ABSTRACT
Micronutrient concentrations are expressed as parts per million (ppm) in this text but in SI units, the terms would be either milligrams per kilogram (mg/kg) or millimoles per kilogram (mmol/kg). Comparative values for the micronutrients in these three units are shown in this example (values selected for illustrative purposes only):
Several of the micronutrients were identiƒed as essential before the criteria for essentiality were established by Arnon and Stout (1939), criteria that have been accepted today by the research community.
