Container Soils - Water Movement and Retention

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From user Al, aka tapla

http://forums2.gardenweb.com/forums/load/contain/msg0622171013552.html

Posted by tapla (http://www.gardenweb.com/auth/nph-logincheck.cgi?action=public_profile&user=tapla) z5b-6a mid-MI (My Page (http://www.gardenweb.com/auth/nph-logincheck.cgi?action=public_profile&user=tapla)) on Sun, Jun 5, 11 at 22:17
I first posted this thread back in March of '05. Thirteen 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 the idea some of 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 2,000 posts at the end of what I have written - just in case you have interest in reviewing them. Thank you for taking the time to examine this topic - I hope that any/all who read it take at least something interesting and helpful from it. I know it's long; my hope is that you find it worth the read.

Container Soils - Water Movement and Retention

A Discussion About Container Soils

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.

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 Water Movement information.

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 improve it's aeration or drainage characteristics by adding larger particulates. Sand, perlite, Turface, calcined DE ...... none of them will work. 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.

My Basic Soils ....

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.

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
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 XIII (http://forums2.gardenweb.com/forums/load/contain/msg0316064615891.html?148AbD7g#post)
Post XII (http://forums2.gardenweb.com/forums/load/contain/msg112015049996.html?148)
Post XI (http://forums2.gardenweb.com/forums/load/contain/msg0309313324323.html?147)
Post X (http://forums2.gardenweb.com/forums/load/contain/msg0214164227031.html?148)
Post IX (http://forums2.gardenweb.com/forums/load/contain/msg0715434425632.html?148)
PostVIII (http://forums2.gardenweb.com/forums/load/contain/msg0521151724775.html?149)
Post VII (http://forums2.gardenweb.com/forums/load/contain/msg0320294031096.html?149#post)

If you feel you were benefited by having read this offering, you might also find this thread about Fertilizing Containerized Plants (http://forums2.gardenweb.com/forums/load/contain/msg0323131520631.html?88) helpful, as well.
If you do find yourself using soils you feel are too water-retentive, You'll find some Help Dealing with Water-retentive Soils (http://forums2.gardenweb.com/forums/load/contain/msg0914024623640.html) by following this embedded link.

If you happen to be at all curious about How Plant Gowth is Limited (http://forums2.gardenweb.com/forums/load/contain/msg0921071615772.html), just click the embedded link.

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

Al

[vacuum]

Thread with discussion: http://forums2.gardenweb.com/forums/load/contain/msg0323131520631.html?88

his 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.
Al

[vacuum]

Thread with discussion: http://forums2.gardenweb.com/forums/load/contain/msg0914024623640.html

Dealing with Water-Retentive Soils

A good friend recently asked me if putting a brick in the bottom of a container interferes with drainage? After reading the question, it occurred to me that there are aspects to the question that I?ve discussed very little here at GW. It also occurred to me that I could use her question to help those who grow in heavy (water-retentive) soils. I?m going to define those soils, but this isn?t about disparaging soil types - it?s about helping you try to squeeze the most plant vitality (and the water) out of them. Heavy soils are based on fine ingredients. If the soil contains more than 30-40% of any combination of peat, coir, compost, or other fine ingredients like builders sand or topsoil, it will retain appreciable amounts of 'perched water' and remain soggy after it?s saturated - and this is about dealing with soggy soils.

Perched water is water that remains in the soil after the soil stops draining. If you wet a sponge & hold it by a corner until it stops draining, the water that is forced out of the sponge when you squeeze it is perched water. From the plant?s perspective, perched water is unhealthy because it occupies air spaces that are needed for normal root function and metabolism. The gasses produced under anoxic (airless) conditions (CO2, sulfurous compounds, methane) are also an issue. The main issue though, is that roots deprived of sufficient oxygen begin to die within hours. You don?t actually see this, but the finest, most important roots die first. The plant then has to spend stored energy or current photosynthetic (food production) to regenerate lost roots - an expensive energy outlay that would otherwise have been spent on blooms, fruit, branch extension, increasing biomass, systems maintenance ?.. Perhaps the plant would have stored the energy for a winter?s rest and the spring flush of growth instead of expending it on root regeneration.

