PWT higher in the pot. This simply reduces the volume of soil available for roots to colonize. Containers with uniform soil particle size from top of container to bottom will yield better and more uniform drainage and have a lower PWT than containers with drainage layers. The coarser the drainage layer, the more detrimental to drainage it is because water is more (for lack of a better scientific word) reluctant to make the downward transition because the capillary pull of the soil above the drainage layer is stronger than the GFP. The reason for this is there is far more surface area for water to be attracted to in the soil above the drainage layer than there is in the drainage layer, so the water “perches”.

I know this goes against what most have thought to be true, but the principle is scientifically sound, and experiments have shown it as so. Many nurserymen are now employing the pot-in-pot or the pot-in-trench method of growing to capitalize on the science.

If you discover you need to increase drainage, you can simply insert an absorbent wick into a drainage hole & allow it to extend from the saturated soil to a few inches below the bottom of the pot, or allow it to contact soil below the container where it can be absorbed. This will successfully eliminate the PWT & give your plants much more soil to grow in as well as allow more, much needed air to the roots.

Uniform size particles of fir, hemlock or pine bark are excellent as the primary component of your soils. The lignin contained in bark keeps it rigid and the rigidity provides air-holding pockets in the root zone far longer than peat or compost mixes that too quickly break down to a soup-like consistency. Conifer bark also contains suberin, a lipid sometimes referred to as nature’s preservative. Suberin is what slows the decomposition of bark-based soils. It contains highly varied hydrocarbon chains and the microorganisms that turn peat to soup have great difficulty cleaving these chains.

In simple terms: Plants that expire because of drainage problems either die of thirst because the roots have rotted and can no longer take up water, or they starve/”suffocate” because there is insufficient air at the root zone to insure normal water/nutrient uptake and root function.

To confirm the existence of the PWT and the effectiveness of using a wick to remove it, try this experiment: Fill a soft drink cup nearly full of garden soil. Add enough water to fill to the top, being sure all soil is saturated. Punch a drain hole in the bottom of the cup & allow to drain. When the drainage stops, insert a wick into the drain hole . Take note of how much additional water drains. Even touching the soil with a toothpick through the drain hole will cause substantial additional water to drain. This is water that occupied the PWT before being drained by the wick. A greatly simplified explanation of what occurs is: The wick "fools" the water into thinking the pot is deeper, so water begins to move downward seeking the "new" bottom of the pot, pulling the rest of the water in the PWT along with it. If there is interest, there are other simple and interesting experiments you can perform to confirm the existence of a PWT in container soils. I can expand later.

I remain cognizant of these physical principles whenever I build a soil. I haven’t used a commercially prepared soil in many years, preferring to build a soil or amend one of my 2 basic mixes to suits individual plantings. I use many amendments when building my soils, but the basic building process starts with conifer bark and perlite. Sphagnum peat usually plays a minor, or at least a secondary role in my container soils because it breaks down too quickly and when it does, it impedes drainage and reduces aeration.

Note that there is no sand or compost in the soils I use. Sand, though it can improve drainage in some cases, reduces aeration by filling valuable macro-pores in soils. Unless sand particle size is fairly uniform and/or larger than about ½ BB size I leave it out of soils. Compost is too unstable for me to consider using in soils. The small amount of micronutrients it supplies can easily be delivered by one or more of a number of chemical or organic sources.

My Basic Soil

I'll give two recipes. I usually make big batches. I also frequently add agricultural sulfur to some soils for acid-lovers or to soils I use dolomitic lime in.

5 parts pine bark fines
1 part sphagnum peat (not reed or sedge peat please)
1-2 parts perlite
garden lime or gypsum
controlled release fertilizer
micronutrient powder (or other continued source of micronutrients)

Big batch:

3 cu ft pine bark fines (1 big bag)
5 gallons peat
5 gallons perlite
2 cups lime or gypsum (you can add more to small portion if needed)
2 cups CRF
1/2 cup micronutrient powder (or other)

Small batch:

3 gallons pine bark
1/2 gallon peat
1/2 gallon perlite
small handful lime or gypsum
1/4 cup CRF
1 tbsp micro-nutrient powder

I have seen advice that some highly organic (practically speaking - almost all are highly organic) container soils are productive for up to 5 years or more. I disagree and will explain why if there is interest. Even if you were to substitute fir bark for pine bark in this recipe (and this recipe will long outlast any peat based soil) you should only expect a maximum of two to three years life before a repot is in order. Usually perennials, including trees (they're perennials too, you know) ;o) should be repotted more frequently to insure vigor closer to their genetic potential. If a soil is desired that will retain structure for long periods, we need to look more to inorganic components. Some examples are crushed granite, pea stone, coarse sand (see above - no smaller than ½ BB size in containers, please), Haydite, lava rock (pumice), Turface or Schultz soil conditioner.

I hope this starts a good exchange of ideas & opinions so we all can learn.

Al Fassezke

Pine bark fines are small pieces of pine bark that are partially composted (see them at the top in the picture). Pine bark is uncomposted bark and comes in various sizes (see them at the left in the picture). You probably don't want to use uncomposted bark larger than what you see in the picture. It will be really difficult to keep watered. Running large particles through a chipper to reduce their size is helpful.

I think we're getting off on the wrong track here. If you cannot find the right products, you're probably better off to stick with what you know. Particle size is important for drainage, aeration, and a soil's ability to hold water. If you try using something with huge particles, I'm afraid you'll be disappointed.

