When talking about soil and fertilizers, what are the primary nutrients in addition to NPK (Nitrogen, Phosphorus, Potassium) that play a major role in plant development?
Primary macronutrients are NPK as you mention.
The secondary [macro]nutrients are calcium (Ca), magnesium (Mg), and sulfur (S).
Micronutrients are those elements essential for plant growth which are needed in only very small (micro) quantities. The micronutrients are boron (B), copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo) and zinc (Zn).