Question
Is there a product that is OMRI registered that will lower the pH of my soil to affect a change in color to my Hydrangeas?

Answer
I was going to reject your question because I do not have a complete and current list of the entire Jan 2008 directory of OMRI registrants.  Without reviewing it carefully (which would also take an awful lot of time) I can't answer you truthfully.

On the other hand, you may not care if there's something on the list.  Because you may simply want to adjust your Soil's pH to make your Hydrangeas Blue or Pink.  You may not know exactly how OMRI makes its list up.  You may simply want a safe and effective system of controlling your Hydrangea color palette.

So I'll tell you what I know.  And you can follow up with another question or maybe clarify it, if you like.  I hope you'll remember that when you submit your rating.  'cause I can just hit 'reject' instead, but I'll try to answer your question instead.

First, OMRI registers commercial products.  If you don't bottle Vinegar and sell it as 'Soil Acidifier' in the garden center, your Vinegar won't have OMRI certification.  My incomplete OMRI list shows no product containing Vinegar for acidifying or amending Soil.  But there are MANY products and substances that are NOT 'Certified' by OMRI that are perfectly acceptable for organic gardening.

Second, OMRI has a philosophy to guide its stamp of approval (or rejection).  That philosophy says that it is VERY important to take good care of your Soil, otherwise nothing you grow in it will be healthy, and nothing you feed that to will be as good for you as it should be.  Traditional gardeners can use several common household products to alter soil pH -- products you could take right out of your kitchen and use in your garden, products you could even EAT or COOK with.

But Organic Gardening is concerned about protecting the SOIL and the life within.  The National Sustainable Agriculture Information Service ('ATTRA') states explains its handbook: 'Failure to support and care for soil biotic life, along with practices that are downright destructive, ultimately leads to its decline.  As a result, plants lose out on the vitamins and other beneficial products these organisms produce, tilth is reduced, and the soil becomes increasingly dependent on synthetic inputs.'  See it here:

http://www.attra.org/attra-pub/PDF/organiccrop.pdf

And ATTRA adds later: 'It is organic farming's approach to soil building and plant fertilization that is the true basis for the belief that organic food and feed has superior nutritional value, much moreso than the absence of pesticide residues, which has drawn the spotlight ever since the 1960s.'  Soil pH is VERY important.  It determines which species of Earthworms -- if any -- is supported in a plot; what Nutrients will and won't be locked up and out of reach; which microbes survive, or don't survive.  Any commercial product intended to alter acidity of the Soil significantly will NOT meet OMRI's strict Organic Standards.

Can you do it?

Well, before you answer that, consider it takes YEARS to provide the constant pH needed for Blue Flowers or Pink Flowers on one of these beautiful hardy shrubs.

Yep, you read that right.  YEARS.

You would not know that from reading the gardening catalogs.  You figure, Oh, I'll just add some vinegar, plant my Hydrangea, and the flowers will turn Blue.

Alas, we know so little about these things.

Enough however to know that you have to get the pH WAY down down down to effect the change you seek.  Blue is so hard to come by in the garden, and these shrubs are so generous with their azure-caerulean blooms.  The whole point of acidifying the Soil they grow in is to make Aluminum available for producing the Blue pigment.  Because Aluminum is soluble ONLY at a pH of 4.0-5.0.

Pink Hydrangeas absorb a LOT LESS Aluminum.  The slightly acidic pH of 6.8 to 7.0 blocks Aluminum absorption.  Hello Pinks.  Goodbye Blues.

Does it matter if OMRI registers a product?  You tell me.  Your followup(s) welcome.  Thanks for writing.