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How Plant Growth is Limited (container forum version)

posted by: tapla on 09.19.2010 at 09:07 pm in Container Gardening Forum


In a recent post, I suffered criticism after I tried to explain why light could not make up for or 'trump' the negative affects of other factors that potentially limit plant growth. Liebig's Law of the Minimum is a universally accepted concept that defines how the growth of plants is limited. Originally the law was viewed by Justus Von Liebig, a German chemist who is often referred to as 'the father of the fertilizer industry', as a fitting way to define the fact that plant growth is not limited by the total of the available resources, but rather, by the single resource in shortest supply.

Though Liebig's focus at the time was on nutrition, his concept was later expanded to include other limiting factors as they were discovered. Not only are each of the elements commonly regarded as essential to plant growth recognized as having the potential to individually limit growth, but the law has also been expanded to recognize the limiting effects of cultural conditions like light, temperature, levels of soil moisture and aeration, insects, disease, and others.

Liebig used a barrel with staves of varied heights, like you see in the picture, to illustrate how his concept worked. Imagine the barrel also had a stave for light, soil moisture/aeration, temperature ..... for each and every potential limiting factor, insects and diseases included. The picture above is illustrating that in this case, N is the limiting factor. The plant is not growing as well as it could be because it is N deficient. When we add more N, and N is no longer the nutrient or potentially limiting factor in shortest supply, something else takes its place as the limiting factor. Even if the supply of N was increased to the point where it was in perfect supply, the least available nutrient or cultural condition would STILL be the limiting factor. We raise the stave representing N, but then another stave representing another resource becomes limiting.

You can see that if light levels are made perfect, it wouldn't compensate for the effects of a N deficiency or a soggy soil. If it could, we would be able to grow our plants in peat porridge with no supplemental fertilization at 32* F in a wind tunnel .... as long as it was a bright wind tunnel .... or we focused on perfecting light levels. The same is true of soils. The most perfect soil we are able to build will not make up for or 'trump' the effects of a nutritional deficiency or poor light.

Our goal then, is to try our best to make sure ALL the cultural conditions are optimum - making ALL the staves taller, as it were. It doesn't do us any good to make all but one stave taller, because it is that pesky short stave that is going to limit growth - EVERY SINGLE TIME! Surprisingly, it is not as difficult as it sounds.

Light and temperature are actually very easy. The onus of learning your plants' preferences for these cultural conditions is on you, but they are very easy to learn and easy to correct, so that issue needs no more attention. Insects and diseases might be a little tougher, but IPM practices are derived from common sense. Identify the pest/disease and use the least noxious remedy possible to reduce the problem to something below your tolerance threshold.

Modern fertilizers make it easy to supply nutrients at near optimum levels and in a ratio to each other that is favorable. Tucked into Liebig's Law is the fact that too much is as bad as not enough, so there is incentive for us not to cater to the idea that because a little is good, more is better. As we look at the barrel example, we can see that increasing the N supply so the N stave is taller than the P or K staves is not going to help. So, using fertilizers with a favorable ratio and applying them wisely is actually something we can all manage.

Because this is the Container Gardening Forum, the most frequent source of trouble and the issues that arise with the most frequency are soil related. Soil moisture and aeration are staves as critical as any other in the barrel. Just as a perfect soil cannot 'trump' the effects of other short staves, optimizing other conditions cannot offset or 'trump' the effects of a poor soil. The necessity of making sure your plants are adequately supplied with water is an obvious given. The effects of excessive water retention and inadequate aeration are widely discussed on the forum. You can learn how to avoid these issues entirely or almost entirely by reading about How Water Behaves in Container Media by clicking this highlighted text; or you can read some tips about
How to Deal With Water-retentive Soils by clicking on this highlighted text.

Keep learning. The more you know about how your plants grow, what cultural conditions they prefer, and the effects varying cultural conditions will have on your plants, the better equipped you are to deal with them, keeping all the staves tall and minimizing limiting effects.



clipped on: 01.12.2014 at 09:42 pm    last updated on: 01.12.2014 at 09:42 pm

Good Growing Practices - An Overview for Beginners

posted by: tapla on 10.21.2011 at 02:00 pm in House Plants Forum

Good Growing Practices -
An Overview for Beginners

My hope is that this thread becomes a gathering place for beginners and the experienced alike, a place where reliable information that is rooted in sound science and horticulture can be found. We will see how that 'gathering' part goes, but I have enjoyed enthusiastic participation on many of my other threads on this and other fora, so I am optimistic.

As I consider what I am going to share with you and how to go about sharing it, I am compelled to offer some background that will hopefully allow some degree of comfort in placing some measure of value on my commentary. I enjoy the growing experience tremendously. I have worked hard toward increasing my skill level for more than 20 years, and I look at sharing what I have learned about the growing sciences as a natural extension of the enjoyment I get from nurturing plants - sort of nurturing people who nurture plants. I am invited to lecture frequently in the mid-MI area, and occasionally beyond. I lecture, conduct workshops, and do demonstrations on a variety of subjects related to growing, but most frequently I talk about things related to container culture, with maintaining houseplants being one of the most requested topics. I also enjoy participating here at Garden Web and at another popular garden forum sites. Hopefully we will be using some links to some of my other offerings here that will help you share some of the confidence others have shown in the reliability of my offerings. Those that know me know I am not after recognition or glory, I simply feel I can help any beginner with a willingness to learn and apply the newfound information, and I get a large measure of personal satisfaction from the feeling I may have helped someone along the path to becoming a better grower.

The first challenge is to offer information that a beginner can digest, and in such a way that he or she feels it is important enough to act on. I am first going to flesh out the main issues that, if understood, will make anyone a better grower and hope I have created enough interest that there will be plenty of questions so I can go into greater detail in the answers. For what it is worth, I tend to look at growing anything in containers from the perspective of what is best for the plant, not what is best for the grower. Far more often than not, the two ideas are mutually exclusive, so if grower convenience is a large priority of anyone reading this, there is not much sense in reading on. Growing well does take a little thought and a little effort.

