Carbon of a plant is derived from that of atmospheric carbon dioxide. The hydrogen contained in the organic material of a plant is derived from the water taken up by roots, while the oxygen is derived both from the water and from the atmospheric sources —CO2 and O2. Plants exhibit many inorganic mineral constituents, which are made available to plant from soil through the root system.

Nitrogen available in atmosphere is inert and cannot be directly used by most of the plants. It combines with oxygen and is brought down to soil with rain. Nitrogen is converted into anionic forms as nitrite and nitrate or reduced to cationic form as ammonium (NH4+) by nitrogen fixers found in soil. The conversion of N2 to NH4+ is accomplished by asymbiotic nitrogen fixation (nitrogen fixation by free living organisms not in association with another).

Nitrogen fixation may be done by organisms found in close association with other i.e. symbiotically and nitrogen is incorporated into amino acids. A relatively large group of plants, the legumes obtain fixed nitrogen through a symbiotic association with soil bacteria of genus Rhizobium. The actual site of nitrogen fixation is the nodules formed in the roots of legume plants as a result of penetration of bacterium Rhizobium. Compounds like nitrite and nitrate enter the plants as nutrients through the root and are assimilated as organic nitrogen. Heterotrophic organisms get their nitrogen supply through these plants. All other elements are made available to plants from soil.

Soils are developed slowly, over many thousands of years as a result of several geological processes. These processes include chemical and physical weathering of rocks, accumulation and decay of living organism etc. Source of these elements are rocks which on weathering add these elements in soil hence called mineral elements. Carbon, hydrogen, oxygen are non-mineral elements. Nitrogen is derived from mineral and non-mineral sources. Although most plants utilize the nitrate form of nitrogen, several plants can assimilate ammonia and certain forms of organic nitrogen.