How is Calcium Found in Soil?

Calcium is primarily found in soil as insoluble calcium carbonate (CaCO3).

Forms of Calcium in Soil

Calcium exists in various forms within the soil, influencing its availability to plants. These forms include:

Insoluble Calcium Compounds: The most abundant form is calcium carbonate (CaCO3), commonly found in alkaline and calcareous soils. Other insoluble forms include calcium phosphates (CaPO4), which can form when high levels of phosphorus are present, limiting its availability.
Exchangeable Calcium (Ca2+): This form is adsorbed onto the surface of soil particles, particularly clay and organic matter. Exchangeable calcium is more readily available to plants than insoluble forms. The amount of exchangeable calcium depends on the soil's cation exchange capacity (CEC) and the relative abundance of other cations (e.g., magnesium, potassium, sodium).
Calcium in Soil Solution (Ca2+): This is the most readily available form for plant uptake. Calcium ions (Ca2+) are dissolved in the soil water and can be absorbed by plant roots. The concentration of calcium in the soil solution is influenced by the solubility of calcium minerals and the exchange reactions occurring on soil particles.

Several factors influence the availability of calcium to plants:

Soil pH: Calcium is generally more available at slightly acidic to neutral pH. In highly alkaline soils, calcium carbonate can precipitate, reducing its solubility.
Soil Texture: Clay soils generally have a higher CEC and can hold more exchangeable calcium than sandy soils.
Organic Matter: Organic matter increases the CEC of the soil, improving its ability to retain calcium.
Nutrient Interactions: High levels of other cations (e.g., magnesium, potassium, sodium) can compete with calcium for uptake by plants. Similarly, high phosphorus levels can lead to the formation of insoluble calcium phosphates.
Liming: Applying lime (calcium carbonate) to acidic soils increases soil pH and provides a source of calcium.

Calcareous Soils: Soils derived from limestone or chalk are typically rich in calcium carbonate.
Acidic Soils: Acidic soils may be deficient in calcium due to leaching of calcium ions. Liming is a common practice to correct calcium deficiencies in acidic soils.