Palm City, if your Sweet Tart is growing in sandy soil, you need to add Calcium to the soil, preferably as Calcium Sulfate (Gypsum). At flowering time you can drench or spray with Calcium Chelate and Boron, or sprayed with Calcium Nitrate.
You need at least twice as much Calcium as Nitrogen, so Calcium Nitrate alone will never get you to the correct ratio, and will easily get you too much Nitrogen.
Har. As far as adding gypsum in sandy soil, can this be inferred for most other varieties of mango? Also, can gypsum be applied via "top dressing" for the first couple years after planting?