Understanding soil pH


Soil pH – What it is and why it’s important

pH tells you how acidic something is. The lower the pH the more acidic it is, and the higher the pH the more alkaline.

A neutral pH is 7. This means that it is neither acidic nor alkaline.

pH < 7 is acidic                            e.g. lemons (pH 2)

pH = 7 is neutral                         e.g. pure water (pH 7)

pH > 7 is  alkaline                       e.g. bicarb soda (pH 9)

Your garden soil or potting mix has a pH, and different plants like soils with different pH ranges. Luckily, almost all plants like to be in soil with a pH between 5 and 7.

Why is pH important? 

pH affects the availability of nutrients. Broadly speaking, if a soil has a pH outside 5 – 7, plants start to have trouble absorbing nutrients in the soil. This leads to a weak plant that is more susceptible to disease.

Some diseases thrive when the soil is alkaline or acidic. For example, Clubroot, which affects members of the Cruciferae family (eg. cabbages, turnips & radishes), is prevalent in acidic soil. Making the soil more alkaline by raising the soil pH can help reduce the incidence of the disease.

Some plants, such as, azaleas and camellias thrive in slightly acidic conditions, while most vegetables are best grown at a pH as close to 7 as possible.

How to test your soil pH? 

The simplest way to test your soil pH is to buy a pH test kit from your local nursery. Most kits will use the following method:

  1. Take a pinch of soil and place it on the white plate provided
  2. Add a small amount of indicator dye to the soil and mix to make a paste
  3. Gently tap the provided white powder (barium sulphate) onto the soil paste
  4. Wait about 1 minute
  5. Compare the colour the white powder has changed to, to the colour chart provided in the kit.

This is the pH indicator card, that comes with your pH testing kit.

How to change the pH of your soil

To raise the pH (make it more alkaline), dig in worm casts from your worm farm. You can also add wood ash or lime – but be careful! It’s easier to raise the pH of a soil than lower it, so continually test the pH as you’re making changes.

To lower the pH (make it more acidic), add compost with animal manure and mix well into the soil. It has been suggested that 2 -3 kgs of raw animal manure per meter squared of soil can lower the pH by 1 pH point (i.e. from a pH of 8 to 7). Egg shells can also lower pH, although you will need a lot of eggshells!


Todays post has been written by soil specialist  Alisa Bryce. Alisa Bryce has extensive knowledge in soil research and consulting.

A Certified Professional Soil Scientist (CPSS), Alisa’s work has been published by ReNEW Magazine, the Journal of Water Science and Technology and CSIRO. Alisa’s strategies focus on creating the best organic soil for your garden.

For more information on how to manage your soil organically, visit www.organicsoilguide.com This recently released ebook details the best way to care for your garden organically, starting with the foundation – the soil. 


3 Responses

  1. Ian

    Hi – The URL above has the “c” and “i” transposed so doesn’t work. Otherwise a very informative post.


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