Will Pouring Tomato Juice On Hydrangeas Affect The Bloom Color?
I have some out of date tomato soup that I could use to increase the acid content if that would have any effect.
Certified GKH Gardening Expert
I wouldn't do that, uncomposted. Besides, tomato juice sitting at, only, a pH of 4 will not acidify the soil enough to make a difference. It may also rot, which could be a death sentence to your plant, potentially.
Lemon juice sits at a pH of around 2 or 3, which is much more appropriate to acidify the soil. Just have care to test the pH of your water after adding, so that a gallon remains around 5, or 5.5 in pH. This will ensure that you don't over-do it, or chelate all of the nutrients out of your soil. That would be very counter-productive.
Here are some tips that will help you to achieve bloom color change: