As a home gardener, it’s important to test your soil pH. Certain plants can only access the soil’s nutrients if the pH is within a certain range. Not even the addition of generous plant food or fertilizer will help if your soil lies outside of a plant’s preferred range.
Technically speaking, a soil pH (potential hydrogen) test measures how many hydrogen ions are in the soil. A pH less than 7 is acidic, 7 is neutral, and anything higher than 7 is alkaline. Acidic or alkaline soil isn’t necessarily bad; it all depends on what you’re growing. Most plants can adapt to soil pH that ranges from 6 to 7.5, but some plants have distinct preferences. For instance, blueberries prefer acidic soil while asparagus tends to do best in alkaline.
What is soil pH?
Soil pH is a way to measure the amount of acidity or alkalinity — in your lawn, garden soil or anything else. It is measured in pH units on a scale from 0 to 14. Extreme acidity is at the low end of the scale, extreme alkalinity is at the top end. Soil at the midpoint, number 7, is neutral soil, neither acidic nor alkaline.
The “p” in pH stands for “potential. The “H” is for hydrogen. While the science of pH gets complicated, everyone is familiar with things that are more or less acidic. Pure water is a neutral 7. Materials such as baking soda are alkaline. Alkalines have less hydrogen ion concentration than pure water, and much less than acidic items such as orange juice.
So pH can be defined as the level of hydrogen ion activity in a substance, including soil.
You may have naturally acidic or alkaline soil. There are many localized exceptions, but as a general rule, soils in the Eastern and Southern United States tend to be acidic. The Midwest tends to be more neutral, while soils in the Southwest and West tend toward alkalinity. The level of acidity originates from three main sources: rain, microbial activity, and nitrogen fertilizers.
- Rain. Rain is intrinsically acidic, says Friedericks. It carries with it a certain amount of nitric, sulfuric, and carbonic acid absorbed from the atmosphere. Your location makes a difference as well. Rain downwind of a metropolitan area can create a soil pH as low as 4.2. That’s because there is more nitric and sulfuric acid in the atmosphere near a city. And in areas that receive more than 25 inches of rain annually, nutrients such as calcium, magnesium and sodium are leached from the topsoil. That, too, creates acidic conditions. All of which translates to: If you live in a rainy area, your soil may tend to be acidic.
- Microbial activity reduces soil pH levels. The decomposition of organic matter releases nutrients into the soil, producing soil acidity. As soil microbes decompose plant and other residues, carbon dioxide is produced and stored in the soil. This reacts with the soil moisture to form carbonic acid and results in increased acidity.
- Fertilizer. Nitrogen fertilizers containing ammonium also lower the soil’s pH, says Friedericks. Ammonium creates acidity when it naturally converts to nitrate in the soil.
An acid soil has a pH less than 7.0, and as the number decreases the acidity becomes more intense. Soil with a pH of less than 5.0 is considered strongly acidic and is a challenging environment for most plants. As the soil pH increases above 7.0 it becomes more basic, or alkaline. A pH above 8.5 is unusual and also is challenging for plant growth.