Garden Gnome:Soil testing reveals interesting facts

So if you want to keep hydrangeas blue, and I do, add aluminum sulfate – carefully – because sulfate can burn plants.

Not to talk bad about rain, but the blooms on our newest mophead hydrangeas are on the ground this week.


It doesn’t matter, though. They still have these beautiful blue flower balls all over. I got an e-mail following last week’s column asking about aluminum and hydrangeas.

It’s a great question, and a chance to discuss the importance of soil testing.

Some folks say that the color of hydrangeas depends on the pH level of the soil, and that’s sort of correct. Hydrangeas growing in our mostly acidic soil are blue. The hydrangea with pink blooms grow in less acidic to alkaline soil. While the color of your hydrangeas might give you a rough idea of the pH level, it’s only a general one.

To turn pink to blue, the pH level needs to be 5.1 to 5.5. While neutral is a pH of 7 and above that is alkaline, the hydrangea flowers will start turning to pink at a pH of 6 to 6.2. The white flowering hydrangeas stay white regardless of pH.

Which brings me back to my hydrangeas. Last year, my three oldest bushes went from blue to pink flowers. Earlier this year, I followed Ted Stephens’ advice, because he is a master of all things green – and if you have doubts go to Nurseries Caroliniana. Stephens said to add aluminum.

See, it is not the pH, it’s the plant’s ability to take in aluminum that turns hydrangea flowers blue. The plant can take in more aluminum at the lower pH level.

That’s why pH is so important and why you should have soil tests done every few years. The plants’ ability to ingest nutrients is directly tied to the pH level.

So if you want to keep hydrangeas blue, and I do, add aluminum sulfate – carefully – because sulfate can burn plants.

A recommendation for 1 tablespoon of aluminum sulfate in a gallon of water sounds reasonable. Water the plant first.

Your county extension office provides soil testing at a reasonable price. In return, you get a report on the pH level and the nutrient levels as well as recommendations for improving your soil.


To learn about hydrangeas from true masters, the public is welcome to attend the 2012 Hydrangea Conference on June 7 at the Savannah Rapids Pavilion. Vince Dooley, who is quite an accomplished gardener and coached a little football, I hear, is the featured speaker. To register, to go