In this article are recipes for soil mixes. The availability of certain specific recommended constituent components will vary from region to region and substitutes will have to be found that serve the same function as the original component. Specifically I am talking about bark. Out West for example it will be a lot easier to find bark nuggets from cedar, fir, yew, hemlock etc. than the recommended pine bark nuggets. They serve the same function: to help aerate the mixture. Next time you're in your favourite local nursery take a close look at the soil mixes being used for the potted perennials. You will find the plants that have been grown in their container for a year or more -1 gallon pots and up- almost always contain mixes heavy in fine bark.

The contents of this page are mostly from a posting to an online forum that I reformatted for the site's moderator to improve readability. I have condensed it somewhat and would like to make it perfectly clear that I am NOT the original writer of this material. You can find the original posting here. In a few select places I have added links to external scientific articles -mostly PDF files- that will provide further information on the subject. A final thought and note: this deals with container gardening, a very different practice from gardening in native soil, requiring a very different approach.

When I first posted this

Potting mixes

Consider this if you will: soil need fill only a few needs in plant culture.

  • Anchorage - A place for roots to extend, securing the plant and preventing it from toppling.
  • Nutrient Sink - It must retain sufficient nutrients to sustain plant systems.
  • Gas Exchange - It must be sufficiently porous to allow air to the root system.
  • And finally, Water - It must retain water enough in liquid and/or vapor form to sustain plants between waterings.

Water Movement in Soils

Most plants could be grown without soil as long as we can provide air, nutrients, and water, (witness hydroponics). Here, I will concentrate primarily on the movement of water in soil(s). There are two forces that cause water movement 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 pot than it is for water at the bottom of the pot. I'll return to that later. Capillarity is a function of the natural forces of adhesion and cohesion. Adhesion is water's tendency to stick to solid objects like soil particles and the sides of the pot. Cohesion is the tendency for water to stick to itself. Cohesion is why we often find water in droplet form - because cohesion is at times stronger than adhesion, water's bond to itself can be stronger than the bond to the object it might be in contact with; in this condition it forms a drop. Capillary action is in evidence when we dip a paper towel in water. The water will soak into the towel and rise several inches above the surface of the water. It will not drain back into the source. It will stop rising when the GFP equals the capillary attraction of the fibers in the paper.

There is, in every pot, what is called a "perched water table" (PWT). This is water that occupies a layer of soil that is always saturated and will not drain at the bottom of the pot. 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 equal the GFP; therefore, the water does not drain, it is "perched".

If we fill five cylinders of varying heights and diameters with the same soil mix and provide each cylinder with a drainage hole, the PWT will be exactly the same height in each container. This is the area of the pot where roots seldom penetrate & where root problems begin due to a lack of aeration. From this we can draw the conclusion that tall growing containers are a superior choice over squat containers when using the same soil mix. The reason: the level of the PWT will be the same in each container, with the taller container providing more usable, air holding soil above the PWT.

Physiology dictates that plants must be able to take in air at the roots in order to complete transpiration and photosynthesis. A given volume of large soil particles have less overall surface area in comparison to the same volume of small particles and therefore less overall adhesive attraction to water. So, in soils with large particles, GFP more readily overcomes capillary attraction. They drain better. We all know this, but the reason, often unclear, is that the PWT is lower in coarse soils than in fine soils. The key to good drainage is size and uniformity of soil particles. Large particles mixed with small particles will not improve drainage because the smaller particles fit between the large, increasing surface area which increases the capillary attraction and thus the water holding potential. Water and air cannot occupy the same space at the same time. Contrary to what some hold to be true, sand does not improve drainage. Pumice (aka lava rock), or one of the hi-fired clay products like Turface are good additives which help promote drainage and porosity because of their irregular shape.

Now to the main point: when we use a coarse drainage layer under our soil, it does not improve drainage. It does conserve on the volume of soil required to fill a pot and it makes the pot lighter. When we employ this exercise in an attempt to improve drainage, what we are actually doing is moving the level of the PWT higher in the pot. This reduces available soil for roots to colonize, reduces total usable pot space, and limits potential for beneficial gas exchange. Containers with uniform soil particle size from top of container to bottom will yield better drainage and have a lower PWT than containers with drainage layers. The coarser the drainage layer, the more detrimental to drainage it is because water is more (for lack of a better scientific word) reluctant to make the downward transition because the capillary pull of the soil above the drainage layer is stronger than the GFP. The reason for this is there is far more surface area in the soil for water to be attracted to than there is in the drainage layer. I know this goes against what most have thought to be true, but the principle is scientifically sound, and experiments have shown it as so.

