What Is Acidic Soil and How Can You Change It?

Acidity Good for Some Plants, Bad for Others

Minuet laurel (image) has reddish-pink flowers. The blooms are impressive.
Mountain laurels, such as the Minuet cultivar, like acidic soils. David Beaulieu

Acidic soils are commonly referred to as being "sour." By definition, acidity is the characteristic of soils that have a pH level of less than 7 (a reading of 7 being "neutral"). The pH scale runs from 0 to 14, so 7 falls in the middle of the scale. Readings over 7 indicate alkalinity.

What Causes the Ground to Become Acidic?

As a gardener, you may be surprised at the results of a soil test showing that the ground in which you garden has become more acidic since the last time you checked. But Mother Nature would be surprised at your surprise. In nature, acidic soil is the norm in many places (including in most wooded areas). As organic matter breaks down, the ground in or under which this decomposition is taking place naturally tends to acidify. But other factors that can cause soil to become acidic include:

  1. The use of fertilizers.
  2. The leaching out of elements such as calcium and magnesium when it rains.

There is some disagreement now over whether or not the type of mulch that you choose to use in your landscape affects soil pH, even though, for years, gardeners have assumed that applying a mulch of pine needles or oak leaves will acidify the ground under it more so than many other kinds of mulch.

How to Change Soil pH (and How to Determine if You Should)

"Sour" is a pejorative term when applied to milk, but it is not necessarily pejorative when applied to earth. Do not automatically assume that it is bad to have a sour soil. It really depends on which plants, specifically, will be growing in the ground in question. So your success in getting all of this right depends on two things:

  1. Check your soil's pH level every few years: You can either send a soil sample to your local extension office to have them perform the test for you or conduct a DIY test.
  2. Research the pH preference of your plants: This is why it is so important to read up on a plant before you decide to grow it. Find out which plants like acidic soil and which do not.

A low pH suits some plants just fine. That is, certain plants thrive in acidic soil (as opposed to alkaline or "sweet" soil), including:

  1. Mountain laurels (Kalmia latifolia), such as the delightful 'Minuet' laurel shrub shown in the picture.
  2. Azaleas and rhododendrons.
  3. Hydrangea bushes.

To satisfy plants that crave acidity (often called "acid-loving" plants), acidity can be raised (meaning soil pH will be lowered) by using commercial fertilizers containing ammonium-N, an ingredient that will be found, for example, in fertilizers sold specifically for azaleas and blueberries.

By contrast, plants that like sweet soil may perform poorly in acidic soils. Soil pH can usually be raised by applying garden lime. Farmers have been sweetening acidic soils for years with lime. Plants to grow in alkaline soils include: