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Bonsai. Soil. Mix. When combined, three words can evoke impassioned, opinionated defense or a feeling of fearful frustration.
To create the best bonsai soil mix, ingredients and proportions depend upon the plant species to be grown, humidity, temperature, wind, light conditions, watering regimen, regional availability and cost, and the horticultural and aesthetic preferences of the bonsai artist.
Table of Contents
- Introduction to Bonsai Soil Mix Basics
- Factors That May Influence Bonsai Soil Mix Choices:
- The Essential Elements of a Good Bonsai Soil
- Step by Step Bonsai Mix’s Field Capacity and Saturation Porosity.
- The Value and Risk of Including Compost in Bonsai Mixes
- The Advantages of Compost in Bonsai Soil Mix
- Increased availability of nutrients to plants
- Limit leaching and compaction
- To facilitate optimized soil pH variability.
- Optimize nutrient cycling and micronutrient uptake
- Increases crop immunity
- Manage pest populations without pesticides
- Concentrate and retain growth forces essential to growth and health
- Increase soil gas and water holding capacity
- Contributes to Healthier Water Reserves
- Maintain optimal ratios of fungi, bacteria, and nematodes in soil
- The Risks of Compost in Bonsai Soil Mix
- Material Option in Bonsai Soil Mixes
- Creating a Bonsai Mix Tailored to Your Needs
- In Summary
Introduction to Bonsai Soil Mix Basics
The art of bonsai is millennia-old and ever-evolving. Therefore, bonsai success preceded the availability of akadama, red lava, perlite, or biochar. Of course, these ingredients make it easier, contributing to the six essentials of growing any plant, which are:
- Moisture management – a balance between drainage and moisture retention
- Air Management – avoidance of anaerobic conditions
- Nutrient management – an ability to store and release essential plant nutrients
- The most undervalued attribute – a hospitable environment for soil microorganisms
- Plant requirements for acidic or alkaline soil (pH requirements)
- Plant anchorage – ensuring the media isn’t so light that the plant cannot remain reasonably erect in winds
Factors That May Influence Bonsai Soil Mix Choices:
- The individual requirements of the plant species
- The depth and shape of the container
- The development stage and maturity of the bonsai
- Environmental factors, including exposure to wind and sun
- How often and when (time of day) can you water
- Temperature extremes in both summer and winter
- Fertilizer usage
- The pH requirements of the plant and your water’s pH
The Essential Elements of a Good Bonsai Soil
Below is an overview of the six key concepts of good soil, whether for bonsai or any other containerized plant. The interplay of these soil factors may vary according to the abovementioned environmental factors. Read on to find out how to ensure the right combinations for specific conditions.
- Soil Moisture Management
- Soil Air Management
- Nutrient Retention and Availability
- Cultivating a Soil Biome
- Plant pH Requirements
- Plant Anchorage
Balancing Moisture Retention and Aeration
A carefully graded soil allows the water to drain to the bottom of the pot, where it builds up a little before escaping from the drainage holes.
The difference (in weight) between added and drained water amounts to the water retained in the soil.
Soil Moisture Management Jargon Terms and Their Meaning
- Field Capacity is water content remaining after saturation and drainage
- Saturation Porosity is the air content remaining in the soil after saturation and drainage
Measuring your Bonsai Mix’s Moisture Management Capacity
Your bonsai wants water without being waterlogged, i.e., a balance between water and air availability.
To measure these interplaying factors, test the soil mix before planting, using a container of known volume and manageable draining holes.
We will use the metric system as it offers a direct relationship between volume and mass, i.e., 1 liter (1 l) of water = 1 kilogram (1 kg). Therefore, the fractional volume of water in a mix at different stages can be determined by weight.
Step by Step Bonsai Mix’s Field Capacity and Saturation Porosity.
- Start with an empty bonsai pot. Ideally, you want to use the pot where you’re going to plant your bonsai.
- Screen and seal the pot’s drainage holes to allow you to unseal them from the outside without affecting the screens. Using duct tape, place mesh over the holes inside to prevent soil loss and duct tape the holes outside to make the container watertight.
- Weighing the empty pot allows you to tare out its weight in later calculations. (M1)
- Fill the bonsai pot with water and weigh it. The difference between the weights of the full and empty container is the weight of the water and, thus the volume (if a metric system is used). Filling the pot to its maximum allows accuracy in later comparisons. (M2)
- Empty the water, and dry the container out. Now fill it to the brim with dry soil mix and weigh it. The mix must be dry. By subtracting the container’s weight from your last reading, you can determine the weight of the soil mix added. Alternatively, weigh a liter of dry soil mix to establish its density. (M3)
- With the dry soil mixed in the pot, add water to the brim, waiting for up to two minutes for the water level to stabilize and for bubbles no longer to be observed. Before weighing, be sure to bring the water level to the brim. (M4)
- Drain the container without loss of solids, waiting up to five minutes for drainage to slow to nothing, and weigh again.