Many nurserymen are now employing the pot-in-pot or the pot-in-trench method of growing to capitalize on the science. If you discover you need to increase drainage, insert a wick into the pot and allow it to extend from the PWT to several inches below the bottom of the pot. This will successfully eliminate the PWT and give your plants much more soil to grow in as well as allow more, much needed air to the roots. Uniform size particles of fir, hemlock or pine bark are excellent as the primary component of your soils. The lignin contained in bark keeps it rigid and the rigidity provides air-holding pockets in the root zone far longer than peat or compost mixes that rapidly break down to a soup-like consistency. Bark also contains suberin, a lipid sometimes referred to as nature's preservative. Suberin is what slows the decomposition of bark-based soils. It contains highly varied hydrocarbon chains and the microorganisms that turn peat to soup have great difficulty cleaving these chains. In simple terms: plants that expire because of drainage problems either die of thirst because the roots have rotted and can no longer take up water, or they starve to death because they cannot obtain sufficient air at the root zone for the respiratory or photosynthetic processes.

To confirm the existence of the PWT and the effectiveness of using a wick to remove it, try this experiment: Fill a soft drink cup nearly full of garden soil. Add enough water to fill to the top, being sure all soil is saturated. Punch a drain hole in the bottom of the cup and allow to drain. When the drainage stops, insert a wick several inches up into the drain hole . Take note of how much additional water drains. This is water that occupied the PWT before being drained by the wick. A greatly simplified explanation of what occurs is: The wick "fools" the water into thinking the pot is deeper, so water begins to move downward seeking the "new" bottom of the pot, pulling the rest of the PWT along with it. Having applied these principles in the culture of my containerized plants, both indoors and out, for many years, the methodology I have adopted has shown to be effective and of great benefit to them. I use many amendments when building my soils, but the basic building process starts with screened bark and perlite. Peat usually plays a very minor role in my container soils because it breaks down rapidly and when it does, it impedes drainage.

Butch's Soil Mix: I'll give two recipes. I usually make big batches.

  • 3 parts pine bark fines
  • 1 part sphagnum peat (not reed or sedge peat)
  • 1-2 parts perlite
  • garden lime
  • controlled release fertilizer and micro-nutrient powder (substitute: small amount of good, composted manure)
    Big batch
  • 3 cu ft pine bark fines (1 big bag)
  • 5 gallons peat
  • 5 gallons perlite
  • 1 cup lime (you can add more to small portion if needed)
  • 2 cups CRF
  • 1/2 cup micro-nutrient powder or 1 gal composted manure
    Small batch
  • 3 gallons pine bark
  • 1/2 gallon peat
  • 1/2 gallon perlite
  • handful lime (careful)
  • 1/4 cup CRF
  • 1 tsp micro-nutrient powder or a dash of manure

I have seen advice that some highly organic soils are productive for up to 5 years. I disagree. Even if you were to substitute fir bark for pine bark in this recipe (and this recipe will far outlast any peat based soil) you should only expect a maximum of three years life before a repot is in order. Usually perennials, including trees (they're perennials too, you know ;o)) should be repotted more frequently to insure vigor closer to genetic potential. If a soil is desired that will retain structure for long periods, we need to look to inorganic amendments. Some examples are crushed granite, pea stone, coarse sand (no smaller than BB size in containers, please), Haydite, lava rock, Turface or Schultz soil conditioner. I hope this starts a good exchange of ideas & opinions so we all can learn.

Footnotes

Since the source material for this was originally part of a forum posting exchange, there is additional valuable information contained within the responses to some of the posters. While the poster's original question is not reflected here, the answers provided are, and that's where the meat of the message is.

