Are you on Acid

August 19, 2008


Acid loving plants could just as easily and probably more accurately be called lime hating plants. It is the presence of lime or calcium in the soil that causes all the problems.

There is quite a range of popular plants that need acidic soil conditions to thrive. These include plants such as Rhododendron, Pieris, Erica, Pernettya, Kalmia  and Pachysandra. They are often termed Ericaceous plants suggesting they all belong to the family Ericaceae (but there are many others).

Compost for lime hating plants is often called Ericaceous compost.

Plants whose natural habitat is alkaline often grow perfectly well in acid conditions but the converse is not often true.

Soil is classified as being alkaline or acidic by its pH value, for example pH 8. (This term comes from the French word ‘puissance’, meaning power or strength and gives us the ‘p’. The H is for hydrogen. And the number refers to the concentration of Hydrogen ions in the soil.) Below pH 7 is considered acidic and above is considered alkaline. Your soil pH can be tested with a kit using solutions mixed in small tubes where it is judged by the colour it turns or, more conveniently, there are inexpensive electronic probes available.

These you simply push into the soil and take a reading.

A much cruder test but ideal for a quick assessment is to see what is growing either in your own or neighbouring gardens. If there are any of the plants mentioned above then the soil is going to be acid.

A word of warning here. Soil conditions can change over a surprisingly short distance so this method is only a guide.

Symptoms of lime intolerance are often called chlorosis.

Plants need to get energy from the sun. To do this they produce a green pigment called Chlorophyll. This is what makes leaves green. To produce this pigment the plant constructs the chlorophyll molecule based on iron before replacing the iron with nitrogen. So iron and nitrogen are important. Undernourished plants often have pale green or yellow leaves because they do not have enough nitrogen. Give them a feed that contains nitrogen and they will soon green up and look healthy once again-easily cured.

However, too much lime in the soil prevents acid loving plants taking up iron. Without iron there is no green chlorophyll production and the plant looks pale, typically showing yellowing between the leaf veins. No matter how much nitrogen you feed to plants under these conditions they will not green up. 

The Cure.

The obvious way to avoid problems of chlorosis is not to grow acid loving plants on alkaline soil. Knowing this, in regions of alkaline soil, many of us choose to grow our Rhodo’s or Pieris in a pot. This is fine as long as you remember to use ericaceous compost and to be aware that if you are continually using hard or limey tap water to irrigate your plants you will gradually build up the levels of calcium in the compost so raising the pH and making life uncomfortable for your plant. It will eventually show signs of chlorosis. Watering with soft or rain water ( I use water from my dehumidifier for my one and only lime hating plant.) prevents this problem.

In the open garden the soil can be acidified by adding sulphur which in combination with water becomes sulphuric acid. Because there is usually such a great reserve of calcium in alkaline soils this is a fairly temporary measure and if you choose this method you will have to repeat the dose  at regular intervals depending on the level of alkalinity.

An alternative is to use chelated iron compounds of which Sequestrene is a well known brand. This provides the iron in a form that can be taken up by the plant. Again this will need to be applied at regular intervals.


Confused by the terminology?  Alkaline, limey, calcium rich and high pH all mean the same thing and that is that you have lime in your soil and you cannot grow rhododendrons.

High pH can have its advantages. If you are a veg grower then a high pH controls the incidence of club root on Brassicas.

If you live in an area with alkaline soil but are desperate to grow acid loving plants then the best long term solution is to create raised beds that can be filled with an acidic peat enriched soil.

Hydrangeas will grow quite happily in alkaline soils but you will not get blue flowers. In limey soils the aluminium that the Hydrangea needs to produce blue flowers is locked up so the flowers turn out pink. Hydrangea colorants contain aluminium that is available to the plant regardless of soil conditions so turning the flowers blue.

Colorants are available from garden centres.

This articles is just one of the articles found on the interactive plant finder, planting advice and pruning guide CD-ROM The CD-ROM contains 3,500 UK garden plants with 9,000 photographs plus illustrated pruning tips