These are elements, that effect soil fertility but are present in tiny amounts, too small to measure without some very sophisticated equipment. Different trace elements favour different plants. We are still discovering what trace elements do in plants. In many cases they are present in such small amounts, it is hard to determine if they are a trace or a contaminant!
A high trace of iron, for example will promote citrus growth, especially oranges. Iron is essential to all plants because it is used to produce chlorophyl – the green compound that plants use to convert sunlight into enough energy to combine carbon dioxide with water to get glucose.
Calcium is also an important trace mineral for plants, just like it is for animals. It is used in the formation of cell walls and a range of other functions.
Molybdenum is vital in the cellular reactions of all plants but especially in nitrogen fixing plants like peas, lupins and clover. A field of pasture of mixed grasses will become dominant with clover if molybdenum is added in trace quantities.
Magnesium is an important trace element in plants. If defficient, the leaves will appear yellow or blotched with yellow patches. The plants will be vulnerable to pest borne diseases. It is required to break down glucose and release energy at the cellular level in all life forms but it serves a double role in plants because it is also required to make chlorophyl.
Cobalt is not regarded as an important trace element in plants but if defficient in grazing animals, they become emaciated because it is a key ingredient in the enzyme rumen bacteria create to digest the cellulose in plants.
There are many other elements that appear as trace elements and we don’t know what they do. Some are thought to assist bacteria in breaking down compounds for use by plants. Others could be used as catalysts by the plants themselves. Because they are present in such tiny amounts and are not actually consumed by the plants it’s difficult to decide if they are contaminants, inert compounds or essential to some function in the plant.