Does baking soda in the soil keep hydrangeas blue?

No, adding baking soda to the soil does not guarantee blue hydrangeas.

The color of hydrangeas is determined by the pH level of the soil. Acidic soil (pH below 7) produces blue flowers, while alkaline soil (pH above 7) produces pink flowers.

To encourage blue hydrangeas, you should lower the pH of the soil by adding aluminum sulphate or other acidifying agents.

Baking soda is a base and will raise the pH of the soil, making it more alkaline and less likely to produce blue hydrangeas.