Article 28 – ‘The pH Factor (Part II)’

Types of soil and their pH content

In the article ‘Bonsai soils’ there is a more descriptive analysis on the different types, but in short there are 6 basic soils:

Peat soils – are comprised of waterlogged partially-decomposed plant material including sphagnum moss, which is very high in organic matter and moisture with a pH starting approximately at 3.0

Sandy soils – contain the largest particles among the different soil types, that is dry, gritty and because there large are particles creating voids, water retention is not possible. However, sandy soil warms more quickly in the spring. Sandy soils have a pH starting at approximately 4.7

Clay soils – are the remnants of certain types of rocks eroded and weathered to form fine particles, that are high in nutrients, but they can be heavy, cold and wet in winter and very dry in summer. Clay soils have pH from 5.5 to 7.1

Chalky soils – are alkaline or lime-rich predominantly made up of calcium carbonate, they are indicative to particular landscapes including. The UK’s south downs and Wiltshire, Portugal, southern France and Spain. Chalk based soils can be either light or heavy and have a pH value from 7.1 to 8.0

Silt soils – are fertile, light and contain larger particles and are more moisture-retentive than sandy soils and drain more effectively than clay. But have a tendency to crack and crumble losing shape and structure; their pH range is approximately 6.5 to 7.

Loams – are mixtures that may include any of the above soil types depending on the needs of a particular species, albeit acidic, neutral or alkaline. For example, ericaceous plants including Azaleas genus Rhododendron family Ericaceae have a preference for higher acidity. Alternatively, Maples – genus Acer prefer a neutral soil composition, whereas the Burning Bush Euonymus alatus family Celastraceae require soil high in alkaline.

But, the acidic, neutral or alkaline content has great bearing on micro organism survival for example. Those found in acid and alkaline soils Acidophiles and Alkaliphiles respectively, are not as varied as the Neutrophiles found in neutral soil compositions. The pH range of 6.6 to 7.5 is more amenable to their needs in order to function. Generally speaking, there are four types of soil compositions including an all-purpose mixture for seeds, acidic, neutral and alkaline. Although they may vary according to the specific pH needs of a particular plant species.

Seed all-purpose mix – with this soil composition there are a lot of discussions as to which type is correct. Some will argue that the following mixture produces the best results:

3 parts organic matter, such as peat, humus or sawdust, 1 part sand, perlite, vermiculite or a combination of all three and 1 part sphagnum moss.
Others contend that seed soil mediums such as John Innes No1., Miracle-Gro, Levington or Thompson and Morgan, available ready mixed are perfectly capable of doing the job. But regardless of which type is preferred, all seeds require a form of stratification. (Described in the article The Stratification of seeds) Because this is the key to induce the embryo to germinate regardless of which soil composition is used albeit acid, neutral or alkaline. Moreover, it can be argued that seeds once stratified can germinate on a piece of damp kitchen paper, in a saucer of water or simply where they fall as shown below.

Scots Pine seedlings Pinus sylvestris

A good seed soil normally consists of heat-treated loam, sphagnum moss, peat, horticultural sand and fertilizers, which will last for approximately 30 days. After which the seedlings will need to be pricked out and transferred to a low nutrient soil mixture and left to develop. This soil composition slows down the growth rate allowing the plant to ‘put-on-weight’ meaning trunk and foliage development. It is also suitable for cuttings, which have taken root.

There are many types of seed soils available world-wide, in Finland a common mixture often used is ‘Musta Multa’, which roughly translated is ‘Black Mould’ a rich composition for most plant types. This product can be likened to the old but familiar ‘grow-bags’ where people who had no gardens or lived in apartments could grow vegetables in confined spaces. On its own this soil compost can be rather soggy if too much water is used, dissipating nutrients via water run off. To prevent this, a little sand, vermiculite, crushed clay or granite chips in the mix, which contain additional micro organisms usually solves this problem. It also allows air to flow through the medium permitting bacteria to function and although retains moisture does not become compact or waterlogged.

Acidic soil – The basic elements which cause acidity in soil are, the organic matter itself and the acidic mineral content within, which break down the matter through time making the soil acidic. Soil can also become acidic due to the loss of key minerals, potassium, magnesium, and calcium that have been leached out of the soil by excessive contact with water, thus the soil becomes acidic. This is a common phenomenon in peat bogs and pine forests and such soil is probably the best to use especially for conifers.