The houseplants we grow are perennials nearly all, capable of growing for many, many years and of being passed from generation to generation. With attention to the areas I will cover in this post, you will discover that you can maintain your plants in good health for as long as you continue to commit to providing favorable cultural conditions. Your plants are all genetically programmed by Mother Nature to grow well and look beautiful. It is only a lack of knowledge and skill in the area of providing the cultural conditions they prefer that prevents them from growing to their potential. That sounds harsh, but it is the truth.

I have never seen anyone other than me discuss growing plants in containers from this perspective, that is (and it bears repeating) your plants are already genetically programmed to grow well and look beautiful, but it is up to you as a grower to eliminate the limitations so often associated with growing in containers. This post is about isolating some of the factors that are commonly the most limiting and helping you to reduce the limiting effects. For more information on the concept of limiting effects, do a search using the words "Liebig's Law of the Minimum" or follow this embedded link to How Plant Growth is Limited additional discussion.

Soil choice - Growers should realize that the most important choice they will make when establishing a new planting or when repotting is their choice of soil. A poor soil is probably behind more than 90% of the issues that growers come to the forums seeking remedial help for. Collapsed or dead plants, spoiled foliage, insect infestations, disease issues are all symptoms usually traceable directly or indirectly to a poor soil. This is so important to understand, that I will devote the bulk of my effort toward making it clear why I offer this contention.

Light is extremely important to plants. Plants make their own food, using water, CO2, and energy from the sun. Inadequate light means the plant cannot make enough food to grow to the potential for which it was genetically programmed. I will not go into great detail about light because when it comes to houseplants; you either have good light or are forced to deal with the limiting effects of inadequate light. If the thread finds favor, we can discuss supplementing light and how to prune to help compensate for the leggy appearance caused by insufficient light, or explore other topics of interest relating to light.

Nutrition supplementation is a requirement for normal growth and good health when growing plants in containers. In the earth, many of the nutrients are supplied by minerals in the soil. Container soils usually have no mineral component (and it is best that they do not in most cases - more later), and the soil components break down so slowly and are washed from the soil so quickly that deficiencies are virtually assured if you do not fertilize. Choice of fertilizer is also an important consideration, as we will see.

Repotting vs. potting up - that there is a difference between the two operations is a concept foreign to most hobby growers. One of the practices ensures your plants will at least have the opportunity to grow to their genetic potential within the limits of other cultural conditions; that would be repotting, with its accompanying root maintenance, complete or partial bare-rooting, and a change of soil. Potting up, on the other hand, only temporarily allows the plant to grow a little closer to its genetic potential before root congestion and a diminished number of fine roots quickly returns the plant to the state of limited growth and vitality it was experiencing before potting up.

Watering habits - extremely important and inextricably linked to soil choice, which is why I saved it until the end of this, the short list - so it would lead me back to the most important consideration - the one most apt to determine the difference between frustration and a rewarding growing experience.

Air is as important as water in all soils plants are to be grown in. Plants absolutely love plenty of air in the root zone, and rebel very quickly at too much water in the soil. I am going to describe what happens when you water plants growing in a soil that retains too much water. There are actually two possibilities. The first is, you water, and a part of the soil near the bottom of the container does not drain. This water has a name, it is called 'perched water', so named because it 'perches' (like a bird) in the soil above the pot bottom. This excess water is critically important because it very quickly begins to kill roots growing near the bottom of the pot - within hours. The first roots to die are the roots that do the lion's share of the work - the very fine roots often referred to as 'hair roots'. The longer the soil remains saturated, the larger the diameter of the roots killed. When air finally returns to this once saturated soil, roots can only then begin to regenerate. This takes energy and is extremely expensive for the plant in terms of energy outlay. During the cyclic death and regeneration of roots associated with excessively water-retentive soils, the plant is actually forced by chemical messengers that tell it to 'grow roots', to direct energy that would have otherwise gone into growing more leaves, branches, blooms, fruit, or just increasing the overall mass of the plant, to replacing the lost roots.

The second thing that might happen when you water if you are using a water retentive soil is, you adopt the practice of watering in 'small sips' so the soil remains damp instead of wet; this, to guard against root rot. It makes sense to only give the plant a little water at a time so the soil never gets soggy - right? That might be a workable option if you have the luxury of using water that has been processed through a reverse osmosis water filtering system, or if you are watering with distilled water, but regular tap water has things dissolved in it, like magnesium, calcium, iron, sulfur, and others. If you water in 'sips', these dissolved solids remain in the soil and build up over time. This has an impact on the plant's ability to absorb water and the nutrients dissolved in water. To illustrate the potential impact these dissolved solids have on a plant, picture in your mind what curing salt does to ham or bacon. It literally pulls water from the cells & dries out the meat. Any solutes (anything dissolved) in the solution surrounding plant roots can have the same potential effect on plant cells. It can make it difficult for plants to absorb water and nutrients, it can make it impossible, and in some cases can actually reverse the flow of water so it moves OUT of cells, effectively collapsing and killing them. We commonly call this 'fertilizer burn', but it does not necessarily have to result from an over-application of fertilizer. When people come here wanting a remedy for foliage that is dying, with dried edges & tips, almost always it is the result of over-watering exacerbated by water-retentive soils and the accompanying limitation that has on root function and metabolism, or as a result of the presence of a high level of dissolved solids from fertilizers and tap water having accumulated in the soil making it difficult for the plant to take up water. Both are so closely related to poor, water-retentive soils we can say the problem is inherent if not addressed directly.

Misting cannot correct a problem related to over-watering or a high level of solutes in your plant's soil. Low humidity can be a contributing factor to the common symptoms of necrotic (dead) leaf tips and margins (edges), but for the actual cause, look to impaired root function from over-watering or a high level of dissolved solids in the soil. BOTH of these conditions are nearly always linked to a poor soil. Misting raises humidity for a few minutes, but there are almost 1,500 minutes in a day. Raising humidity for 10 of the 1,500 has virtually no impact on the plant's ability to keep foliage hydrated. If you have foliage with burned leaf tips and margins, you should look to the soil and the state of root health for the cause.

