Calcium is an extremely important nutrient. The structure and strength of the cell wall is dependent on calcium. Ca2+ forms bridges between various lipids and proteins at the membrane surface, strengthening the cell. Without adequate calcium, cell membrane structure, function, and integrity can be lost, reducing nutrient and water uptake by the roots. Calcium has been shown to boost net primary productivity, leading to the deposition of more above- and below-ground organic matter.

While certain plant tissues may have calcium levels up to 10% of the dry weight, the concentration of calcium within a cell is tightly controlled and typically ranges from 0.1 to 0.2 μM. Cellular calcium concentrations must be so tightly regulated, because calcium is used for stress signaling and signaling during development.

Plants have a group of enzymes called calcium-dependent protein kinases (CDPKs) that are complex signaling hubs. As their name suggests, many of these enzymes require calcium to properly function. CDPKs are associated with GA3, the phytohormone responsible for growth and cell elongation, as well as with biotic and abiotic stress signaling. CDPKs and other proteins bind to calcium or transport Ca2+ into organelles, controlling the concentration of calcium ions in the cytosol. A signal causes the plant to mobilize Ca2+ into the cytosol, where these ions function as a messenger molecule to activate CDPKs. After the processes are complete, binding proteins and transporter proteins remove the excess Ca2+ from the cytosol once again. These enzymes are incredibly fast-acting: CDPKs were shown to be activated as quickly as 1 to 2 minutes after stress exposure.

True calcium deficiencies in soil are rare, and are typically caused by other soil problems such as sandy soils with low CEC or high soil pH. Calcium deficiency typically presents in young tissues, and causes issues like black heart, blossom end rot, and fruit cracking due to lack of structural integrity. A foliar spray of calcium applied to fruits can prevent fruit cracking.

Figure from Rowley, Grand, and Verrecchia (2018).

Calcium (Ca) is a critical nutrient, both in the soil and the plant. In the soil, proper calcium levels buffer the pH and increase cation exchange capacity (CEC), helping to maintain optimal growing conditions. Ca2+ ions are important in the formation and stability of soil aggregates and thus calcium stabilizes organic matter in the soil. As earthworms consume organic matter, calcium in the soil forms microaggregates in their castings, enriching the soil. Additionally, calcium has been shown to bind to complex organic molecules and exudates, forming gel-like structures which stabilize the soil even further. These stable soil aggregates help maintain more pore space, which is crucial for water infiltration.

In addition to stabilizing organic matter in the soil, calcium has also been shown to prevent the decomposition of that organic matter, compounding the effect. Calcium carbonate applied to soils can create crystal structures in the soil, cementing organic matter and improving soil structure. These calcium carbonate crystals trap organic matter, forming inclusions of stable carbon in the soil.

Calcareous soils have a high concentration of free calcium carbonate, which gives them a very high soil buffer capacity. However, excess calcium on calcareous soils may cause calcium toxicity and prevent the uptake of other cations, which may prevent seed germination or stunt plant growth.