But before venturing out to obtain a sack full, which is going to upset the environmentalists and is also possibly illegal, there is a an easier method. Acid soil or Ericaceous soil to give the correct terminology is an entirely organic and peat-reduced compost, available at most horticultural establishments world-wide. Brands include, Thompson and Morgan, John Innes No 2., Bowers, Growise and Verve. Nonetheless, an ericaceous compost mix can lose its acidity over a period of time, depending on the amount used. For example, a large bonsai in a shallow pot will deplete the minerals more quickly than one in a deep pot.

Many horticulturists resort to using acidifying materials to counter this. For example such materials may include, pine needles or sphagnum moss placed on top of the soil allowing their acidic properties to penetrate to the roots. The use of Sulphur, Aluminium sulphate and Ferrous sulphate (sulphate of iron) are also used, but these may take some time to become effective, as they need to be broken down by the bacteria. Other methods are to use only rain water with white vinegar added (1 to 2 tablespoons in 4.5 litres or one gallon of water) as opposed to household tap water that has been treated with alkaline agents.


To make an ericaceous medium the following ingredients can be used;
Organic compost, sand, crushed clay or granite chips. The amount each of these components used depends on the acidic pH value one wants to achieve. Alternatively one can resort to using just Akadama, which many bonsai enthusiasts use for coniferous varieties, for example the Mugo pine Pinus mugo.


Neutral soil – is ready available from garden centres, department and hardware stores and can even be taken from your own garden. Of course the latter will need to be tested to ascertain the pH level, which may be in the region of 6.7 to 7.3 – slightly acid to slightly alkaline – a range that many species of plants can tolerate. But much depends on your particular region and temperate zone.


Arguably, there is no specific chart that states what type of ingredients should be used for this soil composition, as horticulturists world-wide have their own preference; what works for them. However, any neutral soil composition needs to be prepared to facilitate moisture retention, aeration and to allow the micro organisms to function, hence the use of grit, crushed baked clay pellets, sand, perlite or vermiculite and even bark chips can be used.

Alkaline soil – has a high saturation of soluble salts that are known as either sodic or saline with the former being dominated by sodium and the latter dominated by calcium and magnesium. Alkaline predominantly limestone is mixed with topsoil to produce a medium specifically for plants requiring a high pH level that can vary according to one’s particular needs. Manufacturers whom produce this product are for example, John Innes No3., Thompson and Morgan, Bowers and Rolawn. To make an alkaline soil composition, one can use the following ingredients.
Neutral top soil or prepared loam with a pH at approximately 7.0, clay or silt, washed grit and a lime additive. The amount of lime used will depend on the pH level one wishes to achieve. Tap water should be used as it is normally treated with alkaline agents.

The manufacturers instructions are wrong!“We did our research and followed the guidelines, all was fine in the beginning, but now the plant’s leaves are turning yellow and dropping.” Whether you purchased a ready-made soil composition or have mixed your own, it is doubtful that this is the cause for an unhealthy plant. Because reputable manufacturers even those mentioned above have been producing various soil mediums for generations that are tried and scientifically tested. Alternatively, if you have mixed your own soil composition, then you will probably know what ingredients to use and the proportional amount.

The possible causes for this down turn in the tree’s health is stress, which can be attributed to various factors. For example, inadequate drainage, no air circulation in the soil, damaged or compacted roots the latter known as ‘pot-bound’, the lack of nutrients, low light and an incorrect pH. These are the common causes why leaf tissue turns yellow referred to as chlorosis because, the essential green pigment chlorophyll is absent. Arguably the first course of action is to check the soil composition’s pH level, that can be found in the chart in part I of this discussion, which brings us on to the subject of soil pH testing methods.

Soil testing applications – for a true and accurate pH test of your particular soil medium one should opt for a laboratory test, but this can be expensive. Therefore, one resorts to purchasing various readily available test kits from the basic ‘2-test’ test kit (approximately 8€ to 13€ plus postage and packing) to the more expensive multi-test kits. (95€ to 350€+) Other devices for testing soil pH are in the form of meters both analogue and digital, some are purely for pH testing whilst other are multi-functional as shown below.

