Which form of calcium is absorbed by plants from the soil?

Plants absorb calcium from the soil primarily in its ionic form, which is the calcium ion. Calcium ions (Ca²⁺) are soluble forms of calcium that can be readily taken up by plant roots. Once in the ionic state, calcium plays several crucial roles in plant physiology, including cell wall structure, enzyme activity regulation, and signaling functions essential for growth and development.

While calcium carbonate, calcium sulfate, and calcium chloride are all forms of calcium found in the soil, they must undergo changes to release the calcium ion for plant absorption. For instance, calcium carbonate, while common as a soil amendment, can be relatively insoluble in neutral to high pH soils and may require certain conditions to dissolve sufficiently to provide available calcium ions. Calcium sulfate is more soluble compared to calcium carbonate but is less commonly a primary source of calcium for plants. Calcium chloride, although soluble, is often used in specific applications but is not the principal form that plants utilize for calcium uptake.

Therefore, the calcium ion is the correct answer, as it is the specific, absorbable form of calcium that plants uptake directly from the soil solution.