You can see that perched water, from the plant?s perspective, is not a good thing. From our own perspective, we think it?s rather convenient when we only need to water our plantings every 4-5 days, but because we can?t see it, there is a sacrifice in potential growth/vitality for our convenience - like driving on low tires reduces fuel economy. How we choose to resolve this issue is of no concern to me - we all arrange our priorities & few of us are willing to water plants every hour to squeeze the last wee bit of vitality from them. Growing is about compromise in more cases than not. There is no judgment passed here on soil choice.

If you don?t agree that perched water is generally a bad thing in containers, there?s no need to read on. If you?re still interested, I?ll lay a little groundwork here before I outline some things remedial you can do to combat excess water retention. Almost all out-of-the-bag soils retain a considerable amount of perched water after they have been saturated. Each individual soil formulation will retain a specific height of perched water unique to THAT soil. No matter what the shape or size of the container - height, width, round, square ?? the height of the PWT (perched water table) will be the same. You can fill a 1" diameter pipe with a particular soil or a 55 gallon S-shaped drum with the same soil, and both will have exactly the same PWT height.

Let?s do some imagining for the purpose of illustration. Most peat or compost based soils retain in excess of 3 inches of perched water, so lets imagine a soil that retains 3 inches of perched water. Also, imagine a funnel that is 10 inches between the exit hole & the mouth and is filled with soil. Because we are imagining, the mouth is enclosed & has a drain hole in it. In your minds eye, picture the funnel filled with a soil that holds 3 inches of perched water, and the soil is saturated. If the funnel is placed so the large opening, the mouth, is down, you can see the largest possible volume of soil possible when using this container is saturated, the first 3 inches; but, turn the funnel over and what happens? We KNOW that the PWT level is constant at 3 inches, but there is a very large difference in the volume of soil in the lower 3 inches of the funnel after it is placed small end down. This means there is only a small fraction of the volume of perched water in the small-end-down application vs. the large-end-down. When you tip the funnel so the small end is down, all but a small fraction of the perched water runs out the bottom hole as the large water column seeks its 3 inch level in the small volume of soil. In a way, you have employed gravity to help you push the extra water out of the soil.

You haven?t affected the DRAINAGE characteristics of the soil or its level of aeration, but you HAVE affected the o/a water retention of the container. This allows air to return to the soil much faster and greatly reduces any issues associated with excess water retention. OK - we can see that tapered containers will hold a reduced VOLUME of perched water, even when drainage characteristics, aeration, and the actual height of the PWT remain unchanged, but we don?t and won?t all grow in funnels, so lets see how we can apply this information PRACTICALLY to other containers.

Drainage layers don?t work. The soil rests on top of drainage layers, then the water ?perches? in the soil above - just as it would if the soil was resting on the container bottom. Drainage layers simply raises the LOCATION where the PWT resides. But what if we put a brick or several bricks on the bottom of the container? Let?s look at that idea, using the soil with the 3inch PWT again. Let?s say the brick is 4x8x3 inches tall, and the container is a rectangle 10x12x12 inches high. The volume of soil occupied by perched water is going to be 10x12x3, or 360 cubic inches. If we add the brick to the bottom of the container so the height of the brick is 3 inches, it reduces the volume of soil that can hold perched water, so for every brick you add (4x8x3=96) you reduce the volume of soil that can hold perched water by 96 cubic inches. If you add 3 bricks, the volume of soil that holds perched water would be 360-288, or only 72 cubic inches, so you have reduced the amount of perched water in the container by 80% ?.. quite a feat for a brick.

Your job though, is to be sure that what you add to the bottom of the container to reduce the volume of soil that can hold perched water doesn?t create stress later on when the planting has matured. Be sure the container has a large enough volume of soil to produce plants free from the stress of excessive root constriction. You don?t want to trade one stress for another.

