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Potting Soil

Written by:

Ray Nosek


Words from Ray’s Woodshed… (JUL-DEC 2014 Newsletter)


Since I did not have any handouts from my lecture on bonsai soil and components, several members asked me to do a summation in the newsletter. There are four requirements of all good bonsai soils------nutrition, aeration, structure to hold the plant in position, and water or moisture. A fact that really blew my mind is that in a handful of typical garden soil you will find bacteria, fungus, nematodes, protozoa, mites, and micro-arthropods; the greatest majority consists of bacteria. These organisms break down plants and animal materials into simple compounds and thirteen essential minerals which are the building blocks for growing and maintaining healthy plants.


The majority of material that is broken down by the bacteria is consumed by the bacteria for their own processes. The small leftover production of the bacteria is uptake by the plants. So the top of the plant with its leaf system, with the CO2 taken in by the stoma or small pores on the underside of the leaf, along with sunshine acting on the chlorophyll in the leaf, acts in tandem with the water and products aforementioned from the root system. These are the majestic actions that produce life in our bonsai by nutrition.


Aeration—is dependent on uniform particle size to give spaces around and among the root system. Better than 90% of all bonsai soils can be made to consist of particle size 1/4” to 1/16” (screen door size). It should be noted that shohin (less than 8”), mame (less than 4” in height), or plants from nursery cans in very fine soils should be potted in 1/8” to 1/16” screened particle size soils. Smaller particle size just clogs the drain holes and prevents proper drainage and drowning of the roots. This particle size allows for rapid drainage of water but retention of moisture, and with this size particle, watering several times a day during the summer will allow for cooling of the pot without developing root rot.


All materials you will be using should be dried on a tarp for a day or two, then screened using a mask to prevent breathing in the dust particles. After screening, all materials should be washed and dried. The exception is not to wash and dry any organic materials. The physical structure of the particle size will hold the position of the plant so wind and other elements will not tumble the plant out of the soil. In bonsai situations, we will also wire the plant into the pot for support because over time a lot of the large, heavier roots will be removed from the plant to leave room for the smaller roots. These numerous hair projections will be responsible for the exchanges of products in a watery environment. The fourth element of good bonsai soil is the ability to absorb and drain water. So when the plant is watered, evaporation movement by gravity furthers it into the soils and the uptake by the plant forces a vacuum system that moves O2 from the atmosphere into the soil, providing the aeration to the root system.


Proper soil will allow the right drainage with proper watering techniques. Allowing retention of enough moisture prevents the dehydration of the roots, and also allows for better control of the temperature of the root ball. The proper drainage will allow the constant removal of waste products and prevention of accumulation of salts, that at high levels would be toxic to the plant environment.


One other important property of good bonsai soil is proper pH, or acid content.

7.0 is the number for neutral, as you may remember from your school days. Well over 90% of all soils fall into the 6.5 to 7.5 pH.


Soil—The top layer of the earth surface consisting of rock and mineral particles mixed with organic materials. The right recipe for bonsai soil is like the right recipe for spaghetti sauce. Everyone has a slightly different idea of what should go into it, but the basic ingredients generally remain the same. Keep in mind that putting a plant into a pot is a very unnatural foreign environment, so anything we can do to make the situation more tolerable is a definite advantage for our bonsai. The object here is not to describe an exact mixture for making bonsai soil, but rather to discuss the principles and elements necessary for an effective potting mix so that readers can construct a workable medium tailored to their own individual needs.


Soil has two basic components— organic and inorganic


Organic--dead plants, dead animals and manures. The big problem with organic materials is that as if decomposed over time, it has poor structural integrity.  Air spaces are lost, aeration is lost, the plant is drowning from water retention, and we lose structural integrity, so the plant is no longer supported and topples over.  Some very important components of organic material is to help retain moisture, help retain organisms to break down organic for plant food, help retain fertilizers and minerals, and maintain proper pH.


Inorganic-- broken down rock; sand, granite, lava rock, shale, clays, etc. Unlike organic materials, the particles do not break down and the mentioned problems with organic material can be controlled.


75-80% inorganic and 25-30% organic is the most common mixture recommended by bonsai experts. These TWO mixes are recommended because the materials are economical, easy to locate a source, and simple to combine.


(2) parts Black Gold Cactus Potting Mix

(1) part Dry Stall or pumice.


(1) part pumice or Dry Stall

(1) part lava rock (scoria)

(1) part organic, could be Kellogg Amend (rice hulls) or fir bark (orchid bark) or pine bark.


Black Gold Cactus Potting Soil can be purchased at MesquiteValley Growers, Harlow Gardens and ACE Hardware. Check your local nurseries or garden centers to see if they carry it. This should be sifted.


Dry Stall (uniform size pumice) can be purchased at most animal feed stores, the one on East Tanque Verde usually stocks it. Ted Matson prefers this product.


Unsifted pumice, to large sizes, can be bought from garden centers, Acme Sand & Gravel on 22nd Street or fertilizer yards.


Lava rock (scoria) Acme Sand and Gravel on E. 22nd Street, or possibly other sand and gravel sites. Black holds up better than red, and it should be thoroughly sifted before use.


Kellogg’s Amend at Home Depot contains rice hulls, and very little waste or dust.


Orchid bark (fir bark) Mesquite Valley Growers or check with other nurseries in your area. You should specify the finer size bark chips, not the regular size.


This is a review of the show and tell I gave at our last TBS general meeting. I have had a terrible problem with cutter bees where I live on the far east side of town. My bougainvillea’s leaves are ravished by this insect. They cut the leaves to create their nest so they do not ingest the leaf; an insecticide is of no value. The twenty bougainvilleas were kept in the greenhouse all year round to protect against the bees. The plants’ health was going downhill because they really needed to be out of the greenhouse during the summer.


One of the newer members, Lily Rose Krugly, gave me this formula to try:

One tablespoon of ammonia and one tablespoon of liquid flea and tick shampoo in a gallon of water. Spray every day and it will greatly reduce the attraction of the leaf cutters. They are repelled by the mixture. Good for pomegranates, orchid trees, and roses also. The only bougainvillea unaffected is the variegated variety because the leaf is thicker and the bee does not like to cut through the thicker leaf.  Try out the concoction and let me know if you are helped with this remedy.


My bougainvilleas are doing so much better in the extra sun out of the greenhouse, especially with our recent summer rains.


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