The micronutrient needed in smallest amount

The micronutrient needed in the smallest amount is Molybdenum.
Mo is the least abundant essential micronutrient found in most plant tissues and is often set as the base from which all other nutrients are compared and measured. Molybdenum is utilized by selected enzymes to carry out redox reactions.
Molybdenum is required at very low levels where it is known to participate in various redox reactions in plants as part of the pterin complex Moco.