Of the 12 essential plant nutrients which are absorbed by the roots, 6 of them are categorized as micronutrients. That means that only trace amounts are required for normal metabolic functions. Of these, iron is the one required in the highest dosages. Since it has a lot of complications with availability I have devoted an entire blog post to it.
The 6 micronutrients are iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), boron (B), and molybdenum (Mo). The first 4 are generally lumped together since all of these are either fertilized as either sulfates or chelates and they have somewhat similar functions. Molybdenum and boron don't have much in common either chemically or for their functions but do seem to get lumped together in an unofficial "other" category.
The most common function of micronutrients is as coenzymes. An enzyme is a protein which causes different metabolic reactions inside of the cell. Many enzymes require a smaller cofactor to activate it, vitamins serve this function and plants are able to synthesize their own. Some metals are used in these as well and so the plant has a requirement for them. Iron, manganese, and copper are all used in coenzymes for photosynthesis. Zinc is used in DNA transcription reactions, and molybdenum in nitrogen metabolism reactions.
Boron has many Functions
It is a component of cell walls, is involved in the transportation of sugars, and in reproduction (specifically pollination and seed germination).
Boron is non-motile meaning that once it's deposited in a location it will not be moved to other places inside of the plant. This means that any deficiencies will show up in younger tissues first. Since it's associated with cell walls a deficiency will translate to death in the meristems (stem tips where the growth is from).
Molybdenum is Required in Extremely Small Amounts
Looking at the guaranteed analysis of a fertilizer bottle you will see very low concentrations. Holland Secret Micro contains 0.0006% (6 PPM) for example, and once this is diluted the plant is fed about 0.02 PPM.
Molybdenum is involved with enzymes in nitrogen metabolism and so a deficiency will show up with many of the same symptoms as one with nitrogen.
*Did you know*
Molybdenum may have sped up the rate of biological evolution on earth. In the primitive earth which had low oxygen, the rate of change was very slow. Once oxygen became available from photosynthesis it solublized molybdenum which was very helpful in more complex reactions as a coenzyme and consequently the rate of changes increased.
Manganese Fertilizer Stability
The stability of manganese in a concentrated fertilizer can be an issue. It's very light pink in colour. In the hydroponics industry micronutrients are typically found together with calcium in the 'Micro' component of a three part feeding program. Manganese solubility can be destabilized in the presence of calcium and so when using a three part if there is sediment with a pinkish tinge then think twice before using it.