Calcium (Ca) is crucial because it filters and aids the absorption of many other nutrients. Calcium is a base, which can help raise pH if needed. It is most often used during vegetation, with only small amounts still used during the flowering stage.
Magnesium (Mg) is used by the plant to produce chlorophyll during photosynthesis. Magnesium deficiency shows during flowering as yellowing leaves while the leaf veins are still green.
Sulfur (S) is used only in small amounts by the plant. Sulfur deficiency is shown when your new leaves turn yellow.
Iron (Fe) is also only used in small amounts. Iron deficienciy appears very similar to sulfur deficiency, with the new leaves turning yellow. Be sure to also check your pH if you see signs of iron deficiency because high pH can impair the plant's abllity to absorb iron.
Molybdenum (Mb) aids the intake of nitrogen and is used in the conversion of nitrate to ammonium.
Boron (B) is used for carbohydrate transport in the phloem, but may or may not be required. Boron is not usually found in regular soil and must be supplemented if needed. Be very careful not to over use as excess boron can kill the plant.