When using water-retentive soils, it seems almost as though we are on the horns of a dilemma. If we water generously, we risk the soil remaining saturated so long it causes root rot, or at a minimum - impaired root function. If we water sparingly, in small sips, we risk an accumulation of dissolved solids from tap water and fertilizer solutions in the soil - so what to do? Well - I think we should look at an option that solves both issues and makes things much easier for the grower, while also providing the grower with considerably more latitude when it comes to watering and fertilizing.

The factor that determines how water retentive and difficult a soil is to grow in, is the size of the particles it is made from. The smaller the particles - the greater the water retention and the greater the degree of difficulty for growers. Soils made of any combination of peat, coir, compost, sand, topsoil, and other fine particulates are going to be very water retentive, which we know is undesirable from the perspective of the plant, and they cannot be suitably amended to correct drainage or the height of the perched water by adding perlite or other drainage material. If anyone disagrees with that statement, please ask for an explanation before mounting an argument or offering individual observations. Adding perlite to soils reduces the overall water retention of the soil, the reduction usually being a plus, but it does nothing measurable for drainage (flow-through rates) or the height of the perched water table, the later being the critical consideration when it comes to a healthy root zone.

Soils made of a high % of pine bark or other inorganic particles will have plenty of large air spaces called macropores. These are pores that will not hold water, only air, even when the soil is as saturated is it can be. They are critical to a healthy root zone. If you build a soil with plenty of air space, it hardly matters what the soil is made from. What is important is how the soil is structured. I will grow a perfectly healthy plant in a bucket of broken glass on a dare and a wager if anyone is interested in taking me up on it. If you have a soil with a healthy structure, a good nutritional supplementation program, and have good available light, the rest is so easy anyone can do it - honest. I have seen it happen over & over and over again. You will not go wrong if your primary focus is providing a healthy - a truly healthy environment for roots. Roots are the heart of the plant. Roots come first. If you cannot keep the roots happy, there is no chance you can keep the rest of the plant happy. That was a paraphrased quote from Dr. Carl Whitcomb, PhD, who wrote the bible on "Plant Production in Containers".

This ends the beginning discussion about soils. Until you are able to grow plants, the growth rate and appearance of which you are happy with, focusing on removing the limitations placed on your plants by soil choice will almost always constitute the best use of your energies. After reading this far, if nothing else, I hope you take that concept from this offering. It is the most important point and the best piece of advice I can give you. If you are interested in knowing HOW to make soils that will help you remove the limitations, now is the time to ask.

Nutrition is an area that is very misunderstood when it comes to container culture, but it is actually very easy. It is also very easy to become confused because there are so many numbers that represent different fertilizer NPK percentages and so many different kinds of fertilizers. I will need to use some numbers, but I think an understanding of NPK percentages as opposed to fertilizer RATIOS is important. NPK %s tell us how much (N)itrogen, (P)hosphorous pentoxide, and (K) potassium oxide (the symbol for potassium is 'K') are in a fertilizer by weight. So a fertilizer that is labeled "All Purpose 24-8-16" is 24% nitrogen, 8% phosphorous, and 16% potassium. 12-4-8 is also a common "all-purpose" fertilizer. It has exactly half the nutrients of 24-8-16, but both are 3:1:2 RATIO fertilizers. Ratios are a way of describing the amount of nutrients in a fertilizer as they relate to each other. Why is this important? It is important because we know that on average, plants use about 6 times as much N as P, and they use about 3/5 as much K as N, and now I will tell you how we can use this information to our plant's advantage.

The ideal way to fertilize is to supply fertilizer at the same ratio in which plants use the nutrients. The reason is because optimal growth and vitality can be had only when nutrients are in the soil at overall levels low enough that it does not become difficult for plants to take up water and nutrients dissolved in that water. Remember what we said above about a high level of soluble in the soil making it difficult for roots to absorb water and nutrients? Nutrients also need to be present at levels high enough to prevent deficiencies. If we think about it for a second, we can see that the best way to achieve this end is to supply nutrients at the same ratio in which they are used.

I noted that the NPK percentages actually tell us how much phosphorous pentoxide and potassium oxide are in a fertilizer so I can show you how fertilizer manufacturers arrived at a 3:1:2 ratio as their "all-purpose" blend. Only 43% of the P reported on a fertilizer label is actually P, and only 83% of the K reported is actually K. Once you apply these factors to any of the 3:1:2 ratio fertilizers (24-8-16, 12-4-8, and 9-3-6 are all popular 3:1:2 ratios), you will see they supply nutrients in almost exactly the same ratios as the average that plants actually use, and these fertilizers are excellent at keeping the overall level of solubles as low as they can be without creating nutritional deficiencies.

There is no need to use 'specialty' fertilizers; and many specialty fertilizers, like the advertised "bloom boosters" with up to 30 times more phosphorous than a plant could ever use (in relation to the amount of N used), can be (almost always are) moderately to severely limiting because the excess nutrients are a limiting factor.

The question often arises, "Should I use a synthetic or an organic fertilizer"? The answer is: "Use whichever you wish"; but the qualifiers are: Organic fertilizers are actually more accurately called soil amendments. They are mixed into the soil in the hope that at some point soil organisms will digest them and make them available in elemental form so plants can absorb them. The problem with that approach is that the populations and activity levels of soil life populations in containers are erratic and unreliable, making the delivery of nutrients from organic sources just as erratic and unreliable. What you apply today, may not be available until next month, and there is no way to determine what residual amounts of which elements remain in the soil. Soluble fertilizers like Miracle-Gro and others are completely available as soon as applied, and we know exactly what our plants are getting. They are simply much easier to use and deliver nutrients much more reliably than other fertilizer types. You can lump controlled release fertilizers like Osmocote and others in with the soluble synthetic fertilizers. With them, you get an extra measure of convenience but sacrifice a measure of control. As with all fertilizers, it is important to note the NPK percentages to be sure you are supplying the fertilizer in a favorable ratio if you want your plants to be all they can be.

When it comes right down to what occurs at the molecular/cellular levels, plants take up nutrients in elemental form. They cannot absorb the nutrients that are locked in the hydrocarbon chains that make up organic fertilizers until the molecules are broken down into their most basic elemental form. At that point, all nutrients are taken up as salts, and all are in the same form, no matter if they came from compost, a dead fish, or a hose end sprayer. Plants could care less where their nutrients come from, as long as they have a constant supply of all essential nutrients at all times.

