How to Acidify Potting Soil


Some plants can be very picky about their soil pH levels, requiring it to be within a certain range for them to survive, flower or produce fruit. Soil pH can affect the nutrient availability in plants, but many gardeners may fail to even consider that when troubleshooting plant problems. Fortunately, in cases where the soil pH is unacceptable, there are some simple steps you can take to correct it.

Lowering Soil pH

Step 1

Determine whether your soil pH is truly the problem for the plant. In most cases, if the potting soil was bought at the store, this will not be a problem as most potting soils have a suitable pH for house plants.

Step 2

Look up the proper pH value for your plant. Some plants, especially tropical plants and desert plants, have lower pH requirements. For example, cacti and holly ferns require a value as low as 4.5. Orchids also require a pH value near that range.

Step 3

Discover what the pH value of the soil may be using a soil test kit or by taking a sample to a local extension office. Buying a test kit from a garden center or nursery is a better option because it allows for more convenient follow-up tests.

Step 4

Mix the soil you have with sphagnum peat moss, which is naturally very acidic. Mixing 2 inches into the first 6 inches of soil lowers pH by one unit. It can also be placed on the top of the soil, as the acidity will eventually filter down, but it will take longer to see the effects.

Step 5

Mix vinegar in water if sphagnum peat moss is unavailable or you want a quick fix. Adding a teaspoon of vinegar per gallon of water should drop the water's pH by a full point. Test the water's pH one minute after vinegar is mixed in to determine if this is the case. Water the plant once you've achieved the desired pH.

Step 6

Check the soil pH again after the using one of the methods in the above steps. If the soil is still not at the desired level, repeat the process.

Tips and Warnings

  • Be careful about overdoing the pH as lowering it too much can also lead to problems, including the death of the plant.

Things You'll Need

  • Potting soils
  • Soil test kit
  • Plants
  • Vinegar


  • Learn2grow
  • "The Indoor Gardeners Bible;" George F. Van Patten; 2002
  • University of Illinois Extension

Who Can Help

  • Acidity and Plants
  • Acidity and Fertilizers
  • Plant pH Reference List
Keywords: soil pH, troubleshooting plant species, potting soil mixture, peat moss

About this Author

Ken Black is a freelance writer and a staff writer for The Times Republican in Central Iowa. He has written extensively on a variety of topics, including business, politics, family life and travel.