Identifying the Independent Variable in a Growth Experiment

A team of researchers aims to explore the impact of soil pH levels on the growth of a specific type of flower, known as the Bluebell. They set up an experiment with three groups of Bluebells, each exposed to a different soil pH level: acidic (pH 5), neutral (pH 7), and alkaline (pH 9). The researchers keep all other conditions constant, such as light, water, and temperature.

What is the independent variable in this experiment?