Don't see many posts about these but...

I know that Japanese Iris need an acid soil and one sign of them needing acidity is the leaves yellowing. I have mine planted pondside and amended the clay soil with compost and pine bark for organic material last year. They are blooming (beautifully I might add) this year. What do you use to increase the acidity for these beauties?....coffee grounds? aluminum sulfate? Maybe they just need a little bit of nitrogen? Didn't think to take a picture to post.

Thanks for your input.

Mark