Understanding what soil pH means for your garden

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You’ve probably heard or read about soil pH and you know it’s important to gardeners but are not sure why. 

Without going into too many confusing scientific details.  Here are the facts.

pH is a measure of acidity

Remember those chemistry lessons at school where you learnt about acids and alkalis and how they react with other chemicals?

pH is a measure of acidity.  It gives you an idea of how your soil will react with different substances (remember that plant nutrients are essentially chemical compounds) and how strongly that reaction will be.

test tube held against ph testing scale

There are 17 essential plant nutrients and 14 of these come from the soil.  In order to absorb those nutrients, plants need them to be dissolved in water.  The pH of the soil determines how easily nutrients can be dissolved in water and therefore their availability to the plants.

The pH scale goes from 0 – 14.  0 is extremely acidic, 7 is neutral and 14 is extremely alkaline.

pH of commonly found substances

  • Stomach Acid:  2.0
  • Lemon:  2.5
  • Vinegar: 3.0
  • Tomatoes: 4.5
  • Beer: 4.5 -5.0
  • Carrots: 5.0
  • Cabbage: 5.3
  • Cow’s Milk: 6.5
  • Saliva: 6.6 – 7.3
  • Blood: 7.3
  • Eggs: 7.6 – 7.8
  • Sea Water: 8.2
  • Sodium Bicarbonate: 8.4
  • Milk of Magnesia: 10.5
  • Ammonia: 11.1
  • Lime: 12

Testing soil pH

You can easily discover the acidity of your soil using a pH testing kit from the garden centre.  It’s always wise to test different areas of the garden because it’s not unusual to find quite a lot of variation.

Why do you need to know the pH of your soil?

The acidity (or alkalinity) of your soil will tell you what sort of plants will grow in your garden, and what won’t.

When I was much younger and a lot more impulsive that I am now, I once spent a lot of money on a beautiful camellia bush only to have it die within 6 months because I have chalky (alkaline) soil and camellias prefer acidic soils.

Phosphorus is a major plant nutrient that builds strong roots, flowers and fruit.  It doesn’t dissolve well in soil – which is why we gardeners feed it to our tomatoes in liquid fertilisers.  Phosphorus is at its most available in soils of pH 6.5

hydrangea flowers beside a winding stone path

Hydrangea flowers are said to be different colours depending on the acidity of the soil they're growing in

Very acidic soils, those with a pH of between 4 and 5 and have high concentrations of soluble aluminium, iron and manganese.  These are all valuable plant nutrients but just as overdosing on medicine is bad for for humans – too much of some minerals can be toxic to some plants.

Also in strongly acid soils, those wonderful microbes that help decompose dead leaves and roots don’t do so well.  That means that plant nutrients like nitrogen don’t get released from organic matter and can’t be used by the plants.

Some plants do really well in acidic soils, others thrive in alkaline soils but the majority of plants are happiest in soil with a pH of around 6.5


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