N E O P H Y T E   N O O K

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Soil Basics

by Mike Mekelburg, Master Gardener

It's fairly common knowledge that soil pH represents the acidity of soil, but how many of us know what the p and the H stand for, or the math behind the scale?

In a nutshell, pH can be thought of as a proportion (p) of hydrogen ions (H) relative to hydroxide ions. When the two are in balance, the soil is considered neutral (pH 7).

The pH scale is logarithmic. A change of one unit on the scale equates to a tenfold change in the hydrogen and hydroxide ion concentrations. As hydrogen ions increase, acidity increases and the pH number decreases. As hydrogen ions decrease, the opposite takes place.

Our low desert soils are generally tagged with a pH of 8.3, which is near the upper alkaline limit for satisfactory soil for most of the world's plants. However 8.3 is a wonderful pH for most desert-adapted and native flora.

Another important feature of soil is texture, be it sandy, loam, clay, or a combination of these. Our desert soils are primarily clay, which for our purposes means that water will penetrate very slowly and also evaporate very slowly. That's why you'll often hear Arizona master gardeners advising newcomers to, "Water deeply, but not too often."

Soil fertility can be measured with test kits. For the most part non-native landscape plants benefit from applications of nitrogen - as with citrus in February, May, and August - but phosphorous and potassium are abundant in our soil and take a long time to leach out.

So what conclusions should Arizona gardeners draw from this information? If you want to work with nature, choose native or desert-adapted landscape plants, water with a long run-time and wide intervals between irrigations, and apply nitrogen only to plants that absolutely need it.


Maricopa County Master Gardener Volunteer Information
Last Updated July 28, 2003
Author: Lucy K. Bradley, Extension Agent Urban Horticulture, University of Arizona Cooperative Extension, Maricopa County
© 1997 The University of Arizona, College of Agriculture and Life Sciences, Cooperative Extension in Maricopa County
Comments to Maricopa-hort@ag.arizona.edu 4341 E. Broadway Road, Phoenix, AZ 85040,
Voice: (602) 470-8086 ext. 301, Fax (602) 470-8092