It is not going to kill your plants if you use a fertilizer with a less than favorable ratio because plants tend to take the nutrients they need from the soil (solution) and leave the rest, but it is important to understand that it is 'the rest' that constitutes a limiting factor; so avoiding unnecessarily high levels of any one nutrient or nutrients whenever possible is to your (plant's) benefit.

It is important to understand that growing in containers is markedly different than growing in gardens. On a scale of 1-10, with 1 being growing in the garden and 10 being hydroponics, gardening in containers is much closer to hydroponics than gardens, getting a rating of somewhere around 7 or 8. This is why many of the practices that serve us so well in our gardens do not work well in containers. One area that is often a sticking point is the idea we need to "feed the soil". While that is an admirable and productive approach to gardening in the earth, container soils are more about their structure than about any nutrients they might supply. If you concentrate on your soils structure and durability, and more specifically its ability to hold plenty of air, you will greatly increase both the probability of consistent success and the margin for grower error. Well-aerated soils are easier to grow in and offer much greater opportunity for plants that will grow as near to their potential as possible.

As noted above, most growers draw no distinction between 'repotting' and 'potting up'. I have spent literally thousands of hours digging around in the root-balls of containerized plants. Old plants from nurseries of greenhouses are probably the closest examples to what most houseplants are like below the soil line, so I'll offer my thoughts for you to consider or discard as you find fitting.

I have also helped salvage many plants that had been containerized for long periods and were 'circling the drain'. Illustration: Not long ago, our bonsai club invited a visiting artist to conduct a workshop with mugo pines. The nursery (a huge operation) where we have our meetings happened to have purchased several thousand of the mugos somewhere around 10 - 12 years prior and they had been potted up into continually larger containers ever since. Why relate these uninteresting snippets? In the cases of material that has been progressively potted-up only, large perennial roots occupied nearly the entire volume of the container, plant vitality was in severe decline, and soil in the original root-ball had become so hard that in some cases a chisel was required to remove it.
In plants that are potted up, rootage becomes entangled. As root diameters increase, portions of the roots constrict other roots and impair the flow of water and nutrients through them, much the same as in the case of girdling or encircling roots on trees grown in-ground. The ratio of fine, feeder roots to more lignified (woody) and perennial roots becomes skewed to favor the larger, and practically speaking, useless roots.

The initial symptoms of poor root conditions are progressive diminishing of branch extension on plants that branch, loss/shedding of foliage on the parts of branches nearest to the main stems or trunk, often giving the plant a 'poodle look', and reduced vitality. As rootage becomes continually compressed and restricted, branch extension stops and individual branches might die as water/nutrient movement is further compromised. Foliage quality may not (important to understand) indicate the tree is struggling until the condition is severe, but if you observe your plants carefully, you will find them increasingly unable to cope with stressful conditions - too much/too little water, heat, sun, etc. Plants operating under conditions of stress that has progressed to strain, will usually be diagnosed in the end as suffering from attack by insects or other bio-agents/disease while the underlying cause goes unnoticed.

I will mention again that I draw distinct delineation between simply potting up and repotting. Potting up temporarily offers room for fine rootage to grow and do the necessary work of water/nutrient uptake, but these new roots soon lignify, while rootage in the old root mass continues to grow and become increasingly restrictive. The larger and larger containers required for potting-up & the difficulty in handling them also makes us increasingly reluctant to undertake even potting up, let alone undertake the task of repotting/root-pruning, which grows increasingly difficult with each up-potting.
So we are clear on terminology, potting up simply involves moving the plant with its root mass and soil intact, or nearly so, to a larger container and filling in around the root/soil mass with additional soil. Repotting, on the other hand, includes the removal of all or part of the soil and the pruning of roots, with an eye to removing the largest roots, as well as those that would be considered defective. Examples are roots that are dead, those growing back toward the center of the root mass, encircling, girdling or j-hooked roots, and otherwise damaged roots.

I often explain the effects of repotting vs potting up like this:
I will rate growth/vitality potential on a scale of 1-10, with 10 being the best. We are going to say that plants in containers can only achieve a growth/vitality rating of 9, due to the somewhat limiting effects container culture has on all plants. Lets also imagine that for every year a plant goes w/o repotting or potting up, its measure of growth/vitality slips by 1 number, That is to say you pot a plant and the first year it grows at a level of 9, the next year, an 8, the next year a 7. Also imagine please, we're going to go 3 years between repotting or potting up, which is how the illustration is structured.
Here's what happens to the plant you repot/root prune:
year 1: 9
year 2: 8
year 3: 7
year 1: 9
year 2: 8
year 3: 7
year 1: 9
year 2: 8
year 3: 7
You can see that a full repotting and root pruning returns the plant to its full potential within the limits of other cultural influences for as long as you care to repot/root prune.
Looking now at how woody plants respond to only potting up:
year 1: 9
year 2: 8
year 3: 7
pot up
year 1: 8
year 2: 7
year 3: 6
pot up
year 1: 7
year 2: 6
year 3: 5
pot up
year 1: 6
year 2: 5
year 3: 4
pot up
year 1: 5
year 2: 4
year 3: 3
pot up
year 1: 4
year 2: 3
year 3: 2
pot up
year 1: 3
year 2: 2
year 3: 1
This is a fairly accurate illustration of the influence tight roots have on a plant's growth/vitality. You might think of it for a moment in the context of the longevity of bonsai trees vs the life expectancy of most trees grown as houseplants, or the difference between less than 4 years versus more than 400 years, lying primarily in how the roots are treated.

I have not yet mentioned that the dissimilar characteristics of the old soil as compared to the new soil when potting-up; and potentially mixing soils are also a potential recipe for trouble. With a compacted soil in the old roots and a fresh batch of soil surrounding the roots of a freshly potted up plant, it is nearly impossible to establish a watering regimen that doesn't keep the differing soils either too wet or too dry, both conditions occurring concurrently being a limiting factor and the rule rather than the exception.

