Calcium is a nutrient deficiency you’ll see often, and a lot of nutrient makers sell a calcium-magnesium solution because of this. You’ll see a bunch of tiny black dots scattered on top of the leaves. Make sure to look underneath, because if you see them there, then you might have a bug problem. Usually, when you look at the underside of leaves with a calcium def, you’ll see the black dots are translucent through the top of the leaf and not directly on the bottom.
It’s a semi-mobile nutrient, so expect to see deficiencies on leaves that are near the top of the plant or the ones getting direct light. Deficiencies may also look more like brown spot or rust instead of black dots.