Gardening

Find Out Your Soil Type: Understanding & Testing Soil pH

electronic soil tester in the ground

For the gardener, winning the lottery is not as big a deal as having good soil favourable to growing healthy and beautiful plants.

Indeed, to the gardener, having good soil is winning the lottery! Beyond just good soil is ‘Black Gold,’ that smooth, rich, warm, friable, dark soil that looks, smells, and feels like… the jackpot to the ardent gardener.

Is Soil Just Soil?

“Soil is soil!” This is what the casual gardener might very well say. But the dedicated gardener would reply, “There is soil . . . and then there is soil.” True. There are many different types of soil. In general, some soils are beneficial to plant health and growth and others are not. Yet it is also true that one or another specific type of soil may be poor for the large majority of plants yet ideal for a small set of species.

Soil, as such, is composed of varied types of constituent particulate matter in varying proportions. The correct balance between these various constituent materials results in the best soil. Yet that is far from all. Regardless of the kind or type of soil you have, an all-important variable makes the world of a difference in determining whether or not the soil will or will not be beneficial to your plants, and if it will be beneficial, then how much. This crucial variable is pH.

The Building Blocks of Soil

Sand, silt, and clay are the building-block components of soil. These differ primarily in the size of their respective particles, with sand having the largest particles and clay the smallest.

Because of the size of its particles sand cannot retain water or even moisture. On the other hand, clay retains the most water and moisture but if clay particles become tightly compacted, clay may become impenetrable to water. Silt, a sediment, falls in between sand and clay in its size and moisture retention properties.

Often overlooked as a component of soil, gravel has particularly large particles ranging in size between those of sand and pebbles. Indeed, gravel is sometimes modified as ‘sandy gravel’ or ‘pebbly gravel.’

Silt and gravel are both found in river channels, riverbeds, floodplains, and harbours.

An important property separates clay from the other types of particulate matter. This property is that almost all clays have (negative) electrical charge whereas the other particulate matters carry no charge. In turn, this means that clay and soils with clay attract and ‘hold on’ to positively-charged cations such as magnesium, potassium, and calcium, whereas sand and silt cannot do so. This property of clay and clay soils is called ‘Cation Exchange Capacity’ (C.E.C.) and it is crucial for plant growth and health.

The Different Components of Soil

To the above building blocks of soil, we should add chalk. This soft, fine, white material is also of sedimentary origin and it is very common in Southern England’s downs country.

Another component of soil is manure which is essentially decomposed faecal matter. In fact, manure is a saleable by-product of animal farming.

Different types of soil
The make-up of different types of soil

Humus and peat are both naturally-occurring organic materials. Humus is found in wooded and forested areas and it is formed primarily by the organic decomposition of leaves and other plant materials, but also from animal remains. Peat, less crumbly and thicker than humus, is also formed primarily by the organic decomposition of leaves and other plant materials, with some decomposed animal remains, except that it is formed and found in bogs, fens, and swamps.

Because manure, humus, and peat are derived from organic materials, they too carry some or another negative electrical charge and, therefore, have a C.E.C. because of which these soils attract and retain positively-charged cations.

Loam is the ‘ideal’ composite soil. It comprises of about equal parts sand, silt, and clay, with some humus. Depending on its balance, loam is sometimes predicated with qualifiers, e.g. ‘sandy loam’ or ‘clay loam.’ Loam is the most fertile kind of soil.

The Crucial Variable: pH

No matter what kind or type your soil is, it has some or another pH value and also a C.E.C. The C.E.C. depends on both the composition of the soil and the pH of the soil. The pH value is a measure of the acidity-alkalinity of any non-inert chemical substance, including soil. The pH scale runs from 0 to 14. A pH of 7 indicates neutral soil, below 7 is acidic, and above 7 is alkaline. Most plants thrive in soil ranging from slightly acidic to neutral. Just as among soil types, as a general rule loam is the best soil; on the pH scale, as a general rule soil with a pH of 6.5 is the ‘best’ soil.

Acidity-alkalinity levels are a direct result of the concentration and activity of Hydrogen ions. It is measured as the negative of the base 10 logarithm of moles per litre of hydrogen ions. Thus, if the concentration of hydrogen ions in some substance is 10-6, then the pH is 6. The concentration and activity of these Hydrogen ions, defined as pH, play a key role in various properties of the soil as they affect – beneficially and deleteriously – plant health and growth.

A Visual Representation of pH Values of Various Substances
A Visual Representation of pH Values of Various Substances

Why Is Soil pH Important?

Soil pH is extremely important for the health and well-being of your plants because, simply and briefly, pH controls the availability of soil nutrients to the plant. There is a fine balance to be considered here.

Strongly acidic (pH 5.1 to 5.5) soils dissolve Aluminium and make it available to plants through their roots, and Aluminium is wholly toxic to plants. On the other hand, such soils are not conducive to the uptake of magnesium, a plant nutrient. At the same time, although manganese is a plant nutrient, in manganese-rich soils that are strongly acidic, manganese turns into a toxin due to an ‘overdose’ being transported into plants’ vascular systems.

Moderately alkaline (pH 7.9 to 8.4) soils put a crimp on the availability of key nutrients like phosphourous and potassium to plants, leading to mineral deficiencies. Strongly alkaline (pH 8.5 to 9.0) soils interfere with water retention and dispersion through the soil, the capacity of the soil to hold water, and also degrade soil aeration.

The best compromise in terms of soil pH is that happy zone that ranges between Slightly Acidic through Neutral, as defined below. In this range, where the vast majority of plants are concerned none of the harmful effects of soil pH are active, and all of the benefits of soil pH are brought into play. In this pH range, necessary nutrients and Nitrogen are rendered chemically active in the soil and thereby made available to plants and absorbed by the roots, plant toxins are held in check, water is dispersed through the soil, moisture is retained, and aeration is improved. For these reasons it is important to keep soil pH in this ‘happy zone’.

Which pH Do Most Plants Grow Best At?

Most plants – indeed the vast majority of plants – grow best in ‘Slightly Acidic’ through ’Neutral’ soil. These descriptive terms are, in fact, labels codified by the U.S.D.A.’s Natural Resources Conservation Service to denote narrow ranges of pH values, respectively 6.1 to 6.5 and 6.6 to 7.3. To answer our question, if you have to pick a single, precise, pH value as being the ‘best’ for most plants, it would be 6.5.

However, a few plants prefer acidic soil and a few prefer alkaline soil. Gardeners and agriculturalists have called acidic soil ‘sour soil’ and alkaline soil ‘sweet soil’ since times unknown.

Two popular flowering plants, Rhododendrons and Azaleas, are well-known to prefer acidic soil. Magnolia is a flowering tree that thrives in acidic soil. Holly plants and blueberry bushes are further examples of ‘acido-philic’ plants. Among trees, birches and Scots Pine are acid-soil-loving trees.

Many Columbines (Aquilegias), Delphiniums, Day-Lilies, and Foxgloves thrive in alkaline soil. Boston Ivy and Virginia Creeper are among the vines which like basic soils. Among trees, European Beech and European Ash show a preference for alkalinity.

One should bear in mind that the above-named plants and some more are in truth the exceptions that prove the rule as to optimal soil pH.

All this explained, it should be noted that just as the best soil for plants is Slightly Acidic to Neutral, the best soil for agricultural crops, including most vegetables, is Slightly Alkaline to Neutral. And while Moderately Acidic soil is just fine for many plants, it is anything but for agricultural crops, which are better off in Moderately Alkaline soil.

‘Royal Azalea’ Rhododendrons in Acidic Soil
‘Royal Azalea’ Rhododendrons in Acidic Soil

How To Test Your Soil pH

Soil can be tested by using affordable and easy-to-use DIY equipment or by engaging the services of professional labs. The DIY equipment includes outdated approximations, chemical coloured dyes, pH strips, analogue meters, and digital meters.

You can collect a few soil samples from different spots in your garden and from different depths, put them in sterilised bags, mark or label them, and send them off to a professional soil testing lab. The marking and labelling is necessary so that when the lab results are in, you will be able to tell which result corresponds to which soil sample.

In the coloured dye method, soil is mixed with water and then chemicals are added to it. The chemicals dye the soil. The resultant colour is then matched to a colour shade on a chart, and that shade’s pH value is looked up.

pH strips indicate pH level by way of colour shades. These may be thought of as an advanced and specific type of litmus paper that most of us are familiar with from our schooldays. pH strips are of thicker card material than litmus paper and when they react with soil or any other chemically active material, they change colour into one of 15 different shades. They can be inserted directly into soil or into a mixture of soil and water. The shade the strip turns into is matched to the same shade on a chart, and that shade’s accompanying pH is read off the chart.

Amateur-use soil-testing meters, be they analogue or digital, are becoming increasingly popular, mostly because of their convenience and affordability. They are far less expensive than professional equipment but they are also not as reliable at taking precise measurements and giving exact readings. These simple-to-use instruments include a handheld display and a probe that is to be inserted directly into the soil. The pH is shown on the display, and – Voila!

The pH value indicated by a DIY soil-testing meter should be treated as a ballpark number with a plus/minus factor of 1 (which, as pH is measured and represented on the logarithmic scale, actually represents a multiple of ten; for example, a pH of 9 is ten times more alkaline than a pH of 8).

A Close-Up of a Soil pH Testing Meter
A Close-Up of a Soil pH Testing Meter

How To Neutralise Acidic Soil

First, if your soil is Slightly Acidic or even Moderately Acidic be happy – because most plants will be happy in such soil. It is only if your soil is Strongly Acidic (pH 5.1 to 5.5) or lower that you should be concerned and think about neutralising your soil’s acidity.

In gardener’s lingo, reducing soil acidity is called ‘Liming.’ In technical terminology, changing the soil’s pH (or composition or C.E.C.) is known as ‘Amendment’. 

Basically, (pulverised) agricultural lime or (pulverised) dolomite lime is added to the soil. This is a somewhat technical task that an experienced gardener can do by estimation and which a professional service does using measurements and calculations. If your local gardening centre is provided with a few necessary particulars, as outlined below, they can provide pulverised lime and instruct you on how to amend your soil.

Calcium Oxide, Calcium Hydroxide, and Calcium Carbonate are all types of Lime.

First, as the starting point, the soil’s exact pH needs to be known. Although it may seem useful to know the composition of the soil, it is, in fact, the C.E.C. of the soil that should be measured because the amount of lime that will be necessary depends, inter alia, on soil C.E.C. The amount of lime that will need to be added also depends on the lime itself, i.e. on the fineness or size of its particles. The finer and smaller the particles of the lime, the quicker and more efficiently it will reduce soil acidity and, therefore, a smaller quantity of a finely-ground lime will be needed than of a coarsely-ground lime. It is best to do liming (or any soil amendments) in late autumn.

Although lime in its various forms is the most commonly used amendment for acidic soil, wood ash, calcium silicate, and some other substances can also be used to neutralise acidic soil.

Finally, reducing or neutralising soil acidity is usually not a one-shot task; it is an ongoing process as soil tends to revert to its natural pH. Therefore, after amending your soil, test it every two years to see whether your neutralising or alkalinising amendment has ‘held’ or whether your soil needs a ‘booster shot.’

Do Any Other Factors Play a Part?

Although other factors, such as sun, climate, rainfall, and moisture play a very big part in the health and growth of plants, this article pertains only to soil pH. And there is one other soil-related factor that does play a part: earthworms! Earthworms have several beneficial effects on soil, ranging from the nutrient-rich contents of their castings to improving soil porosity and enhancing soil aeration.

While earthworms are not attracted to strongly acidic or strongly alkaline soils, preferring soils that are slightly acidic through neutral through slightly alkaline, whichever soils they inhabit they also play a role in also influencing the pH of that soil. Earthworm castings, which are rich in soil and plant nutrients, have a neutral pH. Thus, the presence of earthworms tends to neutralise soil, be it on the acidic side or the alkaline side, nudging it closer to pH 7.0.

Therefore, if a gardener wants to control moderately acidic or moderately alkaline soils, he/she would do well to check his/her soil for the presence and population density of earthworms and, as necessary, introduce earthworms into the soil. Prep the soil first and make it hospitable for earthworms by adding organic compost and/or mulch.

Earthworms in soil
Get Earthworms Into Your Soil!

About the author

Kersasp 'Kersie' Shekhdar

Kersie learnt the basics of gardening as a toddler, courtesy of his grandfather. In his youth he was an active gardener with a preference for flowering plants. He is a professional and vocational writer and his freelance projects have spanned various kinds of writing.

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