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.
Learn which plants prefer acidity and what soil amendments help adjust soil pH levels.
Plants That Need Acidic Soil
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:
- Most evergreen shrubs
- Pin oak
- Willow oak
- Most evergreen trees
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.
Ways to Make Soil More Acidic
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.
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.
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.
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.
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.
Rahman, Md Atikur et al. Importance of Mineral Nutrition for Mitigating Aluminum Toxicity in Plants on Acidic Soils: Current Status and Opportunities. International journal of molecular sciences vol. 19,10 3073. 8 Oct. 2018, doi:10.3390/ijms19103073
Hazardous Substance Fact Sheet: Aluminum Sulfate. New Jersey Department of Health.
Post-plant nitrogen applications on corn. Iowa State University Extension and Outreach.