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The pH level of your soil is crucial to nutrient uptake and healthy plants.  Get it wrong and your plants will suffer and all your hard work will go to waste.  Here’s how you can make sure you get it right.

Why do I need to know my soil’s pH level?

To ensure you get maximum yield from your plants, test your soil before you plant anything – that includes existing soil and any soil that you are bringing into the garden.

What is pH?

If you’re like me and my chemistry classes are a distant blur, here’s a reminder the importance of pH levels:

The term “pH” is an abbreviation for “power of hydrogen”, where “p” is short for the German word for power (potenz) and “H” is the element symbol for hydrogen found on the periodic table.  The pH of a substance is a measure of how acidic or alkaline/basic the substance is.  

Measured on a scale between 1 and 14, pH is based on the concentration of hydrogen ions in a solution, with 1 being very acidic and 14 being very alkaline/basic.  Water is neutral and has a pH of 7.0.  

In regards to plants and the garden, most plants do well between 5.5 and 7.5 and plant roots only grow in a range of pH levels between 4 and 9.  Only specially adapted plants can survive extreme pH levels.

What does pH do to plants and soil?

Highly acidic or alkaline soil conditions lock up nutrients in soils and make them unavailable for plants, which can waste fertilisers applied and not get the best response from your plants.

Where the pH is lower, the soil is more acidic and has more trace elements such as boron, copper, manganese and zinc.  This makes it harder for plants to absorb calcium and molybdenum (helps to absorb nitrogen from the soil) and can be toxic to the plant.

On the other hand, where the pH levels increase and the soil is more alkaline, there is less copper, iron, manganese and zinc available to the plant.  This impacts the micro-organisms and fungi in the soil.

How to check pH

Easy peasey, lemon squeezy!!  Use a Soil pH Test Kit – the instructions on how to use this are on the back of the packaging – so go easy before recycling this!  Gardening Australia describe the steps on testing the pH levels in soil:

  1. Collect a small sample of soil from your garden (about one matchbox full is ample)
  2. Place the sample on some white plastic or card (often supplied with the tester kit).
  3. Squeeze some of the liquid indicator dye over the soil sample
  4. Add a puff of the powder to over the sample which will react with the indicator dye
  5. Wait (usually less than 30 seconds) for the powder to change colour.
  6. Use the indicator card and match to the closest colour on the chart. This will be your reading.

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There are some exceptions to the rule with some plants preferring alkaline or acidic soils.  These plants will require a bit more TLC through adjusting the pH of the soil.  Check out the plant pH levels for examples.

How can I fix the pH levels in my soil?

The general rule to follow is:

  • Limestone to raise the pH level
  • Sulphur to lower the pH level

There is a little bit of science around this – the difference between a soil pH of 6 and 7 is huge.  A pH of 6 is ten times more acidic than a pH of 7.  So getting the right pH level of the soil is crucial.

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How often do I check the soil pH levels?

Always test the soil pH before designing or planting, or if your plants and vegies have stunted growth or yellowing leaves.  Fertilisers, compost and organic matter can also adjust the pH level of your soil.  A good way to ensure that plants get the nutrients they need is crop rotation and choosing plants that are suited to your soil type.  Check out what the neighbours are planting and that will give you a good idea of what to plant in your garden.