(Editor's Note: This article was originally published on March 25, 2009. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
Because plants only need very small amounts of micronutrients, we often overlook them in a fertilizer program. Once upon a time, our native soils were rich in micronutrients, but we have used them up over the years and now most soils could use a micronutrient boost. We don’t need to get much back into the soil, just a tiny bit works wonders. As an example, cobalt is needed only in minute amounts by plants, animals and humans. Yet without that trace amount, we would die (and too much can cause cobalt poisoning.) Cobalt is the central component in vitamin B12.
Plant nutrition is such a complex subject with so many variables that there are no two gardens alike in nutritional needs. The best we can do is have general guidelines, and an understanding of which nutrient does what. Then by trial and error, we determine what works for us. Once I really got down to business about learning plant nutrition over the past several years, I learned two things that work best for my garden are available calcium, and micronutrients.
The more common of the micronutrients are iron (Fe), molybdenum (Mo), boron (B), copper (Cu), manganese (Mn), zinc (Zn), chloride (Cl) and nickel (Ni). Others include cobalt, silicon, sodium, selenium, uranium, antimony, lead, cadium, chromium, fluorine, barium oxide, titania, scandium, silver, bismuth, palladium, lithium and about 50 other minerals and trace elements.
Our knowledge of the function and/or necessity of these micronutrients is small, but growing. Not all plants need all the micronutrients, and not all soils or plants (even in the same family) are the same. It is important in balancing the nutrient needs of plants to know what these more common micronutrients do, and then test to see what’s already in your soil.
Iron and copper are necessary for photosynthesis (the manufacturing of chlorophyll). Iron when found in the soil is usually in the form of iron sulfate and/or chelated iron. Deficiencies of either iron or copper often show up in yellowing or even necrosis of the leaves.
Molybdenum aids in the utilization of nitrogen and the building of amino acids. A couple of trace elements affect plant sugar production: zinc and boron. Zinc also plays a role in synthesizing RNA from DNA, and in converting carbohydrates; it is a part of the plant system (enzymes) that regulate growth. Zinc is available as zinc oxide, zinc sulfate and zinc chelate. Boron is essential for the production of fruits and seeds and also helps in regulating other nutrients. Boron is also important in cell division, and a deficiency causes stunted growth and necrosis in young leaves. If the soil is low in boron, calcium cannot function and move the calcium into the plant, causing calcium deficiency. You can get boron from balanced organic matter and from borax.
Manganese, like molybdenum, aids in utilizing nitrogen and plays a part in breaking down carbohydrates. It is necessary for building the chloroplasts, which are found in the plant cells working in the complex system of photosynthesis. A deficiency of manganese often shows up in strangely-colored leaves or in spots on leaves.
Nickel is another micronutrient involved in utilizing nitrogen, and in enzyme activation. Silicon (not the same as silicone) is not considered essential for most terrestrial plants although studies suggest it is beneficial to many, if not most of them. Silicon does play a role in the reduction of fungal diseases in some plants.  Silicon also helps protect plants from stress.
Cobalt is essential in some plants like legumes, as a “bio-stimulant” for nitrogen fixing, and beneficial in others. Soils very low in cobalt may grow plants with a cobalt deficiency, and when animals graze on them, the animals suffer from the lack of cobalt.  Cobalt in small amounts is essential to many living organisms, including humans. Having 0.13 to 0.30 mg/kg of cobalt in soils markedly improves the health of grazing animals. Cobalt deficiency is associated with the incidence of Johnne’s disease, the ruminant analog of Crohn’s disease in humans. Johnne’s disease is a huge problem in today’s confinement dairy system. 
Chloride is considered a micronutrient, but plants can take up as much chloride as they do some macronutrients. Chloride has a critical role in plant metabolism, basically along with potassium in opening and closing of the stomata. It also diminishes plant fungal infections. Too much nitrates in the soil affect chloride uptake and have been linked to disease severity. Chloride ions are highly soluble and susceptible to leaching. If you regularly apply muriate of potash (potassium chloride), chances are you don’t have a chloride deficiency.
Selenium and sodium are beneficial, and sodium can substitute for potassium in regulating the stomata. Vanadium may be required by some plants, but in very small amounts. It can also stand in for molybdenum.
“Chloride, nitrate, sulfate, borate, and molybdate are all anions in their available forms, and in that form they are antagonistic to each other. Therefore, an excess of one can decrease the availability of another.” 
Since I first read Bread from Stones several years ago, I have grown to study and understand the importance of micronutrients. I now routinely use Greensand™ and Azomite™. (I have no financial interest nor connection with either company.) See my article on Rock Dust for more information.
Greensand is a hydrous, potassium silicate with varying amounts of calcium, magnesium, phosphorus, iron and a wide variety of 30 + other trace minerals typical of an ancient marine bed. It is mined in New Jersey, and helps bind sandy soils and loosen clay soils (don’t ask me how!). Azomite is a naturally mined volcanic material with over 66 trace minerals and trace elements important for plant nutrition and growth. It improves depleted soils. Apply with compost, humus, manures or other fertilizers to provide additional levels of NPK. Will not burn plants. (Information about both was taken from the bags in my shed.)
Here are links to my other soil nutrition articles:
Iron sulfate photo is in the public domain
Boron crystals photo is in the public domain
Copper sulfate photo is in the public domain
Manganese(II)-sulfate photo is in the public domain
Zinc sulfate photo is licensed under the GNU Free Documentation License
Nickel(II)-sulfate photo is in the public domain
Vanadium oxide photo is in the public domain
Silicon dioxide photo licensed under the Creative Commons Attribution ShareAlike 2.5
Iron Oxide photo is in the public domain
Manganese(IV) oxide photo is in the public domain
Chloroplasts photo is licensed under Creative Commons Attribution ShareAlike 2.0 Germany