The differences in soil pH and what they mean for your garden

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A healthy garden is dependent on the soil acidity, or the pH level. While having soil at a pH that’s not ideally suited to your plants doesn’t kill them, it can prevent or slow growth. Planting in a soil pH that is not ideally suited affects the amount of nutrients that plants can absorb, which is often a factor that gets overlooked. You may even be surprised to find that some plants grow different colours depending on the acidity of the soil. Because every plant prefers its own soil acidity, you may uncover why some of your plants haven’t grown as well as you thought. At the end of this article, we provide you with some quick and easy ways to test your soil pH.

What is soil pH?

Soil can be either alkaline or acidic (sometimes referred to as basicity). The pH of your soil is a measure of the acidity or alkalinity of the soil. Often presented on a colour chart, with red being acidic and blue being alkaline, the pH value measures the hydrogen ion concentration. A logarithmic scale is used to present the level of pH. Very acidic soil (a low pH value) has a higher hydrogen ion concentration than higher alkaline pH values. Most garden soil ranges between a pH level of 3.5 and 10. The three soil pH classifications are:

  • 0 to 6.5 is acidic (anything less than 5.5 is strongly acidic)
  • 6.5 to 7.5 is neutral
  • 7.5 to 14 is alkaline (anything above 11 is a strong alkaline)

To give you an idea of different levels of pH, here’s the pH scale with some examples:

pH scale examples

Based on the pH scale shown on Science News for Students.

The pH of the soil is determined by the rocks from which the soil formed alongside what weathering processes act on the soil. Other factors that can affect the acidity of soils include:

  • Root respiration
  • Plant growth
  • Fertiliser
  • Acid rain
  • Addition of minerals to the soil
  • Water flow

Gardeners should pay attention to the acidity of their soil, as the pH level does affect the nutrients and chemicals that become soluble in water, thereby making them absorbable to plants. Some nutrients absorb better in alkaline soil, while others absorb better in acidic soil. All the essential nutrients required for plant growth are available around the neutral level. In total, fourteen out of seventeen nutrients needed for plants are obtained from the soil. You should aim for a soil pH of between 5.5 and 7.5.

The pH level of the soil also affects the microorganisms within the soil that decompose organic matter. Stronger acidity hinders the bacteria within the soil from decomposing organic matter. That can also reduce the nutrients found in soil.

That’s the reason why you may not get such good blooms or crops. We also mentioned earlier that some plants change colour depending on the soil acidity. Depending on whether they are planted in acidic or alkaline soil, Hydrangeas produce different coloured flowers.

The table below indicates the minerals that are available to plants depending on the acidity of the soil. Green indicates the mineral balance is strong while amber indicates the balance is not as strong.

Find out about the optimum pH levels for different plants.

Altering your soil pH

Depending on the plants you want to grow, you may need to adjust the pH level of your soil. There are a couple of ways you can change the pH of your soils.

  • Fertilisers – certain fertilisers can change the pH of our soil, as well as increase or minimise the nutrients that can be absorbed. If your soil is acidic, nitrate-based fertilisers can help to increase the pH level. Alternatively, if you have alkaline soil, mineral fertiliser that contains sulphur can help decrease soil acidity.
  • Lime – if you have acidic soil and want to increase the pH level, using lime or dolomite can help give you more alkaline-based soil. Depending on the soil you have (such as clay) will determine the amount of lime you need to administer. Using lime also helps add calcium and magnesium to the soil.
  • Wood ash – using wood ash in your soil can help to raise the pH level. As the wood ash has high amounts of potassium and calcium, the acidity of the soil can be reduced. Using wood ash is not as effective as lime but can be used as an alternative if you don’t want to raise the pH level too much.
  • Aluminium – you can use aluminium for alkaline soil should you wish to lower the pH level of the soil. In particular, aluminium sulphate produces acidity once it dissolves in the soil, and will help to reduce pH levels almost instantly.

The effects of soil acidity

Certain plants have become adapted to growing in certain soil acidity levels. In some instances, the correct soil pH can provide plants with a competitive advantage. In other instances, it can prevent growth in particular locations.

Acidic soil

Acidic soil is ideally suited for ericaceous and calcifuge plants. These plants are adapted to growing in acidic soil. These plants don’t like growing in lime heavy soils. Growing ericaceous plants in alkaline heavy soil will produce yellow leaves, a condition referred to as lime-induced chlorosis. Plants that suffer from this condition will not grow and eventually die due to the lack of iron and other nutrients unavailable in alkaline soil. Calcifuge plants are just the same despite alkaline soil.

Ericaceous & calcifuge plants that are ideal for planting in acidic soil include:

  • Rhododendrons
  • Azaleas
  • Heathers
  • Blueberry
  • Camellia
  • Lily
  • Pieris
  • Magnolia

Alkaline soil

Calcicole plants are those adapted for growing in alkaline or lime-rich soil. As with ericaceous plants in alkaline soils, calcicole plants in acidic soil will produce similar effects, including chlorosis, lack of growth and curling leaves. Again, it’s due to the availability of nutrients in the soil. Alkaline soil in particular has higher levels of calcium compared to acidic soil. Alkaline soils are most commonly found in Britain due to the chalk and limestone found here which results in chalky soils.

Calcicole plants that are ideal for planting in alkaline soil include:

  • Ash trees
  • Honeysuckle
  • Lilacs
  • Clematis
  • Syringa
  • Phacelia
  • Polemoniums
  • Lavender

Testing your soil pH

After reading all about soil pH and what it means for your garden, you may want to find out the pH level of your soil. Fortunately, there are easy ways you can find that out.

  • pH meter – possibly the easiest way to determine the pH level of your soil. A pH meter is the best way to determine soil pH as it can provide accurate results for home use. All that’s required with a pH meter is you follow the instructions provided. That usually includes sticking the meter into the ground and turning the meter on. The meter will then give you a reading on the display/dial. It’s that simple.
  • pH test kits – these require a little more chemistry than pH meters. A test kit will provide you with a small amount of testing solution and a colour indicator chart (the pH scale). Inside a provided tube, use a representative piece of soil that has been exposed to any fertilisers and such (soil around 6-8 inches below the surface is ideal). Make sure the soil sample is free from any debris too, such as stone or twigs. You then add the required amount of testing solution and give it a shake. After the liquid clears, you’ll see a colour which will tell you roughly the pH level of your soil using the accompanying chart.
  • Litmus paper – similar to the test kit, litmus paper will provide you with a colour depending on the pH level of your soil. A mixture of your soil sample and water is mixed together inside a test tube. After mixing, you dip the end of your litmus paper into the soil and water solution. Then, the paper will change colour according to acidity.

When so many of your garden plants depend on soil acidity, you should take the time to measure your pH soil levels. If you are starting a new garden or looking to plant vegetables, measuring your pH is especially important. As different plants grow depending on the level of acidity, making sure your pH level is ideally suited for the plants you are considering will help to ensure they grow well and provide a full crop.