Do gardenia plants like acidic soil?

No, geraniums don't prefer acidic soil. But yes, they prefer slightly acidic soil. As with most soil life and cultivated plants, geraniums favor a soil pH of 6.5. They can tolerate a range of 5.8 to 7.0. But they do their best when the pH only is slightly acidic and is bordering the neutral range of 6.5-7.0.