best way to augment the soil?

best way to augment the soil?

I moved into my house a year ago and I inherited garden beds that were more or less left unattended for 2-3 years. I spent a lot of this year just seeing what I had, weeding, mulching, and watching where the sun fell. I have 3 rhododendrons on the side of my house, and all of them show signs of chlorosis. The one closest to my back yard is the most healthy (barely has any signs) and the one closest to the front is the least healthy. The one in the middle is ok-ish. I have a rather large oak in the back, so I suspect that the acidity of the soil close to the back yard is higher than the front... but obviously it's still not enough. Can start to doing something in the fall and winter to correct this problem? And what would be best to use to lower the pH of the soil? I try to stay more organic than not, but I'm up for non-organic products if they're the best bet. The azalea just around the corner from the Rhodis looks fine, btw. It's completely in the back yard.
In Search of Spring!

In Search of Spring!

microtoms

microtoms

As of June 27th

As of June 27th

Chinese Lanterns

Chinese Lanterns

Plant ID!

Plant ID!

Thought I’d share some color

Thought I’d share some color