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, crinkled, and discolored, plant may not set seed. Often boron deficiency is a by product of putting adding too much lime to the soil.
Chloride deficiency – roots are highly branched with stubby tips, leaves wilt then become mottled and tip wilts.
Copper deficiency – leaves are very dark green, and crinkly, stunted growth plants wilt even when given sufficient water. New leaves are small and uniformly yellow. Older leaves curl upward and edges of leaves roll in toward center. Plants are most likely to get fungal infections when copper is deficient. Copper sweetens and strengthens the flavor of fruits and vegetables.
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 can grow much closer together than they should. Black spots can appear on yellowed leaves. Zinc deficiency often occurs when phosphorus levels are high and soil is alkaline.
An excess of one micronutrient usually causes a deficiency of another micronutrient.
Progressive gardens has photos of several deficiencies in plants