Knowledgebase

Compost is too basic #744615

Asked April 16, 2021, 9:15 AM EDT

Hi there, I have a question about compost. We have a very large compost pile that has a pH of 8.4- when tested by MSU. We are very concerned about how to bring down the pH in a safe and sustainable way. Do you have any suggestions? Do people just pour some acid in and try again? Is there a way to calculate how much acid/which acid to add?

Genesee County Michigan

Expert Response

Hello and thank you for using MSU Ask Extension with your composting question.  The pH in compost swings widely while it is still in its unfinished and still in a composting phase.  When finished you have to let the compost sit and "cure" for several weeks.  this should stabilize the pH.  Compost should only be tested in this phase to get a more accurate pH reading.  

Another thing you can do is to take another look at your starting materials and carbon source stocks.  If these are the usual grass and leaves or yard waste and kitchen waste then it is just a matter of being patient until the organisms are finished doing their work. 

Hi Beth, 
Thanks for you quick response.  We have a compost pile that has been sitting since November. When tested in March, it had a pH of 8.4-8.5. (I have a chemistry minor from MSU so I do understand this!) But my confusion is why it hasn’t stabilized in all this time.  I thought it would sort of buffer itself?
Should we have mixed it into the soil and let THAT cure? Have we messed this up?  Is it recoverable?

We also have a low level of nitrogen in it. Is there a way to correct both at once or prevent it in the future?

Thanks so much for your time— I feel like your response has given me a thousand more questions!
Krystal

On Apr 16, 2021, at 10:23 AM, Ask Extension <<personal data hidden>> wrote:


The Question Asker Replied April 16, 2021, 12:27 PM EDT

Hi, 

It should have stabilized if your mix is right. But it is not uncommon for Compost to be a higher pH than expected.  That is the reason we don't recommend using compost mulch around acid loving plants.  Try turning your compost pile well and watering it.  It is entirely possible that the decomposition process stopped over the winter and re-started when it warmed again in March. 

The N-P-K of compost is usually low depending upon the inputs you used.  You can add some nitrogen fertilizer to the finished compost to your compost to increase this nutrient and  help to lower your pH.  https://vegcropshotline.org/article/effects-of-nitrogen-fertilizers-on-soil-ph/. Some people add this nutrient at the start to help "kick start" the pile. Personally, I am a lazy gardener and never add anything or test my compost.  I just use it knowing that it is the organism contribution that I want most of all. -- But that is me.  

You can retest your compost pH easily enough. https://www.thespruce.com/do-it-yourself-soil-ph-test-4125833. The weeds present can also be a clue to soil conditions https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-types-and-weeds.htm

I would discourage adding vinegar or other acid to quickly lower the pH it as this would also kill the organisms that the compost provides in keeping our soil healthy.  

Beth,
Again, thanks for your timely response. It is very informative and helpful. I appreciate the links as well!
Krystal 

On Apr 16, 2021, at 1:06 PM, Ask Extension <<personal data hidden>> wrote:


The Question Asker Replied April 16, 2021, 1:35 PM EDT

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