How to test soil pH and adjust soil acidity

Author
Daniel Weber

16.04.2022

Why is it important to know the pH of your soil, which plants prefer acidic or alkaline soil and how to adjust the acidity of the soil?

Whether you're planning to grow plants in either an edible or an ornamental garden, it is useful to know the pH of the soil in the garden, as different plants are suited to different soil acidity levels (pH). The pH scale describes how acidic or alkaline the soil is. This scale ranges from 0 to 14, with a pH value of 7 being neutral, a pH less than 7 indicating acidity and a pH greater than 7 indicating an alkalinity. The lower the pH number, the higher the acidity and the lower the alkalinity of the soil.

Soil pH affects cultivated plants in several ways. The acidity of the soil primarily affects the solubility of the necessary minerals in the soil and the ability of the plants to extract these nutrients from the soil. This means that even if there are plenty of nutrients in the soil, they may not be available to the plant if the soil pH is not appropriate. For example, in overly acidic soils, plants suffer from a deficiency of potassium, calcium, magnesium, phosphorus or molybdenum. Conversely, in alkaline soils, elements such as iron, zinc, manganese, copper or boron may not be available to the plant. The pH of the soil is also important for the life of soil bacteria and various soil animals, and it also affects soil structure. All these factors influence whether plants will thrive and how resistant they will be to disease and pests.

The pH of the soil depends mainly on the soil composition (clay, sand, limestone and other natural components in the soil), but it's also affected by the way the soil is treated. Soil acidity can be increased over time by using fertilizers (especially synthetic fertilizers) or by green manuring (incorporating green plants or mulch into the soil).

Interesting fact: in chemical terms, acidity means the ability of the molecules of an element to pass on protons and, conversely, alkalinity means the ability of the molecules to receive protons.

How to test soil pH

how to test acidic soil Soil pH is measured using materials that change color as the pH changes. Such materials are referred to as pH indicators. Among the most practical and also the most affordable are litmus paper indicators for pH measuring. They can be found in drugstores or garden centers and are well priced at just cents apiece.

When testing soil pH with litmus indicators, take a sample of dry soil (just a few ounces/grams is sufficient), place it in a jar, and add distilled water (the warmer the better) in a about 1:1 ratio. Stir thoroughly to crumble the soil in the water and leave it to infuse for a few hours. Then dip a disposable litmus paper indicator into the prepared solution. The color of the litmus paper will change within a few seconds, and the given color shade will indicate the pH of the soil, from red (acidic) to yellow and green (neutral) to blue (alkaline). Distilled water is used for testing because its pH is neutral (7), whereas tap water is usually more alkaline and will affect the measurement. Litmus indicator papers for measuring soil pH do not need to have the whole pH scale from 0 to 14. For a soil pH scale, values between 4 and 9 are generally sufficient.

Another option for testing soil pH is to use an electronic soil pH tester, which works by measuring the conductivity of the soil, which is also affected by its acidity. Soil pH testers are both affordable and practical, as you simply stick them into the soil and the device immediately displays the soil's pH value on the screen. However, soil pH testers are not as accurate, as they're affected by the moisture content in the soil as well as the composition of water in the soil.

Soil classification based on its pH value:

pH 5 and less – strongly acidic soil

pH 5 - 6 – acidic soil

pH 6 - 7 – neutral to slightly acidic soil

pH 7 - 8 – alkaline soil

Interesting fact: the pH scale is logarithmic. This means that pH 6 is ten times more acidic than pH 7 and pH 5 is ten times more acidic than pH 6, etc.

How to adjust pH in soil

The acidity of the soil can be increased or decreased as necessary, depending on the types of plants you want to grow in your garden. For overly acidic soils, we can increase the soil pH by adding lime (ground limestone) or wood ash to the soil. The amount of lime needed to raise the pH varies depending on whether it is added to sandy or clay soil. To raise the soil pH from 5 to 6, only 3 oz. per square yard (100 g/square m) is needed for sandy soils, whereas up to three times the amount of limestone is needed for heavy clay soils.

If, on the other hand, your soil is too alkaline and you need to lower the pH, you can add peat, incorporate fresh organic material (needles, leaves, mulch, etc.) into the soil, or add sulfur or fertilizer.

If you would like to test the soil pH again after treatment, it is necessary to wait until the different components and layers of the soil have neutralized each other (approx. 3 months) to get an accurate reading.

What are the appropriate soil pH values for various plant species?

Various plant species are adapted to different soil types, including the pH range in which they can thrive. For example, wild strawberries have a wide range of tolerated pH (5.0 - 7.5), so they thrive in both acidic and alkaline soils, while blueberries typically thrive only in a narrow pH range (4.5 - 5.0).

A neutral to slightly acidic soil with a pH of 6-7 is best suited for most edible and ornamental plants, so these soil pH values are therefore considered ideal. As these are the vast majority of plant species, we won't include them in the list below, which will include plants that prefer a lower, more acidic pH, along with alkaline loving plants that will tolerate a more alkaline environment.