Technically, any soil that has a pH above 7.0 is considered an alkaline soil. Most plants grow best in a slightly acid soil with a pH in the 6.0 to 7.0 range. A few such as Rhododendrons, blueberries, boxwood, heaths, heathers, pin oak and others do best in a more acid soil in the range of pH 4.5 to 5.5. So, excessive alkalinity in the soil can be a problem.

Generally speaking, the soils east of the Mississippi River tend toward an acid reaction while those to the west are more alkaline. This is due to the amount of reliable rainfall in each area. Remember that the H in pH stands for hydrogen and water is H₂0. More rain equals more hydrogen.

However, you should ALWAYS run a soil test through a reputable soil laboratory such as at your land grant university to determine your particular soils pH.

Soils that are Too Alkaline - When the pH of a soil climbs above the neutral level, i.e. pH 7.0 a chemical change takes place which makes it more and more difficult for plants to take up needed nutrients. In other words, if the soil is too alkaline for a particular species of plants, it will show signs of nutrient deficiencies even though a soil test will say that there is plenty of that nutrient in the soil. If the pH is wrong for that plant, it will just not take up that nutrient.

Often plants grown in a soil with a pH that is too high for them will display leaves with dark green veins and yellow between the veins. The older leaves on the plant may remain green.

Finally, microorganisms may not be able to live and thrive in an excessively alkaline soil. This means that they will not be able to decompose organic matter to release the elements for plant use.

Making Soil More Acid - The primary way to make a soil more acid is by adding sulfur. The most common sources are aluminum sulfate and garden sulfur. Certain fertilizers such as ammonium sulfate (21-0-0) will also help acidify the soil. Oak leaves, peat moss and pine needles generally also have an acidifying effect.

You must do a soil test to determine how much sulfur product to apply to make the desired change in pH. The amount you apply will be based on the amount of clay in the soil and the chemical buffering capacity of that particular soil. You cannot just guess on how much to apply to change the pH from 8.4 to 6.5. Soil test!

Note: Raising the pH by using lime is usually a much more predictable and reliable operation than is lowering the pH with sulfur. Soil buffers and other factors seem to making the soil more acid less precise. It can also take several season for the treatments to work...if they work.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.