I can tell you and show you what makes a great soil, but if you don't have appropriate ingredients, then "close" can be frustrating. I'm not trying to insinuate that anyone should use what you guys call "My Mix", or that it will somehow transform you into a fantastic grower overnight. It won't. I just know that it works extremely well & have received the same feedback from easily over 100 people I have helped - lots of evidence of that here on this and other forums. It is very forgiving because it's difficult to over-water, and the high air volume it holds promotes excellent root health, which translates to good vitality in plants growing in it.

So - if you can find the ingredients, you'll be pleased with the results; but if you can't find them, default to what you know.


R - If I said the wheels on the bus go round & round, I'm sure someone would ask for the original data. ;o) It doesn't matter to me if they believe or not. A perched water table is something that is common to the earth's geology. In fact, the idea came to me while having a discussion about the earth's water tables WITH a geologist. I won't go through the effort of convincing naysayers; instead, I'll lay the onus of disproving what I say on them. Perhaps we can learn something in their attempt. These comments are not directed toward you. I know you're convinced the concept is applicable & I respect your views & constructive offerings. Thank you. ;o)

I have pretty regular discussions with people with doctorates in soil science or related fields, and perched water tables & the saturated soil at container bottoms is frequently discussed in detail. The 2.1X gradient differentiation between particulate sizes that determines whether a drainage layer is effective or not was taken from a recent letter from a doctored correspondent and was the result of some research he'd undertaken.

Tell doubters to: Lay a saturated sponge flat on a piece of hardware cloth (1/4 inch mesh wire screen) until it stops draining. Lift the sponge until its longest edges are vertical. A considerable amount of additional water will drain. After it stops, grasp the sponge by a corner & still more water will drain because the perched water has less volume of sponge material to occupy. This easily confirms the existence of a PWT. If that's not proof enough, they can perform the experiment with a Dixie cup full of wet soil & wick.

Donna - I don't use polymer crystals because I make the watering rounds daily and there is no reason to. There are other reasons too, that are mentioned in the last 1/3 of the original post if you would like to review it.

Never mind - I knew where it was, so I retrieved it for you:

" Though it may not be as important for the plants in container culture, we as consumers might want to think about what we use in the way of water absorbing soil amendments and how we use them. They do persists in the environment & have no nutrient value to flora or fauna!

Some of the "extra-absorbent" characteristics mentioned by manufacturers of polymers are exaggerated, & as bio-degradation occurs these polymers actually reverse their effect and hold moisture so tightly it is unavailable to plants. Soils can usually be designed so forest products (bark), peat, and other organic media components that adequately hold moisture can be used with no ill effects. These products, even in containers, provide the plant(s) some nutrient value & fodder for the micro-organisms that polymers inhibit. Some degraded polymer components even have some of the same effects on mammals as female hormones, which can affect mammalian fertility and potency.

Additionally, and as you alluded to, the polyacrylamides in some garden-grade moisture holding polymers are made from (& contain) the monomer acrylamide, a known carcinogen."


Yes, the soils are suitable for wicking applications. I often leave even tall containers full of completely dry soil in a tub of water overnight & by AM the top is moist & the container ready for planting.

I don't want to hurt any feelings, and even though growing in containers is more like hydroponics than growing in the earth, I would like to try to work together and keep the focus of this thread on container soils, please?


Melissa - I use either pine or fir bark in all my houseplant soils (no peat at all) so I know that it works well. If you feel the product you have is too small, keep looking for something like in the pic I'll supply at the bottom, or if you have a friend with a chipper, perhaps you could beg him/her to process it to a little finer mesh for you?

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.

I'll talk a little about EC (electrical conductivity) and TDS (total dissolved solids). They're a measure of what is dissolved in our soil water, so a high level of salt in coir would contribute to elevated levels. I'll explain why this is bad.

Plants take up water best when there are no ions in it - nothing dissolved - no fertilizer - just distilled water. But we cannot do that, or they will starve. We need to add nutrients. There is a fairly narrow band of TDS and EC that is ideal for plants, and it varies by light intensity, temperature, growth rate, and where the plant is in the growth cycle. On the flip side of the coin, if the level of TDS and EC is too high, the plant cannot absorb either water or nutrients, so it could starve or die of thirst in a sea of plenty.

We generally consider the TDS and EC to be an indicator of nutrient levels, but accumulating salts from fertilizers & irrigation water, play an important part, as would latent salts in a coir product (for example). If you want to fertilize at a solution strength of 1,500 ppm, and there is already 700 ppm of accumulated salts in the soil, that means you would only be able to add a solution containing 800 ppm or you would exceed your self-imposed solution strength and possibly create problems in the plants ability to absorb water & nutrients.

I'm not trying to impress you with numbers - pay no attention to them except to help you realize that there is a "ZONE" we hope we can stay in through educated guessing so our plants are at least somewhere between starved & burned up. ;o)

When TDS and EC levels get extremely high, water can be "pulled" from plant cells in exactly the same way that moisture is pulled from ham and bacon. The scientific term for this is plasmolysis, but you and I know it commonly as fertilizer burn.

Ok - now we know there is a zone. We also need to realize that it's important to maintain the right mix of elements and stay within that zone. Why is this important? Well, if you're using the popular 10-52-10 "bloom boosting" blend that's oft touted, you're prolly applying around 30 times more P than the plant can possibly use (in relation to N). You could be fertilizing at the correct zone of TDS & EC, but the extremely high levels of P would automatically guarantee a N deficiency.

