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(click here to read part 2)

ADDITIVES

Additives, for the purpose of this article, refer to the materials incorporated in the growing medium at the mixing stage and before potting. Other additives which are added either in solution or dusted onto the surface after potting are not dealt with here.

In previous articles, we mentioned EC and pH and CEC with reference to characteristics of the growing medium. We are now going to explain what they mean and why they are so fundamental to the wellbeing of our bonsai trees.

EC- Electrical Conductivity

Electrical Conductivity is the measure of salt compounds inside the growing medium. The unit of measurement is µS/cm (or microSiemens per cm) previously measured in µho/cm (or micromhos per centimetre) which are exactly equal at 25ºC.

A growing medium with high EC value affects the ability of the plant to compete with ions in the soil solution for water (physiological drought). The higher the EC, the less water is available to the plant, even though the soil may appear wet. Because plants can only transpire "pure" water, usable plant water in the soil solution decreases dramatically as EC increases.

A fresh medium should ideally have an EC of 750 µS/cm but this will increase as fertilizer is applied. It is very difficult to control EC inside a medium and the general rule is to start off with a medium that has a low EC and to irrigate with water from a reliable non-saline source. That is also one of the reasons why ideally a plant should be re-potted yearly.

CEC - Cation Exchange Capacity

The CEC is the abbreviation for the cation exchange capacity of the substrate.

For those of us who are technically minded . Any element with a positive charge is called a cation and in this case, it refers to the basic cations, calcium (Ca+2), magnesium (Mg+2), potassium (K+1) and sodium (Na+1) and the acidic cations, hydrogen (H+1) and aluminum (Al+3). The amount of these positively charged cations a substrate can hold is described as the CEC and is expressed in milliequivalents per 100 grams (meq/100g) of substrate.

The larger this number, the more cations the soil can hold. The CEC gives an indication of the growing medium’s potential to hold plant nutrients. Increasing the organic matter or the clay content of any substrate will help to increase the CEC.

A substrate with low CEC will starve the plant of nutrients between fertilisation. A way to combat this deficiency is to fertigate, that is to fertilise with each irrigation either by applying a weak fertiliser solution with every watering or placing pallets of slow release fertiliser on the surface of the substrate. Another way is to ensure that the growing medium has a high CEC on re-potting.

The CEC value can vary from 1.5meq/100g for sand and perlite to 125meq/100g for peat and vermiculite.

pH

pH is a measure of the acidity or alkalinity of a substance. A pH less than 7 is considered as acidic, while a pH of more that 7 is considered as alkaline, that of 7 is neutral. Two terms are used to describe the pH of a substrate. Low lime substrates have a pH value below 5.0. These are termed as calcium deficient while high lime substrates are called calcium sufficient.

All plants have a specific pH range in which they thrive. In general a pH level of between 5.0 and 6.5 limits the ability of micronutrients to dissolve. When micronutrient products are applied to plants at the rates recommended by manufacturers, micronutrient levels in the medium are sufficient for healthy plant growth, but below levels needed to create toxicity problems. Low pH (below 5.0 in organic mixes) will lead to a high availability of Zn and Mg which in term results in foliar toxicities from these elements.

The pH scale is logarithmic and as a result, each whole pH value below 7 is ten times more acidic than the next higher value. For example, pH 4 is ten times more acidic than pH 5 and 100 times (10 times 10) more acidic than pH 6. The same holds true for pH values above 7, each of which is ten times more alkaline (another way to say basic) than the next lower whole value. For example, pH 10 is ten times more alkaline than pH 9 and 100 times (10 times 10) more alkaline than pH 8.

In Malta, due to the limestone composition of our ground, water nearly always has a high concentration of calcium dissolved in it. Using this water for irrigation will slowly increase the pH of your substrate due to accumulation. It is therefore very important to start off with a substrate at the lower end of the recommended pH for the specific plant. It is also very important to get into the habit of ensuring that at least 20% of the water drains out of the drainage holes after watering, as this will leach the medium of accumulated calcium and salt deposits.

General observations

In earlier articles, we have discussed the properties of each individual basic material of a potting mix. Now we can discuss how these materials can be mixed with additives to give us an optimum substrate for our plant. Let us first take a look at some of the requirements for growth.

Water, Drainage and Oxygen

A plant is 90% or more water but it also needs oxygen. Water and a large percentage of the oxygen required (perhaps 98%) by a plant comes through its roots. Since both water and air (oxygen) move in, through, and out of soil mixes, both water retention and drainage, and aeration are important considerations in any potting mix.

If the soil is poorly drained, or packed due to fine organic particles, oxygen is cut off from the plant. Also, whenever a plant is watered, it forces air out through the bottom of the pot or it bubbles up to the surface. As excess water drains, in a loose, porous soil, it creates a suction effect which draws in fresh air from the top into the air spaces in the soil.

However, if soil is too moist, or lacks proper drainage, oxygen is excluded and roots gasp for air. Also, a lack of oxygen favours high concentrations of carbon dioxide (a waste product), cutting down on root growth, and certain soil organisms, which like high CO2 concentrations, grow. In the stagnant soil these organisms produce toxic chemicals, which have a disastrous effect on the health of your plant.

AMENDMENTS

Sharp Sand

Sand or River Sand, but definitely not sea sand which is very saline, increases porosity and opens the structure of the growing medium preventing clogging.

The main properties are:
• Increases porosity
• Low CEC
• Very heavy
• Very durable

Gravel

Similar to sand but has a larger grit.

Composted Bark

This is primarily a bi-product of the pulp, paper and plywood industries. Suitable particle size is obtained by hammer milling, composting and screening. This produces a material which is suitable for use in container media. Physical properties obtained from tree barks are similar to those of Sphagnum moss.

Perlite

Perlite is not a trade name but a generic term for naturally occurring siliceous rock. The distinguishing feature which sets perlite apart from other volcanic glasses is that when heated to a suitable point in its softening range, it expands from four to twenty times its original volume.
This expansion is due to the presence of two to six percent combined water in the crude perlite rock. When quickly heated to above 871°C, the crude rock pops in a manner similar to popcorn as the combined water vaporizes and creates countless tiny bubbles which account for the amazing light weight and other exceptional physical properties of expanded perlite. Since perlite is a form of natural glass, it is classified as chemically inert and has a pH of approximately 7.

There are three grades of perlite, fine – for seeds and seedlings, medium – for growing media, and coarse – only used in construction as an insulator.

The main properties are:
• Increases porosity while providing moisture.
• Neutral pH
• Low CEC
• Lightweight
• Inert
• Insulates the medium against temperature fluctuations.

Vermiculite

Vermiculite is a micacious mineral produced by heating to approximately 745oC. The expanded, plate-like particles which are formed have a very high water holding capacity and aid in aeration and drainage. Vermiculite has excellent CEC capacities as well as the ability to supply potassium and magnesium. Although vermiculite is less durable than sand and perlite, its chemical and physical properties are very desirable for container media.

Vermiculite comes in four grades, only grade 2 for seedlings and grade 3 for growing media are used.

The main properties are:
• Increases porosity while providing moisture.
• Neutral pH
• High CEC
• Lightweight
• Inert
• Insulates the medium against temperature fluctuations.
• Not as durable as perlite.

Clay

Clay Slopes at Gnejna Bay

Clay is found naturally in various parts of the Maltese islands. It has a very high CEC and high water retention but unfortunately this water is not easily made available to the plant, thus deceiving you into thinking that your substrate is adequately watered. If used in high proportions in your media, it is likely to starve the roots of much needed oxygen due to its tendency to compact.

While it is fine to add in small amounts, I would rather recommend the use of vermiculite as a nutrient buffer.

Pumice

Pumice is a very light coloured, frothy volcanic rock. Pumice is formed from lava that is full of gas. The lava is ejected and shot through the air during an eruption. As the lava hurtles through the air it cools and the gases escape leaving the rock full of holes. Pumice is so light that it actually floats on water.

Pumice has been used to a large extent as a plant growing medium because it lightens the mix, improves aeration and holds water. Pumice has very stable physical and chemical properties.

• Excellent aeration
• Good drainage
• Neutral PH
• Does not break down, (can be used over and over again).
• Does not rot
• No shrinking or compacting in aging
• Low CEC

Correcting Growing Media

The last things I want to mention are not really additives, but contribute to making a growing medium acceptable for plant growth.
Lime

As we have discussed earlier, plants (generally) need a pH of between 5.0 and 6.5. Since peat and other basics have a lower pH than this, the pH is corrected by adding dolomitic lime. Due to the logarithmic nature of the pH scale, one has to add higher amounts of lime (10 times as much) to rise the pH level between one degree and another.

Wetting Agent

Wetting agents (surfactants) are used so that media will wet out (hydrate) uniformly when watered. This is very important when the mix is relatively dry, as it often is when brand new, or when it fails to get watered on a regular schedule. Many times when a potted plant is watered, the water will form a "pond" on top, or channel down the insides of a container without readily infiltrating the medium. This problem is eliminated by the incorporation of a Wetting agent.

Conclusion

As you may have noticed, I have never suggested any formula for a growing medium. The Bonsai enthusiast, armed with this knowledge is now able to mix his own according to the requirement of his trees.
If anyone requires any clarification on the topics discussed in these articles, please feel free to contact me on joe@greenviewtrading.com.

Happy mixing!