Wood ashes increase soil alkalinity

How to change soil pH

Ph is a scale measuring the acidity or alkalinity of certain elements, in this case, your garden soil. The scale ranges from 0 to 14. Measures from 0 to 7 indicate acidity and 7 to 14 alkalinities. The 7 means neutrality.  


Some plants such as rhododendrons or camellias prefer acid soil whereas others prefer alkaline soil such as lilac. But these are examples for significantly lower or higher pH. The truth is, most plants prefer 6 to 7 on pH scale.

Rhododendron prefers acid soil
Rhododendron prefers acid soil

What is the downside of extremely acid or alkaline soil? 

In simple words, the extreme soil conditions suppress the release of nutrition which means that they are not available to plant roots and your plants will suffer without the external nutrition supply. 


How to determine the soil pH? 

The best way to determine the soil pH is use of a pH kit. This should be widely available in any garden centre in manual or a digital form and its use is extremely simple. Simply follow the instructions provided by the manufacturer. 


Another way to determine soil pH is to watch the behaviour of existing plants. As mentioned earlier, rhododendrons or camellias thrive in acid soil. If you have them in your garden and they seem happy, your soil is likely on the acid side. Hydrangeas tend to change the colour of their flowers accordingly to soil pH. The flowers turn blue when soil is acid, and pink when soil is alkaline. Note that white hydrangeas are not affected.

Hydrangea turning pink in alkaline soil
Hydrangea turning pink in alkaline soil

What affects soil pH? 


  • Soil is formed from a rock breakdown over the centuries and the rock types dictate the soil type. Breakdown of acid rock results in acid soil etc. 
  • Areas with the higher amount of rainfall tend to contain a slightly acid soil due to carbon dioxide in the atmosphere being absorbed by rain. 
  • Regular use of plant fertiliser also lowers the soil pH. 

In this article: 


  • How to increase soil pH 
  • How to lower soil pH 
  • How to maintain soil pH 


How to increase soil pH 

Before you decide to increase the soil pH, it’s worth mentioning that most plants prefer slightly acid soil, between 6 and 7 on the pH scale. But if your soil pH falls below this point, here is what you can do. 



Lime, horticultural limestone 

If you are looking to increase the pH of your topsoil, usage of lime, also known as agricultural limestone, is very easy and the product should be available at your local retailer. When opting for the lime we strongly recommend reading the manufacturer’s instructions carefully.


The amount of applied material will vary from site to site but do not overuse. Overdose can cause an extreme alkalinity which in turn will suppress the release of nutrition available. 


After correct application per metre square, work into soil by using a garden fork or a rotavator. Best time to do this is in the autumn or early winter. Winter weathering conditions will contribute to soil breakdown. Alternatively, upon proper cultivation, you can apply lime two to three months prior to planting.

Lilac grows well in alkaline soil
Lilac grows well in alkaline soil

Wood ashes 

Wood ashes work similarly but are free of charge. They contain high amounts of potassium, the element responsible for fruit and flower production, but are less alkaline than garden lime. But that’s not a downside. That means that you can apply them more often when emptying your fireplace. Just make sure to spread them out in a thin layer. Again, overdose is not beneficial. 


Do not use coal ashes from house or barbeque coal, as they can be more acid than alkaline. For healthy plant and root growth, check the pH yearly when alternating it.

Wood ashes increase soil alkalinity
Wood ashes increase soil alkalinity

How to decrease soil pH 

If your topsoil is too alkaline or you wish to grow ericaceous plants such as rhododendrons, the pH should be decreased. You can achieve this in three ways. 


Sulphur, aluminium sulphate 

Sulphur or aluminium sulphate can be found at your local garden centre. The recommended amount will vary and depends on the current pH of your soil. Therefore. we advise to read the manufacturer’s instructions carefully or seek some advice from your local supplier prior to application. 


Apply the chemicals directly onto the topsoil and work thoroughly using a garden fork or cultivator. Ideal time to do this is autumn or early winter and let the soil be exposed to weathering. This will help to break down the soil clumps. Alternatively, you can do this two to three months prior to planting, providing you break the soil clumps down during work. 


Ericaceous compost 

Ericaceous compost can be used when planting the plants individually or it can be forked into the soil in larger quantities. For best results, spread approximately 10cm (4 inches) on the top of your topsoil and fork in well. For nutrition value, apply well-rotted manure. Manure contains a high volume of micro bacteria helping in soil breakdown, gradually releasing more nutrition. 


You can plant your chosen plants straight away after. To be on the safe side, apply more ericaceous compost around the plant root ball together with organic matter such as farmyard manure which is widely available. 



If you have planned for an ericaceous border in your garden, make sure the conifers are included as bones (fundamentals) of your planting scheme. Dropping needles from cedars, pines or spruces greatly contribute to soil acidity. This will also be your long-term solution for alteration of the soil pH, eliminating the extra yearly cost.  


Sometimes I see conifers dropping needles onto the lawns, resulting in poor lawn growth. In this case, apply garden lime in the affected area. 


Note; when using chemicals, remove any debris landed on the leaves of plants or lawn to prevent any damage. Apply to soil only!

Conifers help to maintain soil acidity
Conifers help to maintain soil acidity

Maintain soil pH 

If your topsoil is a good quality and your plants look happy, you don’t need to do anything special to maintain the existing soil pH. Only make sure to look after the topsoil regularly and avoid overuse of plant fertiliser. If you would like to learn more about soil maintenance, click here. 


Most plants prefer slightly acid soil, that’s true. But with a little knowhow, anyone can alternate the soil pH to desired level and grow their favourite plant regardless of current condition. Just remember to check the pH yearly where applicable.

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