Plant Nutrition :: Mineral Nutrition

Essentiality of Elements in Plant Nutrition

A mineral element is considered essential to plant growth and development if the element is involved in plant metabolic functions and the plant cannot complete its life cycle without the element. Usually the plant exhibits a visual symptom indicating a deficiency in a specific nutrient, which normally can be corrected or prevented by supplying the nutrient. Terms commonly used to describe levels of nutrients in plants:

Deficient: When the concentration of an essential element is low enough to limit yield severely and distinct deficiency symptoms are visible. Extreme deficiencies can result in plant death. With moderate or slight deficiencies, symptoms may not be visible, but yields will still be reduced.

Critical range: The nutrient concentration in the plant below which a yield response to added nutrient occurs. Critical levels or ranges vary among plants and nutrients, but occur somewhere in the transition between nutrient deficiency and sufficiency.

Sufficient: The nutrient concentration range in which added nutrient will not increase yield but can increase nutrient concentration. The term luxury consumption is often used to describe nutrient absorption by the plant that does not influence yield.

Excessive or toxic: When the concentration of essential or other elements is high enough to reduce plant growth and yield. Excessive nutrient concentration can cause an imbalance in other essential nutrients, which also can reduce yield.

Essential Elements

Sixteen elements are considered essential to plant growth. Carbon (C), hydrogen (H) and oxygen (O) are the most abundant elements in plants. The photosynthetic process in green leaves converts CO2 and H2O into simple carbohydrates from which amino acids, sugars, proteins, nucleic acid and other organic compounds are synthesized. Carbon, H and O are not considered mineral nutrients. The supply of CO2 is relatively constant. The supply of H2O rarely limits photosynthesis directly but does indirectly though the various effects resulting from moisture stress.

The remaining 13 essential elements are classified as macronutrients and micronutrients and the classification is based on their relative abundance in plants. The macronutrients are nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca) and magnesium (Mg). Compared to the macronutrients, the concentrations of the seven micronutrients – iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), born (B), chlorine (Cl) and molybdenum (Mo) – are very small. Five additional elements – sodium (Na), cobalt (Co), vanadium (Va), nickel (Ni) and silicon (Si) have been established as essential micronutrients in some plants. Micronutrients are often referred to as minor elements, but this does not mean that they are less important than macronutrients. Micronutrient deficiency or toxicity can reduce plant yield similar to macronutrient deficiency or toxicity.

In fact, plants absorb many nonessential elements, and over 60 elements have been identified in plant materials. When plant material is burned, the remaining plant ash contains all the essential and nonessential mineral elements except, C, H, O, N and S which are burnt off as gases.

Soil, climate, crop variety and management factors exert considerable influence on plant composition. Because many biological and chemical reactions occur with fertilizers in soils, the quantity of nutrients absorbed by plants does not equal the quantity applied as a fertilizer. Proper fertilizer management can maximize the proportion of fertilizer nutrient absorbed by the plant. As plants absorb nutrients from the soil, complete their life cycle and die, the nutrients in the plant residue are returned to the soil. These plant nutrients are subject to the same biological and chemical reactions as fertilizer nutrients. Although this cycle varies somewhat among nutrients, understanding nutrient dynamics in the soil plant atmosphere system is essential to successful fertilizer management.

Table 1. Essential Nutrients for plant growth and their principal forms for uptake

Nutrient	Chemical Symbol	Principal forms for uptake
Carbon	C	CO2
Hydrogen	H	H2O
Oxygen	O	H2O, O2
Nitrogen	N	NH+4, NO-3
Phosphorus	P	H2PO-4, HPO2-4
Potassium	K	K+
Calcium	Ca	Ca2+
Magnesium	Mg	Mg2+
Sulfur	S	SO2-4, SO2
Iron	Fe	Fe2+, Fe3+
Manganese	Mn	Mn2+
Boron	B	H3BO3
Zinc	Zn	Zn2+
Copper	Cu	Cu2+
Molybdenum	Mo	MoO2-4
Chlorine	Cl	Cl-

Table 2 . Relative and Average Plant Nutrient Concentrations

Plant Nutrient	Average Concentration*
H	6.0%
O	45.0%
C	45.0%
N	1.5%
K	1.0%
Ca	0.5%
Mg	0.2%
P	0.1%
S	0.1%
Cl	100 ppm (0.01%)
Fe	100 ppm
B	20 ppm
Mn	50 ppm
Zn	20 ppm
Cu	6 ppm
Mo	0.1 ppm

* Concentration expressed by weight on a dry matter basis.

Table 3 . Functions of Essential Nutrients in Plants

Nutrient	Function
Carbon	Basic molecular component of carbohydrates, proteins, lipids, and nucleic acids.
Oxygen	Oxygen is somewhat like carbon in that it occurs in virtually all organic compounds of living organisms.
Hydrogen	Hydrogen plays a central role in plant metabolism. Important in ionic balance and as main reducing agent and plays a key role in energy relations of cells.
Nitrogen	Nitrogen is a component of many important organic compounds ranging from proteins to nucleic acids.
Phosphorus	Central role in plants is in energy transfer and protein metabolism.
Potassium	Helps in osmotic and ionic regulation. Potassium functions as a cofactor or activator for many enzymes of carbohydrate and protein metabolism.
Calcium	Calcium is involved in cell division and plays a major role in the maintenance of membrane integrity.
Magnesium	Component of chlorophyll and a cofactor for many enzymatic reactions.
Sulfur	Sulfur is somewhat like phosphorus in that it is involved in plant cell energetic.
Iron	An essential component of many heme and nonheme Fe enzymes and carries, including the cytochromes (respiratory electron carriers) and the ferredoxins. The latter are involved in key metabolic function such as N fixation, photosynthesis, and electron transfer.
Zinc	Essential component of servral dehydrogenases, and peptidases, including carbonic anhydrase, alcohol dehydrogenase, glutamic dehydrogenase, and malic dehdrogenase, among others.
Manganese	Involved in the O2 – evolving system of photosynthesis and is a component of the enzymes arginase and phospho transferases.
Copper	Constituent of a number of important enzymes, including cytochrome oxidize, ascorbic acid oxidase, and laccase.
Boron	Involved in carbohydrate metabolism and synthesis of cell wall components.
Molybdenum	Required for the normal assimilation of N in plants. An essential component of nitrate reductase as well as nitrogenase (N2 fixation enzyme)
Chlorine	Essential for photosynthesis and as an activator of enzymes involved in splitting water. It also functions in osmoregulation of plants growing on saline soils.

General absorption and mobility rankings for foliar applied nutrients

Absorption	Mobility
Rapid Urea Nitrogen, Potassium, Zinc Moderate Calcium, Sulfate, Manganese, Boron Slow Magnesium, Copper, Iron, Molybdenum	Mobile Urea Nitrogen, Potassium, Phosphorus, Sulfate Partially Mobile Zinc, Copper, Manganese, Boron, Molybdenum Immobile Iron, Calcium, Magnesium

Source: http://www.plantstress.com/Articles/min_deficiency_m/mitigation.htm