Soil Alkalinity - January 23, 2008
Jeff Schalau, Associate Agent, Agriculture & Natural Resources, Arizona Cooperative Extension, Yavapai County

Arizona soils present many challenges to gardeners and landscapers unfamiliar with the area. Anyone thatís tried to grow a rhododendron or azalea in native soil has probably been frustrated by its poor performance or premature demise. These plants evolved in areas that have acidic soils. Acidic soils develop in humid regions where alkaline materials have been leached from the soil over time. In the arid southwest, alkaline soils develop from calcareous rocks, such as limestone, and persist due to low and intermittent precipitation.

Alkalinity is measured using the pH scale. The pH scale goes from 1 to 14 where 1 is highly acidic, 14 is highly alkaline and 7 is neutral (having a balance between acidity and alkalinity). The abbreviation "pH" stands for "potential of hydrogen" and refers the amount of hydrogen ions in a solution. The pH scale is not linear but logarithmic. That is, a soil with a pH of 8 is ten times more alkaline than a soil with a pH of 7 and a soil with a pH of 6 is a hundred times more acid than a soil with a pH of 8. To give some points of reference using common liquids, lemon juice has a pH of 2, vinegar is 3, milk is 7, sea water is 8.5, milk of magnesia is 10.5, ammonia is 12, and a solution of lye is 13.

The pH of soil refers to the way the saturated soil solution interacts with other soil compounds and nutrients. A near-neutral or slightly acidic soil is generally considered ideal for most plants. Most soils in Arizona are alkaline and have a pH of between 7 and 8.5. Native plants are adapted to these conditions. However, introduced landscape and garden plants often struggle where soil pH approaches 8.5.

Soil pH is critical with respect to nutrient availability. Nutrients such as iron and zinc tend to become less available to plants in alkaline soils when they form insoluble compounds. In our area, iron deficiency is most common. Symptoms of iron deficiency are chlorosis (green veins with yellow or whitish areas in between) on the new growth. Older leaves remain green. These symptoms are especially prevalent on Photinia fraseri (red tip) plants when they are grown in alkaline soils.

Iron deficiency is most common in the spring when daytime temperatures climb, but soil temperatures remain cool. Additions of soil sulfur can temporarily acidify soils to overcome these deficiencies, but it dissipates over time and it is difficult replace it without causing damage to established woody plant roots. In areas with alkaline soils, sulfur can be incorporated where annual crops (flowers and vegetables) are grown. This can be repeated each season.

The fastest way to overcome iron deficiency is to apply chelated (pronounced kee-lated) iron to the foliage. Chelated iron products have been prepared in a specific way to keep them readily available for absorption once they are introduced into the soil. The chelation process prevents them from being rendered unavailable by alkaline compounds in soil. Some gardeners use soil-applied iron amendments, but these rarely correct chlorosis as well as foliar sprays.

Zinc deficiency can also be exacerbated by alkaline soils and is usually seen on deciduous fruit and nut trees (especially pecans). Zinc deficiency is characterized by small leaves that are curved, have wavy edges, have dark veins and yellowish blades, or leaves only in small bunches at the ends of the branches. To correct this, apply a zinc sulfate solution to the foliage when leaves first emerge, and two or more times until all new leaves have developed. The zinc sulfate only affects the young leaves it contacts. As the leaves mature, the thickened leaf cuticle will prevent the zinc from entering.

Remember that our climate and soils are responsible for our unique and beautiful native plant communities. Once understood, we can choose to make alkaline soils more productive by applying the appropriate nutrients/amendments or, better yet, growing locally-adapted and/or native plant species.

The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site:

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: January 15, 2008
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