Introduction, definition, and classification of nutrients

This chapter provides a brief overview of the history of plant nutrition. It is currently accepted that plants require 14 essential mineral elements (nutrients), without which they cannot complete their life cycle. The macronutrients nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), and magnesium (Mg) are required in much larger amounts than the micronutrients iron (Fe), manganese (Mn), boron (B), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni), and chlorine (Cl). Plants also take up beneficial elements, such as sodium (Na), silicon (Si), cobalt (Co), iodine (I), selenium (Se), and aluminum (Al), which can improve resistance to pests, diseases, and abiotic stresses as well as plant growth and crop quality. A scheme to classify plant nutrients according to their biochemical properties and physiological function is presented. A proposal that the current definition of essential nutrients is too narrow to encompass a vision for the future of research in plant nutrition aimed at improving crop production and quality, agronomic practice, and fertilizer use and regulation is discussed. Plant nutrient requirements, and mineral composition, can vary greatly as a consequence of genetic and environmental influences. Extreme variation from the average leaf mineral composition (standard leaf ionome) occurs among some orders of plants: Poales (grasses) are low in Ca, Caryophyllales are high in Mg, and Brassicales are high in S even when sampled from diverse environments. The evolution of such traits has been traced using phylogenetic relationships among angiosperm orders, families, and genera. Variation in the leaf ionome has profound consequences for ecology, mineral cycling in the environment, sustainable agriculture, and livestock and human nutrition.