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This file was used with the kind permission of Phillip Wight.
We have not heard from Phil in a long while and wish him the best.


I am posting below a LONG but important article on Plant Nutrition, 
reproduced with permission of the author.  This is the most complete and 
UNDERSTANDABLE explanation I have ever read of WHY we fertilize, 
WHAT we fertilize with and HOW the plants utilize the nutrition.

Cheers,

Phil

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WHAT IS PLANT NUTRITION?



Plants use inorganic minerals for nutrition, whether grown in the field 
or in a container. Complex interactions involving weathering of rock
minerals, decaying organic matter, animals, and microbes take place to
form inorganic minerals in soil.  Roots absorb mineral nutrients as ions
in soil water.  Many factors influence nutrient uptake for plants.  Ions
can be readily available to roots or could be "tied up" by other
elements or the soil itself.  Soil too high in pH (alkaline) or too low (acid)
makes minerals unavailable to plants. 


FERTILITY OR NUTRITION

The term "fertility" refers to the inherent capacity of a soil to supply
nutrients to plants in adequate amounts and in suitable proportions.  
The term "nutrition" refers to the interrelated steps by which a living
organism assimilates food and uses it for growth and replacement of
tissue.  Previously, plant growth was thought of in terms of soil
fertility or how much fertilizer should be added to increase soil levels
of mineral elements.  Most fertilizers were formulated to account for
deficiencies of mineral elements in the soil.  The use of soilless mixes
and increased research in nutrient cultures and hydroponics as well as
advances in plant tissue analysis have led to a broader understanding of
plant nutrition.  Plant nutrition is a term that takes into account the
interrelationships of mineral elements in the soil or soilless solution 
as well as their role in plant growth.  This interrelationship involves a
complex balance of mineral elements essential and beneficial for optimum
plant growth.   

ESSENTIAL VERSUS BENEFICIAL

The term essential mineral element (or mineral nutrient) was proposed by
Arnon and Stout (1939).  They concluded three criteria must be met for
an element to be considered essential.  These criteria are: 
1.  A plant must be unable to complete its life cycle in the absence of
the mineral element. 
2.  The function of the element must not be replaceable by
another mineral element. 
3.  The element must be directly involved in plant metabolism.  

These criteria are important guidelines for plant nutrition but exclude 
beneficial mineral elements.  Beneficial elements
are those that can compensate for toxic effects of other elements or may
replace mineral nutrients in some other less specific function such as
the maintenance of osmotic pressure.  The omission of beneficial nutrients
in commercial production could mean that plants are not being grown to
their optimum genetic potential but are merely produced at a subsistence
level.

This discussion of plant nutrition includes both the essential and
beneficial mineral elements.

WHAT ARE THE MINERAL ELEMENTS?  

There are actually 20 mineral elements necessary or beneficial for plant
growth.  Carbon (C), hydrogen (H), and oxygen (O) are supplied by air
and water. The six macronutrients, nitrogen (N), phosphorus (P), potassium
(K), calcium (Ca), magnesium (Mg), and sulfur (S) are required by plants
in large amounts.  The rest of the elements are required in trace amounts
(micronutrients).  Essential trace elements include boron (B), chlorine
(Cl), copper (Cu), iron (Fe), manganese (Mn), sodium (Na), zinc (Zn),
molybdenum (Mo), and nickel (Ni).  Beneficial mineral elements include
silicon (Si) and cobalt (Co). The beneficial elements have not been
deemed essential for all plants but may be essential for some.  The distinction
between beneficial and essential is often difficult in the case of some
trace elements. Cobalt for instance is essential for nitrogen fixation in
legumes.  It may also inhibit ethylene formation (Samimy, 1978) and
extend the life of cut roses (Venkatarayappa et al., 1980).  
Silicon, deposited in cell walls, has been found to improve heat 
and drought tolerance and increase resistance to insects and 
fungal infections.  Silicon, acting as a beneficial element, can help 
compensate for toxic levels of manganese, iron, phosphorus and 
aluminum as well as zinc deficiency.  A more holistic
approach to plant nutrition would not be limited to nutrients essential
to survival but would include mineral elements at levels beneficial for
optimum growth.  With developments in analytical chemistry and the 
ability to eliminate contaminants in nutrient cultures, the list of essential
elements may well increase in the future.

THE MINERAL ELEMENTS IN PLANT PRODUCTION

The use of soil for greenhouse production before the 1960's was common. 
Today a few growers still use soil in their mixes.  The bulk of
production is in soilless mixes.  Soilless mixes must provide support, aeration,
nutrient and moisture retention just as soils do, but the addition of
fertilizers or nutrients are different.  Many soilless mixes have
calcium, magnesium, phosphorus, sulfur, nitrogen, potassium and some
micronutrients incorporated as a pre-plant fertilizer.  
Nitrogen and potassium still must be applied to the crop during production.
Difficulty in blending a homogenous mix using pre-plant fertilizers may 
often result in uneven crops and possible toxic or deficient levels of 
nutrients.  Soilless mixes that require addition of micro and macronutrients
applied as liquid throughout the growth of the crop, may actually 
give the grower more control of his crop. 
To achieve optimum production, the grower can 
adjust
nutrient levels to compensate for other environmental factors during the
growing season.  The absorption of mineral ions is dependent on a number
of factors in addition to weather conditions.  These include the cation
exchange capacity or CEC and the pH or relative amount of hydrogen (H() 
or hydroxyl ions (OH() of the growing medium, and the total alkalinity of
the irrigation water. 

