Secondary nutrient is a chemical that is used less, but need not be normal because the soil is already exempt from some of the lands that might be missing that one element, such as iron, magnesium, calcium, either, and boron.
Micronutrients are trace elements used, but required. I usually put in the ground already. Except for some soils may lack any element such as calcium, iron, magnesium and boron.
Minor nutrient element is used less, but need to use. Usually not put due in soil are. Except maybe a lack of any soil elements, such as calcium, iron, magnesium and boron.