Soil pH test kits 5

But before venturing out to purchase a pH test kit, the question is – what type would be suitable and the factors to assist in this decision can be based on two simple questions:

1. The pH range for a particular plant species
2. Was the soil purchased from a horticulture outlet or a self-made composition

1. Most bonsai with the exception of some coniferous and deciduous species are able to tolerate a pH range from 6.7 to 7.3 and have the ability to prosper. For a more descriptive pH requirement for a particular species, refer to the chart in part I of this discussion.
But, any soil composition used in bonsai is an important concern, because this is where the plant receives the nutrients and moisture it needs to thrive.
2. If the soil used was purchased from a store or horticulture outlet, then information as to its composition and pH range will be written on the bag. Alternatively, if you mix your own soil composition/s using ingredients from various sources, then you will ultimately be changing the pH level, which needs to be checked. Therefore, the opening statement of ‘what soil pH test kit would be suitable’, boils down to two factors.

(a) Soil purchased from a reputable outlet will normally state its composition and pH level and even if we add grit, vermiculite, perlite, crushed baked clay etc, the pH level will not change that much. Therefore, investing in pH test kit is not really necessary nevertheless, it is an advantage to have a small one simply for peace of mind and to know what the pH level of a particular soil medium is.
(b) If mixing a particular soil composition using ingredients from various sources, then it is important to know the pH content of each component to ensure that the final medium is acceptable for the species; albeit acidic, neutral or alkaline. Therefore, investment in a testing kit is an advantage.

Soil test kits – Finding a suitable soil test kit does need some research as there are literally countless manufacturers all claiming their particular product/s produce/s the best results. They range from expensive laboratory equipment to analogue and digital probe meters, to relatively small cheap kits you can put in your pocket. The majority of these have been tried and tested with positive and negative results, that can be viewed via the world-wide-web. To give an indication as to the reviews, they have been condensed and listed as follows.

Litmus test – requires a small sample of soil mixed with distilled water in which, the paper is inserted for a few seconds. The paper’s colour change is then compared to those of a colour chart and an estimation of the soil’s pH is then determined. However, this method of testing is not very accurate, because it can only read a pH level in whole numbers for example 5.0, 6.0 and 7.0, which is an approximation. Therefore, as soil pH levels are measured in increments of both whole and half numbers for example 6.0, 6.5, 7.0 and 7.5 this litmus test approach is one to be avoided.


Chemical test – soil tests conducted by commercial laboratories are accurate in that they are able to determine mineral content and compound structures within a soil sample. For example, major nutrients nitrogen, phosphorus and potassium. Secondary nutrients sulphur, calcium, magnesium and minor nutrients; iron, manganese, copper, zinc, boron, molybdenum and chlorine. Such tests can be expensive, but for horticulturists wishing to use this method of testing there are products available. However, low cost ‘2 test’ test kits (top left image shown above) are only able to give an approximation of the pH level and that is the limit of their capability. Moving up the scale (top middle image shown above) are test kits able to determine the presence of major nutrients and are more accurate in reading the pH level. Alternatively, the test kit (top right image shown above) is able to search for both major and secondary nutrients, but has difficulty in ascertaining minor nutrients.


Analogue and digital meters – (shown in the above image) come in various forms from simple pH meters to multi functional probes able to test for moisture content, temperature, light quality and pH. Some manufacturers claim their meters are able to determine the presence of all nutrient categories, major, secondary and minor although this is debatable. Another argument concerning these multi functional meters is why test for moisture, humidity and light. Surely the horticulturist knows whether his or her bonsai has the right water content, knows the temperature/humidity at any given time and is aware of the plants position in relation to the light source.


For bonsai enthusiasts who use readily available soil mediums purchased from reputable outlets, a pH testing kit is not that essential. But for horticulturists making their own soil compositions, investing in a testing kit is advantageous as opposed to not. Because it allows for an accurate pH level to be determined correct for a particular species. If a pH testing kit is desirable then arguably preference should be given to those only dealing with soil pH levels as this is our primary concern. Not multi functional meters with reference to moisture, humidity and light content as these are unnecessary. But whatever test device you opt for, make sure it is suitable to your needs.

And finally, a message to O.T. known as ‘Tiger’ – I have some Maple (Acer) seedlings to give away if you are interested. Until next time BW, N.

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