Friday, January 29, 2010

Soil mechanics and drainage

"Add a layer of gravel or other coarse material in the bottom of containers to improve drainage"

The above statement is pure, unadulterated hogwash! As we read on, we will see that providing such a layer has the opposite effect of what we are trying to accomplish--drainage. In turn, many will argue they serve a useful purpose and will attempt to support their arguments with "the Japanese use them..." This statement in my opinion is overused and overrated.

The aim of this article is to debunk the myth that adding a layer of coarser material at the bottom of a container improves overall drainage. It will be further demonstrated that adding such a layer has the opposite effect of impeeding drainage. The discussion will demonstrate that the saturation zone can in fact be manipulated through the use of varying particulate size advantageously, the latter is not to be construed as “added drainage”. Without further adue, let us see what transpires below the surface.

The Myth
This is just one of those myths that refuses to die, regardless of solid scientific evidence to the contrary! Nearly every book or web site on container gardening recommends placing coarse material at the bottom of containers for drainage. The materials most often recommended for this practice are sand, gravel, pebbles, and pot shards. Other ‘benefits’ often mentioned include preventing creatures from entering through the drain holes, and stabilizing the container.

The Reality
Nearly 100 years ago, soil scientists demonstrated that water does not move easily from layers of finer textured materials to layers of more coarse textured. Since then, similar studies have produced the same results. Additionally, one study found that more moisture was retained in the soil underlain by gravel than that underlain by sand. Therefore, the coarser the underlying material, the more difficult it is for water to move across the interface. Imagine what happens in a container lined with pot shards!

Some have mentioned soil interfaces and their inhibition of water movement. We can see the same phenomenon occurring here: gravitational water will not move from a finely soil texture into a coarser material until the finer soil is saturated. Since the stated goal for using coarse material in the bottoms of containers is to "keep soil from getting water logged,” it is ironic that adding this material will induce the very state it is intended to prevent.
- Dr. Linda Chalker-Scott

The test...
The following experiment was extracted from an article written by Brent Walston “Why the Earth Is Not Like a Pot”. A simple test that can be conducted by anyone, in the comfort of their own home, without any scientific or laboratory equipment.

Material requirement:
A kitchen sponge 2x5x1/2 inch
An oven rack

Place your oven rack over the sink, this will represent your drainage layer. Take a simple 2x5x1/2" kitchen sponge place it on your oven rack over the sink, the sponge represents the soil, any soil, regardless of size.

Now, thoroughly saturate the sponge (soak it in a bowl) then place it on the rack (large surface down) and allow it to drain. When the sponge stops dripping, turn it on it's edge so that it is 2 inches high and see how much water drains out--this is all the additional water that remained behind after a thorough drain. After the sponge has drained, turn it on it's end so the height of the sponge is now 5 inches. See all the water that is still coming out of what we thought was a thoroughly drained sponge? It's amazing and why is that? The sponge's volume never changed during this experiment--it had a given holding capacity. The only way to change the holding capacity of the sponge would be to change it's density. To reduce the holding capacity due to surface tension, one would use a sponge with a greater porosity, not dissimilar to using larger components in our soil.

The water drained because of the height of the water column. Since the weight of the column was greater than the surface tension provided by the sponge, once the weight and tension were equal, the sponge quit draining. In other words, the weight of the water was greater because of the height of the column in any given container etc... Once this weight equals the opposing tension provided by the substrate, it no longer drains as all is in balance.

Now think of the drainage layer at the bottom of the pot as the rack above your sink. A drainage layer will provide a medium which produces less tension--therefore the water will flow right through it. The trapped water (saturation zone or perched water) will still be in the finer course of substrate, regardless of what that size is. Therefore, the "drainage layer" does not provide extra drainage.

The experiment further shows that pot shape will play a large role in water retention. A broad shallow pot will retain more water than a tall narrow pot of equal volume (if using identical substrate). Although this is counterintuitive, a shallow pot will indeed retain more water and “dry out” more slowly than a tall narrow pot of equal volume. Mind you, one can argue that a shallower pot has a greater surface area than a deeper pot of equal volume, and therefore evaporation will be quicker. This is not so! Why? Although, the shallower pot has a greater surface area, it also has retained a greater amount of water (as proven with the sponge), therefore a deeper pot of equal volume has retained less water therefore will evaporate and dry out more quickly.

