Macronutrients - The macronutrients include six major elements-nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and surphur (S)-present as salts that constitute various media. All are essential for plant cell and tissue growth. Culture media should contain at least 25 mmoll-¹ nitrate and potassium.
However, considerably better results are obtained if the source for nitrogen in media is contributed by both nitrates and ammonium (2-20 mmoll-¹) or any other reduced nitrogen source. In case only ammonium is used, there is need to add one or more tricarboxylic acid (TCA) cycle acids (e.g., citrate, succinate, or malate) so that any deleterious effect due to ammonium concentrations in excess of 8 mmoll-¹ in the medium is diluted. When nitrate and ammonia ions are present together in the culture medium, the latter are used more rapidly. Other major elements, Ca, P, S, Mg; at concentrations in the range of 1-3 mmoll-¹, appear adequate provided other requirements are satisfied.