Research Background

     There are three types of plant nutrients: primary nutrients, secondary nutrients, and micronutrients. They are as follows:

Primary
Secondary
Micro
Nitrogen (N)
Calcium (Ca)
Zinc (Zn)
Boron (B)
Phosphorous (P)
Magnesium (Mg)
Iron (Fe)
Molybdenum (Mb)
Potassium (K)
Sulfur (S)
Manganese (Mn)
Chlorine (Cl)
Copper (Cu)

     The primary and secondary nutrients are used to build tissues in the roots, stems, leaves, flowers, seeds, and fruits of plants. They are important for biochemical processes regulating enzymes involved in respiration and photosynthesis, plant energy metabolism, the synthesis of DNA and RNA, and cell division. According to plant physiologist Daniel Arnon and Perry Stout, there are three criteria by which an element is considered to be essential: 1) The element is needed for the completion of the life cycle of the plant, 2) it caused a specific deficiency when unavailable and it cannot be replaced by some other element, and 3) it has a defined role in plant metabolism.

     This unit focuses on three essential elements: nitrogen, sulfur, and iron (one primary, one secondary, and one micro). Nitrogen is part of the plant's amino acids, proteins, and DNA. It is a key ingredient of chlorophyll, the light-gathering pigment that captures the energy of sunlight during photosynthesis. This is the nutrient that is most often deficient in soils. Nitrogen is mobile in the plant and will translocate to new growth when a deficiency occurs. Sulfur is required in the formation of proteins and is present in amino acids. This element is immobile during deficiency. Iron, one of the micronutrients, is needed in small amounts; however, it is very essential for energy transfer during photosynthesis and respiration. It exists in proteins and is immobile during a deficiency.

     If the concentration of an essential nutrient element in plant tissue drops below a level necessary for optimal growth, the plant is said to be deficient in that element. When a plant is deficient, substantial changes in metabolism and growth result (metabolism is slowed resulting in stunted growth). No other deficiency is as dramatic as nitrogen deficiency in non-legumes. Symptoms include general chlorosis (yellow appearance of leaves, retarded or slowed growth, and a spindly appearance. Nitrogen deficiencies first appear in older tissues of the plant. Deficiencies in sulfur resemble those of nitrogen, except they appear in new growth. The plants are chlorotic, spindly, and grow poorly. An iron deficiency results in a general chlorosis of young leaves and interveinal chlorosis. Interveinal chlorosis occurs when the leaf veins remain green before eventually becoming chlorotic.