found this on the net.
Certain fertilizers generally cannot be mixed at high concentrations. Fertilizer compounds containing sulfate, (for example magnesium sulfate), are not compatible in solution with calcium nitrate because a reaction occurs where insoluble calcium sulfate (gypsum) will form as a precipitate (solid). If a blended fertilizer contains both calcium and magnesium, then the sources have to be calcium nitrate and magnesium nitrate. Similarly, calcium nitrate and monoammonium phosphate cannot be mixed in the same concentrated solution because insoluble calcium phosphate will form as a precipitate.
Sorry, but you asked. ;o)
The short version is that the chemicals used in soluble fertilizers tend to cause Ca to precipitate (fall out) of solution as an insoluble solid where is if no value to the plant, so why include it?
A few fertilizers, like Foliage-Pro 9-3-6 DO contain good amounts of Ca and Mg in the proper proportions. It should be an excellent fertilizer for houseplants.
here is some more info,
Hi, Joel. You're right. Even though Calcium (Ca) is generally found in plant tissues at about 1/10 as much as N. it is extremely important. Ca is virtually immobile in plants (this means the plant cannot rob it from old leaves to help make the new leaves, like it can do with Nitrogen), so plants need a continual supply of Ca as new leaves form. The stability and function of cell membranes requires Ca to act as sort of a "glue" in the form calcium pectate, which binds adjacent cells together. If adequate amounts of Ca are not transported during cell formation, tissues become less stable and prone to disintegration. As a Ca deficiency arises, concentrations of Ca will be higher in older, already formed foliage, so it is the newest growth which first shows deficiency symptoms. Often tip burn and/or twisted or cupped growth are indicators.
Ca also plays a role in activating enzymes and messenger chemicals in the plant, regulating the flow of water movement in cells and is essential for all cell growth and division. It also acts as a buffer when excesses of other elements are in soils, making it an important component of root vitality.
Al