I'm going to stop here so you can catch up & absorb this - it's important to understand. I'll leave a pic for Melissa see if there are any questions.


First, you're almost totally responsible for supplying nutrients to your plants in container culture. Container culture is completely different from growing in the garden, and is much closer to hydroponics than garden culture. I think you'd do well to forget the granular fertilizers, except in isolated cases & only then if you're well-versed in what you're doing. I'll probably get all kinds of flack for saying this, but I'm equipped & ready to debate this point: If you're confused about fertilizers for container plantings - all you really need to know is that the closest ratio on the market to what plants ACTUALLY use is 3:1:2 - N:P:K. That isn't the % of NPK - it's the ratio. The two fertilizers you should look for are: 24-8-16 or 12-4-8. Miracle gro makes both in easy-to-use granular/soluble, or liquid, and they come with some of the micronutrients. I use them by choice on 99% of my plants & I have a broad diversity of plant material - especially in summer. If you're at all unsure of how to tweak your nutrient supplementation program, you won't go far wrong if you use this formula for everything. It's been around for years & there's a good reason that MG labels it as "All-Purpose Fertilizer". ... uide.produ... Miracle-Gro Granular soluble 24-8-16 with micronutrients. Click on "read label" for more info ... Miracle-Gro liquid 12-4-8 with micronutrients. Click on read label for more info. This is what I use almost always.


Melissa - First - it's pretty much a must, that you supplement a containerized planting if you expect it to grow w/o nutritional deficiencies, so the sooner you settle on some kind of a nutritional supplementation program for your houseplants & others in containers, the happier both you and your plants will be. ;o)

Second - I promise I'll do my best to never peddle "snake oil" to you. I try to make sure that everything I say here is firmly rooted in plant physiology or one of the sciences, and that I can always call on science to reinforce my observations and opinions.

I'm not sure how you'd become well-versed in the physics of container soils, I kind of did that on my own, but as far as physiology, plant husbandry, fertilization, that sort of thing, good quality text books abound & they're all filled with as much knowledge as we can possibly absorb. If you're really interested, I'll name a few, but I think they would be pretty dry & technical to suit most tastes. Let me know ...?

You said you haven't fertilized and wondered "How can one fertilizer be all purpose?"

Well, look carefully at the chart I made. It shows the range of nutrients found in the living tissues of almost all plants. I gave Nitrogen, because it's the largest nutrient component, the value of 100. Other nutrients are listed as a weight percentage of N.

N 100
P 13-19 (16) 1/6
K 45-80 (62) 3/5
S 6-9 (8) 1/12
Mg 5-15 (10) 1/10
Ca 5-15 (10) 1/10
Fe 0.7
Mn 0.4
B(oron) 0.2
Zn 0.06
Cu 0.03
Cl 0.03
M(olybden) 0.003

How to interpret the chart: The first set of numbers is the range of the nutrient as it occurs in plant tissues for every 100 parts of N. So there are 13-19 parts of P, and 45-80 parts of K in plant tissues for every 100 parts of N. The second number, in parenthesis, is simply the average number for the range. Be patient - this is going somewhere. ;o) So, plants average 16 parts of P and 62 parts of K for every 100 parts of N. The last number, the fraction, represents how much of each nutrients are in living tissues when compared to N. There is approximately 1/6 the P and 3/5 the K in living plant tissues as there is N.

If we want to see how these averages compare to the 3:1:2 ratio of fertilizer I suggested above, we need to only divide the value of all the averages by 3.33. if we divide 100:16:62 by 3.33, we come up with approximately a ratio of 3:.5:2 - N:P:K that is actually IN plant tissues. This is extremely close to the 3:1:2 ratio I suggested, and even though it is still a little high in P, plants will tolerate it well, From this you can do a few calculations and see that a 20-20-20 blend supplies (on average) about 6 times more P than plants could ever use, and almost twice the K.

This chart, based on a % of N is the basis of how commercial greenhouse fertigation programs are structured. They determine how much N is needed, and all the other nutrients are added as a % of N. When commercial operations fertilize, they often use sophisticated tissue analysis to determine which of the three primary macronutrients, secondary macronutrients (magnesium, calcium, sulfur), and/or micronutrients are in tissues in excess, or are deficient. When they are deficient, they will adjust the fertigation program to raise the level of that nutrient in tissues to the proper range - the opposite for excesses. If tissue analysis shows there is no deficiency or excess, all is well (unless there is intentional manipulation of nutrients to achieve a specific end - often the rule) and the blend will be very close to the 3:1:2 blend mentioned above

Since we, as hobby growers, haven't the wherewithal to track our plant's nutritional needs so closely, we need to take an informed shotgun approach. Since plants use the major nutrients in very close to a 3:1:2 ratio of N:P:K, doesn't it make sense to supply those nutrients in as close to that exact ratio as possible? I'll be bold & answer that question for you - YES, of course it does. ;o)


I-guy - Since (with a few exceptions like tropical hibiscus) plants use more N than any other element, fertilizer under controlled conditions is applied as a function of the amount of N used. It's indisputable that plants use approximately 6 times more N than P, and about 1.5 times more N than K. If we apply a 1:1:1 fertilizer at a rate that guarantees no N deficiency, we are automatically applying 6 times more P and 1.5 times more K than the plant will use.

