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Dear Spaggs, what is the pH of limestone?   Sunday 07 July, 2013

Specialist Aggregates explore the use of Limestone in ecologically sensitive environments.


Dear Spaggs, what is the pH of limestone?

What seemed to be a simple question became a real poser and even stumped one of the largest supplies of agricultural lime in the country.
Our client wished to use limestone for a footpath in an ecologically sensitive area of Chalk Down in Southern England and wished to know if crushed limestone would alter the pH balance of the local soils.

We discovered that if a pH test is carried out on a piece of pure limestone the value returned is neutral ( ie pH 7), the reason being that limestone is effectively insoluble in pure water. So why do soils in areas of Chalk and Limestone test as alkaline?

To find the answer Specialist Aggregates turned to swimming pool treatment specialist Derek Watts of pHlocrite who pointed out that in natural environments rain water is not pure, it is slightly acidic with dissolved carbon dioxide.
The reaction between acidic rainwater and limestone or chalk is the classic GCSE geography / chemistry explanation of how these rocks are dissolved to form caves and Karst landscapes.
In everyday terms, If you take limestone rock or chalk (calcium carbonate ) plus rainwater water with dissolved carbon dioxide they will react together to produce soluble Calcium Bicarbonate. The added observation as Derek pointed out the dissolved bicarbonate in this situation is slightly alkaline. Hence when soils which have developed in chalk or limestone environments are tested for pH they will give readings of 7.5 to 8 showing them to be alkaline.

The reaction also explains why it is difficult to change the pH of an alkaline soil. If organic rich compost or neutralising chemicals like natural sulphur are added to an alkaline soil they will initially neutralise the “bicarbonate ” but themselves will rapidly become neutralised by the huge reservoir of Calcium Carbonate held as limestone or chalk fragments within the soil.

Finally to answer the question……… Whilst it is not possible to directly measure the pH of solid limestone, typically established uncultivated soils overlying limestone (and chalk) soils will show an alkaline pH of around 7.5 to 8. If limestone is used for footpaths in areas of alkaline soils it will have no effect on pH levels.

Our images show Steph and Charlotte taking in the views from the top of the White Chalk Cliffs of Dover. ( Note the very thin soil covering and poor quality grass - Staff members we just there as a scale!!) together with an image taken of steeply dipping ckalk beds in Lulworth Cove nearly 170 miles to the West.
Our final image shows dove grey limestone being utilised for a new section of dry stone walling, discovered near Brassington in Derbyshire.

Extra Information:

For more information, please visit this related link:  Limestone Rockery.

Dear Spaggs, what is the pH of limestone ?
Dear Spaggs, what is the pH of limestone ?
Dear Spaggs, what is the pH of limestone ?

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