Shkolnik (1950) suggested a convenient classification of minerals needed by plants, which is given below
(a) Structural Elements : Carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulphur (S), phosphorus (P) and magnesium (Mg).
Bio-catalytical Elements : They form part of biocatalysts e.g., iron (Fe), copper (Cu), manganese (Mn) and cobalt (Co).
(c) Indispensable Elements : They are potassium, calcium, boron and molybdenum.
(d) Radio-active Elements: Uranium, thorium, radium, actinium and rhodium are its examples.
(e) Stimulating Elements : Sodium, arsenic, titanium,
nickel, bromine, rubidium, chlorine, cobalt and iodine are examples of these.