An amusing slant...
Many folks use a drainage layer because the Japanese still use them. What I find amusing is that for years now Japanese enthusiasts have moved away from deep cascade pots. Their reason for doing so was/is:

"you do not see tall pot used for cascade any more in japan. tall stand is used because the tree grow on the cliff. tall pot is difficult to use. gravity will pull water to the bottom. the surface will stay dry for that reason. and when it get root bound, it is very difficult to take the tree out of the pot. i hope the explanation make sense. it is personal taste. i used to grow my cascade in tall pot when i started bonsai. it is because i was told and the reason at that time seems logical. but i grow out of it. i find better logical reason"
- Boon Manakitivipart

Once again, think of the sponge. The saturation zone is at the bottom of the pot. Many times the root mass does not occupy the entire depth of the pot. It is said they prefer a shallow root mass because a deeper root mass becomes a re-potting challenge. To counter this challenge, they have opted for taller stands with shallower pots for cascading bonsai. Was/is there another solution to their dilemma? The answer would be yes. With regards to personal taste, we will discuss aesthetics further on.

I believe we have become so preoccupied with growing bonsai artistically, that we might have forgotten many important horticultural practices in the process. Probably because bonsais are “special”. Cultivating and caring for bonsai is no different in principle than container gardening. Now, some may argue this point, but is a bonsai pot not a container? If so, then you are container gardening, for all intents and purposes. Although the myth of drainage layers has been debunked for years, even centuries, it persists. So what is happening beneath the surface?

It goes without saying that a soil with coarser components will not only drain faster, retain less water but evaporation will be greater when compared against a soil that utilizes finer components.

Watering Containers
A few seconds after you start watering a container, all pores are filled with water, displacing the air from the pores. Drainage occurs through the holes at the bottom of the container (Fig. 5.1). After you stop watering, drainage continues and the wet profile slowly moves downward while air moves inside the pores at the top. After drainage has stopped (Fig. 5.2-5.5), the lower part of the mix remains saturated with water. The depth (or height) of this saturation layer is not determined by the number and size of the drainage holes. Saturation is determined by the pore sizes of the mix, which is determined by particle size or texture. A pot filled with a coarse mix (with large pores) will have a smaller saturation zone (Fig. 5.5) than a pot filled with a fine (smaller pores) mix (Fig. 5.6). This is strictly a physical phenomenon called capillarity.

Fig 5 (Courtesy Ohio State University)

Figure 5. The watering of containers. 1) Saturation of the medium due to watering. 2-4) Drainage occurs; water moves out of the containers through the bottom holes allowing air to move in at the top. 5) After drainage has stopped, the lower portion of the mix is still saturated while in the upper portion of the mix the particles are surrounded by water but pores have air. 6) If the same pot is filled with a finer mix, the saturation layer would be larger (higher).

In a saturation zone, all pores are filled with water and no air, while the mix above has pores containing both air and water. Unless roots remove the water from the saturation zone, water remains at the bottom of the pot for a long time, because evaporation through the top of the mix is a very slow process.

Lack of oxygen puts root systems of some plants under severe stress, making them more susceptible to diseases. Furthermore, pathogens such as Pythium and Phytophtora require a water-saturated environment to infect roots. Minimizing the size and duration of the saturation layer should be a high priority.

The idea still persists that drainage from containers can be improved by adding a layer of coarse material, such as gravel, to the bottom of the container. In reality, this makes matters worse because the saturation layer is simply moved up, reducing the unsaturated portion of the container (Fig. 6).

Fig 6 (Courtesy Ohio State University)

The saturation zone
As seen in Figure 6, adding a course of larger particles to the bottom of the pot for improving drainage has the inverse effect of not improving drainage, but merely raising the saturation zone. Should you have roots extending to the bottom then, as can be seen, the lower portion of your root mass will be sitting in water. In WWI many veterans contracted and suffered from "trench foot" after prolonged exposure. What is happening to your roots, more important, where is the oxygen?

Bonsai Lindsay Farr's WorldOfBonsai episode 18 from Vimeo.

The video not only depicts the use of a "drainage layer", but the use of several courses of varying component size. What is the apprentice trying to achieve? At minute 1:48 in the video we see him adding a layer of large particles. At minute 2:00 he adds a medium size layer which he mounds nearly to the top of the pot. At minute 3:30 he adds medium size soil to the outside rim of the pot. He then works that in with a chopstick. At minute 4:00 he adds a fine layer almost to the rim of the pot, once again using a chopstick to fill all the root cavities, followed by a layer of mountain moss on the surface.

Based on what can be seen and has been written, why did he use multiple layers of varying sizes, when we know that the saturation zone will be in the fine layer of soil? Is that a good thing?

Bonsai Aesthetics
A lot of what we do in Bonsai circulates around the artistic side or aesthetics. Pots are chosen for their complementary aspect to the tree and when done right, they frame each other nicely in perfect harmony. Is this practice wrong? I would have to say no.

