Gardening Gardening Basics

Acidic Soil: What It Is and When to Change It

How to Create the Right Soil pH Level for Your Plants

Minuet laurel with reddish-pink flowers.
Minuet mountain laurels like acidic soils. David Beaulieu

Acidic soil is commonly referred to as being "sour" and tends to have mineral deficiencies that can affect plant growth. By definition, acidity is the characteristic of soil that has a pH level of less than 6.5 (a reading of 5.5 means the soil is very acidic). Readings over 7 or 7.5 indicate alkalinity. A soil's pH reading of 7 is considered neutral. Read on to learn more about acidic soil, what it means for your plants, and how you can amend your soil for your plants if necessary.

What Causes Acidic Soil?

As a gardener, you may be surprised by the results of a soil test showing that your garden has become more acidic since the last time you checked. In nature, acidic soil is the norm in many places, including most wooded areas. This is why many woodland plants, such as the native North American ground cover, bunchberry (Cornus canadensis), prefer growing in acidic soil. Three main factors can cause the soil to become acidic over time:

  • Chemical fertilizers: The use of high-nitrogen fertilizers acidifies soil.
  • Heavy rainfall: Water moving rapidly through soil (leaching) due to excessive rain removes beneficial elements, such as calcium and magnesium, and acidifies the soil.
  • Organic matter: As organic matter breaks down (whether in your yard or the forest), the ground in or under which this decomposition is taking place naturally tends to acidify.

There is some disagreement over whether or not the type of mulch that you choose to use in your landscape affects soil pH. 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 Test Your Soil

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. Test kits are available at most home improvement stores and many garden centers. Research the soil pH preference of your plants: It is important to read up on a plant before growing it to see which plants prefer acidic or alkaline soil.

How to Make Soil More or Less Acidic

It's important to check your soil pH regularly because it takes time to change the pH levels of soil. Here's what to do to amend soil:

Raising Soil Acidity

You can raise the acidity of the soil to satisfy plants that crave acidity (often called "acid-loving" plants). In doing so, you lower the soil's pH. The best way to raise acidity is to use commercial fertilizers containing ammonium-N, an ingredient that will be found, for example, in fertilizers sold specifically for azaleas and blueberries.

Lowering Soil Acidity

By contrast, plants that like sweet soil may perform poorly in acidic soils. You can reduce your soil's acidity by raising the soil's pH with the application of garden lime. Farmers have been sweetening acidic soils for years with lime. Lime is the white powder that you may see spread out over a patch of agricultural land.

If a soil test indicates a need to raise soil pH, include an application of garden lime in your tasks for lawn and garden care in the fall. You will begin seeing results over the course of the next growing season.

Plants for Acidic vs. Alkaline Soil

Certain plants thrive in acidic or sour soil, including:

Plants that like to grow in alkaline or sweet soils include:

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  1. Acid Soils. Food and Agriculture Organization of the United Nations.

  2. Causes and Effects of Soil Acidity. Oklahoma State University Extension.