The Orchid House
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
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 1960s 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
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 other organic 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
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
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
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 to carbohydrate 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
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
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
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 - Reproduced with permission
of the author.
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