When choosing a pot, we are driven by certain fundamentals, artistically speaking. It is these fundamentals that determine the pot size in reference to the tree, nothing else (for the moment). It is further evident in the video that the pot complimented the tree and was of appropriate size to accommodate the tree. The depth as we know is determined by the trunk diameter at the base with no reference to the depth of the root pad, so to speak. We know in the beginning the root pad could be quite thick until we have pruned the renegade roots to suit bonsai culture. Once the unruly roots are tamed, the root pad thickness is quite shallow by comparison. In this particular case (video), the root mass is too shallow for such a deep pot. So it was indeed necessary to build the soil level in the pot so the height of the tree sitting in the pot was just right.

Most of the water in the pot is removed from the pot by absorption by the roots and not by evaporation. If you over-pot, it will take a long time for roots to colonize the bottom of the container and consequently it will take much longer for the saturated layer to become fully aerated ((the only factor at work is evaporation remember this)). Over-potting will generally lead to overly wet conditions and eventually root rot.

Water will drain from a pot until the lowest level of saturated soil (that can be supported) is reached. At this point drainage stops and this saturated layer remains saturated, no more water will drain out, ever. The height of this column of soil depends on the nature of the mix. A coarser soil will have a lower (shallower) column or layer of saturated soil than a finer mix. The total retained amount of water is less for a coarser soil.

Water can be removed from this saturated layer in two ways: evaporation (the water will be wicked upward as water evaporates from the surface), or by the absorption of water by the roots (powered by foliage transpiration). Of these two, removal by transpiration is by far the most effective. To prove this to yourself, just place two pots of identical soil next to each other, one with an established plant in it, the other with no plant. Water them thoroughly and then compare the weight of the pots over the period of one hot summer day.

If the plant is not root established, it cannot remove very much water by transpiration. This leaves too much water in the parts of soil without roots. In the short run, this is not much of a problem. In a proper environment, the plant will grow and will root establish quickly so that the saturated level is wicked dry in a day or two after a few weeks or months of growth.

However, ifthe pot is so large that the saturated level cannot be removed by normal root colonization, problems begin. This is not dependent on the soil type. With coarse soils a larger pot could be tolerated, but there are still limits to the space that can be quickly root colonized.

If you are using an organic amendment such as bark, you will experience accelerated soil composting. This means that you will lose youreffective soil particle size more quickly than if you used a smaller pot which is wicked dry daily. This is the most common effect. You use a pot that is too large and it stays too wet. The organic amendment quickly decays in this wet environment, particle size decreases, soil collapses, the saturated level increases, even more water is retained, roots eventually remain in standing water, root failure occurs with, or without, the presence of a pathogen. Using only stable inorganic amendments would avoid this scenario, but there are other problems.

Even if the above doesn't occur, what kind of root growth occurs in a volume that is not wicked dry daily? When you water properly, a new charge of air is pulled into the pot by the volume of water draining from the drain holes. Carbon dioxide and other gases are purged from the soil. The longer you leave these gases in the soil, and the longer you wait to introduce a fresh charge of oxygen, the poorer the roots will be. If you create a situation such as over-potting that doesn't require daily watering, then you don't obtain an optimal soil growing environment.
- Brent walston

Back to the video...
We previously discussed that for aesthetic reasons the pot was of the appropriate size for the tree. We have also seen/read the dangers of over-potting. Although, aesthetically, the pot was the proper size, the pot was/is not suitable for horticultural reasons. Why is that? The same reason the Japanese moved away from deep cascade pots. The soil would dry out too quickly. This can be compensated by reducing the size of the components but at the detriment of drainage and oxygen-holding capacity of the soil. So what is the solution? Move the saturation zone closer to where it is required, at the roots, and that is what the video depicts, not increasing drainage.

In the video, it can clearly be seen that the size of the mesh would not permit the use of the soil he used for the top layer as it would simply fall through the mesh. So, he used a large size component as his first layer. The advantage of such a layer was to occupy a large portion of the bottom of the pot and provide a barrier for subsequent layers. He continued to fill the pot with medium size soil which he mounded at the center. It should be noted here that the level of soil used is/was commensurate with the proper final tree height after wiring the tree to the container.

Now, he could have very well continued filling the soil with medium sized soil particles, but that would have moved the saturation zone to the bottom of the pot, away from the root pad as seen in the previous discussion. So, one might ask, why did he fill the outer circumference of the root pad with medium sized particles? Quite simply, once again, to contain the saturation zone where needed: at the roots. If we remember the video, the tree is sitting on medium size particles and the depth of the medium size particles he adds to the circumference of the pad is only as thick as the root pad itself. All these layers merely form a container within a container.

He finishes off with the last layer of finer particles which acts as a top dressing, more or less. This layer is not very thick--as a matter of fact if we think of it, very little soil is in the pot, figuratively speaking. We could stop here and all would be fine but, watering frequency would be excessive as a result of the evaporation rate of the soil, due in a large part to tree uptake and a significant, although lesser, surface evaporation. To resolve this, he added mountain moss to slow down surface evaporation and to keep the root pad relatively cool (think of refrigeration here). The massive root pad is more than capable of keeping its planting zone moist and not sodden until the tree is once again watered.

