Acidity and alkalinity of soil are measured by "pH," or "the 'potential' of the 'hydrogen' ion in water," according to Clemson University. Soil's pH can range from 1 to 14, with a reading around 7.0 indicating neutral soil. Below 7.0, soil is acidic; above 7.0, soil is alkaline. Soil pH can vary from region to region, and can vary in different parts of your yard. It's wise to learn what type of soil your plants favor: for example, tomatoes, citrus and azaleas prefer acidic soil. You can lower pH and increase your soil's acidity by adding sulfur or an acid-based fertilizer.
Increasing Soil Acidity
Test your soil by using an inexpensive soil pH test kit, available at garden supply centers. If the test results show that your soil pH is above 7.0, you'll need to increase the acidity by lowering the pH.
Mix ground rock sulfur into your planting area: To reduce the pH of sandy soil by one point, use 1.2 oz. of sulfur for every square yard of surface area. For loam and clay soils, use 4.6 oz. of sulfur per square yard. Thoroughly mix the sulfur into the soil before you plant.
Add sawdust, composted leaves, wood chips, cottonseed meal or peat moss to also help lower your soil's pH, thereby increasing its acidity.
Spread a 2-inch layer of mulch consisting of oak leaves and/or pine needles: This mixture will benefit rhododendrons and other acid-loving plants by providing a continual source of acidity whenever it rains or you water the area.
Apply an acid-based fertilizer to acid-loving plants such as blueberries, azaleas, rhododendrons, citrus trees and tomatoes whenever you feed them, which will help to improve soil acidity and give your plants the acidity they need.
Watch the condition of your plants. If leaves begin to turn yellow with dark green veins, this can indicate a lack of iron, which occurs in soils that are becoming alkaline (pH over 6.0). Spray your plant with iron sulfate--1 oz. of this product mixed with 2 gallons of water is the correct mixture.