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Understanding Soil pH

It doesn’t matter how healthy the plant from the nursery is or how much fertiliser or water it is given, if the soil has a pH that doesn’t suit its requirements, it just won’t thrive.   And, as tempting as it might be to stop reading now thinking that this is a boring subject, if you love your garden you owe it to yourself to find out a little bit more so that it will be full of the most glorious flowers, foliage and fruit. 

Many gardeners skip the most important step in gardening which is doing a pH soil test so that they can get to the planting stage.  Knowing the pH is essential to understanding what is happening in the soil to know if anything needs to be added to get the best results.

Soil pH is a measure of the acidity or alkalinity of the soil.  It is affected by the chemistry and biological life in the soil as well as physical factors such as the water in the soil or the aeration levels within the grains.

Every area of the state is different and in fact, two spots in the garden just a few metres apart can have two very different pH levels meaning that plants might thrive in one spot and not survive in the other.

A lot of money is spent on buying plants to grow a beautiful garden but it doesn’t matter how great these plants look in their pots, if they are planted into a soil which has a pH that doesn’t suit them, they will soon struggle and perhaps not even survive.

Some plants can grow in a very wide range of pH levels but there are others that are far too sensitive and will not survive.  Camellias, for example, require an acidic soil and will never thrive in soil that is alkaline.

The reason for this is that the soil is made of many elements such as iron and phosphorous which all plants need but in differing amounts.  Some might need more iron than others; some may require more phosphorous.  But, the nutrients are released at different rates depending on the pH of the soil.  For example, more iron is released in an acidic soil than an alkaline soil so if a plant that requires extra iron is planted into an alkaline soil, it won’t receive enough of this element that is so vital to it thriving.

Put simply, a plant that is grown in soil that does not suit its pH requirements can’t take up the nutrients that it needs.  Copious amounts of fertiliser can be added but it just won’t make any difference as the nutrients will be ‘locked up’ to the plant until pH of the soil is adjusted or it is moved to a more suitable spot.  In essence, the plant will starve.

Whether the plant is a vegetable, flower or shrub, if the leaves are turning yellow or its growth is stunted, there is never any point in applying fertiliser or any treatments until the pH of the soil is tested.  It would be better to pour it down the drain as it’s possible that it’s never going to get taken up by the plant.  Once the pH is known, the problem can be solved and the plant will return to full health.

Taking a pH test is not that tricky and only takes a few minutes.  Testing kits are quite cheap and are always available at Better Pets and Gardens.

The first step is to take the soil sample.  Use a clean spoon to take a few tablespoons of soil from between 8 and 10 centimetres below the surface which is where the roots are.   Put the sample into a clean container or sandwich bag and label where the soil was taken from.  It is best to take samples from several locations around the garden as they can all be different but put each of them in different containers and test them separately.

The kit will come with a plate onto which a small amount of soil is placed.  Add 3 to 5 drops of the pH dye indicator liquid and stir it with the paddle provided.  Dust the soil with the white powder in the kit and wait for one minute.  The powder will change colour which can then be matched to those on the sample card indicating the pH measurement of the soil.

Repeat this same test for each of the soil samples taken but be sure to wash the testing plate and paddle thoroughly between each test as this may affect the results.

The team at Better Pets and Gardens are also able to conduct soil pH tests for you in-store and can then give advice based on what is to be grown in the garden.

The pH scale ranges from 1 to 14 with ‘neutral’ being 7 which is the same as fresh water.  From 7.1 to 14 is classed as ‘alkaline’ with a substance such as baking soda being 8.5.  Anything less than 7 is ‘acid’ with lemon juice being 2.6.

Most plants will grow best in soil that is slightly acidic, neutral or slightly alkaline as this is when most nutrients are at their maximum availability.  However, if the soil is too acidic or too alkaline, there are only two options.  Adjust the soil to make it more suitable to the plants in the garden or put in plants that enjoy those conditions.  The latter is probably much easier in the long run and will give a better result.

What is fascinating is that hydrangeas will grow in both acid and alkaline soils.  In acid soil with a pH of 5 or less, hydrangeas are usually blue.  In neutral soils, hydrangeas turn mauve and in alkaline soils they are pink.







If the soil is acidic and needs to be raised to neutral or slightly alkaline, the most common approach is to apply agricultural lime (not hydrated lime) or dolomite.  How much to add and how often to add it is difficult as there are many factors that influence the results.  Coarse textured soil like sand needs less lime than fine textured soil such as clay and soil with very little organic matter also needs less than soils high in organic matter.

The best approach is to apply the amount of agricultural lime suggested in the pH test kit or on the packet to the top 15cm of the soil and then test the soil in a month to give it time to have an effect.  Further applications may be necessary and continue to test every 6 weeks to ensure that the level is maintained.

Mushroom compost often has lime added to it so this will also help to raise the pH of soil.

If the soil is too alkaline, the easiest solution is to add lots of organic matter to the soil such as compost (not mushroom), green manure and animal manure.  These organic materials produce hydrogen ions as they decompose bringing the pH of the soil down but only use animal manures that have not had lime or dolomite added as some manufacturers do this to reduce their smell.

Again, the amount to be added can be difficult to judge but start with 2 to 3 kg of sheep or cow manure per square metre.  Chicken manure and dynamic lifter is much more acidic than other animal manures so far less is required.  Cover the soil with mulch and let the garden settle for a month and then test to see if further applications are necessary.  Continue to test every six weeks to maintain the pH of levels.

Powdered sulphur, also known as elemental sulphur, will also help to reduce pH.  Apply 40g per square metre for sandy soils and 100g per square metre for clay soils.  It’s important not to confuse this with lime sulphur which is a fungicide and a very different product.