6 Methods for Increasing Soil Acidity

Diagram showing five ways that you can add acidity to your soil

The Spruce / Xiaojie Liu

Soils vary in their mineral content according to local geology, and these variations can in turn affect the relative acidity or alkalinity of the soil—a level that is measured by a pH scale. The pH scale rating scale runs from zero to 14, with a pH of seven representing a neutral level. Lower numbers are acidic, higher numbers are alkaline. Maintaining the proper pH level is essential for a plant's ability to absorb the nutrients it needs from the soil—iron absorption is especially affected by soil pH levels.

Some Plants Need More Acidity

The ideal soil pH for most landscape plants and turf grasses is around 6.5, which is considered slightly acidic. But there are some plants that require a more acidic soil in order to thrive. Here are some of the plants that need acidic soil:

Shrubs:

Trees:

Flowers:

Vegetables:

Fruits:

Plants that require a more acidic soil than they are growing in will often signal this by developing iron chlorosis—a deficiency that causes the leaf veins or entire leaves to turn yellow. If this happens, your plants may require that you add a soil amendment to the soil they are growing in, or that you supplement their need for acid with a fertilizer designed to provide it. A variety of soil amendments and fertilizers are available to provide this assistance to acid-loving plants. Most are readily available at garden centers and home improvement stores.

Before applying any soil amendment, have your soil tested to determine the types and amounts of soil amendments required.

Add Sulfur

Sulfur will take some time to lower the soil pH, so it should be added the year before you want to plant. In many ways, though, it is the best option. It lasts for years in the soil and does a better job of acidifying than most other amendments. It's best to apply sulfur in the summer or fall before the following spring planting season, digging it deep into the soil. It does not work very well to try and dig in sulfur around existing plants. As with any amendment, you need to have a soil test conducted in order to determine how much sulfur to apply in order to reach the desired pH.

Sulfur on hand-held shovel added to soil for more acidity

The Spruce / Jayme Burrows

Add Iron Sulfate

Iron sulfate lowers pH but requires a much larger volume of product to produce the same results as sulfur. It is often used to treat specific symptoms of iron deficiency. Iron sulfate will provide faster results than sulfur (in three or four weeks) but can damage plants if over-used. It can be dug into the soil as a powder or applied in solution and watered over leaves for absorption.

Add Sphagnum Peat Moss

When used in large amounts as a soil amendment, sphagnum peat moss will slightly acidify the soil while also adding organic material. When preparing your soil for planting, place four to six inches of acidic peat moss on your topsoil and till it to a depth of six inches. This will acidify the soil for about two years.

Sphagnum peat moss held in hand closeup

The Spruce / Jayme Burrows

Use Acidic Fertilizer

If your acid-loving plants are isolated among other non-acid plants, it may not be practical to amend the soil, since the increased acidity might then affect other plants. Here, the best option is to fertilize with one of the many water-soluble products available, such as Miracid. Begin with mild solutions until you understand the impact on your plants.

Pink flowers in green container sprayed with acid fertilizer

The Spruce / Jayme Burrows

Add Aluminum Sulfate

Powdered aluminum sulfate has been a standard soil additive for gardeners growing blueberries and many other plants since it is quick-acting and convenient to dig in around individual plants. However, there are recent concerns about the possibility of aluminum toxicity, which can be especially damaging to children. Aluminum can be absorbed from drinking water, and excessive use of aluminum sulfate as a soil amendment can contribute to the contamination of groundwater supplies. Many experts now recommend that aluminum sulfate is used only on hydrangeas, where the aluminum helps create the vivid blue flowers that are prized. For other plants, safer options are available, such as ammonium sulfate.​

Warning

In high quantities or in its pure form, aluminum sulfate is considered a "hazardous substance." Use this product with an abundance of caution. Chemically, when combined with water, aluminum sulfate becomes corrosive sulfuric acid.

Add Ammonium Sulfate

This is a good alternative to aluminum sulfate. It can be dug into the soil around the base of plants to increase sulfur levels in the soil. It requires some care, however, because it can burn plants by increasing acid levels too quickly.

Article Sources
The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Rahman, Md Atikur et al. Importance of Mineral Nutrition for Mitigating Aluminum Toxicity in Plants on Acidic Soils: Current Status and OpportunitiesInternational journal of molecular sciences vol. 19,10 3073. 8 Oct. 2018, doi:10.3390/ijms19103073

  2. Hazardous Substance Fact Sheet: Aluminum Sulfate. New Jersey Department of Health.

  3. Post-plant nitrogen applications on corn. Iowa State University Extension and Outreach