I'm not saying that you can't have perfectly healthy plants by using a 1:1:1 ratio judiciously, but it does hamper your ability to apply more fertilizer when a N deficiency becomes apparent, and limits o/a flexibility. I won't say that a 1:1:1 blend cannot be a good choice, but I will say that with an extremely high % of plant material, a 3:1:2 blend is a better choice.

For example: In weekly fertilizing of geraniums, N should be applied at 480 ppm. If we want to limit our TDS level of our fertilizer water to 1,500 ppm (a very realistic goal), using a 1:1:1 blend, we have 480 ppm each of NPK or 1,440 ppm of JUST those 3 elements. If we add in 250 ppm for TDS in the water and another 250 ppm for micronutrients (both conservative), we're already well over our limit (1,940 ppm) & into the range where we may be inhibiting water & nutrient uptake because of elevated EC and TDS levels.

If we applied a 3:1:2 ratio fertilizer, we supply all the N the plant needs, all the other secondary macronutrients, all the micronutrients, have allowed for water hardness, and we still come in at 1,460 ppm - room to spare if we needed to correct for a nutritional deficiency.

Our goal should be to keep all nutrient levels in soil solution somewhere between "adequate" and "luxury" levels. At the same time, we want to keep the level of TDS and EC at it's lowest while supplying those nutrients. Using a 1:1:1 ratio fertilizer like 12-12-12 or 20-20-20 inhibits our ability to maintain luxury levels AND to minimize the level of TDS and EC.

I mentioned hibiscus in my opening comment because it's one of the few plants that prefers a little more K than N in its diet. I compensate for this by adding a tbsp of potash (supplies K) per gallon of soil when I pot/repot and still use a 3:1:2 ratio fertilizer.

Melissa - I knew that you didn't mean anything offensive & I was smiling as I mentioned the snake oil. I didn't take any offense. ;o)

If you want to learn about soil composition and fertilization, I'll suggest a very good and reasonably easy to understand text: Water, Media, and Nutrition for Greenhouse Crops - edited by David Reed, published by Ball Publishing ISBN 1-883052-12-2

Here is an excellent online source for info on plant physiology, listed by subject. It should keep you busy until the book comes. ;o)

Take care.


TLC asks: "One last question and then I'll go away; is it true that you shouldn't fertilize your over wintered plants."

To answer the question, I would need to qualify the response. There is usually no real need to fertilize dormant deciduous plants. For conifers taking a cold quiescent (resting) break, there is some advantage in maintaining a low level of the full compliment of nutrients necessary for growth, simply because the plant is capable of some growth while over-wintering in a cold place.

Houseplants and other (usually perennial) plants that are not truly dormant (they may be resting in a cool place) go about the business of living and adjusting their metabolic needs according to certain internal rhythms (search circadian and endogenous rhythms for more info) and cultural conditions. Their internal clocks and lowered light levels are key factors in the marked slowdown most of us observe in our plants in winter; however, slowed growth cannot simply be offered up as proof of "dormancy". Just because we can't see plants growing or we think they are not growing is insufficient cause for certainty in the matter. In fact, in winter, our houseplants are carrying on photosynthesis and respiration - keeping their systems orderly, and going about their metabolic processes in a "business as usual" manner. They are just doing it at a much-reduced rate.

Why then, would we deprive plants of the building blocks they need (fertilizer) to produce the energy (make food) to carry on their metabolism? In nature, do the nutrients just disappear from soils whenever a plant's internal clock or cultural conditions cause slowed growth? Of course not - and the idea is absurd. Even if you cannot SEE plants growing, they are STILL producing and storing photosynthate to be used in a later push of (spring?) growth. Withholding fertilizer, LIMITS the plants ability to carry on this important part of its growth cycle. Plants are efficient users of nutrients, but they cannot make something from nothing.

If you were striving for ultimate growth and best vitality, it would be REQUIRED that plants should ALWAYS have a full compliment of ALL the nutrients essential for growth in a solution strong enough to supply all nutrients in the adequacy range, but not so strong that it makes it osmotically difficult for the plant to absorb water and nutrients. This bold part is key.

The reason it is so often parroted that we should refrain from fertilizing in winter isn't because the practice itself is bad for plants (simple science and a little knowledge of plant physiology is all that's needed to dispel that myth); it's because so many of us are growing in a soil that simply will not allow us to fertilize in a way that is best for the plants.

Remember, I'm often at odds with growers who support a practice out of convenience or a necessity based on cultural limits they have either placed on themselves or that they must work within. Soo often you'll find me saying that grower convenience and plant vitality are often at odds with each other and are often mutually exclusive.

Where am I headed? Well, if we KNOW that availability of low levels of all nutrients at all times is best, even in winter, why are we so often admonished against winter fertilizing? It's because of the soils we use. Even without the addition of fertilizer to our irrigation water, the level of salts and total dissolved solids (TDS) in our soils (for most of us) continues to accumulate over winter because of watering habits necessitated by slow soils. Some limit themselves by soil choice and then try to tell others that ARE using a soil that allows them to fertilize appropriately that they are doing something wrong. This, because the some lack adequate understanding about what is really happening with regard to plant's actual nutritional needs.

