Olivia noticed that the pH of her garden soil decreased after it rained. Did her garden soil become more acidic or less acidic

The garden soil become more acidic.

Explanation:

The pH is defined as -log [H⁺]. An acidic solution contains an important amount of [H⁺]. When the pH increases, H⁺ decreases doing the solution more basic. In the other way, an increasing in H⁺ concentration decreases the pH doing the solution more acidic.

The pH of the garden soil of Olivia decreases after it rained. As pH decreases, [H⁺] increases become the garden soil more acidic.