Soil pH – How to Recognize Alkaline and Acidic Soils

Many gardeners are aware that soil pH is an important factor in plant growth. We’ll discuss what soil pH has to do in terms of acidic and alkaline soil, as well as how you can influence soil acidity.

What is the Soil pH value?

Soil pH is a single number that indicates whether the soil is more acidic, neutral, or alkaline. The letters pH stand for “potential of hydrogen“, and it relates to the hydrogen content and activity in the soil. The higher the concentration of hydrogen in the soil, the more acidic it is, and thus the pH of the measured soil is also lower.

But what does the pH of the soil, or even the concentration of hydrogen ions, matter to a gardener?

The pH value has an impact on a variety of chemical and biological activities in the soil. The acidity of the soil influences nutrient availability, soil life activity, crumb structure, and thus plant growth. It is not for nothing that an optimal soil pH value for all crops may be determined, at which they can grow to their full potential.

#Tip: Central European as well as North American soils often have a slightly acidic pH range of 5.5 to 6.5, which is also ideal for a major part of our crop and decorative plants. Exotic or highly specialized plants, on the other hand, may require a more acidic or alkaline soil. Furthermore, there are several regions in Central Europe where soil acidity levels depart significantly from this criterion.

So it’s worthwhile to investigate the pH of your own soil, with the purpose of promoting plant life in your garden as much as possible.

How to Find the pH of your Garden Soil

It is especially interesting to know the pH level of the soil if you have recently acquired a garden or a plot of land. Because based on this figure, you can make an informed judgment about whether plants will thrive here and whether the acidity should be adjusted at all.

#Tip: For many gardeners, the grass, in particular, is an important part of the garden. The pH of lawns should be around 5.5 and 7.5, based on the soil type. Problems with moss, other weeds, or just inadequate lawn growth are frequently caused by an abnormally high acidity level. So, if you verify the acidity level of your own soil ahead of time, you can save yourself a lot of trouble. The soil beneath established lawns can also be enhanced in this manner, specifically by liming the lawn.

There are various ways of calculating soil pH values. These methods are fairly easy for garden owners to use:

Soil pH Test

There are a variety of soil testers available for use, including simple and rather inaccurate soil pH tests with indicator papers. At the same time, you can also find sophisticated test kits that reveal pH by the colour change of a liquid. There are also digital pH metres available, as well as highly professional laboratory tests that evaluate soil pH, nutrient levels, and soil type.

The truth is that the test method you use has an impact on the outcome. To measure pH, simple tests use water or distilled water; laboratories typically employ calcium chloride solutions. Digital metres can even detect the pH of soil directly by monitoring the current flow between two measuring electrodes and analyzing the findings.

Measurements from specialised laboratories are the most trustworthy and comparable. All other approaches can only be regarded as approximations, which are usually adequate for hobby gardening.

#Tip: If you wish to test the pH of your soil personally with the help of a test kit or an indicator strip, use pure water and keep the suspension as warm as 25 °C as feasible. This is due to the fact that the pH value measured is impacted by the temperature and compounds dissolved in the water.

Using Plants which indicate pH of the Soil

As previously stated, plants always favour a specific pH range. They perish or do not germinate in soil that is far too acidic or alkaline, yet at appropriate pH levels, they flourish and even proliferate abundantly.

One advantage is that some invasive plants or weeds can be utilised as indicators of soil qualities. Of course, good indicator plants (“bio-indicators“) are only those genera and species that are highly specialised in a restricted pH range. Common plants that appear to thrive everywhere, such as dandelion (Taraxacum sect. Ruderalia) or chickweed (Stellaria media), are not good indicator plants since they endure a wide range of environments.

Bio-Indicators for Acidic Soils
Black Medic or Yellow Hop Clover (Medicago pupulina)
Cornflower or Bachelor’s button (Centaurea cyanus)
Red Sorrel or Sheep’s Sorrel (Rumex acetosella)
Creeping tormentil (Potentilla reptans)
Garden burnet (Sanguisorba minor)
Rabbitfoot or Stone clover(Trifolium arvense)
Pansy (Viola tricolor)
Marsh valerian (Valeriana dioica)
Acidic Soils Bio-Indicators
Bio-Indicators for Alkaline Soils
Hyssop – Evergreen garden herb (Hyssopus offcinalis)
Charcoal or Field Mustard (Sinapsis arvensis)
Forking or Rocket Larkspur (Consolida regalis)
Alkaline Soils Bio-Indicators

If you notice one of these plants flourishing in your garden in a healthy and abundant manner, you can use it to determine whether your soil is acidic or alkaline:

What does pH of my Soil indicate?