So YES, many readers are limited to being unable to fertilize adequately because of soil choice, and just because plants carry through winter w/o additional fertilizer supplements over the dark months, is not an indication of anything except that plants will usually tolerate it. Is it the end of the world if you don't fertilize in winter - or you can't? Not really, but you can see that there really is a better way than simply withholding nutrients from an already stressed plant.

Dr. Alex Shigo: " ... correct the stress, which will lead to strain, that if uncorrected will lead to the death of the organism."

As you might guess if you've followed this thread, I use fast soils that drain freely & I fertilize with my own concoction (which is basically MG 12-4-8 with micronutrients + some STEM + some Sprint 138 Fe chelate [an iron supplement for high pH water applications] + MgSO4 + vinegar) at EVERY watering, and it works extremely well for the plants I over-winter. For winter watering, when I add the TDS of my water and what I add to it, I'm applying the right mix of nutrients at every watering at a rate of less than 300 PPM of TDS which puts me on very sound horticultural ground. In summer, the same plants will be fertilized at somewhere near the rate of 1,500 - 1,800 PPM weekly - a big difference.

I'm not suggesting that you adopt a "fertilize at every watering" routine like I have, but there is no reason that you cannot fertilize regularly during the winter if you're using a well-aerated and fast draining soil. If you're using a slow, water-retentive soil that necessitates your watering in "sips" instead of watering copiously and flushing the soil at every watering, the salt build-up from irrigation water and fertilizer increases the risk factor for elevated levels of TDS and EC and eliminates your ability to make sure the right nutrients are available in the right proportions.

Tip: Often, around this time of year (Feb usually) when you think your plants are suffering because of low humidity levels in your home, what actually is happening is this: You've been watering in sips, maybe even fertilizing a few times since last Fall, so almost ALL the fertilizer salts and metal salts dissolved in your irrigation water have accumulated in the soil. If you read the post upthread that explains that high levels of salts in the soil (EC & TDS) inhibits the plants ability to absorb nutrients and water, you'll understand that though humidity is probably playing a part in the plant's suffering, it's more likely you should attribute the suffering to a poor soil and accumulating salts in that soil because you risk root rot if you flush the soil at each watering.

Take good care.


Steve - yes, absolutely. I get lots of mail this time of year from folks with plants exhibiting tip-burn and dry/necrotic leaf margins. Almost universally, they attribute the malady wholly to low humidity levels in the home, but as you note, there is more at work.

Growing in a soil that requires you to water in small 'sips' to prevent saturated soil conditions and accompanying root rot, instead of watering so that the soil is flushed at every water, promotes the accumulation of both fertilizer and irrigation water salts in the soil. There is an inverse relationship between accumulating dissolved solids (salts) in soils and the plant's ability to absorb water (and the nutrients dissolved in the water). As the level of salts increases, the plants ability to take up water decreases. Plasmolysis (fertilizer burn) to varying degrees is the extreme result, and can occur even if you are not applying fertilizers.

As Steve pointed out (if you can't/don't want to grow in a fast medium that allows you to flush the soil at each watering) flushing the soil intermittently is a good remedy. I think around a month would be a reasonable interval between flushing operations and to insure minimal accumulation of salts, flushing soils regularly should continue for as long as you're forced to water in sips.

This is a very effective way to flush soils and still minimize the chance of root rot:

Completely saturate the soil and allow it to drain. In succession, and at about 15 minute intervals, pour approximately the same volume of water the plant's container would hold through the soil. Do this 3 or 4 times and it will remove a huge % of the salt accumulation. To help remove any extra water in the soil, you can unpot the plant & set it on a newspaper overnight (or as long as you feel it reasonable - o/a soil mass will determine what is prudent). The newspaper will pull water from the soil and it will evaporate. You can undertake this whole procedure in the shower & wash foliage at the same time for added benefit. If you do not wish to unpot the plant, you can push a wick through the drain hole, up into the soil & allow it to dangle below the container. This will also remove a high % of water the soil might 'try' to retain.

Thanks, Steve - and thank you again for the recent link. I enjoyed the read. ;o)


Yes, dolomite/dolomitic lime is what you need & it generally comes with a ratio of about 4:1 Ca:Mg, which is considered a favorable ratio. The Ca(NO3)2 will supply N and Ca, but then you'll need to find a way to supply needed Mg (Epsom salts) in something close to a favorable ratio. I generally lime at the rate of 1/3 - 1/2 cup of dolomite per cu ft of container soil.


If you want a soil that will see extended use (more than 2 growth cycles), and if you do not wish to worry about aeration/drainage/soil collapse, you can solve the problem by using about 2/3 inorganic components. For my long term soils, I use equal parts of pine bark fines, Turface, and crushed granite. I generally don't use these soils for more than three years in one container, but I could, and without worry about collapse, as long as I was able to turn the soil over each spring before planting and add additional pine bark as the old decomposes.

When building soils for long term applications, the three most important considerations are particle size, durability/longevity of the particles, and the water holding ability of the soil as a whole, as well as the individual particles - in that order. I try to maintain an average particle size of just under 1/8", with the range in size from about 1/16 to 3/8". The heaviest concentration pf particles would ideally be in the 1/16 - 1/8" size, with few particles being larger than 1/4 inch.

