Soil pH
Soil acidity
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Identification
There is a lot of nonsense written about soil acidity (pH) for cacti on the internet with many sites - even governmental ones! recommending alkaline soils and adding lime for cacti.
(If you think you need to add lime, add it as Gypsum, you won't affect the ph)
Cacti grow best undoutedly in an acid soil, with a pH value of between 5 and 6.5.
If your plants are just not growing after say 12 months in a compost, it may be that you have water in your area which has a very high pH (Alkaline) and this water raises the alkalinity of your soil. This prevents cacti from absorbing the nutrients which they need to be healthy.
A neutral pH is 7.0 ie. neither acid nor alkaline. The pH in my tap water in Surrey I measured at 8.5 which is highly alkaline and may explain why my plants are not growing as well as they did when I lived in Devon (Where the water had a pH of 7.5)
Prevention
Water with rainwater whenever possible, rainwater is acid and will keep your soil pH just right.
Cure
If you are at all serious about growing your cacti well, I would suggest testing the pH of your tap water. The most accurate way if you don't want to spend too much is to visit your local garden centre or aquarium shop and buy a pH testing kit.
If your pH is found to be above 7.00, I would advise reducing it by adding acid to the water before you give it to your cacti. This can be done by just adding vinegar to the water.
As a very rough guide, if you find your pH to be 8.00, you will need to add four teaspoons (x5ml) of vinegar to each 5 gallon (25 litre) bucket of water to bring the pH down to 5.00.
You will need to experiment with your own tap water, gradually adding vinegar until you reach pH 5.00. Make a note of the result - it will be different for everyone and can vary greatly even from one side of town to another.
On a larger scale, economically , you can use citric acid (From a chemist or grocery store) or sulphuric acid (This is the cheapest - battery acid - available from your local garage). However, if you use sulphuric acid, you must water your plants from below or a dusting of chalk (gypsum) will form on your plants and spoil their appearance.
Try it! You will notice a phenominal difference in growth and appearance of both adult cacti and seedlings!
An excellent, (but rather technical in parts!) pdf article on pH for cacti can be found here