How else might we ?trick? the water in the container into leaving? Let?s think about the following in 2 dimensions, because it?s easier to visualize. If you look at the side view of a cylindrical or rectangular container, you see a rectangle, so imagine a cylinder or rectangle 10 inches wide or 10 inches in diameter and 8" deep. Both side views are rectangles. Now, draw a horizontal line 3 inches above the bottom to represent the level of the PWT. Remember, this line will always remain horizontal and 3 inches above the bottom. Now tip the container at a 45 degree angle and notice what happens. The profile is now a triangle with an apex pointing downward and the base is of course the line of the PWT 3 inches above the bottom. Can you see there is a much lower volume of soil in the bottom 3 inches of the triangle than in the bottom 3 inches of the rectangle? The PWT line is level at 3 inches above the apex, so by simply tilting your containers after you water, you can trick a large fraction of the unwanted perched water to exit the container. Sometimes it helps to have a drain hole on the bottom outside edge of the pot, but not always. Only when the location of the hole is above the natural level of the PWT when the pot has been tilted does it affect how much additional water might have been removed.

On the forums, I?ve often talked about wicks, so I?ll just touch on them lightly. If you push a wick through the drain hole and allow it to dangle several inches below the bottom of your container immediately after watering, the wick will ?fool? the perched water into behaving as though the container was deeper than it actually is. The water will move down the wick, seeking the bottom of the container and will then be pushed off the end of the wick by the additional water moving down behind it.

A variation of the wick, is the pot-in-pot technique, in which you place/nest one container inside another container with several inches of the same soil in the bottom and fill in around the sides. Leaving the drain hole of the top container open allows an unobstructed soil bridge between containers. Water will move downward through the soil bridge from the top container into the bottom container seeking its natural level; so all of the perched water the soil is capable of holding ends up in the bottom container, leaving you with much better aeration in your growing container.

The immediately above example employs the soil in the lower container as a wick, but you can achieve the same results by partially burying containers in the yard or garden, essentially employing the earth as a giant wick. These techniques change the physical dynamics of water movement and retention from the way water normally behaves in containers to the way water behaves in the earth. Essentially, you have turned your containers into mini raised beds, from the perspective of hydrology.

What I shared doesn?t mean it?s a good thing to use water retentive soils, simply because you have tricks to help you deal with them. For years, I?ve been using highly aerated soils and biting the ?water more often? bullet because I?ve seen the considerable difference these durable and highly aerated soils make when it comes to plant growth and vitality. Many others have come to the same realization and are freely sharing their thoughts and encouragement all across the forums, so I won?t go into detail about soils here.

It should also be noted that roots are the heart of the plant, and it is impossible to maximize the health and vitality of above-ground parts without first maximizing the health and vitality of roots. Healthy roots also reduce the incidence of disease and insect predation by keeping metabolisms and vitality high so the plant can maximize the production of bio-compounds essential to defense.

The soil/medium is the foundation of every conventional container planting, and plantings are not unlike buildings in that you cannot build much on a weak foundation. A good soil is much easier to grow in, and offers a much wider margin for error for growers across the board, no matter their level of experience. But regardless of what soils you choose, I hope the outline here provides you with some useful strategies if you DO find yourself having to deal with a heavy soil.

Al

[vacuum]

Thread http://forums2.gardenweb.com/forums/load/contain/msg0921071615772.html

How Plant Growth is Limited
Posted by tapla (http://www.gardenweb.com/auth/nph-logincheck.cgi?action=public_profile&user=tapla) z5b-6a MI (My Page (http://www.gardenweb.com/auth/nph-logincheck.cgi?action=public_profile&user=tapla)) on Sun, Sep 19, 10 at 21:07
http://i26.photobucket.com/albums/c149/afassezke/LiebigsLaw.gif (http://s26.photobucket.com/albums/c149/afassezke/?action=view%C2%A4t=LiebigsLaw.gif)
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 (http://forums2.gardenweb.com/forums/load/contain/msg0309313324323.html?126) by clicking this highlighted text; or you can read some tips about

How to Deal With Water-retentive Soils (http://forums2.gardenweb.com/forums/load/contain/msg0914024623640.html?37) 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.


Al

[gunDriller]

i think one problem people make is forgetting that roots need oxygen.

so many plants do way better if they are moist but not flooded.

that means religiously emptying the pan beneath a pot, so that the roots are not stuck in a swamp. (aerobic)


unless you're growing bamboo or aquarium weed or something that is meant for immersion of the roots. (an-aerobic)