You'll notice that the Turface and granite will last indefinitely, and the pine bark will have the longest life of any of the readily available organic components that are used for soils. The bark and Turface are excellent at holding water within the particles at the same time their size and shape allows for plenty of macropores in the soil. The granite adds little in the way of water holding ability, serving primarily to promote drainage/aeration. By increasing the volume of Turface and reducing the granite, you can adjust the soil's ability to hold water, while still maintaining the 2/3 inorganic component of the soil. You can also reduce water held by increasing granite and reducing Turface.

Does that cover your questions? Feel free to keep asking until all is clear to you. ;o)


I keep trying to follow along, but so many of the pots I have are annuals and I don't care if the soil fully drains or is perched high or low . . .

You really should care. The physics involved allows no differentiation between plant types. Saturated soil is as damaging to annuals as it is to perennials. Perhaps the rest of my reply will offer opportunity for better understanding.

*Is* it possible to have a layer of inorganic stuff in the lower 1/2 and just refresh the upper part of the soilless mix in a large pot? In one place you sort of say you can, but in another place, you indicate the PWT would be too high.

I think something of what I said might have been lost in your summation of what I said, so let me explain: Any soil made of small particles will support a perched water table (PWT), which is manifest in a saturated layer of soil, usually at the bottom of the pot, but which can also perch on top of material added to the pot as "an aid to drainage". This PWT will always be consistent in height, no matter the size/shape of the container the soil is in. With this knowledge, we can say that a soil that supports a 3" PWT, used in a 3" deep container will remain 100% saturated at container capacity. Container capacity is the state of the container soil after it has been fully saturated and is just at the point where drainage has stopped. So, a 6" deep container will have 50% saturation and a 12" deep container will have only 1/4 of the soil saturated at container capacity; therefore, it should be easy for us to draw the conclusion that taller containers are easier to grow in, based on soil saturation levels. "Skinny" or fat makes no difference, the PWT height using any given soil will remain consistent from one container to another.

Container shape has no effect on the height of the PWT, but it can/does have an effect on the total volume of water in the PWT. The easiest way to understand this concept is by illustration. A container that tapers toward the bottom will have less soil o/a in the bottom 3" than one with vertical sides; thus, the lesser volume of soil will hold a lesser volume of perched water. Here, we can draw the conclusion that, based on soil saturation levels, containers that taper toward the bottom will be easier to grow in.

Since the PWT height diminishes with increasing particle size, until at around a particle size of just under 1/8" the PWT disappears, there is much to be gained from using a coarse, well-aerated soil. Those that I use and suggest are designed to eliminate or nearly eliminate any perched water and associated concerns.

We know that when we use a "drainage layer" under any soil that will support a PWT, the particles in the "drainage layer" must be less than 2.1x the size of the particles in the layer above, or water will perch. Therefore, if we use the 3" water saturation level we refer to above, and if you use a coarse drainage layer below it, water dispersement will be situated like this: You will have the depth of the "drainage layer" that remains quite full of air and relatively free of water. On top of/above that, you will have 3" of saturated soil, and above the saturated soil, you will have soil with an aeration level unaffected by the perched water.

The soils I suggest using are large enough in particle size that they eliminate or nearly eliminate the perched water. This allows the entire container to be filled with soil w/o worry over perched water. If some does occur, it is so minimal that it is dispersed throughout the soil quickly (by diffusion) as the plant uses water & some evaporates. It takes 6 times longer for a 3" water table to diffuse than a 1/2" water table, and during that time, roots that are deprived of O2 are dying.

My other question is about this passage you wrote:
"If you discover you need to increase drainage, insert a wick into the pot & allow it to extend from the PWT to several inches below the bottom of the pot. This will successfully eliminate the PWT & give your plants much more soil to grow in as well as allow more, much needed air to the roots."

The question is, how do I get a wick to extend several inches below the pot if the pot is sitting in the ground or on a saucer or piece of slate?

If you have the container resting somewhere the water can puddle around the container, the wicking ability is negated. It becomes more effective when the drained water can flow away from the container, over a hard surface like the slate or a concrete patio with a slope. Most effective is if the wick can dangle below the container or if the wick is in contact with soil below the container. When the wick contacts the soil, the earth becomes a giant extension of the wick and will absorb water saturating the wick until the physical forces working inside and outside the container are equalized.

I have more questions, one about the link to the Miraclegro which I cannot find. Here is the link you posted, but they don't appear to list the exact product you mention -- Miracle-Gro Granular soluble 24-8-16 with micronutrients. The liquid you linked to is a Canadian website. I am only frustrated because I cannot find it here locally.

Both MG 12-4-8 liquid and 24-8-16 granular soluble are extremely common. The label will say "All-Purpose Fertilizer" on it. Within the last week, I've seen it at Lowe's, Menard's, and two nurseries near me. You'll have no trouble finding it if you look at the analysis on the labels. For some reason, MG seems to HIDE the analysis - as if it's unimportant! ;o)

You posted about Foliage-Pro 9-3-6. On the same webpage I found they also offer a blooms fertilizer with a totally different ratio than 3-1-2. For somebody wanting a big pot of petunias, are you sure I don't want a high middle number?

Yes, I'm sure. Don't wild flowers bloom beautifully year after year w/o the help of someone sprinkling them with additional P? ;o) Plants can only use so much P, and we know that in natural settings or if all nutrients are provided in containers at adequate levels, that plants use about 6 times more P than N. Since a 3:1:2 ratio has only 3X the N than P, you can see that it is already a high P formula, with twice as much P as the plant needs (in relation to N).

