Why Cobalt is an Essential Element for Plants

Cobalt (Co) is a metallic element, neighbouring manganese, iron, nickel, copper and zinc in the periodic table of elements. It was established long ago that cobalt is essential for living beings that belong to different genera, from microorganisms to mammals.

Some of its central roles in their lives stem from being an essential component of vitamin B12, and of some specific enzymes found in nitrogen-fixing bacteria, whether or not living symbiotically within the nodules of legume plants (e.g. rhizobia). Cobalt application is occasionally reported to benefit crop growth; however, its essentiality for higher plants' physiology cannot be proven, perhaps because the plant requirement for it, measured in parts-per-billion, is always found in the soil or as a contaminant of other nutrients.  

Cobalt is found in most soils at low concentrations. Its incidence depends on soil's parent material, soil's texture and its contents of organic matter. Sorption by mineral surfaces and organic matter decreases with rising pH, in a similar way to that of zinc (Zn) and nickel (Ni). Weathered, coarse-textured soils are generally low in cobalt (Co) because they don't resist its leaching. Finer-textured soils, and soils containing higher levels of organic matter, tend to have greater Co concentrations. And soils developed from minerals such as olivine and pyroxene have ample Co. It is largely present as Co2+. Its highest masses are required in trace amounts by atmospheric nitrogen fixing microorganisms. 

Cobalt (Co) is an element which is not commonly thought to play as crucial of a role in the health of a plant. However, is one of the elements which is classified as an essential micronutrient!