Most who read this would have great difficulty showing me a containerized plant that is more than 10 years old and as vigorous as it could be, unless it has been root-pruned at repotting time; yet I can show you hundreds of trees 20 years to 200 years old and older, and many of my very old houseplants/succulents that are in perfect health. All have been root-pruned and given a fresh footing in new soil at regular and frequent intervals, the same treatment all my houseplants get.

Thanks to any/all who made it this far. This is only an overview, but with even a rudimentary understanding of how to go about reducing the effects of the limiting factors that restrict growth and vitality, I know you can improve on how well your plants can grow, as well as on the degree of satisfaction you get from your growing experience - my only reasons for writing this. Hopefully the offering leaves you with many questions.



clipped on: 01.12.2014 at 08:02 pm    last updated on: 01.12.2014 at 08:02 pm

Fertilizer Program for Containerized Plants II

posted by: tapla on 03.11.2009 at 11:13 pm in Container Gardening Forum

This subject has been discussed frequently, but usually in piecemeal fashion on the Container Gardening forum and other forums related. Prompted originally by a question about fertilizers in another's post, I decided to collect a few thoughts & present a personal overview.

Fertilizer Program - Containerized Plants II

Let me begin with a brief and hopefully not too technical explanation of how plants absorb water from the soil and how they obtain the nutrients/solutes that are dissolved in that water. Most of us remember from our biology classes that cells have membranes that are semi-permeable. That is, they allow some things to pass through the walls, like water and select elements in ionic form dissolved in the water, while excluding other materials like large organic molecules. Osmosis is a natural phenomenon that is nature�s attempt at creating a balance (isotonicity) in the concentration of solutes in water inside and outside of cells. Water and ionic solutes will pass in and out of cell walls until an equilibrium is reached and the level of solutes in the water surrounding the cell is the same as the level of solutes in the cell.

This process begins when the finest roots absorb water molecule by molecule at the cellular level from the surface of soil particles and transport it, along with its nutrient load, throughout the plant. I want to keep this simple, so I�ll just say that the best water absorption occurs when the level of solutes in soil water is lowest, and in the presence of good amounts of oxygen (this is where I get to plug a well-aerated and free-draining soil), ;o). Deionized (distilled) water contains no solutes, and is easiest for plants to absorb. Of course, since distilled water contains no nutrients, using it alone practically guarantees deficiencies of multiple nutrients as the plant is shorted the building materials (nutrients) it needs to manufacture food, keep its systems orderly, and keep its metabolism running smoothly.

We already learned that if the dissolved solutes in soil water are low, the plant may be well-hydrated, but starving; however, if they are too high, the plant may have a large store of nutrients in the soil, but because of osmotic pressure, the plant may be unable to absorb the water and could die of thirst in a sea of plenty. When this condition occurs, and is severe enough (high concentrations of solutes in soil water), it causes fertilizer burn (plasmolysis), a condition seen when plasma is torn from cell walls as the water inside the cell exits to maintain solute equilibrium with the water surrounding the cell.

Our job, because you cannot depend on an adequate supply of nutrients from the organic component of a container soil, is to provide a solution of dissolved nutrients in a concentration high enough to supply nutrients in the adequate to luxury range, yet still low enough that it remains easy for the plant to take up enough water to be well-hydrated and free of drought stress. Electrical conductivity (EC) of, and the level of TDS (total dissolved solids) in the soil solution is a reliable way to judge the adequacy of solutes and the plant�s ability to take up water. There are meters that measure these concentrations, and for most plants the ideal range of conductivity is from 1.5 - 3.5 mS, with some, like tomatoes, being as high as 4.5 mS. This is more technical than I wanted to be, but I added it in case someone wanted to search "mS" or "EC". Most of us, including me, will have to be satisfied with simply guessing at concentrations, but understanding how plants take up water and fertilizer, as well as the effects of solute concentrations in soil water is an important piece of the fertilizing puzzle.

Now, some disconcerting news - you have listened to all this talk about nutrient concentrations, but what do we supply, when, and how do we supply them? We have to decide what nutrients are appropriate to add to our supplementation program, but how? Most of us are just hobby growers and cannot do tissue analysis to determine what is lacking. We can be observant and learn the symptoms of various nutrient deficiencies though - and we CAN make some surprising generalizations.

What if I said that the nutritional needs of all plants is basically the same and that one fertilizer could suit almost all the plants we grow in containers - that by increasing/decreasing the dosage as we water, we could even manipulate plants to bloom and fruit more abundantly? It�s really quite logical, so please let me explain.

Tissue analysis of plants will nearly always show NPK to be in the ratio of approximately 10:1.5:7. If we assign N the constant of 100, P and K will range from 13-19 and 45-70 respectively. (I�ll try to remember to make a chart showing the relative ratios of all the other 13 essential nutrients that don�t come from the air at the end of what I write.) All we need to do is supply nutrients in approximately the same ratio as plants use them, and in adequate amounts to keep them in the adequate to luxury range at all times.

Remember that we can maximize water uptake by keeping the concentrations of solutes low, so a continual supply of a weak solution is best. Nutrients don�t often just suddenly appear in large quantities in nature, so the low and continual dose method most closely mimics the nutritional supply Mother Nature offers. If you decide to adopt a "fertilize every time you water" approach, most liquid fertilizers can be applied at � to 1 tsp per gallon for best results. If you decide that�s too much work, try halving the dose recommended & cutting the interval in half. You can work out the math for granular soluble fertilizers and apply at a similar rate.

The system is rather self regulating if fertilizer is applied in low concentrations each time you water, even with houseplants in winter. As the plant�s growth slows, so does its need for both water and nutrients. Larger plants and plants that are growing robustly will need more water and nutrients, so linking nutrient supply to the water supply is a win/win situation all around.

Another advantage to supplying a continual low concentration of fertilizer is it eliminates the tendency of plants to show symptoms of nutrient deficiencies after they have received high doses of fertilizer and then been allowed to return to a more favorable level of soil solute concentrations. Even at perfectly acceptable concentrations of nutrients in the soil, plants previously exposed to high concentrations of fertilizer readily display these symptoms.

You will still need to guard against watering in sips, and that habit�s accompanying tendency to allow solute (salt) accumulation in soils. Remember that as salts accumulate, both water and nutrient uptake is made more difficult and finally impaired or made impossible in severe cases. Your soils should always allow you to water so that at least 10-15% of the total volume of water applied passes through the soil and out the drain hole to be discarded. This flushes the soil and carries accumulating solutes out the drain hole.

