“Macro” and “micro” nutrient fertilizers refer to the quantity of nutrient needed by plants, not to theirimportance to plant growth. The following statements define the categories more specifically:
Macronutrients are divided into two classes: primary and secondary. The three primary macronutrients are nitrogen (N), phosphorus (P), and potassium (K); all are required in relatively large quantities by plants.
The secondary macronutrients, calcium (Ca), magnesium (Mg), and sulfur (S), are required in lesser quantities relative to the primary category.
Micronutrients are required by plants in far smaller quantities than any macronutrient. Micronutrients required by plants include zinc (Zn), iron (Fe), manganese (Mn), copper (Cu), boron (B), molybdenum (Mo), and chlorine (Cl). Relative to the macronutrients, micronutrients are required in trace quantities.
Remember that the words “primary” and “secondary” relate to the quantity needed, and not their importance to plant growth; all are equally important.