When the pH of the soil is analyzed, it essentially tells you how much hydrogen is present and how active it is. Different compounds of these two elements are discovered in the soil solution based on the quantity of hydrogen present in the soil in relation to oxygen: Oxonium ions (H3O+), water (H2O), and hydroxide ions (OH-) in particular are always present, albeit in different proportions depending on the acidity of the soil.

So an,

  1. Acidic soil is high in oxonium.
  2. Neutral soil is high in neutral water.
  3. Basic or Alkaline soil is high in hydroxide.
Acidic SoilsNeutral SoilsAlkaline Soils
pH Values3 – 6.5Around 77.5 – 9
Presence of Hydrogen (H+ ions)Oxonium H3O+ Water H2OHydroxide OH-
Hydrogen to Oxygen Ratios3:12:11:1
Soils – pH values – Corresponding Hydrogen Ions

Finally, the existence of these three chemicals is responsible for the effects on the soil. They have an indirect impact on the availability of nutrients, the functioning of soil organisms, and the degree of humus formation, for example.

Acidic Soils (pH < 5.5)

In acidic soils, which have a high concentration of reactive oxonium, nutrients that are otherwise hard to obtain dissolve easily, salts and minerals disintegrate, and some trace nutrients, such as iron, manganese, and zinc, become more readily available.

Excessive acidity, on the other hand, can cause the production of chemicals that are toxic to most plants, such as aluminium. Fungi and fungal microorganisms thrive in slightly acidic soils, whereas many soil bacteria cannot.

This has an impact on the soil’s humus-formation processes. On acidic soils, lighter, lower-quality humus of reddish colour is generated with the help of soil fungi.

Why does the Soil become Acidic?

Acidic soils arise when the minerals in the soil are unable to compensate for the acidifying action of slightly acidic precipitation, decaying organic matter, acidic fertilisers, and hydrogen-secreting plant roots. This is referred to as the soil’s buffer capacity.

Sandy soils, for instance, are less “buffered” compared to clay soils and hence acidify more quickly and severely – this can be seen in peatlands, which are exclusively organic soils with no rock components. Because no materials, such as limestone, exist to buffer the acidification, the bog gets progressively acidic.

In addition, water saturation and waterlogging generate acidity in the soil because carbon dioxide from the air combines with oxygen to form carbonic acid – another reason why very wet bogs are highly acidic.

Which Plants Grow in Acidic Soils?

Only a few plants can adapt to extremely Acidic soil conditions. They are,

  • hydrangeas (Hydrangea)
  • blueberries (Vaccinium)
  • rhododendrons
  • azaleas (Rhododendron)
  • lavender heaths (Pieris)
  • heathers (Calluna, Erica)
  • numerous sweet grasses (Poaceae)
  • skimmias (Skimmia)
  • bluebells

Alkaline Soils (pH > 7)

Bacterial soil organisms prefer alkaline soils, which contain more hydroxide, to acidic soils. As a result, many biologically stored nutrients, such as nitrogen, phosphates, and potassium, become more available at high pH levels due to the release of soil organisms.

Rising soil pH often improves soil structure, making it more crumbly and loose. However, critical trace nutrients are scarce in excessively alkaline soils, causing plant growth to be hampered and fertility to decline.

Why does Soil become Alkaline?

Alkaline soils form when soil grows on highly calcareous rock, most commonly limestone. Soils with a high clay or silicate content can always (over)compensate for released hydrogen, resulting in a soil solution with a low proportion of oxonium.

The exceptionally fertile black soils and brown soils, for example, have a high pH due to the significant loess concentration in the mineral portion of the soil.

Which plants grow on alkaline soil?

Only just a few plants thrive on alkaline soils. Mountain plants like cowbell (Pulsatilla), strawflowers (Helichrysum), and bulbous (Allium), black salsify (Scorzonera), as well as lavender (Lavandula) prefer neutral to slightly alkaline soil, whereas garden plants like bulbous (Allium), black salsify (Scorzonera), and lavender (Lavandula) demand high pH.

Neutral or Weakly Acidic Soils (pH 5.5 – 7)

The soil structure is considered good when it is weakly acidic, weakly alkaline, or neutral soil, and all nutrients are readily available. It’s no surprise that most plants prefer this environment. Those who claim slightly acidic or neutral soil as their own are usually no longer required to use lime or other agents and can begin fertilising and planting immediately. A considerable portion of German soil is mildly acidic, and a large portion of our native plants are similarly adapted to it.

Why are some Soils Weakly Acidic or Neutral?

Soils with a balanced acidity can buffer acidifying forces by neutralising hydrogen with clay minerals along with metal oxides (such as iron oxide), allowing the pH to remain constant between 5 and 8. Small amounts of limestone are also common. Furthermore, the humus functions as a buffer and helps to keep the pH stable. Loamy, humic soils over limestone, as is common in Germany, are frequently slightly acidic to neutral.

Which plants grow on Weakly Acidic or Neutral Soils?

The majority of farmed and ornamental plants demand a mild acidity level. Only specialized plants in bogs, desert regions and in the tropics, etc are accustomed to extremely harsh conditions can thrive in weakly acidic to neutral soils.

Soil Strategy for Gardening

If the soil in your garden is not just somewhat acidic or neutral, but rather extremely acidic or neutral, you have two options: To begin, you can experiment with changing the soil by mixing suitable additives. Changing the pH of vast regions can be expensive, especially if the pH is excessively acidic or alkaline.

#Tip: Plan and Go with the soil instead of against it

Furthermore, occasionally the shift does not last, but the soil returns to its natural pH through the use of soil-building components. Second, folks with severely acidic or alkaline soil may consider growing specific plants that thrive in these conditions and growing other plants in pots, containers or raised beds.

How to Adjust the Soil pH

Is your soil slightly too acidic or alkaline for your needs, or has it become out of balance as a result of years of poor management? If you want to adjust the pH of your soil permanently, you need to check it once a year, and here’s what you can do to change it.

If the Soil is very Acidic

Lime can quickly rectify soil that is somewhat excessively acidic, for example, with a pH of 5. The sort of lime that should be used, however, is determined by the soil type: sandy soils, for instance, are more easily influenced than clay-rich soils.

The correct type and amount of lime are also influenced by the humus concentration. The lime application is usually divided into two treatments, i.e. lime is applied two years in a row. Carbonic acid lime works effectively on sandy and peaty soils.

Quicklime is extremely fast-acting, even caustic, and can help even heavy, clayey soils acquire a higher pH. Only a proper soil pH test can tell you how much lime to apply. Tests for recreational purposes frequently provide helpful hints on how to proceed.

Another technique to enhance soil pH is to utilise alkaline primary rock flour manufactured from diabase and other forms of basalt on a regular basis.

If the Soil is very Alkaline

Making a soil acidic is a little more complicated than making it alkaline. Because the rocks that comprise the soil are the cause of alkaline soil, decreasing the pH rapidly becomes a war against windmills that must be done every year.

Soils can be acidified using elemental sulphur, commonly known as a sulphur blossom or sulphur bloom since bacteria in the soil convert it to sulfuric acid. A dose of 50 to 100 g of sulphur flower per square metre can lower the pH of a medium-heavy soil by more than one unit.

Light soils respond more sensitively and rapidly become acidic; heavy soils, particularly highly humus-rich soils as well as compost soils, react considerably more slowly and require much more sulphur flower to become acidic. The use of grape marc, acidic granite primary rock meal, or regular mulching are various natural strategies to acidify the soil.

Those who wish to preserve peat in the garden but do not want to give up peat entirely can irrigate plants on a regular basis with peat extracts: Soak two handfuls of peat in a 10-litre watering can for a week to accomplish this.

The peat is subsequently extracted or filtered, and the newly acidic watering water is provided to acid-loving plants. Nevertheless, the effect of this procedure is temporary and must be repeated on a monthly basis.

The type of soil has a strong influence on the pH of the soil. In our unique post, we explain how to assess the soil type in your garden.