Reviewed By COLIN SKELLY
Colin is a Horticulturist and Horticultural Consultant with experience in a range of practical and managerial roles across heritage, commercial and public horticulture. He holds the Royal Horticultural Society’s Master of Horticulture award and has a particular interest in horticultural ecology and naturalistic planting for habitat and climate resilience.
Contributions From BEN RASKIN
With 20 years of commercial growing experience under his belt, Ben Raskin is now the Head of Horticulture & Agroforestry at the Soil Association. He is also the author of several gardening books, including Zero Waste Gardening and The Woodchip Handbook. Ben is a full member of the Garden Media Guild.
IN THIS GUIDE
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!” the casual gardener might very well say. But the dedicated gardener would reply, “There is soil…and then there is soil”.
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.
“Farming and horticulture had been very chemically focused until quite recently,” shares Soil Expert Ben Raskin.
“People have been advised to apply artificial fertilisers, but if you get the biology right, your plants can cope with mostly anything.
“If you have healthy soil, plants can communicate with each other through fungal networks. There’s lots of stuff going on in there that we are only beginning to understand.”
Colin Skelly, Master Horticulturist, adds:
“The concept of soil health brought soil ecology into consideration of soil rather than just its mineral and chemical composition and structure.
“Soil supports life, which then influences the nature of the soil, which then influences the balance of life in the soil – a complex network of relationships and cycles.
“The evidence points to reducing chemical inputs, adding organic matter and keeping soil covered with vegetation is key to optimising soil life and nutrient cycles. The old adage of feeding your soil not your plants holds true!”
As Ben and Colin suggest, this 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 Crucial Variable: pH
No matter what kind or type your soil is, it has some or another pH value.
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.
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.1Bojórquez-Quintal, E., Escalante-Magaña, C., Echevarría-Machado, I., & Martínez-Estévez, M. (2017). Aluminum, a Friend or Foe of Higher Plants in Acid Soils. Frontiers in Plant Science, 8. https://doi.org/10.3389/fpls.2017.01767
Moderately alkaline (pH 7.9 to 8.4) soils put a crimp on the availability of key nutrients like phosphorous and potassium to plants, leading to mineral deficiencies.2Soil pH and the Availability of Plant Nutrients. (n.d.). Nutrient Stewardship. Retrieved March 27, 2023, from https://nutrientstewardship.org/implementation/soil-ph-and-the-availability-of-plant-nutrients/
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.3Soil pH and Nutrient Availability. (n.d.). Laqua Horiba. Retrieved March 27, 2023, from https://www.horiba.com/int/water-quality/applications/agriculture-crop-science/soil-ph-and-nutrient-availability/
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 USDA’s Natural Resources Conservation Service to denote narrow ranges of pH values, respectively 6.1 to 6.5 and 6.6 to 7.3.4European Commission Directorate General for Health and Consumers. (2010, May 31). Evaluation of the Community Plant Health Regime. Retrieved March 27, 2023, from https://food.ec.europa.eu/system/files/2016-10/ph_biosec_rules_annexes_eval_en.pdf
To answer our question, if you have to pick a single, precise, pH value as being the ‘best’ for most plants, it would be around 6.5.5Kluepfel, M., & Lippert, B. (1999, June). Changing the pH of Your Soil. Home & Garden Information Center | Clemson University, South Carolina. Retrieved March 27, 2023, from https://hgic.clemson.edu/factsheet/changing-the-ph-of-your-soil/
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, Acers, 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.
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.
1) Lab Testing
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.
2) pH Strips
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.
3) Soil Testing Meters
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).
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 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.6Lime types and their meanings explained in our Glossary! (2020, May 22). Lancaster Lime Works. Retrieved March 27, 2023, from https://lancasterlimeworks.com/learning-center/types-of-lime/
First, as the starting point, the soil’s exact pH needs to be known.
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.’
Introducing Earthworms
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.7Earthworms and soil health. (2019, June). Farming Connect. Retrieved March 27, 2023, from https://businesswales.gov.wales/farmingconnect/sites/farmingconnect/files/documents/cff_earthworms_and_soil_health_eng.pdf
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 also play a role in also influencing the pH of that soil.8How earthworms can help your soil. (n.d.). Department of Primary Industries. Retrieved March 27, 2023, from https://www.dpi.nsw.gov.au/agriculture/soils/guides/soil-biology/earthworms
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.
The Building Blocks Of Soil
“When I look at some of the best growers I visit, they have been nurturing their soils for over 30 years and their plants are so vibrant, even within a commercial setting,” Ben adds.
“Even in our home gardens, we have such an opportunity to do that easily, as we are not under the same commercial pressures and can really look after our soils, but I think it’s not seen as easy.”
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.9Ball, J. (n.d.). Soil and Water Relationships. Noble Research Institute. Retrieved March 27, 2023, from https://www.noble.org/regenerative-agriculture/soil/soil-and-water-relationships/
On the other hand, clay retains the most water and moisture but if clay particles become tightly compacted, clay may become impenetrable to water.10Clay soils. (n.d.). Royal Horticultural Society. Retrieved March 27, 2023, from https://www.rhs.org.uk/soil-composts-mulches/clay-soils
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.11Rivers & Deltas. (n.d.). Geological Society. Retrieved March 27, 2023, from https://www.geolsoc.org.uk/ks3/gsl/education/resources/rockcycle/page3604.html
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.12Cation Exchange Capacity (CEC). (2010). Agronomy Fact Sheet Series: Cornell University. Retrieved March 27, 2023, from http://nmsp.cals.cornell.edu/publications/factsheets/factsheet22.pdf
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.13Brown, K., & Lemon, J. (n.d.). Cations and Cation Exchange Capacity. Soilquality.org. Retrieved March 27, 2023, from https://soilquality.org.au/factsheets/cation-exchange-capacity
This property of clay and clay soils is called Cation Exchange Capacity (CEC) and it is crucial for plant growth and health.14Mengel, D. (2014). Fundamentals of Soil Cation Exchange Capacity (CEC). Agronomy Guide. Retrieved March 27, 2023, from https://www.extension.purdue.edu/extmedia/ay/ay-238.html
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.15Chalk. (n.d.). South Downs National Park. Retrieved March 27, 2023, from https://learning.southdowns.gov.uk/geology-landscapes/chalk/
Another component of soil is manure which is essentially decomposed faecal matter. In fact, manure is a saleable by-product of animal farming.
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.16Humus. (n.d.). National Geographic. Retrieved March 27, 2023, from https://education.nationalgeographic.org/resource/humus/
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.17What is peat? (2019, July 2). International Peatland Society. Retrieved March 27, 2023, from https://peatlands.org/peat/peat/
Because manure, humus, and peat are derived from organic materials, they too carry some or another negative electrical charge and, therefore, have a CEC because of which these soils attract and retain positively-charged cations.18Soil Chemistry. (n.d.). Retrieved March 27, 2023, from https://web.njit.edu/~kebbekus/Soil%20chem%20notes.htm
Loam is the ‘ideal’ composite soil comprising equal parts sand, silt, and clay, with some humus.19Maurya, N. (n.d.). Different Types of Soil – Sand, Silt, Clay and Loam. Madhav University. Retrieved March 27, 2023, from https://madhavuniversity.edu.in/soil-types.html
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.
