Soil is formed when rock is broken down by climate, organisms, and vegetation over a period of time. It is made up of weathered rock fragments and decaying remains of plants and animals (organic matter). It also contains varying amounts of air, water, and micro-organisms. It furnishes mechanical support and nutrients for growing plants. Most Arizona soils developed under desert or scrub vegetation. However, because of the wide variation in elevation and climatic conditions found throughout the state there are wide differences in the types of soil profiles found. Interesting, only tropical soils are not found within Arizona. Important physical properties of a soil is color, texture, structure, drainage, depth, and surface features. These properties can vary greatly in one's own backyard. The color of the soil can be a good indicator of drainage. Red/brown soils usually indicate good drainage while yellow soils have medium drainage and gray soils usually have poor drainage.

Here in the High Desert, we have alkaline soils. The relative acidity or alkalinity of soils is expressed as pH, which is a measure of the relative number of free hydrogen ions. The pH scale goes from 1 through 14, with 1 being the most acid and 14 being most alkaline, and 7 being neutral. Each number represents a tenfold change in acidity or alkalinity. For example, a soil with the pH of 9 is ten times more alkaline than one with a pH of 8. Generally, soils are acidic where it receives more than 20 inches of rainfall in a year. Areas with less than 20 inches of rainfall in a year, such as the High Desert where average rainfall is between 12-16 inches, the soils are alkaline. This is due to the calcium carbonate content of the soil which is not leached through the soil because of our limited rainfall.

Why is this important to the gardener? Because we need to learn to work with the soil on our property. It is much easier and cost effective to work with Mother Nature than against her. Gardeners in Cochise County can pursue two possible strategies. The first is to attempt to alter the pH of the soil by the use of amendments so they can grow acid-loving plants. To illustrate what amending the soil would take lets assume that a cubic foot of soil weighs 100 pounds and has a 1 percent calcium carbonate content. This means that there would be one pound of calcium carbonate present. It takes one pound of sulfuric acid or its equivalent to neutralize one pound of calcium carbonate. Therefore, to decrease soil pH from alkaline to neutral or acid it would require equal mixing of one pound or equivalent of sulfuric acid throughout the cubic foot of soil. If the soil had 10 percent of calcium carbonate then 10 pounds of sulfuric acid or its equivalent would be needed to neutralize the soil! The second choice is to go native and grow plants that are adapted to our soils. Not only do these plants grow well in our High Desert soils but they are adept to cope with harsh climate conditions of heat, wind, and drought. There are also many non-native adapted plants which do well in Cochise County.

Although attempts to lower the pH of soils can be successful in the short term, they require repeated additions of amendments to maintain the change in pH. Ultimately the gardener will lose to Nature. The best strategy is to leave the soil pH alone and plant those plants that are adapted to the soils of our area.

Feature article for the Sierra Vista Herald/Bisbee Daily Review
Sunday, November 8, 1998
Author: Gary Gruenhagen, Cochise County Master Gardener