Calcium in soil originates from a combination of natural geological processes, decomposing organic materials, and human-introduced agricultural amendments. This vital nutrient is predominantly supplied by the slow breakdown of calcium-rich minerals, making it readily available for plant uptake.
Mineral Weathering and Native Rocks
The primary natural source of calcium in soil is the weathering of various minerals found in the Earth's crust. As rocks and parent materials break down over long periods, they release calcium into the soil solution. Among the most common and significant calcium-bearing minerals are:
- Calcite (calcium carbonate): A major component of limestone and marble, calcite is a prevalent natural source and a widely used liming agent.
- Dolomite (calcium magnesium carbonate): Similar to calcite, dolomite also contributes calcium (and magnesium) to the soil as it weathers.
- Gypsum (hydrous calcium sulfate): Found naturally in sedimentary deposits, gypsum is a more soluble calcium mineral.
- Anorthite (a plagioclase feldspar): This silicate mineral contributes calcium as it undergoes chemical weathering.
- Apatite: A phosphate mineral that contains calcium.
Calcium released from these minerals is a dominant cation on the soil's cation exchange capacity (CEC). This means it readily attaches to negatively charged soil particles. From these exchange sites, calcium can easily desorb (detach) and replenish the soil solution, ensuring a continuous supply for plant roots as needed.
Decomposing Organic Matter
Organic matter, derived from the decomposition of plant and animal residues, also contributes to the soil's calcium pool. As leaves, stems, roots, and other organic materials break down, the calcium absorbed by the living organisms is released back into the soil, becoming available for future plant growth. This process is particularly important in surface horizons where organic matter accumulates.
Agricultural Amendments and Inputs
Human activities, especially in agriculture, introduce significant amounts of calcium into the soil through various amendments:
Liming Materials
Liming is a common practice to raise soil pH and supply calcium. The most frequently used liming agents include:
- Agricultural Lime: Primarily composed of ground limestone (calcite) or dolomitic limestone (dolomite), which both supply calcium carbonate.
- Quicklime (calcium oxide) and Hydrated Lime (calcium hydroxide): These are more reactive forms of lime used in specific agricultural contexts.
- Gypsum (calcium sulfate): While not a liming agent (it doesn't significantly alter soil pH), gypsum is a popular calcium amendment, particularly useful for improving soil structure in sodic soils and supplying sulfur.
Fertilizers
Many commercial fertilizers contain calcium, either as a primary nutrient or as a secondary component:
- Calcium Nitrate: A direct source of both calcium and nitrogen.
- Superphosphate (e.g., triple superphosphate): Contains calcium as part of its composition, supplying calcium along with phosphorus.
- Calcium Chloride: Used in some specialty fertilizers and foliar sprays.
Irrigation Water
Depending on its source, irrigation water can also contribute dissolved calcium to the soil. Water that has passed through calcium-rich rock formations (like limestone) will carry varying concentrations of calcium, depositing it in the soil with each application.
Summary of Calcium Sources
| Source Category | Examples | Contribution |
|---|---|---|
| Mineral Weathering | Calcite, Dolomite, Gypsum, Anorthite, Apatite | Primary natural supply, slow release from rock and mineral breakdown. Dominant cation on CEC. |
| Decomposing Organic Matter | Plant residues, animal waste | Releases calcium sequestered by living organisms back into the soil. |
| Agricultural Amendments | Agricultural lime (calcite, dolomite), gypsum, calcium nitrate, superphosphate | Intentional application to raise pH, provide calcium, and improve soil structure. |
| Irrigation Water | Groundwater, surface water (depending on source geology) | Carries dissolved calcium, depositing it with water application. |
Understanding these diverse sources is crucial for managing soil fertility and ensuring adequate calcium availability for healthy plant growth.