Large greenhouse operations use a method of fertilizing that includes injecting fertilizer into the irrigation water. It's called fertigation. They do tissue analysis on the plants to find what nutrients are present at either deficient or toxic levels, and adjust what they inject to correct the deficiency of all the 13 nutrients plants don't get from air/water.

If all nutrients are available in the soil at 'adequacy' levels, the plant will use approximately 1.5 parts of P and 7 parts of K for every 10 parts of N they use. Usage of N:P:K = 10:1.5:7. I'm going to do some division and reduce this ratio to 1/3 of what it is now, for later reference. If I divide 10:1.5:7 by 3.33, it comes out to be 3:.5:2.

Greenhouse growers have learned that they can control plant growth by adjusting the amount of N available, as long as all the other nutrients are available at at least adequacy levels, and they make it their business to be sure they are. You mentioned that they started their plants on a 20-10-20 blend, which is a 2:1:2 ratio.

Lets go back to the division I did a little up the post. We see that plants want to use nutrients at the rate of 3:.5:2, but the greenhouse is fertilizing them with a low N diet, only 2/3 of the N they want, and twice the P. (2:1:2 vs 3:.5:2 yields 2/3 the N and twice the P). The reason for this is pretty simple. The reduced N slows vegetative growth substantially, but it doesn't affect photosynthesis or the amount of photosynthate produced. The plant can't grow leaves & stems, so what does it do with all the extra food it is producing? It makes flowers/fruit with it.

So here's the deal: A 1:1:1 blend like 20-20-20 or 14-14-14 supplies 6 times more P and almost twice as much K as the plant can use, so is probably not the best o/a choice for containerized plants. The surplus nutrients just unnecessarily raise the level of dissolved solids in, and the electrical conductivity of the soil, which makes it increasingly difficult for the plants to absorb water and the nutrients dissolved in the water. A 2:1:2 ratio fertilizer like the 20-10-20 is still a high P fertilizer in relationship to the amount of N applied (greenhouse fertilizer programs usually furnish all the other nutrients as a % of N) and will keep plants compact while still allowing them to bloom well. If you want the plants to grow lusher foliage, & grow closer to their natural growth pattern, a 3:1:2 blend like MG 24-8-16 granular soluble or 12-4-8 liquid is preferred.

I would also advise you that before you attempt to manipulate growth, you'll need to be sure your plants are getting enough of the secondary macronutrients (Ca & Mg are very important - S is usually never a problem in container culture) and a full complement of the other minor elements (Fe, Mn, Zn, Cu, are the others to look for). For best results, you should incorporate an insoluble source of the minors like Micromax into the soil before planting, or add a soluble supplement like STEM to your irrigation water every time you fertilize.

I hope this wasn't too confusing or complicated, but it's very difficult to get the message across w/o using the numbers. I hope you take time to digest what I said, as it will give a better understanding of nutrient supplementation for container culture in general, as well as for your specific application.

Take good care.


How do you decide whether to use lime or gypsum, and how much do you use?
How much CRF?

I use gypsum in the gritty soil because it's initial pH is somewhere around 6.0-6.5 and that's in the ideal range. If I/you add lime, it raises the pH, and generally, pH continues to rise as soils age, and that can create pH induced nutrient deficiencies. I also use gypsum in the soil for plants that have difficulty absorbing some nutrients at pHs of 6.0 - 6.5 and above. We usually refer to these plants as acid lovers. Using gypsum usually keeps the pH of the 5:1:1 mix described above under 5.0. Though the pH of the medium is less important than the pH of the nutrient solution, it helps to try to keep media pH in a range favored by the plants.

I use lime and gypsum at the same rate. 1/2 cup per cu ft or 1 tbsp per gallon of soil. By coincidence, the 'medium' application rate of most CRFs (4-5 lbs per cu yd) figures out to about 1/2 cup per cu ft or 1 tbsp per gallon, also.

Here's the deal on the elemental sulfur. If I add it to soils, I get greener plants, so I usually include a tsp per gallon in my soils. I don't often mention it because I'm not sure why it works. It's supposed to be insoluble & ineffective at lowering pH in container soils. It may just be because there is no sulfur included in most of the popular soluble fertilizers like MG. Pine bark and peat soils both are low in sulfur, so it's very possible that what I'm seeing is a deficiency correction. It could also be that it actually DOES lower soil pH and make elemental Fe, Mn, and others more available. If you're using a fertilizer that includes it, or if you are using Gypsum and Epsom salts as your Ca & Mg source, disregard it - both contain sulfur.

Is elemental sulfur the same as garden sulfur?

Garden and agricultural sulfur are the same as elemental sulfur. They are all about 90%.

How much micronutrients? Is manure an acceptable substitute?

How much micronutrient mixture depends on what you're using for fertilizer. If you are using something like the Dyna-Gro Foliage-Pro 9-3-6, it's complete and has all the nutrients. I can help you if I know what you're using or what you want to grow - or with a basic program, but I can't give you anything definitive with no info. Manure supplies 'some' nutrients, but it's high in salts and often full of seeds. It also breaks down quickly, so (imo), what little it offers in nutrient value is offset by its end detriment to soil structure.

Hi, Rev - Glad to know you found the wicking trick useful. ;o)


The average pH of various sawdusts is around 4.0, which is not much different than sphagnum peat (higher, actually), so I don't think that IF a person was to include sawdust in their container soils they would need take any more care than they would adding peat. Simply adding a liming agent would/does solve the problem nicely, just as it does for peat.

How sawdust would react in a container soil as far as pH is concerned: As the organic acids formed during sawdust decomposition break down, ammonia is released and combines with H to form ammonium and the pH rises. As the ammonium is consumed by soil biota, H ions are released and pH falls, stabilizing around 7.0. When compared to the possible harm from N immobilization and the phenol generated, not to mention the compaction/water retention issue, the pH factor is actually pretty insignificant.

I can't agree that "wood chips" are a good choice as a soil ingredient, unless they are conifer bark chips (or unless you seek out a particular species known to be slow to decompose and not prone to producing phytotoxins during decomposition, like redwood or eucalyptus). The hydrocarbon chains in sugars & proteins within cell walls of whitewood/sapwood (and hardwood bark) are very quickly broken down, leaving primarily cellulose/hemicellulose which is also quickly cleaved by microorganisms. The explosion in microorganism populations typically creates severe competition for N between the microorganisms and the plant. Since the microorganisms are more efficient at extracting N from soil solutions, N immobilization/ drawdown/ deficiency, is the result. Additionally, the high micropopulations are very efficient at quickly reducing soil particulate size, diminishing aeration and increasing water retention. The hydrocarbon chains in conifer bark are very difficult for microorganisms to cleave, supporting fewer and and a more stable population of microorganisms, which slows decomposition, reduces N drawdown, and helps to insure structural stability in container soils.


No. Actually the (dolomitic) lime serves 2 purposes. It adjusts the initially acidic pH of most peat and bark soils (as it would soils with sawdust in them too, if you were using it) and it also supplies needed Ca and Mg in a favorable ratio. It is not a required additive to stabilize pH, but it is used to bring pH into a more favorable range. Stabilization would happen (gradually/eventually) whether or not you added a liming agent. The organic acids formed as the hydrocarbon chains break down will be broken down or leached from the container. Eventually there will be nothing left of the woody material but small particles of lignin and the pH will be about 7.0.

"Is this why adding lime to a soil/medium takes a while to neutralize (7.0 pH) the soil/medium, due to the fact that the H has to be released first and it takes a period of time?"

No. Adding lime to raise soil pH is chemically complex. As hobby growers, we make our best guess & hope for the best. In applications where it is important to use liming materials to help achieve a target pH, you must take into account several things. The initial pH of the media, its buffering capacity (resistance to change), the target pH, and the effects of irrigation water. Dolomitic lime is slow to react simply because of its low solubility. BTW, the H doesn't come from the dolomitic limestone, CaMg(CO3)2, as you can see by the formula for the compound.

Tigerlily mused: "There is something missing here that definitely pertains to the discussion, esp regarding using some sort of wood chips/mulch and that is time. How long is the plant going to remain in this mixture before either being planted in the ground or having to be moved up to a larger size pot and thus a new mixture? In other words-does it really matter, in terms of wood chips/mulch breaking down/compaction (not PH) if by the time it happens, the plant is already rootbound and/or is going into a new medium?"

We generally don't care what inert soil components are made of as long as their combination as a whole holds air water (and to varying degrees - nutrients) in a favorable arrangement. You'll remember how I stress structural stability in the original post? When the organic component of a soil breaks down quickly, it supports huge populations of microorganisms that out-compete the plant for N. Adding sufficient N is a challenge because the microorganisms are better at absorbing N from the soil solution, so the N simply creates larger and larger populations of microorganisms. It's extremely difficult to the point of being almost impossible to balance N applications in soils made of whitewood chips. You either get in trouble with carryover toxicity when microorganisms die & give up their N which is then added to your recent N applications; or, if you are using the more common container fertilizers (say 20-20-20) you cannot supply enough N to satisfy the plant because the P & K levels go sky high and raise the levels of TDS (total dissolved solids) so high the plant experiences fertilizer burn (plasmolysis).

N immobilization is immediate when using whitewood chips, and lasts throughout the composting process , so the reasoning that the plant will only be in them for a year won't hold up to scrutiny. I'm certain you can do better by changing your soils to either a much higher % or inert/inorganic components or changing the organic component to some type of appropriate size conifer bark.

"Am I right in remembering that you grow bonsai? What about these plants, whose tops are restricted in growth? Does that also restrict the root growth in proportion to the leaf output?"

I am a bonsai practitioner, yes. I think we've left the subject of soils here? Reducing the green portion of plants reduces the amount of photosynthate (food) produced, which limits root growth. The plant will always try to reach a balance between the number of roots and shoots.

If you reduce the canopy only, a tree will focus primarily on replacing the lost foliage. The response varies though, according to when the reduction occurs. In many plants, if you prune in late summer, the plant is not so eager to backbud & grow new leaves as it would have been if the pruning occurred in early spring.

Tree reactions to root pruning only is highly variable by plant & season, so I'm not going to expand. The same is true of simultaneous pruning of roots & canopy - varies by plant & season; but if you have specific questions, I'll answer by D-mail or you can start a thread.

I'm glad to see there's some renewed interest in this thread, it's been pretty quiet for awhile. ;o)


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Re: Container Soils: Water Movement and Retention II

Post by Izhar » August 6th, 2012, 8:45 am

I have always read Al's articles with interest ... those who used his recommended 5:1:1 mix have the healthiest plants.. but the problem is the availability of the materials used in it... any help will be very appreciable in this regard...

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