Soil Science: How to Test Your Soil pH

Learn how to test your soil’s pH and how to apply the results of your findings for a more productive garden.


| November/December 2015



Organic compost

Adding organic material is the best way to maintain healthy soil with a near-neutral pH.

Photo by iStock

In late fall and early winter, before the ground is frozen solid in most of the country, it’s the perfect time to brush up on knowledge that can help us build a better garden come spring. One way we can help ensure our garden crops grow to their maximum potential is to gain an understanding of our soil’s pH. The pH describes the relative acidity or alkalinity of the soil’s makeup, and it has important implications for plant health and growth. Soil pH impacts beneficial fungi and bacteria in the soil and influences whether essential minerals are available for uptake by plant roots.

What is Soil pH?

All pH is measured on a scale from 0 (most acidic) to 14 (most alkaline, or basic); 7.0 is neutral. Most food crops prefer a pH of 6.0 to 6.5, but you can have a productive food garden at pH levels between 5.5 and 7.5. A difference of just 0.5 may not seem like much, but the pH scale is logarithmic, which means, for example, a pH of 7.0 is actually 10 times less acidic than a pH of 6.0. Potatoes and most berries, which grow best in more acidic soil, are the main exceptions to the average preferred pH range.

A soil’s pH results from interactions among native rocks, plants and weather conditions over many years, and it varies with climate and physical surroundings. In moist climates that support dense forests, such as those east of the Mississippi River and along the Pacific Coast, soil tends to be acidic, with a pH between 4.0 and 5.5. The grasslands of the comparatively dry Midwest often have slightly acidic soil (6.0 to 6.5), while most arid regions, such as the Rocky Mountains, have alkaline soil (7.0 to 7.8).

However, local differences can cause huge variations within these general patterns—for example, weathered limestone can create alkaline patches in otherwise acidic landscapes. Soil is often severely disturbed during construction; sometimes native topsoil is completely lost. Some synthetic fertilizers—mainly those high in ammonium or sulfur—can also acidify soil, as can tillage methods that reduce soil’s levels of organic matter. Acid rain caused by coal combustion began to acidify streams and soil during the late 1800s, and continues to push soil in some regions into the acidic range every time it rains. Finally, some organic matter, such as peat moss and pine needles, acidifies during decomposition.

Alkaline soil occurs in places where soil is formed from limestone or other calcium-rich minerals, and high water-evaporation rates common in arid climates aggravate the problem by loading topsoil with accumulated salts. Many plants can still thrive when grown in alkaline soil that has been generously enriched with organic matter, which also improves soil’s ability to retain water. Mulches also will slow buildup of salts in plants’ root zones by reducing surface evaporation.

How to Test Soil pH

Soil chemistry is complex, so how can we boil it down to help us in our gardens? If your crops seem to be thriving, you probably don’t need to worry about your pH. But if you find that plants just don’t seem to be growing as well as you’d like, you should probably have your soil checked with a pH test. The cost for a basic soil evaluation by a state soil-testing lab ranges from free to $25, depending on the state, and typically includes a pH test along with results for major and sometimes minor nutrients. Soil-test kits with detailed instructions are usually available at extension service offices, or you can order them by mail.





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