Secondary nutrients are necessary for plant production, but are needed at lower levels than the major nutrients (NPK.)
Calcium strengthens cell walls, helping to reduce bruising and disease in the crop. A good supply of calcium produces food crops which are less susceptible to damage and have a longer shelf life. Fruit and vegetables will also have a higher nutritional value. Crops deficient in calcium will have growth disorders.
Magnesium is important for crop quality and is an important component of leaf chlorophyll and the enzymes that support plant growth. Deficiency in magnesium leads to reduced photosynthesis, which limits crop yields.
Sulfur is an essential part of many amino acids and proteins, and is particularly important for ensuring the protein content of cereal crop grains. A lack of sulfur and magnesium will slow the growth of crops and cause the leaves to turn pale or yellow.
Information from Yara.