I have recently switched to a liquid fertilizer with micronutrients in a 12:4:8 NPK ratio. Note how closely this fit�s the average ratio of NPK content in plant tissues, noted above (10:1.5:7). If the P looks a little high at 4, consider that in container soils, P begins to be more tightly held as pH goes from 6.5 to below 6.0, which is on the high side of most container soil�s pH, so the manufacturer probably gave this some careful consideration. Also, P and K percentages shown on fertilizer packages are not the actual amount of P or K in the blend. The percentage of P on the package is the percentage of P2O5 (phosphorous pentoxide) and you need to multiply the percentage shown by .43 to get the actual amount of P in the fertilizer. Similarly, the K level percentage shown is actually the level of K2O ( potassium oxide) and must be multiplied by .83 to arrive at the actual amount of K supplied.

To answer the inevitable questions about specialty fertilizers and "special" plant nutritional requirements, let me repeat that plants need nutrients in roughly the same ratio. Ratio is an entirely a separate consideration from dosage. You�ll need to adjust the dosage to fit the plant and perhaps strike a happy medium in containers that have a diversity of material.

If nutrient availability is unbalanced - if plants are getting more than they need of certain nutrients, but less than they need of others, the nutrient they need the most will be the one that limits growth. There are 6 factors that affect plant growth and yield; they are: air water light temperature soil or media nutrients. Liebig's Law of Limiting Factors states the most deficient factor limits plant growth and increasing the supply of non-limiting factors will not increase plant growth. Only by increasing most deficient nutrient will the plant growth increase. There is also an optimum combination?ratio of the nutrients and increasing them, individually or in various combinations, can lead to toxicities.

When individual nutrients are available in excess, it not only unnecessarily contributes to the total volume of solutes in the soil solution, which makes it more difficult for the plant to absorb water and nutrients, it also often creates an antagonistic deficiency of other nutrients as toxicity levels block a plant's ability to take up other nutrients. E.g., too much Fe (iron) can cause a Mn (manganese) deficiency, with the converse also true, Too much Ca (calcium) can cause a Mg (magnesium) deficiency. Too much P (phosphorous) can cause an insoluble precipitate with Fe and make Fe unavailable. It also interferes with the uptake of several other micro-nutrients. You can see why it�s advantageous to supply nutrients in as close to the same ratio in which plants use them and at levels not so high that they interfere with water uptake. I know I�m repeating myself here, but this is an important point.

What about the high-P "Bloom Booster" fertilizers you might ask? To induce more prolific flowering, a reduced N supply will have more and better effect than the high P bloom formulas. When N is reduced, it slows vegetative growth without reducing photosynthesis. Since vegetative growth is limited by a lack of N, and the photosynthetic machinery continues to turn out food, it leaves an expendable surplus for the plant to spend on flowers and fruit. Plants use about 6 times more N than P, so fertilizers that supply more P than N are wasteful and more likely to inhibit blooms (remember that too much P inhibits uptake of Fe and many micro-nutrients - it raises pH unnecessarily as well, which could also be problematic). Popular "bloom-booster" fertilizers like 10-52-10 actually supply about 32x more P than your plant could ever use (in relationship to how much N it uses) and has the potential to wreak all kinds of havoc with your plants.

The fact that different species of plants grow in different types of soil where they are naturally found, does not mean that one needs more of a certain nutrient than the other. It just means that the plants have developed strategies to adapt to certain conditions, like excesses and deficiencies of particular nutrients.

Plants that "love" acid soils, e.g., have simply developed strategies to cope with those soils. Their calcium needs are still the same as any other plant and no different from the nutrient requirements of plants that thrive in alkaline soils. The problem for acid-loving plants is that they are unable to adequately limit their calcium uptake, and will absorb too much of it when available, resulting in cellular pH-values that are too high. Some acid-loving plants also have difficulties absorbing Fe, Mn, Cu, or Zn, which is more tightly held in alkaline soils, another reason why they thrive in low pH (acid) soils.

So, If you select a fertilizer that is close in ratio to the concentration of major elements in plant tissues, you�re going to be in good shape. Whether the fertilizer is furnished in chemical or organic form matters not a whit to the plant. Ions are ions, but there is one major consideration. Chemical fertilizers are available for immediate uptake while organic fertilizers must be acted on by passing through the gut of micro-organisms to break them down into usable elemental form. Since microorganism populations are affected by cultural conditions like moisture/air levels in the soil, soil pH, fertility levels, temperature, etc., they tend to follow a boom/bust cycle in container culture, which has an impact on the reliability and timing of delivery of nutrients supplied in organic form. Nutrients locked in hydrocarbon chains cannot be relied upon to be available when the plant needs them. This is particularly an issue with the immobile nutrients that must be present in the nutrient stream at all times for the plant to grow normally.

What is my approach? I have been very happy with Miracle-Gro 12-4-8 all purpose liquid fertilizer, or 24-8-16 Miracle-Gro granular all-purpose fertilizer - both are completely soluble. I incorporate a granular micro-nutrient supplement in my soils when I make them (Micromax) or use a soluble micro-nutrient blend (STEM). I would encourage you to make sure your plants are getting all the micro-nutrients. More readily available than the supplements I use is Earth Juice�s �Microblast�. Last year, I discovered a fertilizer by Dyna-Gro called Foliage-Pro 9-3-6. It is a 3:1:2 ratio like I like and has ALL the primary macro-nutrients, secondary macro-nutrients (Ca, Mg, S) and all the micro-nutrients. It performed very well for me.

When plants are growing robustly, I try to fertilize my plants weakly (pun intended) with a half recommended dose of the concentrate at half the suggested intervals. When plants are growing slowly, I fertilize more often with very weak doses. It�s important to realize your soil must drain freely and you must water so a fair amount of water drains from your container each time you water to fertilize this way. This year my display containers performed better than they ever have in years past & they were still all looking amazingly attractive at the beginning of Oct when I finally decided to dismantle them because of imminent cold weather. I attribute results primarily to a good soil and a healthy nutrient supplementation program.