CEC OR CATION EXCHANGE CAPACITY

The Cation Exchange Capacity refers to the ability of the growing medium
to hold exchangeable mineral elements within its structure.  These cations
include ammonium nitrogen, potassium, calcium, magnesium, iron, 
manganese, zinc and copper.  Peat moss and mixes containing bark, 
sawdust and otherorganic materials all have some level of cation 
exchange capacity. 

pH:  WHAT DOES IT MEAN?

The term pH refers to the alkalinity or acidity of a growing media water
solution.  This solution consists of mineral elements dissolved in ionic
form in water.  The reaction of this solution whether it is acid, 
neutral
or alkaline will have a marked effect on the availability of mineral
elements to plant roots.  When there is a greater amount of hydrogen H+
ions the solution will be acid (< 7.0).  If there is more hydroxyl OH-
ions the solution will be alkaline (>7.0).  A balance of hydrogen to
hydroxyl ions yields a pH neutral soil (=7.0).  The range for most crops
is 5.5 to 6.2 or slightly acidic.  This creates the greatest average level
for availability for all essential plant nutrients.  Extreme fluctuations
of higher or lower pH can cause deficiency or toxicity of nutrients. 


THE ELEMENTS OF COMPLETE PLANT NUTRITION

The following is a brief guideline of the role of essential and beneficial
mineral nutrients that are crucial for growth.  Eliminate any one of these
elements, and plants will display abnormalities of growth, deficiency
symptoms, or may not reproduce normally. 

Macronutrients

Nitrogen is a major component of proteins, hormones, chlorophyll, 
vitamins
and enzymes essential for plant life.  Nitrogen metabolism is a major
factor in stem and leaf growth (vegetative growth).  Too much can delay
flowering and fruiting.  Deficiencies can reduce yields, cause yellowing
of the leaves and stunt growth. 

Phosphorus is necessary for seed germination, photosynthesis, protein
formation and almost all aspects of growth and metabolism in plants. It 
is essential for flower and fruit formation.  Low pH (<4) results in
phosphate being chemically locked up in organic soils.  Deficiency
symptoms are purple stems and leaves; maturity and growth are retarded. 
Yields of fruit and flowers are poor.  Premature drop of fruits and
flowers may often occur.  Phosphorus must be applied close to the 
plant's roots in order for the plant to utilize it.  Large applications of
phosphorus without adequate levels of zinc can cause a zinc deficiency.

Potassium is necessary for formation of sugars, starches, carbohydrates,
protein synthesis and cell division in roots and other parts of the 
plant. 
It helps to adjust water balance, improves stem rigidity and cold
hardiness, enhances flavor and color on fruit and vegetable crops,
increases the oil content of fruits and is important for leafy crops.
Deficiencies result in low yields, mottled, spotted or curled leaves,
scorched or burned look to leaves.. 

Sulfur is a structural component of amino acids, proteins, vitamins and
enzymes and is essential to produce chlorophyll.  It imparts flavor to
many vegetables.  Deficiencies show as light green leaves.  Sulfur is
readily lost by leaching from soils and should be applied with a
nutrient formula.  Some water supplies may contain Sulfur.

Magnesium is a critical structural component of the chlorophyll molecule
and is necessary for functioning of plant enzymes to produce
carbohydrates, sugars and fats.  It is used for fruit and nut formation
and essential for germination of seeds.  Deficient plants appear
chlorotic, show yellowing between veins of older leaves; leaves may 
droop. 
Magnesium is leached by watering and must be supplied when feeding.  It
can be applied as a foliar spray to correct deficiencies. 

Calcium activates enzymes, is a structural component of cell walls,
influences water movement in cells and is necessary for cell growth and
division.  Some plants must have calcium to take up nitrogen and other
minerals.  Calcium is easily leached.  Calcium, once deposited in plant
tissue, is immobile (non-translocatable) so there must be a constant
supply for growth.  Deficiency causes stunting of new growth in stems,
flowers and roots.  Symptoms range from distorted new growth to black
spots on leaves and fruit.  Yellow leaf margins may also appear. 

