The answer to this question is often, “Yes!” Under certain conditions soils can be laden with alkali (a general term for mineral salts) and be alkaline (have a pH value above 7.0). However, I hear these terms frequently used interchangeably, but they are not equivalent. In this first post, I will explore the nuance of separating these common Utah soil conditions.
As a review, the pH scale used to measure the acidity or alkalinity of a medium, ranges from a low of 1.0 (highly acidic) to a high of 14.0 (highly basic or alkaline) with a value of 7.0 being neutral. Most soils worldwide range from about pH 5.0 to 9.0 (6.5 to 8.5 in Utah). Soils with pH values above 7.0 are considered alkaline and often are buffered at these higher pH values by various carbonate minerals, especially calcium carbonate (called calcite or lime) in Utah and Intermountain West soils.
Alkaline soils high in lime content (often termed “calcareous soils”) may or may not be high in soluble salt content. Soluble mineral salts accumulate through evapotranspiration over time from our arid and semi-arid soils where water removal from the surface layers exceeds precipitation, leaving the salt behind. High precipitation washes such salts out of soils in humid environments. Accumulated soluble mineral salts are collectively (and I might add, archaically) termed “alkali”, particularly salts of the basic cations, or “alkali metal” elements, i.e., calcium, magnesium, potassium and sodium.
While alkalinity is determined by pH, soluble salt (or, alkali) content is measured by soil electrical conductivity (EC). Dissolved salts in soil solutions conduct electrical current, and proportionally so, hence higher EC is proportional to higher salt content. We will explore the effects of salts in soils in an upcoming post.
Any soil with a pH value above 7.0 is “alkaline” and any soil that contains high soluble salt content is said to contain “alkali”, but either condition can exist, and often does, independent of the other. Therefore, both measurements (pH and EC) are required to know which terms apply.
Grant Cardon – USU Extension Soils Specialist
“White Alkali” is generally used to refer to any salt deposits (cream colored to white) on soil surfaces. The term “Black Alkali” is used to refer to soils high in sodium carbonate content. Sodium carbonate buffers pH levels in soils above 9.0. At that pH level, soil organic matter (or, humus) in soils can dissolve and wick up to the soil surface giving salt deposits on the soil surface a dark brown to black coloration.
For Further Study:
- Saline Soils and Their Management (UN-FAO online documents)