You must water Rhododendrons, Azaleas and Camellias with rainwater.
I’ve heard this many times in hard water areas, based no doubt on the fact hard water contains lime and lime is used to reduce the acidity of soils and composts. While it is true Rhododendrons and other ericaceous need acidic growing conditions the amount of lime in the hardest drinking water, such as my local one, is not going to be sufficient to effect the plant you are growing. Nurseries and garden centres large and small happily grow rhododendrons etc. and I have never come across one that didn’t use the ordinary tap water regardless of how hard or soft it is.
This is a water based solution which has a Ph above 7. Most alkaline soils (also called basic soils) lie in the range 7 to 9.
This is a water based solution which has a pH below 7. Most acid soil lie in the range of 7 to 5 though some peat soils may be lower.
In simple terms this is how acid or alkaline something is – only a water based solution can have a pH. A pH of 7 is neutral, below 7 is acidic and above 7 is alkaline. The only way to know the pH of a soil is to measure it, either by adding it to a solution which changes colour according to the pH or using a pH meter. Both have there draw backs, with the solution it can be hard to check the colour as the soil discolours the liquid while on the other hand pH meters; if they are to be reliable they are expensive and need constant recalibration with a buffer solution. I’m very dubious about how reliable the ph meters for the domestic market are and I would say for your own garden the kits of indicator solutions are probably better.
Don’t get too hung up on the absolute accuracy of individual test as the pH of soil is vary variable and you may well find slightly different readings in different parts of your garden The advent of pH metes has lead to people publishing the recommended for plants down to a tenth. I don’t really see this has any practical value.
When plants are grown in a soil which is to alkaline for them they suffer from iron deficiency as a result of the effect the pH has on the nutrients which are available to plants growing in it. Sequestered iron is iron in a form which is not affected by the pH and so it remains available to the plants. It is really only a short term measure so it has to be regularly reapplied to the plants.
The pH confusion
People get terribly worried about lime near Rhododendrons, to the extent that some people insist that you should never water ericaceous plants with hard water. The problem is the whole soil pH thing courses all sorts of confusion. So let’s ignore all that’s been said before and start from the very beginning. First off soil contains water but of course its not pure water it contains allsorts of dissolved minerals the plants need to grow. These are called plant nutrients and their presence is not enough; they must be in a form which makes them available for the plants to take up. The soil water will also have a pH; all water based solutions have one. How it is calculated isn’t important for our purposes but it effect is.
The sugar analogy
For a minute let us imagine we are dissolving sugar in water, we can carry on adding more and more sugar until no mater how long we stir the water no more sugar will dissolve. Now imagine we start to heat the sugar and water gently on a stove, the sugar will all now dissolve and we can dissolve still more. Now imagine the temperature of the water is the pH and the sugar is one of the plant nutrients. As the pH is altered the amount of the nutrient that is available in the soil water also changes. To further complicate things, as if it was needed, the availability of different nutrients changes differently as the pH changes.
The result of all this is that in alkaline conditions there is a lot less Iron, Manganese, Zinc, Copper and cobalt available for plants to take up than there is in acidic soil conditions. Just adding these nutrients to the soil is not going to help as the same process that had made them unavailable in the first place is going to act on the additional nutrients making them unavailable; if a pint pot is full pouring more water in it still leaves a pint of water.
Manipulating the pH
This leaves two options: change the pH or provide the missing nutrients some other way. Changing the pH of a soil is not that easy as the complex system that makes up soils tends to resist the change moving the pH back to the original level when to try to change it. Moving a soil towards an alkaline pH is the easier than towards an acid pH one. One is to add an acidic compost to the soil but the only readily available one is peat and that is becoming increasingly problematic because of the environmental concerns surrounding it. A second means of making the soil more acidic is to add a chemical to it. Three chemicals are generally recommended: sulphur, aluminium sulphate and ferrous sulphate. The other option is to provide an alternative source of the missing nutrients for the plants and this can be achieved by using sequestered iron which provides the missing iron in a form that remains available even if the pH would make it unavailable.
After all of this you may have noticed lime has not been mentioned since the first sentence. Lime is in practice mainly calcium carbonate with some other chemicals mixed in according to the source. The calcium is a plant nutrient but its carbonate has the effect of make the soil water more alkaline and it is this change to the soil water, not the lime, which has the effect on the ericaceous plants.
This is a catch all term for plants which need acidic soil to grow in.