Set up an experiment to see how soil acidity and minerals affect flower color. You can pour minerals on to any of four hydrangea beds. Then press Play to see the plants grow and flower for that season.

Data format

When a season finishes, the sim measures every plant and sends all of the rows to the data table together. There are four beds with eight plants in each, so one season gives you 32 rows. Every row records the season number, which bed the plant grew in, which variety it was, how acidic the soil was, how much of each mineral you added, and how blue the flower turned out.

Blueness is a number from 0 to 100. A fully pink flower is 0 and a deep blue one is 100. The white variety has no blue pigment to change, so it stays near 0 no matter what you add to its bed.

Model details

Hydrangea flowers get their color from a pigment called an anthocyanin. The plant makes this pigment whatever the soil is like. On its own the pigment looks pink. It only looks blue once aluminum from the soil reaches the flower and attaches to it. Nothing switches a gene on or off in this model. Scientists call this phenotypic plasticity, which means the same genes can produce a different appearance in different conditions. It is not epigenetics.

Soil acidity does not affect the pigment itself. What it changes is how much aluminum dissolves in the soil, and only dissolved aluminum can be taken up by the roots. Below about pH 5.5 most of the aluminum is dissolved and available to the plant. Above about pH 7 most of it has turned solid again and the roots cannot reach it. That is why acidity and aluminum are two separate tubs on the bench.

Soil already holds a little aluminum before you add any, so an acidic bed will turn part way blue with nothing added and aluminum sulfate takes it the rest of the way. In a bed between pH 6.5 and 7, adding aluminum sulfate still changes the color a great deal even though the acidity has not moved. Above about pH 7.2 almost none of it dissolves, so no amount of aluminum will turn an alkaline bed blue.

Iron behaves the same way as aluminum. It also stops dissolving in alkaline soil, and a hydrangea that cannot get enough iron turns pale yellow-green between the veins of its leaves. Adding iron to the soil fixes the leaves and does not affect the flowers at all.

A few things are simplified. Real aluminum sulfate makes soil more acidic as well as adding aluminum, but here it only adds aluminum, so that you can change one thing at a time. Real sulfur takes months for soil bacteria to turn into acid, while the probe in this sim responds as soon as you pour. Each plant takes up slightly more or less aluminum than its neighbors, which is why the plants in one bed do not all come out the same color.

The three varieties differ in how much aluminum they move into the flower and in how blue they are able to get. They differ in nothing else you can see. Plain macrophylla hydrangeas cannot be told apart by their leaves, which is why the cuttings on the bench all look the same.

Standards

HS-LS3-3 Apply concepts of statistics and probability to explain the variation and distribution of expressed traits in a population.

Design intent

The four beds give a class a population to study, an environment they control, and one trait measured on every individual. The goal is the claim in HS-LS3-3: how a trait is spread across a population depends both on the genes the organisms carry and on the conditions they grew in.

• Soil acidity is in the sim so that students have to work out which variable is actually causing the flower color to change. Acidity does not affect the pigment. It only controls how much aluminum dissolves. A student who changes acidity and nothing else will find a strong relationship in their data and the wrong explanation for it. Holding acidity steady while adding aluminum, or adding phosphate to lock the aluminum away, is what separates the two.
• The three varieties provide the genetic side of the standard. Given identical soil, one turns strongly blue, one turns only part way blue however much aluminum it gets, and the white one never changes at all.
• Iron is in the sim to give students a reasonable thing to try that turns out not to affect the flowers. It cures the yellow leaves of an alkaline bed and changes nothing about the color.

Discussion prompts

• Two beds have the same soil acidity, but one is blue and the other is pink. What is different about them?
• Plot Blueness against Soil pH, then against Aluminum Sulfate. What has to be true about how you set up your beds before either graph can tell you anything about cause?
• The white variety never changes color. Does that mean the soil has no effect on it?
• All eight plants in a bed grew in the same soil. Why did they come out different colors?
• Predict what a bed at pH 7.5 will do if you give it six scoops of aluminum sulfate, then run it and check.

How do I explore?

1. Drag a scoop out of a tub and drop it on a bed. That adds one scoop of that mineral to the bed.
2. Drag a tray of cuttings on to a bed to replant it with a different variety. That bed also gets fresh soil, so it is how you undo what you poured.
3. Press Play to grow a season. When it finishes, every plant is measured and the rows are added to the data table for you.

What should I do/notice?

• Give each of the four beds a different amount of sulfur or lime, so that they end up at four different pH readings. Which end of the range turns blue?
• Now set two beds to the same pH reading, and give only one of them aluminum sulfate. Which one changes color?
• Watch the soil while a season is growing. In some beds the specks of aluminum travel up the stems. In others they turn into gray crystals and stay in the soil. What did you pour on those beds?
• Add lime to a bed until its pH reaches about 7.5 and look at the leaves. Then add iron to that bed. What changed, and what stayed the same?
• Plant the white variety and add as much of everything as you like. Why do its flowers never change?

What about the data?

Put Blueness on the up-and-down axis and Soil pH along the bottom, then fit a straight line through the points. Look at how steep that line is and how closely the points follow it. Now do the same with Aluminum Sulfate along the bottom instead, and compare the two.

Color the points by Cultivar, which is the plant variety, to see how the three varieties answer the same soil differently. Use Bed and Season to tell your runs apart.