Herself’s Houston Garden

In addition to Secondary plant nutrients, and primary plant nutrients. Micronutrients are essential for plant growth but only in extremely small amounts. Usually just adding a little organic matter ( worm castings, compost etc ) will take care of all your plants micro nutrients. This is more of an issue in food crops for humans and animals. Lack of micronutrients in food crops can cause diseases related to these deficiencies in humans and animals. So be especially sure your herb and vegetable garden get a bit of organic matter each year.

Signs of micronutrient deficiencies in plants:

Boron deficiency - newest buds die, stems may crack and split, young leaves are distorted in shape, plant may not set seeds.

Chloride deficiency - roots are highly branched with stubby tips, leaves wilt then become mottled and tip wilts.

Copper deficiency - leaves are very dark green, stunted growth plants wilt even when given sufficient water.

Iron deficiency - yellow leaves with green veins, yellowing begins at center of leaf where it attaches to the stem

Molybdenum deficiency - older leaves yellow, newer leaves are light green

Zinc deficiency - yellow and brown mottling of leaves, green veins, also small leaves that are distorted.

An excess of one micronutrient usually causes a deficiency of another micronutrient.

See also:
Secondary plant nutrients
Primary plant nutrients

More information:
Progressive gardens has photos of several deficiencies in plants