Determining Density, Saturation, and Field Capacity
- The soil mix density or specific gravity (SG). Water is the standard with an SG of 1 (ice is 0.9, and therefore floats on water, less dense). Specific gravity is a factor of weight to volume. Therefore, the density of your soil mix is its dry weight divided by its volume.
- The Saturation Porosity (SP) is the difference between the fully saturated mix and the drained mix as a factor of its volume. Let’s assume the following:
- The volume of our container is 5 liters (5kg of water)
- The weight of the saturated mix is 5kg
- The weight of the drained mix is 4.5kg
- The SP (the fractional air content) is (5-4.5)/5 = 0.1 or 10%. Typical farming soil ranges between 2 to 7%. An SP below 6% indicates anaerobic conditions where roots cannot breathe.
- The Field Capacity (FC) is the difference between the drained wet weight and the dry weight of the mix divided by the volume. Let’s assume the following:
- The weight of the dry soil mix in the container was 1.5kg
- The weight of the drained mix is 4.5kg
- The FC (fractional water content) is (4.5-1.5)/5 = 0.6 or 60%. Typical farm soil is in the region of 50%.
To bring some practical perspective to the above calculations, akadama has an SP (air availability) of 31% and an FC (moisture retention) of 21%.
Inexpensive yet effective components with good drainage and water absorption include perlite, calcine clay, bark, pea gravel, and granite grit.
|Material||Saturated Porosity (SP) – Air||Field Capacity (FC) – Water|
|Calcine Clay (Turface or Haydite)||28%||40 – 60%|
The Value and Risk of Including Compost in Bonsai Mixes
Some bonsai growers believe that no organic components are required for a successful bonsai soil mix.
They say that drainage and aeration are the two critical components of a healthy bonsai mix and that it is your duty as the grower to provide your bonsai with whatever nutrients and moisture it may need.
This is unquestionably accurate, but you must admit it sounds a lot like hydroponics. I prefer adding compost at a minimum ratio of one part of cured compost to four parts of other ingredients. The advantages are well established:
The Advantages of Compost in Bonsai Soil Mix
The addition of compost to a potting soil introduces a soil food web to the root environment. There are hundreds of benefits to their inclusion, but here is a list of 10 of them:
Increased availability of nutrients to plants
The soil food web development promoted by composting provides a tiered hierarchy of consumers.
Nitrogen is absorbed and stored in lower-order organisms (bacteria & fungi) as they consume materials.
At the same time, carbon is consumed and burnt off, releasing CO2. The result is the sequential reduction of essential minerals to a form that plants can use.
Limit leaching and compaction
Studies have shown that building sites, notorious for compaction by mechanical action can fully recover by merely adding a top composting layer.
Compost aids in the formation of aggregates, which are clusters of soil particles that aid in the structure of the soil. Air, moisture, and nutrients are all held together in the soil’s many tiny air channels and pores.
To facilitate optimized soil pH variability.
Aerobic composting has a pH buffering function able to adapt soil acidity to environmental requirements. The microorganisms cause the pH to fluctuate between 5.5 and 7.2.
Optimize nutrient cycling and micronutrient uptake
During composting, organic materials do not merely undergo processes of degradation. Microorganisms reduce complex compounds into monomers (monosaccharides, amino acids, etc.), reconstituting these into bioavailable compounds. The activity also involves re-synthesis and polymerization reactions to produce a product that is intricately beneficial to soil health and plant life.
Increases crop immunity
Beneficial microbes in compost can suppress soil and foliar pathogens. Making compost tea and using this diluted form is a barrier to leaf infections.
Manage pest populations without pesticides
Compost reduces the need for chemical pesticides as the microorganisms protect plants from pests and diseases. The predator/prey sequence extends to the roots and the plant’s shoots.
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Concentrate and retain growth forces essential to growth and health
Soil health is critical to the preservation (and remediation) of our waterways. Increased water retention and reduced runoff are two benefits of composting. Effective microorganisms have demonstrable effectiveness at remediating polluted water sources.
Increase soil gas and water holding capacity
Microorganisms aid in forming aggregates (clusters of soil particles) that benefit soil structure and reduce erosion. Tiny channels and pores are formed, increasing the soil’s capacity to retain air, moisture, and nutrients.
Contributes to Healthier Water Reserves
Nitrogen and pesticide runoff from yards and farms cause untold damage to our water reserves. Compost enriches the soil and retains fertilizer more effectively, providing less fertilizer runoff. Compost also acts as a buffer, neutralizing alkaline and acidic soils and bringing pH levels to an optimized range for plant nutrient availability.
Maintain optimal ratios of fungi, bacteria, and nematodes in soil
Ecosystems can, if not interfered with, self-regulate toward stability. For instance, some fungi destroy pathogenic fungi; actinomycetes produce antibiotics that limit the development of bacteria; and nematodes can act as transporters of bacteria, ensuring their better distribution.
