Growing Media pH
What is pH, and how do I obtain the proper pH for my rhododendrons?
The term "pH" refers to the acidity of a material. Technically, it is a measurement of the hydrogen ion content. The pH scale ranges from 0 to 14 and pHs of 0 to 7 are acidic - pHs of 7 to 14 are referred to as being basic or alkaline. A pH of 7 means the material is neutral.
The experts indicate rhododendrons prefer an acidic medium. The preferred pH should be between 5 and 6.5.
It is almost impossible for a layperson to determine the pH of the potting medium they use. There are pH meters on the market...but in my experience the ones that cost less than $100 are practically worthless. I have yet to try one that is better than plus or minus 1 pH.
But...all is not lost. It is actually fairly easy to get your pH in the desired range. Fir or hemlock bark is almost always in an acceptable range and, therefore, an ideal medium to use. It is best if the bark has decayed or mulched for six months prior to use.
The reason pH is important for plants has to do with the intake of minerals and nutrients. If the pH is too low, yes, soil can be too acidic, the plants have difficulty taking in the nitrogen and phosphorous they need for growth.
The foliage will not be the rich green that you expect. Adding lime to the medium or the soil will raise the pH and help this condition. Too low pH can occur when soil has been fertilized heavily for years. The fertilizer frequently increases the acidity of the soil...that is...it lowers the pH.
If the planting medium of soil is too alkaline, i.e. the pH is too high, it usually causes iron and/or manganese deficiencies. These deficiencies result in chlorosis - a condition where the veins may remain dark green - but the spaces between the veins will be yellow...the leaves are said to be chlorotic. To remedy this condition sulfur is often applied for a quick fix. Good mulching will also help in the long run. A decomposed mix that would not use up the nitrogen in your fertilizer is best.
In summary, pH is important but your plants will tell you if you have a problem. Generally, it is always best to use bark in pots and bark or pine needle mulch as an additive for your soil...and you will rarely have a problem.