Like with your soil, compost pH levels are important. pH is the chemical measure of acidity(sourness) or alkalinity(sweetness). A neutral pH level is 7.0. The pH scale ranges from 0 to 14. A value of “0” is highly acidic, while “14” is highly alkaline. Is adjusting compost pH in your future? Test the pH level of the finished product to find out.
Garden soil has a pH, and varies by type of soil and what is in it. Like your garden soil, compost has a pH level, too. This level will also vary, depending upon what you put into your composter. Early in the decomposition process, compost tends to be acidic.
The ideal pH for most plants is at or near the midpoint: 6.5 – 7.0 If compost or soil is too acidic, or too alkaline, plants can not grow.
A wide variety of materials goes into composters. The contents often vary from batch to batch. Most (but not all) plants and weeds have a pH near or slightly below 7. Other compostable items can vary widely. So, you need to have some knowledge of the pH value of items your place in your composter, and ultimately into your garden soil.
Do not make any adjustments until the compost is finished decomposing. After your compost has decomposed, and before you mix it into your garden, we recommend a quick home soil test for ph levels. See Electronic Soil Testers
For more information, see: Soil pH
Before using your fresh compost, you can adjust the pH level. Add a little wood ash or ground limestone to acidic compost. To lower alkaline levels, you can use sphagnum peat, elemental sulfur, aluminum sulfate, or iron sulfate.
Garden Tip: If you are making compost for a specific plant, you may not want to have a neutral pH. First, identify the correct pH level for the specific plant. Then, test the compost. Finally, adjust as needed.