Fall Science Part 3 - Anthocyanins, Soil pH, and Autumn Colors

Remember our mystery tree from last week (fall science part 2)?

Well, in all our reading, we discovered there's another factor, that might help us predict what color the tree's leaves will change to this fall - the pH level.

According to Dr. Joseph Zeleznik, of North Dakota State University, the pH of the cell sap of the leaves, is one of the factors affecting the exact color produced by anthocyanins. An acid pH tends to bring about a more reddish color, while an alkaline pH, leads to darker purples.

Knowing that our tree leaves have a heavy dose of carotenoids (the yellow pigment), and the presence of what we believe are anthocyanins - at least, according to our leaf chromatography experiment, we decided to see if we could determine the pH balance of the leaves.

We started with the soil near the tree. I'd read somewhere that you could test soil pH, really basically, with a vinegar, and baking soda test, and that sounded interesting.

The children brought in two soil samples, taken from the same spot, near the tree.

We poured a half cup of vinegar into one sample...

...and it fizzed!

We thought, that probably meant we had an alkaline soil. But, just for kicks, we decided to pour some water, mixed with baking soda, into our second soil sample...

...it fizzed, too. So, bummer - our results were less than conclusive.

But, we still thought it was pretty neat, that baking soda, and/or vinegar, can react with dirt. After shaking the jars, we thought we were getting more of a reaction with the vinegar, but just to be sure, we decided to try testing the actual leaves.

So, we tore up a couple of the leaves, and let them soak in water, until we could see that some of the pigments were mixing with the water.

Then, we dipped in some old litmus paper, we had left over from a science kit. We tested the papers first, with baking soda water, and vinegar, to make sure they worked. With baking soda water, we saw a very dark purple. And, with our leaf water, we saw a very light (but clear) purple.

So, between our soil test, and our leaf test, we're thinking the tree has a slightly alkaline pH in the sap of the leaves. Meaning our anthocyanins, should lean towards a darker red, to purple. Mix that with the yellow of the carotenoids, and you have what? Maybe, a golden orange?

Of course, there is still the matter of the weather. A lot of rain, will leach away the anthocyanins. An early frost could kill the leaves before all the chlorophyll has even stopped being produced, causing the leaves to go from green, straight to brown. And, we haven't even stopped to consider tannins, a bitter, bug repelling, sun screening, substance in leaves, that also has a brown color.

While we're waiting on pins, and pine needles for the leaves to change, we just might have to go ahead an identify the tree, so we can find out what its normal (if there is such a thing) fall colors are.

Click here to see the mystery revealed.

It's great to be a homeschooler.