I don't have an answer for your question but I have a question for your question;

I'm curious about the focus on pH. I have a hydrangea on my property in Idaho where the soil pH is above 8. It's the buffering provided by humic and fulvic acids that is allowing that plant to absorb iron and create a beautiful blue blossom despite the high pH.

There are a number of factors that affect the availability of Iron in soils as well as a plant ability to utilize that iron. The soils pH is one, amount of Phosphorus could be another, soil temperature and moisture could be more, and the amount of organic matter in the soil is another.

Here is a link that might be useful: Iron availability in soil

Thanks, kimmsr. I actually did find that article already. It was a good primer. I'm now looking for some sources that go into a little more depth

idaho_gardner, I'm not sure I understand your question. Do you mean that pH isn't an important variable for determining iron availability?

Ferro - yes, pH matters. If the pH were too high, no amount of buffering would make metals available to plants. But the presence of organic matter in most soils will serve as a buffer that allows plants to absorb iron, chromium, selenium, magnesium, and all the other metals they require.

The alkaline clay soil in the Boise, Idaho area can be amended with humic shale ore and compost and it turns the sticky clay into something like the chocolate cake sometimes described in gardening books; crumbly, brown loamy. The organic material breaks up the clay and makes the metals available, yet the pH of the soil is not changed much.

If you're in a hurry and can find the humic shale ore-based amendment, it's a very good way to go and it's not expensive. Does not burn plants, does not stain, and can be applied any time.

Paul

It's aluminum that turns hydrangeas blue, not iron.

rhizo that's news to me. I don't know here I got the idea that iron is what makes the blooms blue. But the point remains; buffering is what allows the metals to be available.

A soils pH is what determines the blossom color of Hydrangeas, and Aluminum is often used to lower a soils pH. One problem with that is Aluminum can reach toxic levels in soils fairly quickly.

Here is a link that might be useful: About Hydrangea blossom color

Aluminum sulfate is just a convenient and water soluble salt that is often used to acidify the soil. I too thought it was the pH not the aluminum that does it.

It's NOT soil pH that determines bloom color or intensity, it's cellular pH.

Three different pigments - chlorophyll, flavonoids, and carotenoids - mixed in different proportions, give color to plants. By mixing and matching the three pigments, an endless variety of colors can be created. E.g. most reds are the result of mixing orange carotenoids with magenta flavonoids. Cellular pH (not to be confused with soil pH) has a profound effect on plant color.

Even most experienced gardeners think that lowering soil pH produces blue blooms in hydrangea. Technically, it is only a part of the equation. Lowering soil pH makes aluminum ions more readily available for plant uptake. The aluminum is then available to bond with pigment compounds (anthocyanin, the colored component of flavonoids) changing the way color is reflected. Our perception is blooms changing from pink to blue.

The anthocyanidin group is what makes apples, autumn leaves, roses, strawberries, and cranberries red. They make blueberries, cornflowers, and violets blue. They also make some grapes, blackberries, and red cabbage purple.

One of the things that changes the color of anthocyanins is the level of acid or alkali (the pH) in the cell surrounding the pigment. As cellular pH increases, the pigment changes structure and reflects different wavelengths of light. The anthocyanin reflects bright pink in acid cell environments, reddish-purple in neutral and green in more alkaline cells.

Al

Well, I'll be hornswoggled. I have not seen the biochemical explanation in such detail before. Thanks for posting! So it is indeed the aluminum. Color me reddish!

Soil Fertility and Fertilizers, Tisdale et al