Zinc

Zinc is a very important trace element because it not only is the only metal present in all six classes of enzymes but it is also an enzyme cofactor (meaning it enables the plant to function properly). It is  linked to many important processes including protein synthesis, cell membrane maintenance and DNA transcription. It is the second most abundant transition metal found in organisms after iron and as such has a relatively high typical concentration of < 125 ppm. Unfortunately 90% of this is insoluble and biologically unavailable and that is why the zinc concentrations in plants vary between only 0.02-0.04 ppm. 

The symptoms of zinc toxicity include leaf chlorosis (yellowing), necrosis of old leaves and decrease in length and number of roots. Zinc deficiency however is much more common and is thought to affect 30% of agricultural crops worldwide. It is such a major threat because of the vast range of processes zinc is involved in, and common symptoms include decreased photosynthetic rates, leaves turning red-brown and decreased leaf size. Zinc is usually added as a foliar spray in the form of zinc sulfate heptahydrate (ZnSO4·7H2O) or zinc chelate (Zn-EDTA) where a soil application of 5-10 kg/ha is recommended. 

Hooda, P. (2010). Trace elements in soils. Wiley.

Sillanpää, M. (1972). Trace Elements in soils and agriculture. FAO of the UN.