Micronutrients

Iron is necessary for many enzyme functions and as a catalyst for the
synthesis of chlorophyll.  It is essential for the young growing parts of
plants.  Deficiencies are pale leaf color of young leaves followed by
yellowing of leaves and large veins.  Iron is lost by leaching and is 
held in the lower portions of the soil structure.  Under conditions of high 
pH (alkaline) iron is rendered unavailable to plants.  When soils are
alkaline, iron may be abundant but unavailable.  Applications of an acid
nutrient formula containing iron chelates, held in soluble form, should
correct the problem. 

Manganese is involved in enzyme activity for photosynthesis, 
respiration, and nitrogen metabolism.  
Deficiency in young leaves may show a network of
green veins on a light green background similar to an iron deficiency.  
In
the advanced stages the light green parts become white, and leaves are
shed.  Brownish, black, or grayish spots may appear next to the veins.  
In
neutral or alkaline soils plants often show deficiency symptoms.  In
highly acid soils, manganese may be available to the extent that it
results in toxicity. 

Boron is necessary for cell wall formation, membrane integrity, calcium
uptake and may aid in the translocation of sugars.  Boron affects at
least 16 functions in plants.  These functions include flowering, pollen
germination, fruiting, cell division, water relationships and the 
movement
of hormones.  Boron must be available throughout the life of the plant.
It is not translocated and is easily leached from soils.  Deficiencies
kill terminal buds leaving a rosette effect on the plant.  Leaves are
thick, curled and brittle.  Fruits, tubers and roots are discolored,
cracked and flecked with brown spots. 

Zinc is a component of enzymes or a functional cofactor of a large
number of enzymes including auxins (plant growth hormones).  
It is essential tocarbohydrate metabolism, protein synthesis and 
internodal elongation (stem growth).  
Deficient plants have mottled leaves with irregular chlorotic
areas.  Zinc deficiency leads to iron deficiency causing similar 
symptoms. 

Deficiency occurs on eroded soils and is least available at a pH range
of 5.5 - 7.0.  Lowering the pH can render zinc more available to the point
of toxicity. 

Copper is concentrated in roots of plants and plays a part in nitrogen
metabolism.  It is a component of several enzymes and may be part of the
enzyme systems that use carbohydrates and proteins.  Deficiencies cause
die back of the shoot tips, and terminal leaves develop brown spots.
Copper is bound tightly in organic matter and may be deficient in highly
organic soils.  It is not readily lost from soil but may often be
unavailable.  Too much copper can cause toxicity. 

Molybdenum is a structural component of the enzyme that reduces nitrates
to ammonia.  Without it, the synthesis of proteins is blocked and plant
growth ceases.  Root nodule (nitrogen fixing) bacteria also require it.
Seeds may not form completely, and nitrogen deficiency may occur if
plants are lacking molybdenum.  Deficiency signs are pale green leaves
with rolled or cupped margins.

Chlorine is involved in osmosis (movement of water or solutes in cells),
the ionic balance necessary for plants to take up mineral elements and
in photosynthesis.  Deficiency symptoms include wilting, stubby roots,
chlorosis (yellowing) and bronzing.  Odors in some plants may be
decreased.  Chloride, the ionic form of chlorine used by plants, is
usually found in soluble forms and is lost by leaching.  Some plants
may show signs of toxicity if levels are too high.

Nickel has just recently won the status as an essential trace element
for plants according to the Agricultural Research Service Plant, Soil 
and Nutrition Laboratory in Ithaca, NY.  It is required for the enzyme
urease to break down urea to liberate the nitrogen into a usable form for
plants. 

Nickel is required for iron absorption.  Seeds need nickel in order to
germinate.  Plants grown without additional nickel will gradually reach 
a deficient level at about the time they mature and begin reproductive
growth. If nickel is deficient plants may fail to produce viable seeds. 

Sodium is involved in osmotic (water movement) and ionic balance in
plants. 

Cobalt is required for nitrogen fixation in legumes and in root nodules 
of nonlegumes. The demand for cobalt is much higher for nitrogen fixation
than for ammonium nutrition. Deficient levels could result in nitrogen
deficiency symptoms. 

Silicon is found as a component of cell walls.  Plants with supplies of
soluble silicon produce stronger, tougher cell walls making them a
mechanical barrier to piercing and sucking insects.  This significantly
enhances plant heat and drought tolerance.  Foliar sprays of silicon 
have also shown benefits reducing populations of aphids on field crops.

Tests have also found that silicon can be deposited by the plants at the
site of infection by fungus to combat the penetration of the cell walls 
by the attacking fungus. Improved leaf erectness, stem strength and 
prevention or depression of iron and manganese toxicity have all been 
noted as effects from silicon.  
Silicon has not been determined essential for all plants but may be 
beneficial for many.  

_____________________

 Written by Dorothy Morgan.  Staff Horticulturist employed by Dyna-Gro
Corporation.  Dorothy holds a B. S. Degree in Horticulture from Delaware
Valley College of Science and Agriculture and Penn State University.
Her experience has included managing commercial greenhouses, nurseries,
hydroponics, and teaching vocational agriculture 
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Reproduced with permission of the author. 

Philip F. Wight