The nine major nutrient elements (macronutrients) were identified by the mid-1800s and classified into three groups.

Hydrogen, carbon and oxygen are obtained from air and water.

Nitrogen, phosphorus and potassium became the "primary" nutrients supplied by fertilizers.

Calcium, magnesium and sulfur became the "secondary" elements, often supplied in fertilizers or in gypsum, limestone or dolomite. These elements are not secondary in importance or in the role they play in plant nutrition, but rather the amounts of these elements applied to the soil as fertilizers are generally less than those of the primary nutrients - nitrogen, phosphorus and potassium.

(Reference: The Fertilizer Handbook / The Fertilizer Institute - pp 8,45,91)

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