The Risks of Compost in Bonsai Soil Mix
It is essential to know the difference between fresh and cured compost. In the latter, the microorganisms have fully degraded the organic material to humus.
Fresh compost still has carbon that has not been processed, mainly because nitrogen is unavailable.
Adding fresh compost to the soil will reduce the nitrogen available to the plant as the organisms use it to degrade the carbon content. This will stabilize after about six weeks but can be avoided using cured compost.
You should also avoid using anything that might be too “hot” (too much nutrient) for tender young roots, such as cow manure or fresh compost.
Material Option in Bonsai Soil Mixes
|Akadama & Kanuma:||Akadama and Kanuma are particles of pelletized clay that the Japanese use extensively for potting their bonsai. It should be included but does not need to replace all ingredients. They have excellent water retention and drainage properties and are unique in that roots grow through the material, not just around them. They also supply important minerals (especially for azalea and wisteria)|
|Biochar:||It improves moisture retention while allowing good drainage. Watch for high CEC and pH increases. It also retains nutrients from fertilizers and promotes beneficial fungal and microbial activity, improving growth, health, and pest resistance. Lightweight, frost-proof, and resistant to decay or breaking down.|
|Calcine Clay (Turface or Haydite):||A mined clay that has been solidified and given a strong capacity to absorb water after being burned at a temperature just above its melting point. Commonly used on sports grounds and in container gardening.|
|Compost:||An effective way of introducing microorganisms but can degrade and cause compaction. Promotes beneficial fungal and microbial activity, improving growth, health, and pest resistance. See the topic above.|
|Composted hardwood:||A different type of organic additive to bark is denser, less absorbent, and slower to decompose. It is made from composted, shredded hardwood mulch.|
|Expanded Shale||Increases soil porosity while offering slow decay and pH stability. It is lightweight, economical, and readily available. See its saturated porosity and field capacity indicators above.|
|Granite-grit:||These are mall pieces of sharp-edged granite commonly used for poultry and wild bird gizzard health. Grains measure 1.5-4 mm in the “Grower” variety.|
|Lava:||Lava is a reddish or black, porous volcanic rock that occurs naturally. It provides some minerals and is taken out of mines.|
|Pea gravel:||Smooth gravel with rounded surfaces, deep gray, and 2-6 mm in size. Sold for construction purposes.|
|Perlite:||It floats on water, has a very low density, and is covered in numerous small bubbles, some of which can hold water. Due to its capacity to enhance drainage, it is widely used in container gardening.|
|Pine Bark:||Made from shredded bark but must be sifted to remove fines.|
|Pumice:||Naturally occurring porous volcanic rock. Provides some minerals.|
|Silica sand (coarse):||This sand has a uniform grain size of about 1 mm and doesn’t require screening.|
Creating a Bonsai Mix Tailored to Your Needs
A bonsai soil MUST have all of its components screened. You will want various screens with mesh sizes ranging from one-half inch to one-sixteenth.
The components mentioned above should be screened to produce particles around a quarter inch or less in diameter for a typical bonsai pot (about 11 inches wide and two deep).
Mixtures for Mame and Shohin class plants, which are smaller bonsai, should be screened to generate particles between one-eighth and one-sixteenth inch in size. Fine dust particles should be eliminated in both situations.
When selecting which soil mixture is ideal for you, using a little common sense is the best course of action.
All types of evergreens and the majority of deciduous bonsai can grow well in a mixture of three-quarters aggregate and one-quarter organic elements.
It’s a good idea to alter the components and ratios in a mix to accommodate specific plant species. More organic material, which holds more water, may be required if the plant (such as larches and bald cypress) prefers a damp soil mixture.
More aggregate might be advised if the tree (pine, ficus, or juniper) favors dry soil.
Examine the growing environment in your backyard.
If your location is shaded, you can constantly experience issues with pots being soggy. In this situation, adding extra aggregate to your soil mix can be good so that it dries out faster.
However, if your yard receives a lot of sunlight, you might want to add extra organic materials so that the soil can hold more moisture.
Bonsai Basic Fundamentals
Bonsai is an art; to become good at bonsai, you must master the fundamentals. These include the following but are not limited to. Click the links in the table to learn more about each subject.
|Placement||Pruning||Style / Form|
|Repotting||Deadwood||Growing from Cuttings|
|Soil||Surface Roots||Growing from Seed|
The art of bonsai is millennia old and ever-evolving. Generally, a composition of 75 percent inert aggregate and 25 percent organic material is a good starting point. Take the time to measure your mix’s field capacity (water retention capacity) and saturation porosity (air availability) using the guidelines provided.
Finally, consider the climate and growing conditions in your backyard and create a soil mixture that will fit you and your bonsai’s specific needs.
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