Think like a plant when measuring soil pH

By Judy Scott

Photo by Michael Allen Smith

Some meters and methods are more accurate than others.

Soil pH can make a big difference to the plants in your garden. To understand how, you must "think" like a plant.

"Think of yourself in a swimming pool," said Sam Angima, a soil scientist for the Oregon State University Extension Service. "If the water is too cold, or the chlorine makes your eyes hurt, you don't want to be there for long."

Roots in the soil are just like you in the pool, Angima said. If the soil pH is too high or too low, plant roots don't want to be there. "They don't grow normally and they may not take up enough nutrients to support plant health."

Scientifically speaking, pH measures the concentration of hydrogen ions in soil solution. It is measured on a scale from 0 to 14, where values below 7.0 indicate acidic soil, and those above 7.0 indicate basic or alkaline soil. Each unit change e.g. from 6.0 to 5.0, is a 10-fold difference in the concentration of hydrogen.

"That means if your soil pH is 6.0 and your neighbor's soil pH is 5.0, your neighbor's soil pH is 10 times more acidic than yours," explained Angima.

Soil in western Oregon has a natural tendency to be acidic. Centuries of leaching the soil with the abundant winter rainfall removed chemical constituents such as calcium and magnesium, making the soil acidic. The natural process of soil acidification is accelerated by the addition of nitrogen fertilizers, and general farming or gardening in general.

"As soil pH decreases, the solubility of iron, zinc, manganese and aluminum increases,” Angima said. “The concentration of some of these metals can reach levels that are toxic to some plants. Alfalfa, garlic and many garden vegetables are particularly sensitive, whereas blueberries and rhododendrons are quite tolerant."

Standard soil tests can determine the pH level of your soil. If it is too acidic, the addition of lime can help raise the pH. To understand how lime works, Angima again helps us think like a plant.

"We will begin with coffee," he said. "With a pH of about 5.5, coffee can be too acidic for some people. How do you reduce the acidity of coffee? You add cream. The action of cream in coffee is the same as adding lime to soil. And just as you would need to stir the cream into the coffee, so you need to mix the lime into the soil before planting."

The goal of putting lime in your garden or cream in your coffee is not to neutralize all hydrogen or raise the pH to 7, rather it is to reduce acidity to a tolerable level, Angima said.

The pH of a liming material is not as important as its ability to combine with hydrogen, the primary component of soil acidity. Just as an oyster shell in your coffee cup would do little to neutralize the acid, so some materials react differently with the hydrogen in the soil. Effective liming agents such as calcium carbonate (agricultural lime) or dolomitic lime (calcium magnesium carbonate), bind with hydrogen and remove it from soil solution, which in turn reduces acidity.

For more information about testing your soil's pH, check with your local county office of the OSU Extension Service or read the "Measuring Soil pH" research online.

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