How can I make my compost less acidic?

QUESTION: How can I make my compost less acidic?

ANSWER: Compost tends to have a pH level in the neutral to acidic range while the microbes in the compost are still working to decompose the materials. (The average pH range of compost in which microbes are still breaking ingredients down is between 5.0 and 8,) It is healthy and beneficial for the compost to be a bit acidic while the microbes are working. The pH level is considered acidic when it is less than seven. 

The acids in the compost will be neutralized during the decomposition process, and when the compost is complete, the pH range tends to be near the neutral zone, or between 6 and 8. So if you are concerned about the pH level of compost that is still decomposing, the best thing you can do is wait until the microbes have finished working, because the pH level of your compost may not need to be corrected at all. 

There are a few different reasons that compost can be too acidic once it has finished the decomposition process. If you’ve added more wet “green” materials that are rich in nitrogen without balancing them adequately with the drier “brown” ingredients that are full of carbon, the compost can increase in acidity and may also develop an unpleasant smell and break down more slowly than normal.

Acidity can also develop in compost if the materials used in the compost are themselves high in acid, such as if you’ve added lots of citrus fruits. You can correct both these issues by mixing in plenty of dry “brown” materials that are rich in carbon to balance out the acidity, especially if the compost is also too moist. Add a small amount of the wetter “green” ingredients when you balance things out so the compost will be triggered to start the process of decomposition again.

You should also be aware that the pH level of your compost is likely to vary depending on where in the pile the reading is taken from, just as the temperature does. To get an accurate pH test from your compost, take readings from samples in several areas and then either mix the samples together and test the combined sample or test each sample individually to get an idea of the range of pH levels that are present in your compost.

Although it may seem like a good idea to add the pH levels from each sample together and divide by the number of samples to determine the average, the average will not be accurate because pH levels are expressed as a logarithm.

If your compost becomes anaerobic, which means the compost  has no oxygen in it, the organic acids that naturally occur in compost will pile up instead of being broken down by the microbes. (This is because the microbes are unable to do much decomposition at all when the conditions in the compost pile are anaerobic. They need oxygen to survive and to perform their role of breaking the materials down.)

A compost pile that has become anaerobic may be wet, smelly, and black in color instead of rich brown. To resolve acidity in an anaerobic compost pile, simply 

turn or mix the compost to aerate it. For best results, you should be turning your compost every few weeks. Turning the compost on this schedule keeps oxygen available for the microbes and ensures your compost will be completed as efficiently as possible.

Experts warn against adding lime (also called calcium carbonate) to your compost in an attempt to correct an acidic pH balance and recommend using other solutions instead. When you use lime in your compost, ammonium nitrogen evaporates into the air as ammonia gas and is lost. This evaporation of ammonia when lime is used causes compost piles to stink and also decreases the level of nitrogen the compost will contain—and one of the reasons you’re making compost in the first place is to provide your plants with nitrogen.

However, other sources recommend adding ground limestone to your compost as a way of balancing out acidity despite this drawback. If you need to add something to your compost to reduce the acidity, we recommend using wood ash instead of lime. Incorporate the wood ash by adding several layers of ash interspersed with your compost every 18 inches.

There are a few different methods you can use to test the pH level of your compost. Whichever method you choose, make sure to measure the pH immediately after sampling so your reading isn’t affected by chemical changes that may take place once the sample is separated from the pile. 

  • You can use any type of soil test kit on your compost without changing the way the kits are used. Just follow the instructions that are provided for testing soil to get a pH reading from your compost. 
  • You can also opt for a pH indicator strip (also called pH paper) if your compost is moist but not sludgy or oversaturated. Simply push the pH strip into the compost pile, let it remain there a while to absorb moisture, then use the color comparison reading to measure the pH level.
  • You can use pH paper or a calibrated meter to measure your compost’s pH by taking a reading from compost extract. Create the extract as follows: Spread compost in an oven-safe pan in a thin layer, then place the pan in an oven at 105 to 110 degrees Fahrenheit for 24 hours. Once the compost has dried in the oven for 24 hours, weigh out samples of five grams each into individual small containers. Add 25 ml of distilled water to each sample. Stir each sample vigorously for five seconds, then allow them to rest for 10 minutes before taking your pH reading with the pH paper or calibrated meter. The pH level of a sample tested in this way should be expressed as compost pH in water (pHw).