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Soil pH meters: reasons for measuring soil acidity and how to change it

A soil pH meter

Do you know the importance of soil acidity? Soil acidity is essential to maintaining a healthy and blooming flower display. Despite the importance of soil acidity, most people are unaware of how it can affect plant health and growth.

We explore why you should measure soil acidity and how to change it for the plants you want to grow.

What is soil acidity?
Measuring soil acidity

Soil acidity is a measurement for determining the concentration of hydrogen ions in the soil. Using the pH scale, a pH measurement indicates whether the soil is acidic or alkaline. Soil with higher concentrations of hydrogen ions in the soil is acidic, with a lower pH.

The logarithmic pH scale determines the extent of acidity using a score of 0-14. The lower the score (0-6), the higher the acidity. Higher scores (8-14) indicate alkalinity concentrations. A score of 7 is neutral and is neither acidic nor alkaline.

The ideal acidity for soil should be between 6 (slightly acidic) and 8 (slightly alkaline). When displayed on a pH chart, the stronger the acidity, the darker the colour red. The higher the alkaline level, the stronger the purple colour. Neutral is indicated with a green colour.

Here is an example of a pH chart with examples:
The pH meter with examples

Why should you measure soil acidity?
A soil pH meter displaying the light, acidity, and moisture levels

Measuring and monitoring soil acidity is something every keen gardener should do. Everyone can benefit from measuring soil acidity regardless of the size of their garden or plant production. Here are the reasons why you should measure soil acidity.

One of the biggest reasons you should measure soil acidity is to determine whether plants can optimally absorb nutrients from the soil. Depending on the acidity of the soil will affect what nutrients plant roots can absorb.

Many gardeners may plant flowers in soil different from the optimal pH for nutrient absorption. In this case, plants may struggle to grow or yellow plant foliage. It can also affect the quality of fruit and vegetable crops.

Here is a chart to indicate the nutrients available at different soil acidity levels.
Soil acidity and nutrient absorbency chart

The soil's pH level also affects the soil's microorganisms. The optimal pH level improves the decomposition of organic matter. High acidity levels hinder microorganisms in the soil and their effectiveness. A pH level around the neutral level is ideal for microorganism processes.

Plant growth may stall if certain nutrients necessary for plant growth are not readily available. If this is the case, methods for changing soil acidity are required. You can adjust soil acidity in several ways to increase acidity and alkalinity.

How can you change soil acidity?

To measure soil acidity, a soil pH meter provides the most accurate results with just the press of a button. Other methods to test the soil pH level include litmus paper and pH test kits. However, these tests don’t provide accurate numerical readings like a soil pH meter.

Taking a soil pH reading within three months of adding fertiliser to the soil can affect results. After measuring acidity levels using a soil pH meter, you may find that it is necessary to adjust the pH levels. Fortunately, you can increase the acidity and alkalinity of the soil.

Several factors affect soil acidity, including:
  • - The rock source from which the soil was formed
  • - Weathering processes acting on the soil
  • - Fertilisers
  • - Soil content (for example, clay soils are more resistant to pH changes)
  • - Root respiration
  • - Organic matter decomposition
  • - Acid rain

Increasing soil acidity

Firstly, are the methods you can use to increase soil acidity. These tips are suited for those with a low pH soil level (or alkaline soil).

Lime and dolomite

The most popular technique used for raising soil pH is to apply lime or dolomite to the soil. Both solutions will effectively and quickly increase the alkalinity of the soil. This method suits those wishing to grow lilacs, lavender, and honeysuckle.

The amount of lime or dolomite you apply to the soil depends on the soil type. Poor drainage soil, such as clay, will require more lime to alter pH levels effectively. Sand and silt soils will require much less lime to change the pH levels.

Wood ash

However, lime is only an effective solution for some plants. Rhododendrons and Camellia growth are affected by adding lime to the soil. Instead, adding wood ash to the soil is a more suitable alternative to increase soil acidity levels.

Wood ash is an effective method to alter pH levels. Using wood ash adds potassium and calcium to the soil. Wood ash also has a similar effect to lime for increasing pH levels. You can also use any leftover ash from wood fires to spread over shallow ground or dig into the ground.

Increasing soil alkalinity

Secondly, are the methods you can use to increase soil alkalinity. These trips are suited for those with a high pH soil level (or acidic soil).


Adding sulphur to the soil is one of the most popular choices for increasing soil acidity. For faster increases in soil alkalinity, use finer sulphur particles. Soil bacteria can convert sulphur into sulfuric acid much quicker, benefiting the soil sooner.

Consider choosing sulphur dust compared to sulphur chips for much quicker outcomes but more expensive. However, you will still need to wait a few weeks before any effects occur. This is longer during the colder winter months.

Increasing soil acidity through sulphur is ideal for growing blueberries, heathers, lilies, magnolias, and azaleas. Adding sulphur is also considered the least likely to harm your plants.

Aluminium sulphate

Another choice for increasing soil’s alkalinity is aluminium sulphate or ferrous sulphate. Compared to sulphur, a build-up of aluminium can be harmful and toxic for plants. Large quantities of ferrous sulphate may also interfere with phosphorus availability.

However, both aluminium sulphate and ferrous sulphate have a faster turnaround time for reducing soil pH levels than sulphur.

It should be noted, though, that the Royal Horticultural Society (RHS) no longer recommends using peat to lower soil pH. Sulphur is still the safest and most effective method for increasing soil alkalinity.

Knowing the importance and reasons for measuring soil acidity is important for ensuring healthy flower blooms. For those who want to alter soil pH, following the above methods will benefit healthy flower blooms.