Back to our cascade pot...
I understand and support the artistic idea that a tall stand represents a cliff, it is logical and makes a lot of sense when compared to a tall pot. Based on the aforementioned discussion, is it still possible to use a deep cascade pot without the chance of the soil drying out too rapidly? The answer would be yes, by manipulating the saturation zone as was done with the Azalea.

Adding coarser material to the bottom of the pot, to a height just below the root mass, would ensure that the saturation zone would be at root level, would it not? If this latter statement seems doubtful, go back to the sponge.

OK! it could be argued then, why aren't the Japanese doing this? More than likely because, outside of artistic reasons, it just is not practical to do so. There are advantages to using a massive "drainage layer": added weight down low in the pot provides for greater pot stability on a bonsai bench; it counterbalances the weight of the cascading branch (some of these are quite large); it also allows for the saturation zone to be much higher in the pot (at root level); and reducing watering frequency (compared to a full depth cascade pot, which retains very little moisture because of the height of the water column.) The disadvantages to such a practice are: cost of potting material and weight. It would prove to be very impractical to carry out this method with “large bonsai”, but nonetheless quite suitable for much smaller bonsai. Also, not all trees are displayed and hence supported/elevated by stands, and not everyone has "monkey poles" as part of their landscape.

Soil composition
What is the perfect soil? The perfect soil is: one that drains readily; provides good oxygenation; has the ability to retain moisture (read moisture not water); and provide our trees with the required nutrients. I would like to touch on the last point for a second...

Based on the data evidenced herein, where does the excess fertilizer go in an improperly constructed substrate or with an over-potted bonsai incapable of ridding the pot of accumulated water in the saturation zone? It could be argued that, with subsequent watering, the unused portion of fertilizer would be diluted and eventually carried away. But is it? We haven't talked about capillary action of the soil or wicking.

In the ideal world (with of the right soil composition,) when fertilizing our trees, what was not used by the tree or retained by the soil would drain away, preventing the accumulation of unwanted salts in our pot.

What has been discussed in the past is that we underfeed our trees due to the use of modern substrate. We have come to find out that we need to feed our trees at a greater frequency, to compensate for the lack of the Cation Exchange Capacity (CEC) of modern substrate. In other words, because we have moved to almost inert soils, we need to feed our trees at lesser intervals than practiced in the past.
- Walter Pall

Because we feed our trees at a greater frequency when using modern substrate, the accumulation of unwanted salts is quite possible in an improper draining substrate or, with an inadequate pot carrying a saturation zone greater than what can be processed by the tree and evaporation. Because of this, not all fertilizer was washed away between feedings. In the latter instance, the danger does exist for over-fertilization.

I believe it is more important to find a balance in "soil construction" versus manipulating the saturation zone for proper moisture retention between watering. As discussed, this can be easily achieved by manipulating particle size: larger for shallow pots; and finer for deeper pots. We can also assist the water retentive capacity of any substrate by the addition of a greater portion of water retentive matter e.g. shredded Sphagnum Moss, Lava Rock etc... or by reducing the amount of inert particulate (grit) vice reducing the area occupied by the oxygen in favour of finer particles, which in turn increase the saturation zone.

In conclusion
The addition of a "drainage layer" does not improve drainage. This is a myth and it has been debunked for quite some time, although many still believe in the practice.

The size and height of the saturation zone is commensurate with the granular size of the medium used and the overall depth of the pot.

We can manipulate the height of the saturation zone by varying the size of our potting components to our benefit when usin deeper pots.

It is better to use a substrate with coarser components and a smaller saturation zone than it is to have a finer soil which in turns has a larger water carrying capacity with less oxygen.

It is better to add water retentive components to our substrate rather than decreasing its size in favour of moisture retention. The latter assist in moisture retention without an increase in the size of the saturation zone.

Although this article has made reference to the use of "drainage layers", it was written to debunk the "drainage myth" and to provide evidence that the addition of a drainage layer in the bottom of a container does not improve drainage, but merely increases the height of the saturation zone in any given container/bonsai pot and in turn, increases the water carrying capacity of the substrate rather than decreasing it. In other words, the same pot without a drainage layer will hold less water because of the height of the column of water prior to the impermeable layer (the bottom of the pot). In closing, I hope I was able to shed some light on what transpires below the surface in a bonsai pot.

The myth of drainage layers... - Dr. Linda Chalker-Scott
Physical Characteristics of growing mixes - Dr. Claudio C. Pasian Ohio State University
Physical properties of container media - Dr. James Altland Oregon State University
Factors influencing growth in containers - Donald T. Krizek and Stephen P. Dubik
Evergreen Gardenworks - Brent Walston

No comments: