Sixteen chemical elements have been classified as plant nutrients.
Major elements are nitrogen, phosphorus and potassium -- which plants generally obtain from the soil -- and carbon, hydrogen and oxygen, which plants obtain from the air.
Plants need secondary elements -- calcium and magnesium -- in smaller amounts and obtain them from the soil.
Plants need still smaller amounts of the micronutrients boron, iron, manganese, copper, zinc, molybdenum and chlorine. Plants use these elements to manufacture food, but they are not "food" themselves.