Tech Brief #2: Solubility

Not all sources of calcium sulfate are equal in plant availability

Calcium sulfate fertilizers, like most other nutrient amendments, do not report differences beyond nutrient analysis. Nutrient analysis is not the only factor that affects how fast a source of calcium sulfate will become available to the plants. Liming materials, however, are an example which reports the effective calcium carbonate equivalent (ECCE), not just the per cent of calcium carbonate.

The ECCE measures the effectiveness of liming materials and is calculated as the product of purity value (CCE) and the fineness value divided by 100. For example, if the purity is 80 percent and the fineness value is 75 percent, then: ECCE = (80 x 75)/100 = 60%. The lack of ‘effectiveness’ reporting in calcium sulfate products does not mean nutrient composition alone will explain nutrient effectiveness.

In this fact sheet, “Tech Brief #2: Solubility,” we illustrate how different sources of calcium sulfate can vary in how they become available for plant uptake.

Dissolution and Particle Dispersion

If you assume the same amount of calcium sulfate is spread uniformly in a field, there are two main factors – ‘dissolution’ and ‘particle dispersion’ – that can vary between sources in a way that affect availability.

The dissolution factor is the process by which the small solid particles mix at the molecular level with the water (dissolve) to become part of the soil solution. This process must occur for a plant root to take it up.

The particle size of the calcium sulfate can make a significant difference in how fast the particles dissolve into the soil solution. Mined sources of calcium sulfate are typically much more substantial in size compared to sources from flue gas (such as SUL4R-PLUS® products). These differences are essential because calcium sulfate is similar to lime in how readily it dissolves, meaning the more substantial the particle size, the slower it becomes part of the soil solution. Below is a picture showing the differences in particle size with an electron microscope.

The differences in calcium sulfate products are measured using screens of different sizes to see how much of each calcium sulfate material passes through – as they do for lime. The table below shows the results of various screenings when measuring differences in particle sizes. Notice the finer particles of the SUL4R-PLUS product compared to other options.

Note: the sieve size is the number of wires in an inch, so the higher

the number, the finer the particles are passing through it.

Particle dispersion is defined as the process of the fertilizer granule, breaking down into smaller particles. Calcium sulfate, the tiny particles of calcium sulfate, are usually ‘glued’ together with a binder that forms a ‘pellet’ or ‘granule’ to make it spread better and reduce dust.


A few factors can affect how easily the pellet can release the particles. The type of binder, the size of pellet and the amount of binder can affect how easily the pellet can release the particles.

Below is a 16-second video showing the difference in how easily the ‘pellets’ break down between two commercially available sources of calcium sulfate.


Notice how the other product on the left does not quickly disintegrate even after stirring in water, but the SUL4R-PLUS product on the right does.

For most ag applications you want the pellets to break down so the small particles can react with soil solution and become available for plant uptake. Ag field conditions typically have a limited water availability so we would expect even more considerable differences in the pellet breakdown.


The factors of dissolution and particle dispersion are essential when comparing different sources of calcium sulfate for use in crop production to help growers and agronomists make informed decisions when making their product selection. At SUL4R-PLUS LLC, we believe our products provide superior performance, and this document explains two of them. Please see our other Tech Briefs to learn more about other distinctions of interest to growers and agronomists.

For more information, visit