General Information

Plant nutrients in fertilizers are classified as Macronutrients (N, P, K), Secondary nutrients (Ca, Mg, S) and Micronutrients (B, Cu, Fe, Mn, Zn, Mo). 
Macronutrients and Secondary nutrients are used in relatively large amounts, but Micronutrients are used in small amounts and are sometimes referred to as Trace Elements.

Micronutrients

BORON:
Boron primarily regulates the carbohydrate metabolism in plants.  It is essential for protein synthesis, see and cell wall formation, germination of pollen grains and growth of pollen tubes.  Boron is associated with sugar translocation.  Alkaline soils have reduced updake of Boron due to high pH.  Leached soils may be Boron deficient because of low Boron reserves.  Sandy soils, organic soils and some fine textured lake bed soils are most frequently deficient in Boron.

COPPER:
Copper is essential for plant growth and activation of many enzymes.  A copper deficiency interferes with protein synthesis and causes a buildup of soluble nitrogen compounds.  Without Copper all crops can fail to grow.  Copper applied to soil is not easily leached.

IRON:
Iron is a constituent of many organic compounds in plants.  It is essential for synthesizing chlorophyll, which gives plants their green color.  Iron deficiency can be induced by high levels of Manganese.  High Iron levels can also cause a Manganese deficiency.

MANGANESE:
Manganese is absorbed by plants in the Mn++ ionic form.  Manganese may substitute for Magnesium by activating certain phosphate-transferring enzymes, which in turn affect many metabolic processes.  A high Manganese concentration may induce an Iron deficiency in plants.  Manganese availability is closely related to the degree of soil acidity.  Deficient plants are usually found on slightly acid or alkaline soils. 

ZINC:
Zinc is essential for plant growth because it controls the synthesis of indoeacetic acid, which dramatically regulates plant growth.  Zinc is also active in many enzymatic reactions and is necessary for chlorophyll synthesis and carbohydrate formation.  Because Zinc is not readily translocated in the plant, deficiency symptoms first appear on younger leaves.  Soils associated with Zinc deficiency are usually neutral to alkaline in reaction.  The more alkaline the soil the greater the need for Zinc.

Secondary Nutrients

CALCIUM:
Calcium is an essential part of the plant cell wall structure and provides normal transport and retention of other elements and strength in the plant.  It is thought to counteract the effect of alkali salts and organic acids within the plant.  Calcium is absorbed as the Ca++ ion.

SULFUR:
Plants take up Sulfur as Sulfate ions (S04=).  It is reduced and assembled in the plant as organic compounds.  It is required for nodulation and nitrogen fixation in legumes.  As the Sulfate ion, it may be responsible for activating some enzymes.

MAGNESIUM:
Magnesium is a part of the chlorophyll molecule in all green plants and is essential for photosynthesis and helps activate plant enzymes needed for growth.  Magnesium is relatively mobile and is absorbed as the Mg++ ion.  It is readily translocated from older to younger plants.

Source:  Michigan State University Extension