What would I recommend to someone who asked what to use as an all-purpose fertilizer for nearly all their container plantings? If you can find it, a 3:1:2 ratio soluble liquid fertilizer (24-8-16, 12-4-8, 9-3-6 are all 3:1:2 ratio fertilizers) that contains all the minor elements would great.

How plants use nutrients - the chart I promised:

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
To read the chart: P - plants use 13-19 parts of P or an average of about 16 parts for every 100 parts of N, or 6 times more N than P. Plants use about 45-80 parts of K or an average of about 62 parts for every 100 parts of N, or about 3/5 as much K as N, and so on.

If you're still awake - thanks for reading. It makes me feel like the effort was worth it. ;o) Let me know what you think - please.

Here is a link to the first posting of A Fertilizer Program for Containers

Another link to information about Container Soils- Water Movement and Retention


clipped on: 01.02.2014 at 05:53 pm    last updated on: 01.02.2014 at 05:53 pm

Container Soils - Water Movement and Retention XVI

posted by: tapla on 12.18.2012 at 09:34 pm in Container Gardening Forum

I first posted this thread back in March of '05. Fifteen times it has reached the maximum number of posts GW allows to a single thread, 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 nearly 2,500 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. 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 recently, 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 soil?"

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 hill.

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 soils.

We become better growers by improving our ability to 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.

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. ~ Al

Container Soils - Water Movement & 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.

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 simply drain better and hold more air. 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. An illustrative question: How much perlite do we need to add to pudding to make it drain well?

I already stated I hold as true that the grower's soil choice when establishing a planting for the long term is the most important decision he/she will make. There is no question that the roots are the heart of the plant, and plant vitality is inextricably linked in a hard lock-up with root vitality. In order to get the best from your plants, you absolutely must have happy roots.

If you start with a water-retentive medium, you cannot effectively amend it to improve aeration or drainage characteristics by adding larger particulates. Sand, perlite, Turface, calcined DE ...... none of them will work effectively. To visualize why sand and perlite can't change drainage/aeration, think of how well a pot full of BBs would drain (perlite); then think of how poorly a pot full of pudding would drain (bagged soil). Even mixing the pudding and perlite/BBs together 1:1 in a third pot yields a mix that retains the drainage characteristics and PWT height of the pudding. It's only after the perlite become the largest fraction of the mix (60-75%) that drainage & PWT height begins to improve. At that point, you're growing in perlite amended with a little potting soil.

You cannot add coarse material to fine material and improve drainage or the ht of the PWT. Use the same example as above & replace the pudding with play sand or peat moss or a peat-based potting soil - same results. The benefit in adding perlite to heavy soils doesn't come from the fact that they drain better. The fine peat or pudding particles simply 'fill in' around the perlite, so drainage & the ht of the PWT remains the same. All perlite does in heavy soils is occupy space that would otherwise be full of water. Perlite simply reduces the amount of water a soil is capable of holding because it is not internally porous. IOW - all it does is take up space. That can be a considerable benefit, but it makes more sense to approach the problem from an angle that also allows us to increase the aeration AND durability of the soil. That is where Pine bark comes in, and I will get to that soon.

If you want to profit from a soil that offers superior drainage and aeration, you need to start with an ingredient as the basis for your soils that already HAVE those properties, by ensuring that the soil is primarily comprised of particles much larger than those in peat/compost/coir/sand/topsoil, which is why the recipes I suggest as starting points all direct readers to START with the foremost fraction of the soil being large particles, to ensure excellent aeration. From there, if you choose, you can add an appropriate volume of finer particles to increase water retention. You do not have that option with a soil that is already extremely water-retentive right out of the bag.

I fully understand that many are happy with the results they get when using commercially prepared soils, and I'm not trying to get anyone to change anything. My intent is to make sure that those who are having trouble with issues related to soil, understand why the issues occur, that there are options, and what they are.

We have seen that adding a coarse drainage layer at the container bottom does not improve drainage. It does though, reduce the volume of soil required to fill a container, making the container lighter. When we employ a drainage layer 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 using the same soil with added 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 on soil particles 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 employ 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 in the container to a few inches below the bottom of the pot, or allow it to contact soil below the container where the earth acts as a giant wick and will absorb all or most of the perched water in the container, in most cases. Eliminating the PWT has much the same effect as providing your plants much more soil to grow in, as well as allowing more, much needed air in the root zone.

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 suffer/die because there is insufficient air at the root zone to insure normal root function, so water/nutrient uptake and root metabolism become seriously impaired.

To confirm the existence of the PWT and how effective a wick is at removing 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 and allow the water to drain. When drainage has stopped, 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. The water that drains is water that occupied the PWT. A greatly simplified explanation of what occurs is: The wick or toothpick "fools" the water into thinking the pot is deeper than it is, 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 in the thread.

I always remain cognizant of these physical principles whenever I build a soil. I have not used a commercially prepared soil in many years, preferring to build a soil or amend one of my 2 basic mixes to suit individual plantings. I keep many ingredients at the ready for building soils, but the basic building process usually starts with conifer bark and perlite. Sphagnum peat plays a secondary role in my container soils because it breaks down too quickly to suit me, and when it does, it impedes drainage and reduces aeration. Size matters. Partially composted conifer bark fines (pine is easiest to find and least expensive) works best in the following recipes, followed by uncomposted bark in the <3/8" range.

Bark fines of pine, fir or hemlock, 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, more scarce as a presence in sapwood products and hardwood bark, dramatically slows the decomposition of conifer bark-based soils. It contains highly varied hydrocarbon chains and the microorganisms that turn peat to soup have great difficulty cleaving these chains - it retains its structure.

Note that there is no sand or compost in the soils I use. Sand, as most of you think of it, can improve drainage in some cases, but it reduces aeration by filling valuable macro-pores in soils. Unless sand particle size is fairly uniform and/or larger than about BB size, I leave it out of soils. Compost is too fine and unstable for me to consider using in soils in any significant volume as well. The small amount of micro-nutrients it supplies can easily be delivered by one or more of a number of chemical or organic sources that do not detract from drainage/aeration.

The basic soils I use ....

