From a management perspective only, the primary nutrients are N, P, and K, because they are most often limiting from a crop production standpoint. All of the other essential macronutrient elements are secondary nutrients because they are rarely limiting, and more rarely added to soils as fertilizers.
The ability of soils to supply secondary nutrients to plants indefinitely is
is subject to the law of conservation of matter and is therefore dependent upon nutrient cycling.
Continued crop removal of Ca, Mg, and S requires replentishment just as surely as primary nutrients,
but most likely less frequently. Calcium and magnesium are often supplied by mineral weathering,
either of natural soil materials or of aglime, ground limestone added to correct soil acidity.
Sulfur is often added to soil as either atmospheric deposition (associated with air pollution) or
as impurities in fertilizers, particularly common P fertilizers.
Whether a macronutrient or micronutrient, or whether a primary or secondary nutrient, the Law of the Minimum holds: the most growth-limiting nutrient will limit growth, no matter how favorable the nutrient supply of other elements. For example, a deficiency of Fe or Mn (most common in soils containing calcium carbonate) can severely limit plant growth in spite of adequate N, P, and K.