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Optimal pH for Composting with Worms

ph strip aciditySuccessful composting with worms involves keeping the worm bin low-odor and safe for the worms. One crucial factor is worm bin acidity. The worm bin bedding needs a pH level within a specific range. Too acidic, and the worms can get sick or even die. Composting worms thrive in a neutral pH. How can you help maintain a neutral pH in the composting bin? Is it necessary to measure bin acidity? Which food scraps raise bin acidity?

Worms have sensitive skin. They live in the bedding. Therefore, they are sensitive to the bedding and any food scraps you add. And, they have nowhere else to go. Acid can cause burns, making the worms sick. They could even die.

Acidity Versus Alkalinity

“pH” literally means “Power of Hydrogen.” The more hydrogen atoms in a solution, the more alkaline it is. Less hydrogen means the solution is more acidic. The pH scale usually runs from 0 to 14.

  • Stomach acid has a pH of 0.
  • Lemon juice has a pH of 2.
  • Water has a pH of 7, which is neutral.
  • Baking soda has a pH of 9.
  • Lye has a pH of 13.

The ideal pH in a worm bin is 6 to 7. However, the worm bin will naturally lean toward becoming more acidic. The breakdown of organic matter creates ammonia. Ammonia itself is alkaline, but various chemical processes associated with ammonia breakdown make the bedding more acidic. Over-feeding the worms leads to acidic conditions.

Directly adding acidic foods and liquids to the worm bin also increases acidity. Keep reading to find a list of acidic foods to avoid.

How to Measure pH

The average composting worm owner should not worry too much about measuring the pH of the bedding. Check the smell of the bin when you feed the worms. It should have an “earthy” aroma. If the smell is musty, rotten, or mildew, something is wrong. The problem might be pH, or it might be something else. See our worm bin troubleshooting guide.

If you want, purchase a pH meter and test the bedding in several spots. School projects may include pH measurements as part of the record-keeping. Farmers raising large numbers of worms will need to test the pH. Note: Some inexpensive solar-powered pH meters also check moisture – they might be called “soil meters.”

How to Maintain a Healthy pH

There are three easy ways to help keep the pH in your worm bin neutral.

Firstly, put only the right foods in the worm bin. Adding acidic foods lowers the pH, causing problems. Acidic foods include all types of citrus (oranges, lemons, limes), pineapple, and large amounts of tomatoes. See our article “What to Not Feed Worms” for a complete list.

Secondly, do not over-feed the worms. Before adding food, make sure they have started eating the last feeding. Cut the food up small so it breaks down faster. A food processor or chopper helps. If you find large amounts of uneaten scraps, remove them.

Finally, add crushed eggshells once or twice a month. Eggshells contain calcium, which is alkaline. Similar to poultry, worms need grit for digestion. Rinse out eggshells and leave them to dry. Crush or pulverize them to make a powder. Sprinkle about half a cup of eggshell on top of the worm bin. If you want, stir gently near the top of the bedding. Wait a few weeks. When the specks disappear, add more.

If your worm bin is too acidic, add eggshells more often. Make sure you are feeding the right foods, and not too much at once. Also look into the bin’s moisture levels.


Take care of your worms, and they will take care of your kitchen scraps. The worms will also produce free organic fertilizer for your plants and garden. With little effort, you can keep your worm bin a neutral pH. A healthy worm bin is more productive and sweet-smelling.

At Uncle Jim’s Worm Farm, we carry composting worms, mealworms, books about worm compostingcomposting bins designed specifically for vermicomposting, and accessories.


17 comments on “Optimal pH for Composting with Worms

    • Hello Phlica;

      Thank you for the question. Coffee grounds are safe to put in your worm bin in moderation as too many can become acidic and that will burn the worms and could possibly kill them. the amount of coffee grounds in your bin should never be more than 1/8 of the bedding amount. I hope this is helpful.

  • What about tea leaves, are the acidic also? Is it helpful to mix egg shells with coffee grounds before adding to worm bin?

  • I placed my worms in the bottom level of the CAN O’ Worms. It has been a few months, The food is above in the upper bin, With damp peat moss. Why haven’t they moved up?

  • Bought my euros from you and great service. Worms are doing well but I cant get the ph from 8 down to the six or 7 range. I know dolomite lime will get the ph up but how do I get it down?


  • I’m no expert, but based on previous posts it looks like adding coffee grounds, tomatoes, pineapple will each contribute to the acidity of the bed.

  • I bought worm food from Uncle Jim’s Worm Farm when I bought my worms. What is it made of and how much do I put in at a time? Also, do I sprinkle it on top or bury it?

  • pH literally means the negative logarithm of the hydronium ion concentration. The “p” is shorthand notation for the negative logarithm. It has nothing to do with total hydrogen content. Also in the following sentence, even though “hydrogen” is not correct, I think you meant to say the higher the concentration, the lower the pH, not the other way around. Looking forward to receiving my first order of worms from you today, great website overall!

  • I just ordered some worms and they should be coming tomorrow. I started adding bedding and kitchen scraps to my bin about 2 weeks ago and I found maggots in them today. I added a little more eggshells and leaves. Is this environment too acidic for the worms to go in tomorrow? Help!! Thank you!!

  • Water (filtered w/o chlorine) has a p/H of 7 (neutral), check moisture, if your worm bed is on the dry side that’s what is creating the acidic bedding. I used to have this problem and I was keeping the bedding too dry.

  • Use water and lemon juice, mix juice and water testing the pH after each small addition of juice until the solution is at 6 pH. Lightly apply to bedding surfaces, mixing each additional portion of the solution into the bedding and check pH often. Repeat until the desired pH is reached.

  • High concentrations of hydrogen ions yield a low pH (acidic substances), whereas low levels of hydrogen ions result in a high pH (basic substances). … Therefore, the more hydrogen ions present, the lower the pH; conversely, the fewer hydrogen ions, the higher the pH.


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