Becky - If you were watering with a wick and fighting gravity, the material would need to be absorbent enough to "pull" water the distance between the source of water and the bottom of the pot. Since in this case, we're removing water from the pot, gravity is our friend and we can use almost anything absorbant that is made from something the little bugs (microbes) won't eat up too soon. The mop strands Lydia mentioned above (from synthetic mop heads) work great. I have even used braided nylon as a wick material (ties from citrus fruit bags). The material itself holds no water at all, but the capillary action allows it to drain pots nicely. If you're using only 1 drain hole, the wick should be small enough so that there is still good drainage of the water that would normally drain without a wick. If you use more than 1 drain hole (I always do, but there really isn't much advantage except as a guard against 1 clogging, or in this case, the other is full of a wick) the wick can fill the entire hole while the other(s) provide drainage of water that would normally drain.

Next question: Double potting can be done a couple of ways. You can use a socket pot, which is a pot buried in the ground. Your pot should nest inside of the socket pot & a portion of the bottom of the socket pot and the space between the two is filled with soil. This method should drain all of the perched water every time, unless the surrounding soil is at saturation level and the water will not flow laterally. The second method is essentially the same except that the pots are above ground. In order for this method to be effective, the depth of the soil in the larger pot (not the one you grow in) should be as deep as or deeper than the depth of the perched water in your grow pot. BUT - you can add a wick to the large pot and insure that all perched water drains from both pots. Take note that using a wick accomplishes exactly the same thing as pot-in-pot growing. The biggest benefit of pot-in-pot (for our purposes) might lie in its temperature modifying effects. Most plants roots begin to suffer when actual root temperatures get around 90º F and shut down around 95º F. If you grow in dark colored containers, they experience much higher temperatures from solar gain. Aluminum foil, white pots, or pot-in-pot growing all help to moderate the high temperatures. Shading the pots helps a great deal as well. I read that pot-in-pot growing can lower root temperatures significantly - more than 15º F. in some cases. Remember, wicking is particularly valuable when we are growing over-potted plants (plants in a pot that is probably too large to be healthy for the roots) and when we first plant our containers. As an example, you might have a 5 gallon bucket with 1 squash plant in it in anticipation of how large that plant WILL be by harvest, or maybe a dozen annuals in a large pot with a root mass the size of golf balls. In these instances, it's likely the soil remains too wet. Roots growing in poorly aerated media are weaker, less succulent and more susceptible to micro-nutrient deficiencies and root rot pathogens such as Pythium and Phytophthora than roots growing in well-aerated media. Anaerobic conditions (without oxygen) do not allow the roots to obtain energy from the respiratory process and encourage disease development. Energy is required for root growth, proper hormone balance and nutrient uptake as well as maintenance of cell membranes and other plant apparati needed for basic physiological processes. So, you can grow well in a soil that wants to retain lots of water, you just don't let it - you wick it. Later in the year, when roots have colonized the entire pot and transpiration is placing extreme demand on roots to provide water - remove the wick. It's almost like having the advantage of growing in a different soil without transplanting. Sand is useless as a drainage layer. By all means, use pot shards or something over the drain holes to keep soil from running out the holes. I use insect screening. If your intent is to reduce the amount of soil in your pot, fill the bottom with something non-absorbant, but don't expect it to improve drainage. It will only raise the level of saturated soil higher in the pot. Use as many drainage holes as you wish. I have wooden grow boxes that have 20 0r 30 holes in them, not necessarily for drainage, but to increase gas exchange at the rootzone. If you use a wick that sort of plugs the center hole, it's best to have other holes to drain water that would normally drain w/o the wick. Use whatever you wish for soil, I outlined why I build mine the way I do & what I use. You decide if you would like to give it a try. I hope you do - I think you will like it. The wick only needs to go into the pot far enough to keep it from falling out. The water needs to drain away from the pot. It can be elevated or set on the ground, where it will be absorbed. If a puddle forms around the bottom of the pot, the wick will be less effective. "Large pots" of 35 gal isn't enough info. You need be less concerned about drainage in deep pots and more concerned about it in shallow pots. More holes or wicks will not change anything except that more wicks will remove the PWT a little faster - not much advantage there, though. That recommendation is for 2 reasons. The first is because it can harbor gnawing rodents. The second is it makes for a wet environment against the bark where decay organisms can multiply. The outcome is the eventual decay of bark, followed by exposure and decay of cambial tissues, effectively girdling the tree. Roots however, are not effected, so if your tree is planted at the proper depth - with the basal flare above soil line - there is no cause for concern. I guess I wouldn't call this my idea or method. I just adopted it from different sources, modified it to suit my needs and shared it here. I almost never use a wick because my soils drain extremely fast. I try to design every planting so, in summer, it barely goes a day without needing water. Wicks aren't necessary in that kind of soil. I grow lots of tropical & sub-tropical trees in containers, too - probably 75 - 100 right now. For those, I use a soil that is at least 70% inorganic.

