The color of hydrangea flowers can change from pink to purple and blue by tweaking the soil.

How do other plants react to those changes?

Sydney Kozak explored just that. Her project, “Does changing the pH in soil affect a plant’s growth?” won third place in the junior plant science division in the 2021 North Museum Science and Engineering Fair.

Sydney’s parents, Matt and Debbie, like to garden at their East Hempfield Township home so she’s grown up around plants. Years ago, she played under a tent or in a bouncy seat while they built stone walls in the steep back yard. Later, she built stick homes in the woods with neighbors and dug quartz for playtime currency.

Science fair Sydney Kozak

When it was time to pick a science fair project as an eighth grader at St. Leo the Great School, Sydney wanted to focus on something with plants, she says. When she’s not winning awards in the science fair, Sydney, 14, likes drawing, reading fantasy books and caring for her pet rabbit, Sugar.

Sydney looked into pH, an important factor in plant health that allows nutrients to be made available to plants. Soil might have great nutrients, but that’s not enough if the pH isn’t right.

“Usually when we see a micronutrient deficiency in a plant, it is not because there is not enough of the nutrient in the soil,” according to Penn State Extension. “It is because the soil pH has limited the availability of that nutrient.”

So does changing the pH in soil affect a plant's growth? Sydney hypothesized that plants growing in neutral soil would grow the largest. She made that guess after seeing that most plants prefer neutral soil, halfway between acidic soils and alkaline soils. 

Finding an alternative test

Using fast-growing pea plants, Sydney planned to grow plants in acidic soil, neutral soil and alkaline soil. However, her pH meter didn’t pick up subtle changes during the first tests.

Instead, she used red cabbage juice to test the acidity of the soil. She added red cabbage juice to soil samples. Acidic soil turned the juice red. Neutral soil stayed the same color and alkaline soil turned blue.

She used this test for soil, but it’s also a quick way to test the acidity in things like food, making sure it’s safe to eat when pickling or canning produce. Harvard University explains how to do this at home in a video at lanc.news/CabbagepH.

After a month under grow lights in the dining room, Sydney made her final measurements. The plants in acidic soil had the highest average plant growth because all of the plants grew.

The plants in the neutral soil had the tallest plant and grew the fastest. However, one of the plants didn’t grow.

The plants in alkaline soil didn’t grow much. Only one sprouted.

“I learned that pH can have a bigger effect then I would have thought it did,” she says. “And that peas are very delicate.”

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