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Note: Originally posted by Delta at another site.

Action Mode, Deficiency and Toxicity Symptoms of the 17 Essential Nutrients
MACRO NUTRIENTS
Nitrogen (N)
Action Mode
Absorbed as NO3-, NH4+; responsible for rapid foliage growth and green color; easily leaches from soil, especially NO3-; mobile in plant, moving to new growth
Deficiency
Reduced growth, light green to yellow foliage (chlorosis); reds and purples may intensify with some plants; reduced lateral breaks; symptoms appear first on older growth
Excess
Succulent growth, leaves are dark green, thick and brittle; poor fruit set; excess ammonia can induce calcium deficiency
Comments
The best NH4+/NO3- ratio is 1/1; high NH4+ under low light can cause leaf curl; uptake inhibited by high P levels; indoors, best N/K ratio is 1/1 unless light is extremely high; in soils with high C/N ratio more N should be supplied.


Phosphorus (P)
Action Mode
Promotes root formation and growth; affects quality of seed, fruit and flower production; increased disease resistance; does not leach from soil readily; mobile in plant, moving to new growth
Deficiency
Reduced growth; leaves dark green; purple or red color in older leaves, especially on the underside of the leaf along the veins; leaf shape may be distorted; thin stems; limited root growth
Excess
Shows up as micronutrient deficiency of Zn, Fe, or Co
Comments
Rapidly "fixed" on soil particles; when applied under acid conditions, fixed with Fe, Mn and Al; under alkaline conditions fixed with Ca; high P interferes with micronutrient and N absorption; used in relatively small amounts when compared to N and K; availability is lowest in cold soils.


Potassium (K)
Action Mode
Helps plants overcome drought stress; improves winter hardiness; increased disease resistance; improves the rigidity of stalks; leaches from soil; mobile in plant
Deficiency
Reduced growth; shortened internodes; margins of older leaves become chlorotic and burn; necrotic (dead) spots on older leaves; reduction of lateral breaks and tendency to wilt readily; poorly developed root systems; weak stalks
Excess
Causes N deficiency in plant and may affect the uptake of other positive ions such as Mg and Ca
Comments
High N/low K favors vegetative growth; low N/high K promotes reproductive growth (flower, fruit); calcium excess impedes uptake of potassium


Magnesium (Mg)
Action Mode
Absorbed as Mg++; leaches from sandy soil; mobile in plant
Deficiency
Reduction in growth; yellowish, bronze, or reddish color of older leaves, while veins remains green; leaf margins may curl downward or upward with a puckering effect
Excess
Interferes with Ca uptake; small necrotic spots in older leaves; smaller veins in older leaves may turn brown; in advanced stage, young leaves may be spotted
Comments
Mg is commonly deficient in foliage plants because it is leached and not replaced; epsom salts at a rate of 1 teaspoon per gallon may be used two times a year; Mg can be absorbed by leaves if sprayed in a weak solution; dolomitic limestone can be applied in outdoor situations to rectify a deficiency


Calcium (Ca)
Action Mode
Absorbed as Ca++; moderately leachable; limited mobility in plant; essential for growth of shoot and root tips; reduces the toxicity of aluminum and manganese
Deficiency
Inhibition of bud growth; roots can turn black and rot; young leaves are scalloped and abnormally green; leaf tips may stick together; cupping of maturing leaves; blossom end rot of many fruits, pits on root vegetables; stem structure is weak; premature shedding of fruit and buds
Excess
Interferes with Mg absorption; high Ca usually causes high pH which then precipitates many of the micronutrient so they become unavailable to the plant
Comments
Ca is rarely deficient if the correct pH is maintained; too much or too little water, can affect Ca relationships within the plant causing deficiency in the location where Ca was needed at the time of stress

Sulfur (S)
Action Mode
Absorbed as SO4-; leachable; not mobile; contributes to odor and taste of some vegetables
Deficiency
Rarely deficient; general yellowing of the young leaves then the entire plant; veins lighter in color than adjoining interveinal area; roots and stems are small, hard and woody
Excess
Sulfur excess is usually in the form of air pollution
Comments
Sulfur excess is difficult to control but rarely a problem.


MICRONUTRIENT
Iron (Fe)
Mode of Action
Absorbed as Fe++, Fe+++; accumulates in the oldest leaves and is relative immobile in the phloem; necessary for the maintenance of chlorophyll
Deficiency
Interveinal chlorosis primarily on young tissue, which may become white; Fe deficiency may occur even if Fe is in the soil when: soil high in Ca, poorly drained soil, soil high in Mn, high pH, high P, soil high in heavy metals (Cu, Zn), oxygen deficient soils or when nematodes attack the roots; Fe should be added in the chelate form; the type of chelate needed depends upon the soil pH; foliar fertilization will temporarily correct the deficiency; may be deficient in centipedegrass where pH and P are high
Excess /Comments
Rare except on flooded soils

Boron (B)
Mode of Action
Absorbed as B(OH)3-; important in enabling photosynthetic transfer; very immoble in plants
Deficiency
Failure to set seed; internal breakdown of fruit or vegetable; death of apical buds, giving rise to witches broom; failure of root tip to elongate normally; young leaves become thick, leathery, and chlorotic; rust colored cracks and corking on young stems, petioles and flower stalks (e.g. heart rot of beets, stem crack of celery); breakdown occurs at the base of the youngest shoots
Excess /Comments
Tips and edges of leaves exhibit necrotic spots coalescing into a marginal scorch (similiar to high soluable salts); oldest leaves are affected first; plants are easily damaged by excess application

Zinc (Zn)
Mode of Action
Absorbed as Zn++; enzyme activity
Deficiency
Young leaves are very small, sometimes missing leaf blades; short internodes; distorted or puckered leaf margins; interveinal chlorosis
Excess /Comments
Sever stunting, reddening; poor germination; older leaves wilt; entire leaf is affected by chlorosis, edges and main vein often retain more color; can be caused by galvanized metal.

Copper (Cu)
Mode of Action
Absorbed as Cu++, Cu+; enzyme activity
Deficiency
New growth small, misshapen, wilted; may be found in some peat soils; in some species young leaves may show interveinal chlorosis while tips of older leaves remain green.
Excess /Comments
Can occur at low pH; shows up as Fe deficiency

Manganese (Mn)
Mode of Action
Absorbed as Mn++
Deficiency
Interveinal chlorosis with smallest leaves remaining green producing a checkered effect; grey or tan spots usually develop in chlorotic areas; dead spots may drop out of the leaf; poor bloom size and color; induced by excessively high pH.
Excess /Comments
Reduction in growth, brown spotting on leaves; shows up as Fe deficiency; found under strongly acid conditions

Molybdenum (Mo)
Mode of Action
Absorbed as MoO4-
Deficiency
Interveinal chlorosis on older or midstem leaves; twisted leaves (whiptail); marginal scorching and rolling or cupping of leaves; nitrogen deficiency symptoms may develop
Excess /Comments
Intense yellow or purple color in leaves; rarely observed

Chlorine (Cl)
Mode of Action
Absorbed as Cl -
Deficiency
Wilted leaves which become bronze then chlorotic then die; club roots
Excess /Comments
Salt injury, leaf burn, may increase succulence

Cobalt (Co)
Mode of Action
Absorbed as Co++
Deficiency
This need by plants recently established; essential for Nitrogen fixation
Excess /Comments
Little is known about its deficiency or toxicity symptoms

Nickel (Ni)
Mode of Action
Absorbed as Ni+
Deficiency
This need by plants recently established; essential for seed development
Excess /Comments
None