Most minerals important to biological life follow a natural cycle that tracks how they enter the soil and their fate once there. Understanding these cycles is important because it helps you manage the mineral and influence the way it interacts with other factors on the farm. Let’s take a look at the calcium cycle.
Calcium is primarily present as rock, minerals or as structural calcium built into mineral crystal lattices of soil particles and is not readily available. Calcium can also be added as fertilizer, lime or by-products. Water can carry calcium into the soil through weathering and natural dissolution.
When in the soil, most of the calcium is in an insoluble form until it is ‘weathered off’ of minerals or when organic matter is broken down by microbes into soluble calcium. However, some of the calcium are held loosely or tightly on soil’s cation exchange complex (CEC) or in the soil solution and are available to plants and microorganisms.
When animals, microorganisms, or plants decay, their bodies decompose and the calcium is mineralized and released back into the soil. Roots also regularly leak minerals, sugars, and other compounds back into the soil including calcium.
Since calcium is a positively charged ion, it is adsorbed in the soil to the surface of clay and organic particles which are negatively charged. Positively charged ions (cations) adsorb to soil particles and are termed “exchangeable ions” because they can be exchanged by other ions present in the soil solution.
When absorbed by plants or microorganisms, calcium enters an organic phase. In this form, calcium is continually recycled between the plant roots, microorganisms, and soil. After a plant, animal, or soil fauna dies, decomposers break down the organism and calcium is released back to the soil in a soluble form. Calcium routinely moves back and forth between the soluble (and available) and the insoluble (unavailable) phases.
The calcium cycle is also influenced by soil pH. Calcium is more available to plants from a pH at 7 .0 to 8.5. Soils with a pH above 7.5 may contain calcium carbonate, known as free lime. Calcium competes with other positively charged ions, such as sodium (Na+1), potassium (K+1), and magnesium (Mg+2). Applying too much of these positively charged ions can decrease calcium uptake by plants as sodium ions can replace the adsorbed calcium, damage soil structure, and decrease calcium availability.
When nitrate fertilizers are applied to soil, calcium absorption increases in the plant. In turn, higher soluble calcium content increases ammonium uptake. Increases in soluble calcium (Ca+2) in soil displaces soluble aluminum (Al+3) content in acid soils, as well as displacing sodium (Na+1) in sodic soils. However when soluble aluminum (Al+3) increases in acid soils it will cause a decrease in calcium uptake.
Calcium will also precipitate as a mineral. Soluble (free) calcium in the soil solution forms insoluble compounds with phosphorous and carbonates. Consequently, calcium and phosphorous availability are both diminished.
Plants take up calcium in its cation form (positively-charged ion) as Ca+2. In plants it moves in the xylem (vascular system) with water, up through the plant through stems and petioles to leaves. It only slightly mobile in the soil, generally adhering to soil particles or carbonates.
In the plant kingdom, calcium is referred to as a secondary macronutrient because of the amount a plant needs relative to nitrogen, phosphorus and potassium (primary macronutrients). Calcium joins magnesium and sulfur in this class of secondary nutrients.
In plants, calcium is required for cell wall rigidity, cell division of meristems and root tips, normal mitosis, membrane function, acts as a secondary messenger or signaling compound, aids in storage of phosphates in vacuoles, actively involved in photosynthesis and involved in sugar transport. It has some of the same functions in microorganisms and mammals and also plays an important role in bone structure.
When you understand how plants fit into the calcium cycle, you can better manage the amount and availability of calcium in your soil. For more information about how gypsum can provide calcium for your soil and crops, contact us.
For more about how the calcium cycle works in agriculture, visit our new calcium cycle infographic illustrating the key steps in the process.
Dr. Daniel Davidson – EcoGEM Agronomist. Dr. Daniel Davidson is a nationally recognized agronomist. He served most recently as Director of Strategic Research for the Illinois Soybean Association. Dr. Davidson has also served in various capacities at GEOSYS, Cargill, Agri Business Group and Agri Growth, Inc. He holds a Ph.D. in Agronomy from Washington State University and an MS in Agronomy from the University of Missouri.
Dr. Davidson posts articles on soil health and management related subjects. If you have suggestions for topics or questions, feel free to contact him at [email protected] or call 402-649-5919.