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The Truth About Acidic and Alkaline Soil

Last Updated Jul 13, 2010 · Written by Craig Gibson

Are you a keen gardener but finding that your plants aren’t growing as well as you’d hoped?  Well, the problem may just lie in the pH of your soil.  If your soil’s pH is incorrect, then your plants will be unhappy.  Thankfully, there are some things that you can do to correct the pH levels of your soil.

What is pH?

pH is a tool to measure the acidity or alkalinity of your soil.  When you are measuring the pH of your soil, you are actually measuring the number of potential hydrogen (H) ions – hence, the abbreviation, pH.  The more hydrogen ions there are the more acid the soil is.

The pH is measured on a scale of 0 to 14.  A perfectly neutral pH is 7.0.  If the pH is less than 7.0, the soil is said to be acidic, and if the pH is more than 7.0, the soil is said to be alkaline.  To use a common example of something acidic, lemon juice has a pH level of 2.0.  Baking soda is alkaline at 8.5.  When it comes to soil, moist climates tend to be more acidic, while drier climates are alkaline.

Why is pH Important for Gardening?

pH is vital in your gardening as the acidity or alkalinity of your soil will affect the way that your plants grow.  If the pH level is incorrect, the plants will not be able to absorb the nutrients that they need from the soil.  The roots of a plant will only absorb nutrients when they are dissolved in water.  If the soil is too acid or too alkaline, some of the nutrients will not be dissolved and thus they won’t be available to the plant.  The nutrients that plants need are most easily available when the pH of the soil is between 6.0 and 7.5.  If the soil has a pH lower than 6.0, nitrogen, potassium, and phosphorus are not as available to the plant.  If the pH is higher than 7.5, iron, manganese and phosphorus are not as available.

Testing your Soil pH

It is easy to test the pH of your soil yourself.  Testing kits are available at your local nursery and these are supplied with all instructions on their use and many also include advice on how to adjust the pH of your soil.  Many nurseries will even test your soil for you, taking all the hassle away.

If you are testing the soil yourself, most kits will come with a test tube, a test solution and a colour chart for comparison.  Soil is placed into the tube, some test solution is added, and the tube is shaken and left for a short period of time to fully settle.  Once the sample has settled, the solution will change colour to indicate the pH level of your soil.  Match the colour in the test tube to the colour on the colour chart.

Raising Soil pH

If you need to raise the pH of your soil, that is make it more alkaline, powdered or ground lime is an often recommended choice.  Ground limestone is less potent and slower acting than powdered limestone.  How much limestone you need will depend on the texture of your soil (i.e. is it clay or sandy soil?) and some other factors.  Other options for raising soil pH include wood ashes and oyster shells.

Lowering Soil pH

To lower the pH of your soil or make it more acidic, sulfur is the cheapest choice.  However, depending on the circumstances, ferrous sulfate and aluminium sulfate may also be recommended.  Ferrous sulfate also adds iron to the soil and it is valuable for plants that have yellow leaves and appear to be in poor health. 

If you have time, you can lower the pH of your soil by regular application of organic matter such as compost (link to compost article) or manure.

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