Soil pH

A Busy Gardener’s Guide to pH: Soil Science and Preparation

by Don Boekelheide

Since the pH scale was first devised 100 years ago, it has inspired endless debate and garden folklore (nobody remembers for sure exactly what that "p" was supposed to stand for, but everyone's got a theory.) But there's no doubt that the pH scale is a very useful gardening tool.

The pH scale is a way chemists and soil scientists measure the acidity or alkalinity of solutions. The scale runs from 0 to 14, extremely acidic (think concentrated battery acid) to extremely basic (think lye), respectively. Seven (7) is the neutral midpoint, represented by pure water at 77 degrees F. (25 C). Each step in the scale is logarithmic, which means pH 5 is ten times more acidic than pH 6.

Why do gardeners care?

A soil solution pH of about 6.5 to 7 is ideal for making plant nutrients available to garden vegetables. Since our Southern soils are typically acidic, in the pH 5 to 5.5 range, adding powdered limestone to bring the pH up to 6 to 6.5 can pay off in healthier plants and higher yields.

The best way by far to determine the pH in your soil is through a lab-based soil test. Since natural forces keep pushing pH lower, you'll need to add lime from time to time, typically every three years. Adding compost helps by buffering the soil solution and keeping the pH more stable, even though it does not itself alter pH.

Liming isn't for everything, however!

Native plants, such as blueberries, have ecologically adapted to low pH. They actually prefer and grow best in more acidic soils. In this case, hold the lime, please!

Silent and enigmatic, she gazes at us from a sfumato world of earth tones—chocolate, terra cotta and sand. Leonardo da Vinci sometimes used soil to create his palette, so his masterpiece might well contain bits of clay from the da Vinci broccoli patch.

In the series of three articles that accompany this soil science topic we’ll take a closer look at active zone soil preparation: the nuts and bolts of digging a vegetable bed, doing transplant preparation and dealing with buying soil. (Go to www.mastergardenersmecklenburg.org, Garden Articles, and find the articles in this Soil Science & Preparation series.)

Don Boekelheide

For the past five years, Don Boekelheide, has taught a hands-on food gardening class at Central Piedmont Community College, modeled on his Peace Corps training. Don holds a Master of Science degree in agriculture from Cal Poly and formerly served as a Peace Corp Ag teacher in Togo. He is a former Extension Master Gardener Volunteer for Mecklenburg County, NC.

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