Arizona soils present many challenges to gardeners and landscapers unfamiliar with the area. It is important to realize some of these challenges so that we can spend our time efficiently and be productive in trying to improve them. Soil alkalinity, or rather the effects of it on plants, vexes landscapers and home gardeners alike.
Alkalinity is measured using the pH scale. You can skip this paragraph if you understand pH, or just want to know that neutral is good and really don’t need to know why. The pH scale goes from 1 to 14 where 1 is highly acidic, 14 is highly alkaline or basic and 7 is neutral (having a balance between acidity and alkalinity). The abbreviation “pH” stands for “potential of hydrogen” and refers to the amount of hydrogen ions in a solution. The pH scale is not linear but logarithmic. That is, a soil with a pH of 8.5 is ten times more alkaline than a soil with a pH of 7.5 and a soil with a pH of 6.5 is a hundred times more acid than a soil with a pH of 8.5.
To give some points of reference using common liquids, lemon juice has a pH of 2, vinegar is 3, milk is 7, baking soda and sea water are 8.5, milk of magnesia is 10.5, ammonia is 12, and 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. With some notable exceptions, a soil pH of 6.0 to 7.0 requires no special cultural practices to improve plant growth. 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 as the pH approaches 8.5.
Soil pH is most critical with respect to nutrient availability. Nutrients such as iron and zinc tend to become less available to many plants in alkaline soils. In our area, iron deficiency is most common.
Iron deficiency is most common in the spring when daytime temperatures climb, but soil temperatures remain cool. Additions of soil sulfur can help acidify soils to overcome these deficiencies, but it is very difficult to apply enough to make a significant difference, especially over the long term. Soil sulfur should be applied to alkaline soils where annual crops 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 Ironite applied to the soil to overcome iron deficiency. This is usually ineffective, because Ironite cannot release enough iron to overcome an iron deficiency.
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.
Once understood, we can make alkaline soils more productive by applying the appropriate nutrients, building raised beds, or planting less susceptible drought adapted landscape species.