The Scoop on Soils, Part 1

By David DeGroot, as presented to the Bonsai Society of Portland November 1994
Summarized by Anne Spencer (A video is available in the club library)

The Soil
There are three things that good soil will do for a bonsai:

1) Plant support: Soil holds a plant up. Using a lightweight soil mix such as peat and perlite doesn't do the job well. Soil has to be heavy enough to sift down in among the roots. Use something with substance like sand or grit to add weight to your mix.

2) Fluid Availability: Soil makes air and water available to the plant.

3) Nutrient Availability: A good soil structure also makes nutrients available to the plant.

What's a good soil?
There are as many good soils as there are successful bonsai growers. Soil is just one small part of an equation that describes the entire environment in which the tree exists. Whether or not a given soil works will depend on many factors: Climate, environment, whether it is wet or dry, hot or cold, bright or shady, windy or sheltered. It depends on the fertilizing habits of the owner, watering needs of the plant, and what variety it is. All these aspects affect whether a soil will work well. There is no universally good soil. For instance, a California Juniper needs fast drainage while a bald cypress doesn't require any. So there is great variety in a plant's needs and the way in which it responds to soil.

Pore space in soil
Much research has been done on the proportions of solid matter and pore space in a good soil. Pore space (the air space between the soil particles) is required for any good mixture. About half should be solid mass and half pore space.  There are two different kinds of pore space. Total Pore Space (TPS) is all the space that exists between and around the soil particles. The most important type to us is Drainable Pore Space (DPS).

Dave demonstrated DPS by filling matching pots with various soils and saturating the pot with water so that all the air is driven out. He then let the water drain out (while keeping the pot carefully horizontal). The amount of air left in the soil is called DPS. DPS is important because all parts of a plant breathe, including the roots. If the roots are constantly surrounded by water, they can't release carbon dioxide or find oxygen. It's like having a plastic bag over your head. The tree suffocates without enough DPS.

Predicting DPS
How are we able to predict the amount of DPS in a soil mix? We can look at the shape of the available soil ingredients, especially if we have sifted them to take out the dust, and see how much pore space they have. The shape of the soil particles makes the difference. 

Clay particles are flat and plate-like and are so small that a microscope is required to actually see them. They stack very close together with almost no space between them. The result is that clay is very dense and has very little DPS.

Next are silts, which are very fine but larger than clay. They are flat, crystalline forms that pack together tightly like clays. Silts have little DPS.

Sands are hard minerals in more irregular shapes, not flat plates. When tumbled (river sand) they get smooth and roundish. When roundish shapes are put together, there is more poor space.

Crushed granite has flat sides which lets it pack tighter with less poor space than sand.

Lavas, fired clays, crushed clay, pumices, organic matter, chopped bark, etc., are irregular in shape and have a lot of DPS.

Soil particle size
The size of the particles in the soil mix has importance too. Small particles and large particles have about the same TPS. But the larger particles will have more DPS than the smaller particles. The reason: Water is attracted to soil particles. The attraction of the water to the soil particles is stronger than that of gravity. Water around a larger particle exists only as a thin film around the particle but no water is retained between the particles. Around the smaller particles the water stays suspended because of the smaller spaces between particles.

A characteristic known as perching where water stays suspended in soil rather than running through was demonstrated using a clear plastic pan full of sand and blue colored water. Truth: if you have a fine soil, moisture will tend to remain suspended in it....perched in it. Dave's demonstration also showed that there is considerable difference in the amount of water perching in fine, versus medium, versus large particle soil (More perches in fine than coarse)

Using different sized particles
Dave uses 1/16, 1/8, 1/4 inch screens. Whatever falls through the smallest screen or stays above the 1/4 in screen he throws out. The fine was what stayed between the 1/16 to 1/8 inch screens. Average mix was 1/16 to 1/4, and large was 1/8 to 1/4. (Reread that last part. Notice that the medium grade actually uses fine and coarse screenings) Use different particle sizes so you can fool around with proportions to mix soils that drain faster or slower according to your needs.

Pot depth
Particle size and shape are not the only factors which affect the drainability of soil. The pot depth also affects the amount of water remaining perched in the soil. The pot is selected not only for its aesthetic appeal, but also for its horticultural value. Another experiment demonstrated that deep pots drain better and more completely than shallow pots. Conclusions: Deep containers drain well because the column of water and the pore spaces are tall and heavy so gravity pulls the water out. Shallow containers do not drain very effectively because the column of water is short. The water's attraction for the particles is greater than that of gravity so the water stays suspended.

Important applications: In our cool cloudy climate a shallow container will tend to stay wetter than the same soil in a deep container. So if you use a shallow pot, use a coarser soil texture so it drains better, unless, of course, the type of plant needs extra water, in which case, use the finer soil mix. Get the idea? If you have a plant that needs a lot of water and needs full sun, and has a shallow pot, use a fine mix so there will be excess water for the plant to use.

Use moss or even top-dress the soil with a thin layer of finer soil from a 1/20 inch screen to help prevent evaporation from the soil surface. In rainy weather you should tip the pot so the excess water drains out better. With a deep pot you can use fine soil and have good drainage. A deep pot with large particles usually drains too well for most plants.

Dave's 'ULTRA SECRET" soil mix
Just kidding. Dave uses 40 % bark, 20% each of sand, lava, fired clay. Remember to sift the dust out of the sand, lava, etc.
(Editor's note:  This article was written before the use of Akadama became common.  Please see article on Boon Soil Basics and Recipe.)

Sand: The sand he uses is really 1/16 inch crushed rock from Manufacturer's Minerals in Renton, WA. We have it available in Portland at LaGrand Industrials Supplies, 503-244-5800, at 2620 SW First. (On Macadam about 2 blocks south of the Ross Island Bridge.) Called filter sand (bridge topping), it is made for filter systems and is similar to #2 poultry grit available at feed and seed stores here in Portland. (Later on in the evening he made a reference to #4 blasting sand from Manufacturer's Minerals.) Using fine sand doesn't work well for him.

Fired clays: The fired clay (expanded shale) he uses is from American Soil Products at 222 Third St. (at Bancroft) Berkeley, CA 94710. 415-540-8011, and also located in San Rafael, CA. A fired clay called Isolite is used by golf courses as an underlayment for their greens. It's too fine for bonsai soils. Turface (Profile) (called a soil conditioner) is a fired (baked) clay product now available in Portland at most garden stores. It holds more water than red lava. Fired clay has more interior pore space so it enhances water retention, which may or may not be an advantage.

Red Lava: Crushed red lava acts like fired clay, holds water, and drains well. It's available via Dave and Donna Burnett, Portland Bonsai Society members living in Battleground, WA 360-687-5641. Their lava comes washed and screened to a medium to large particle size. Call regarding delivery to meetings).

Unfired clays: Oil Dry is a dry absorbent clay found in building supply stores and auto supply stores, A kitty litter called Cat's Pride has no additives (verify this, please), is an unfired clay, and can be used in bonsai soils. It holds lots of moistureure.  Test unfired clays to learn if they melt or not. Test them by soaking them in water a while. Good bonsai particles should hold their shape.