I am answering questions for Ask A Master Gardener online and I got this one, so verbatim:
“I have a composting pile where I put mostly cottonwood dry and yellow leaves as the brown. Does the tannin present in the green leaves affect the ph of my compost?”
In researching this on the web, I have found that Cottonwood leaves are high in tannins which are acidic, so it will raise the pH (make it more acidic?) of the humus you are creating in the compost. Since our soils in NM tend toward the alkaline, is that problem mitigated? I see it is recommended to break the leaves up, so they decompose faster. I think what she wants to know is how to put her resource of Cottonwood leaves to optimal use. I don’t know what she is using for her greens.
A Master Gardener intern in Santa Fe
I will answer to the best of my ability. To be clear about the pH scale: “raising“ the pH scale would make it more alkaline, not acidic. (See attached image of PH scale.)
The decomposition (composting) is a neutralizing process over time, so adding an organic which is a bit acidic will not inhibit the process. That assumes that one is diluting that which is acidic with other organics which are not, eg. used paper products, vegetable scraps, yard & garden clippings and leaves from a variety of trees. So mixing a variety of ingredients into the leaves with tannins would “dilute” the acidity and composting process itself will neutralize the end product (finished compost) to a pH of about 7. So I would use the cottonwood leaves, shred them if possible, blend them with other organic materials, maintain moisture in the setup at all times, then compost (decomposition) happens.
Hope that this is useful. Get back to me if you need more info.. All the best.
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