The 5:1:1 mix:

5 parts pine bark fines (partially composted fines are best)
1 part sphagnum peat (not reed or sedge peat please)
1-2 parts perlite
garden lime (or gypsum in some cases)
controlled release fertilizer (if preferred)

Big batch:
2-3 cu ft pine bark fines
5 gallons peat
5 gallons perlite
2 cups dolomitic (garden) lime (or gypsum in some cases)
2 cups CRF (if preferred)

Small batch:
3 gallons pine bark
1/2 gallon peat
1/2 gallon perlite
4 tbsp lime (or gypsum in some cases)
1/4 cup CRF (if preferred)

I have seen advice that some highly organic (practically speaking - almost all container soils 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) should be repotted more frequently to insure they can grow at as close to their genetic potential within the limits of other cultural factors as possible. 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, fine stone, VERY coarse sand (see above - usually no smaller than BB size in containers, please), Haydite, lava rock (pumice), Turface, calcined DE, and others.

For long term (especially woody) plantings and houseplants, I use a superb soil that is extremely durable and structurally sound. The basic mix is equal parts of pine bark, Turface, and crushed granite.

The gritty mix:

1 part uncomposted screened pine or fir bark (1/8-1/4")
1 part screened Turface
1 part crushed Gran-I-Grit (grower size) or #2 cherrystone
1 Tbsp gypsum per gallon of soil (eliminate if your fertilizer has Ca)
CRF (if desired)

I use 1/8 -1/4 tsp Epsom salts (MgSO4) per gallon of fertilizer solution when I fertilize if the fertilizer does not contain Mg (check your fertilizer - if it is soluble, it is probable it does not contain Ca or Mg. If I am using my currently favored fertilizer (I use it on everything), Dyna-Gro's Foliage-Pro in the 9-3-6 formulation, and I don't use gypsum or Epsom salts in the fertilizer solution.

If there is interest, you'll find some of the more recent continuations of the thread at the links below:

Post XV

Post XIV


Post XII

Post XI

If you feel you were benefited by having read this offering, you might also find this thread about Fertilizing Containerized Plants

If you do find yourself using soils you feel are too water-retentive, you'll find some Help Dealing with Water Retentive Soils by following this embedded link.

If you happen to be at all curious about How Plant Growth is Limited, just click the embedded link.

Finally, if you are primarily into houseplants, you can find an Overview of the Basics that should provide help in avoiding the most common pitfalls.

As always - best luck. Good growing!! Let me know if you think there is anything I might be able to help you with.



clipped on: 01.02.2014 at 05:27 pm    last updated on: 01.02.2014 at 05:28 pm

RE: succulent soil mix (Follow-Up #2)

posted by: tapla on 11.02.2007 at 04:10 pm in Container Gardening Forum

Below, you'll find a few thumbnails to click on if you're interested. Sorry that I didn't take much time to take especially pretty photos. ;o) Just wanted you to see what I grow houseplants and succulents in. If you look past the hail damage, you can see the plants are very pleased with what their feet are in.

I always have a mix of equal parts of Turface, crushed granite (grower/turkey grit), and fir bark on hand because it's my basic bonsai soil mix. I add some very coarse sand, and vermiculite, along with a little gypsum & dolomitic lime to it to round things out. Here's the formula:
3 parts Turface
3 parts crushed granite (farm feed store)
3 parts pine or fir bark (see photo for size)
1 part coarse silica sand (masonry supply company)
1 part vermiculite
CRF (18-4-9 is what I use, but anything with a high first # or close to a 3-1-2 ratio works well)
Dolomitic lime & gypsum
Micronutrient granules


The Turface or a similar product is important.

Notice the lack of a large variance in particle size.

You could eliminate either the granite or sand by varying ratios or with a similar product.

The CRF is not necessary if you're diligent about your nutrient supplementation program.

I can help you determine how much dolomite & gypsum to add when I know your batch size.

You don't need peat in your mix, and the mix you're looking at is approx 75% mineral/inorganic. I grow some plants with a 0% organic component, so don't let that bother you. This soil is extremely consistent in how it performs over time and very structurally stable.



clipped on: 01.01.2014 at 06:13 pm    last updated on: 01.01.2014 at 06:14 pm

RE: What is this Al's gritty mix? (Follow-Up #1)

posted by: tapla on 02.18.2010 at 11:18 am in Container Gardening Forum

Equal parts by volume of:

pine or fir bark in 1/8-1/4" size
Screened Turface (may substitute calcined DE, such as NAPA floor dry)
Gran-I-Grit (crushed granite in grower size or #2 cherrystone)




clipped on: 01.01.2014 at 06:03 pm    last updated on: 01.01.2014 at 06:04 pm

RE: Al's Gritty Mix Questions (Follow-Up #1)

posted by: smishgibson on 11.12.2013 at 08:43 am in Container Gardening Forum

First off, welcome e-w-031!

I do believe you posted this in the correct section. I will answer part of your questions, as i know nothing about ferns.

The Manna pro is great stuff! Some people use it unscreened, just washing it to get the dust off. Most of the time this is fine, i tried screening the 2, 25lb bags i got over an insect screen and nothing fell through. If you screen over a 1/8inch screen you will lose about half but this is not necessary. I would highly suggest trying to find a tractor supply store near you.

They sell a 25 pound bag for 8.99.

The Turface would work great, however, again I would consider finding a local source, a good place to try is John deere landscapes.

And ask for Turface Allsport. I had to wait 2 weeks for them to order it and get it in for me, but after sifting, i got 12 quarts of "large"(1/8th inch or larger) and 20 quarts of "medium"(1/8th inch - 1/16 inch) for 16 dollars!

Nothing wrong with buying them off amazon, just wanted you to be aware of your options elsewhere, as they will be much more economical, especially if you are making a larger batch. If you let us know what area, or major city you are near, someone here may already know a good local source.

Hopefully someone else will have some more info on the ingredients, and also know more about ferns. Do you know what type of fern(s) you have? Different types my like different soils.

Again welcome to the forums and best of luck with your gritty mix!


clipped on: 01.01.2014 at 05:57 pm    last updated on: 01.01.2014 at 05:57 pm