Calcification in soil refers to the process of calcium carbonate (CaCO3) deposition, accumulation, and concentration, typically occurring in soils found in arid and semi-arid environments characterized by low precipitation and high evaporation rates. This leads to a water deficit in the soil.
Understanding the Process
Calcification is a pedogenic process, meaning it's a soil-forming process driven by environmental conditions. Here's a breakdown of how it works:
-
Source of Calcium: Calcium ions (Ca2+) originate from the weathering of calcium-rich minerals in the soil parent material or through atmospheric deposition.
-
Dissolution and Transport: Rainwater, although scarce in arid regions, dissolves some of these calcium ions. This calcium, along with carbonates (CO32-), is transported downward through the soil profile via soil water.
-
Evaporation and Precipitation: As the water moves upward due to capillary action driven by high evaporation, it reaches a point where the water evaporates. This evaporation increases the concentration of calcium carbonate. When the concentration of calcium carbonate exceeds its solubility threshold, it precipitates out of solution.
-
Accumulation: Over time, repeated cycles of dissolution, transport, and precipitation lead to the accumulation of calcium carbonate within specific soil horizons. This accumulation can manifest in various forms, such as nodules, concretions, or a hardened layer called a caliche.
Environmental Factors
- Aridity: The most crucial factor is aridity. Low rainfall minimizes leaching (removal of soluble materials), while high evaporation concentrates the calcium carbonate.
- Temperature: High temperatures accelerate evaporation, promoting calcium carbonate precipitation.
- Parent Material: Calcium-rich parent materials, such as limestone or calcareous shale, provide an abundant source of calcium.
- Vegetation: Sparse vegetation cover reduces evapotranspiration, allowing more water to penetrate the soil and dissolve calcium carbonate.
Forms of Calcium Carbonate Accumulation
- Nodules: Small, rounded accumulations of calcium carbonate.
- Concretions: Larger, irregular-shaped accumulations of calcium carbonate.
- Caliche (also called hardpan): A hardened layer of calcium carbonate that can restrict root growth and water infiltration.
- Soft powdery accumulation: Less consolidated form of calcium carbonate that is easily broken apart.
Effects of Calcification on Soil Properties
- Increased Soil pH: Calcium carbonate is alkaline, so calcification typically raises the soil pH.
- Reduced Nutrient Availability: High pH can reduce the availability of certain micronutrients, such as iron, zinc, and manganese.
- Restricted Root Growth: Caliche layers can physically impede root penetration and water infiltration.
- Improved Soil Structure (in some cases): Moderate levels of calcium carbonate can help to aggregate soil particles, improving soil structure. However, excessive accumulation can lead to cementation and reduced porosity.
Example
A common example of calcification is the formation of caliche layers in the southwestern United States and other arid regions. These hardened layers can be several meters thick and significantly affect land use and water resources.
Summary
In summary, calcification is a significant soil-forming process in arid and semi-arid regions where calcium carbonate accumulates due to low precipitation and high evaporation, influencing soil properties and land use.