Lydia - The larger the container, especially taller containers, the denser the mix should/can be. The reason is the level of saturated soil is constant in all containers as long as you use the same soil. If you have a container that is deep (say 3 ft) All water will drain from the large pores (macro-pores) and the only water in the top 9/10 (approx) of the soil will be that retained in the smaller (micro) pores. A denser soil in a tall container will hold more water & still provide drainage. I think supplying this info might be starting to split hairs, though. As long as you have aeration, you can easily compensate by watering more often after roots colonize the soil.

Hi, Ken. In a container, the water that is most "free" or most readily drainable, is the water in larger pores between particles in the portion of soil near the bottom of the pot. This is the water that drains when using a wick for drainage. The water that is attached to soil particle surfaces and captured in micro-pores in soil particles remains because it is too tightly held. This includes the tightly held water above the saturated layer, too. A wick makes the free water in the saturated layer of soil "think" the pot is deeper, so it moves down the wick until only the free water is drained. The benefit of this is it drains macro-pores in soil at the bottom of the pot and makes that environment suitable for root colonization. A container full of dry soil, with a wick extending into a water reservoir, will moisten the soil in the entire pot. Here's why. The particles of soil themselves have a great attraction for water where soil particles touch and the thousands of tiny pores in the organic portion of the soil are the most powerful in capillary attraction. Remember, the cells in the portion of your soil that was once living (bark, peat, etc.) were once mostly water, and water readily finds its way back into these dead cells. This strong capillarity is how giant sequoia trees pull water 300 feet into the canopy, so an 18" pot should be a snap. ;o) While the wick provides a means by which the perched water can be drained, it also provides a bridge between the reservoir and the pot. Once the water reaches the bottom of the pot, the strong capillary pull of particles will pull water through the rest of the pot. You can do your own experiment by filling a soft drink cup with soil and inserting a wick. Allow the wick to dangle in a reservoir. Come back later, and the top of the soil will be moist. You only need to be sure the wick is absorbent enough to pull water high enough to contact the soil. This process could take an hour or several hours, but eventually, the entire volume of soil will be moistened. If you use it, this type of watering can cause excessive fertilizer salts build-up, so the soil should be flushed regularly to remove it. Same holds true for earth box type containers.

Hi, Vetivert. You're lucky to have this tree so plentiful where you are. We have very few specimens here and those that remain are infected with pitch canker, which leaves them weak, so bark beetles are also a major problem. Anyway - a story for another time. Re-wettability of container soils depends in part on our not allowing them to go below a certain threshold of soil moisture. When the water content of pine bark is allowed to drop below 35% (by volume) it becomes difficult to re-wet. However, this figure depends on how advanced is the state of decomposition of the soil particles. Peat in soils shows similar characteristics. The obvious answer here is not to allow soils to dry to below these thresholds or - water more frequently. I've read your posts and know you are an experienced gardener, so it's likely this info is not helpful to you. If you have learned to recognize the problem batches of bark, perhaps constructing your soil so it is initially higher in peat content than usual for a pine soil. The peat will act as a water retaining component in the soil. As the peat begins to break down into fine particles and wash out of drain holes in the second and third years, the cellulose in the bark should be well on the way to breakdown, making for greater bark porosity and so better wettability. Although there are many surfactants and surfactant/polymer combinations to enhance the wettability and water-holding capacity of container media, the effectiveness of the surfactants decreases over time, requiring additional applications. I've never needed to use them, so I've done little research on their use, but of some additional consideration is the fact that some of the wetting agents are phytotoxic to woody plant materials. Double watering is an effective way for me to be sure important plants don't end up with dry areas in the soil where portions have become hydrophobic for one reason or another. During the heat of summer, I often water containers and then, about 5 minutes later, give another thorough soaking. Another method would be to water from above, but also allow plants to remain in a saucer of water for a period to take advantage of capillarity. With this regimen, you run little risk of salt build-up. Perhaps partially composting the bark you intend to use for a year before using it?

Sign our guestbookGuestbookView guestbook
contact us

Pages last updated 1 May 2013