Calcium as a Plant Nutrient
Calcium is generally considered a secondary nutrient. This is not because it is any less important than any other required nutrient, but because it is required in lesser amounts than the macronutrients. Calcium is absorbed as the Ca2+ ion with a normal concentration in plant leaves between 0.2 to 1.0%. Calcium plays an essential role in cell division and elongation. It also plays an important role in the structure and permeability of the cell membranes. Without calcium, the cell membranes break down, causing a loss of diffusible compounds (increasing the risk of disease). Calcium also enhances uptake of some nitrogen forms, as well as transport and retention of other nutrients within the plant.
Calcium’s Role as a Soil Amendment
Soluble calcium (Ca2+) plays a critical role in reclaiming saline soils and in establishing and maintaining good soil flocculation. We are all aware of the potential negative effects of excessive salts in soil: poor growth, reduced yields, and, if salt levels are high enough, inability to grow a crop. Likewise, a soil that has “broken down” will become impermeable to air and penetration.
The question then is “What role does soluble calcium play in mitigating these problems?” To answer this question, it is first necessary to understand how sodium acts in the soil.
Sodium is a monovalent cation (it has one positive charge) which will be attracted to the surface of soil particles that are negatively charged. If high levels of sodium are present, then sodium will be present on many of the exchange sites. The sodium ions will not be attracted to the other soil and the particles will not form aggregates or crumbs, which are desired for good soil structure. Instead, the soil structure will break down, leading to a soil that is impermeable to air and water. In addition, the sodium ion itself can be toxic to plants at high concentrations.
Calcium is a divalent cation (it has two positive charges), meaning it can be attracted to two different soil particles and “bond” them together. It can be thought of as a bridge between the soil particles. Many soil particles can be linked together in this way to form aggregates or crumbs. This creates a well flocculated soil that allows water and air penetration. Because calcium is a divalent cation, it has a greater attraction to the soil particles than a monovalent cation such as sodium, and it will replace sodium on the exchange site.