Mineral nutrients absorbed in the inorganic form are indispensable for plant growth and development. The various processes underlying yield formation in crops such as biomass accumulation and its partitioning, are directly regulated by nutrient supply. Plants essentially require 17 elements for the production of optimum biomass and yield, while some additional elements are beneficial for their survival under stress and/or improvement in quality of economic produce. The relationship between nutrient supply and yield was well established by Mitscherlich (1947) and is known as the ‘Law of diminishing yield increment’. This law states that if the supply of a particular nutrient is increased, it will cause non-availability of other nutrients or the limitation of genetic potential of crops, resulting in no increase or even a decrease in yield. Furthermore, it is observed that when a particular nutrient is supplied in abundance, the decrease in yield could be due to physiological factors, toxicity or induced deficiency of other nutrients within the plant, or in the soil, and interactions with other elements. For example, excess supply of nitrogen (N) to cereals causes lodging thereby reducing yield. Another example of induced toxicity is where excess nickel (Ni) supplementation displaces magnesium (Mg2+) ions from Rubisco, resulting in a loss of enzyme activity (Wildner & Henkel, 1979; van Assche & Clijsters, 1986). Likewise, higher tissue concentrations of zinc (Zn) reduce the uptake of phosphorus (P) and vice-versa (Mohammed et al. 2021). Nutrient availability in soil is primarily dependent on the pH of the soil solution, which is very often altered by the presence of excess amounts of some nutrients. For example, in acid soils with high aluminium (Al3+) and iron (Fe3+) content, the availability of inorganic P to plant roots is restricted. Therefore, to achieve optimal growth and potential yield of crops, not only nutrient deficiencies but also excesses or toxicity stresses need to be taken into consideration. The concentration of any particular nutrient in plant tissues beyond the critical threshold level, either in the deficit or the toxic zone, will result in decline in yield and quality.
