Soluble versus Extractable Calcium – soil science
Calcium is secondary nutrient in plants and is an important part of cell walls. Calcium is often times abundant in the soil as an element but can be tied up with carbonate – forming calcium carbonate and other secondary minerals. There is also some calcium as part of the structural lattice of soil particles and organic matter. But as far as the plant and soil organisms are concerned, exchangeable and soluble calcium are most important.
Exchangeable calcium is the Ca2+ attached to the cation exchange complex on soil particles or organic matter, which carries a negative charge. Soluble calcium is the calcium salt that is free in the soil solution that surrounds soil particles. When you send a soil sample to a laboratory and they measure calcium they will measure and report extractable calcium. To get soluble calcium, you have to request a water extract and they will measure all ions. To get total calcium, which includes all forms, you need what’s called a harsh digest procedure to liberate and measure all forms of calcium.
I pulled a composite soil sample from two adjacent 40 acre fields, separated by a creek but have different landscapes. You will note from the table that extractable calcium was 60 to 70% of the total calcium and carbonate calcium was 20 to 30% of the total calcium while soluble calcium is only 1 to 2% of total calcium. What was not specifically reported below is the calcium in organic matter, the soil lattice, or other minerals so it comprises the residual amount in the total.
| Calcium form | Field A | Field B |
| Calcium, extractable, ppm | 3407.0 | 2802.0 |
| Calcium, water soluble, ppm | 96.4 | 58.3 |
| Calcium carbonate, % | 0.28 | 0.22 |
| Calcium as carbonate, ppm | 1670.0 | 823.0 |
| Total calcium, ppm | 5804.0 | 3989.3 |
In the majority of soils, calcium is available for plant uptake from the soil solution and exchangeable calcium. Calcium is often abundant occupying 70% of the exchangeable sites on the cation exchange complex of soil. However calcium is adsorbed very strongly on the soil exchange complex. The reason is because of its divalent (+2) charge and has a small hydrated radius. In addition the calcium ion doesn’t hydrate as strongly as monovalent ions like sodium or magnesium and is held closer and tighter to the clay or organic matter particles.
Nutrient uptake in the soil is achieved by cation exchange, where root hairs pump hydrogen ions (H+) out of the plant and into the soil through proton pumps. These hydrogen ions displace cations such as calcium attached to negatively charged soil particles so that the cations are available for uptake by the root. Plants are required to exchange something to get extractable calcium. However soluble calcium is free to the plant and readily available. Unfortunately most calcium is tied up on the exchange complex (not readily available) or with carbonates (unavailable).
Dr. Davidson posts articles on soil management and subjects to gypsum. If you have suggestions for topics or questions, feel free to contact him at [email protected] or call 402-649-5919.
