Table of Contents
What Is Soil Acidity?
Soil acidity refers to the measure of how acidic or alkaline the soil is. Acidity or alkalinity is determined using a scale from 0 to 14, where 7 represents neutral soil. Numbers below 7 indicate increasing acidity, while numbers above 7 represent increasing alkalinity.
Typically, soil pH ranges from a highly acidic level of 3 to a highly alkaline level of 10. Changes in soil pH can result from several factors, including the use of nitrogen fertilizers, rainfall levels, and the cultivation of certain plants such as soybeans and alfalfa.
Why Is Knowing Soil pH Important?
Finding the right soil pH is crucial for plant health, as it determines how effectively plants absorb nutrients. Plant growth and vitality are strongly influenced by soil pH levels. For example, azaleas, rhododendrons, blueberries, and conifers thrive in acidic soils with a pH between 5.0 and 5.5.
Vegetables, herbs, and most ornamental plants prefer slightly acidic soil with a pH between 5.8 and 6.5. Some species—such as ferns and asparagus—grow best in neutral to slightly alkaline soil. If soil pH drifts above or below the ideal range, plant growth weakens and nutrient absorption becomes less efficient.
How to Raise Soil pH
Soil pH can be increased using one of the following methods:
1. Adding Lime or Limestone-Based Compounds
These materials are widely available in garden centers in powder, liquid, or pellet form. Choose the suitable type depending on soil texture and moisture level. Some lime products contain micronutrients, such as dolomitic lime (a mix of calcium and magnesium carbonate). Dolomitic lime should only be used when the soil is deficient in magnesium.
2. Adding Wood Ash
Although wood ash is less effective than lime, it provides valuable micronutrients such as calcium, potassium, phosphate, and boron. Over time, it may raise soil pH significantly. However, avoid direct contact between ash and plant roots or young seedlings, as it may damage them.
Note: Always test soil pH before attempting to adjust it. This helps determine the correct type and amount of amendment needed. Soil tests can be done at home or through a specialized lab.
How to Lower Soil pH
To decrease soil pH, you can choose one of the following:
1. Aluminum Sulfate
Aluminum sulfate lowers soil pH immediately because it increases acidity once dissolved in the soil.
2. Sulfur
Sulfur does not act instantly. It takes time to convert into sulfuric acid with the help of soil bacteria. The speed of this process depends on factors such as sulfur fineness, soil moisture, temperature, and bacterial activity. Under less-than-ideal conditions, this process may take months. For faster results, many gardeners prefer aluminum sulfate.
Note: After applying either material, the soil should be tilled and mixed well for increased effectiveness. Rinse plant leaves if any material gets on them to prevent leaf burn.
