Soils vary in their mineral content according to local geology, and these variations 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 0 to 14, with a pH of 7 representing a neutral level. Lower numbers are acidic, higher numbers are alkaline. The proper pH levels are essential for a plant's ability to take up the nutrients it needs from the soil. Iron absorption is especially affected by soil pH levels.
The ideal soil pH for most landscape plants and turf grasses is around 6.5, which is 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
- Most evergreen trees
- Pin oak
- Willow oaks
- Japanese iris
- Bleeding heart
- Sweet potatoes
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 leave 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.
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.
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.
Sphagnum Peat Moss
When used in large amounts as a soil amendment, peat moss will slightly acidify the soil while also adding organic material to the soil. When preparing your soil for planting, place 4 to 6 inches of acidic peat moss on your topsoil and till it into a depth of 6 inches. This will acidify the soil for about two years.
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 fertilizer with one of the many water-soluble products available, such as Miracid. Begin with mild solutions until you understand the impact on your plants.
Aluminum Sulfate: Use With Caution
Powdered aluminum sulfate has been 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 possible dangers 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 contamination of groundwater supplies. Many experts now recommend that aluminum sulfate be 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.
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.