Primary: Hydrogen, carbon, nitrogen, oxygen, phosphorus.
Secondary: Calcium, magnesium, sodium, potassium, iron, sulphur, chlorine.
Micro: Manganese, copper, boron, silicon, aluminum, fluorine, iodine.
Then there are another eighteen elements at least variably necessary to life, viz:
(1) Variable Secondary Elements: Zinc, titanium, vanadium and bromine.
(2) Variable Micro-Elements: Lithium, rubidium, caesium, silver, beryllium, strontium, cadmium, germanium, tin, lead, arsenic, chromium, cobalt and nickel.
Elements in Soils Essential for Plant Growth
It is furthermore safe to state at the present time that fertile soils should contain at least the following twenty elements: Nitrogen, phosphorus, potassium, calcium, magnesium, sulphur, hydrogen, carbon, oxygen, iron, sodium, chlorine, aluminum, silicon, manganese, copper, zinc, boron, iodine, and fluorine.
Until quite recently many scientists believed that only the first ten elements were necessary for growth and maturing of crops; that only the first three should be considered as fertilizer ingredients, and that the others were supplied by soil, air and water, or were present as natural fillers in manures and fertilizer raw materials.
The modern agronomist, however, takes all these twenty essential elements into consideration, and many so-called "complete" fertilizers contain at least sixteen to eighteen, if not all of the elements mentioned above. Cobalt, essential to animal nutrition, can also most economically be supplied through the soil, even though crops grow without it.