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 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 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 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 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 treated plants also produced higher levels of chlorophyll
and increased photosynthesis rates at lower light levels.
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.
The Elements of Complete Plant Nutrition (more)
Complete nutrition rewards with superior plant growth. Why choose
anything less for your plants? There are 20 elements necessary for
plant growth. Air and water supply carbon, hydrogen, and oxygen.
Six macronutrients are required by plants in large amounts.
Micronutrients are required in trace amounts. Eliminate any of these
elements, and explains the importance of complete nutrition in plants.
Macronutrients
(N)-Nitrogen: Part of proteins, hormones, chlorophyll, vitamins and
enzymes. Promotes stem and leaf growth. Too much can delay fruiting.
Deficiency (Def.): reduced yields, yellowing of leaves, stunted growth.
(P)-Phosphorus: Seed germination, photosynthesis, protein formation,
overall growth and metabolism, flavor and fruit formation. Def.: purple
stems and leaves, regarded growth and maturity; poor flowering and
fruiting. Apply close to roots. Large amounts without zinc cause deficiency.
Low pH <4 ties-up phosphates in organic soils.
(K)-Potassium: Formation of sugars, carbohydrates, proteins, cell division.
Adjusts water balance; improves stem rigidity, cold hardiness; enhances
flavor, color and oil content of fruits; important for leafy crops.
Def.: spotted, curled, or burned look to leaves; lower yields.
(S)-Sulfur: Part of amino acids, proteins, vitamins, enzymes. Essential
for chlorophyll. Imparts flavor to many vegetables. Def.: light green
leaves. Leached by watering. Water supplies may contain Sulfur.
(Mg)-Magnesium: Critical part of chlorophyll; functioning of enzymes of
carbohydrates; sugars and fats; fruit and nut formation; germination of
seeds. Def.: yellowing between veins of older leaves; chlorosis; leaves
droop. Leached by watering. Foliar spray to correct deficiencies.
(Ca)-Calcium: Activates enzymes; structural part of cell walls;
influences water movement; cell growth and division. Required
for uptake of nitrogen and other minerals. Leached by watering.
Immobile-requires a constant supply of growth. Def.: stunting of new
growth in stems, flowers, roots; black spots on leaves and fruit; yellow
leaf margins.
Micronutrients
(Fe)-Iron: Enzyme functions; catalyst for synthesis of chlorophyll; essential
for new growth. Def.: pale leaves; yellowing of the leaves and veins.
Leached by water and held in lower parts of soil. High pH soils may have
iron present but unavailable to plants.
(Mn)-Manganese: Enzyme activity for photosynthesis, respiration, and
nitrogen metabolism. Def.: young leaves pale with green veins similar
to iron def.; advanced stages are white, leaves are shed. Brown, black,
or gray spots appear next to veins. Plants in neutral or alkaline soils
often show def. Acid soils may increase uptake causing toxicity.
(B)-Boron: Affects at least 16 functions: flowering, pollen germination,
fruiting, cell division, water relationships, movement of hormones, cell
wall formation, membrane integrity, calcium uptake, movement of sugars.
Immobile, easily leached. Def.: terminal bud dies, causing rosette of thick,
curled, brittle leaves or brown, discolored, cracked, fruits, tubers and roots.
(Zn)-Zinc: Functional part of enzymes including auxins (growth hormones),
carbohydrate metabolism, protein synthesis, stem growth. Def.: mottled
leaves, irregular yellow areas. Zinc deficiency leads to iron deficiency.
Occurs on eroded soils; least available at pH of 5.5-7.0. Lower pH can
cause availability to the point of toxicity.
(Cu)-Copper: Found in roots; necessary for nitrogen metabolism; component
of enzymes-may be part of enzyme systems that use carbohydrates and
proteins. Def.: die back of shoot tips; terminal leaves develop drown spots.
Bound tightly in organic matter. May be deficient in highly organic soils.
Not readily lost from soil but may be unavailable. Too much can cause
toxicity.
(Mo)-Molybdenum: Structural part of the enzyme that reduces nitrites to
ammonia; without it, synthesis of proteins is blocked, plant growth ceases.
Required by nitrogen fixing bacteria. Def.: pale leaves with rolled, cupped
margins. Seeds may not form; nitrogen def. may occur if plants are
lacking Mo.
(Cl)-Chlorine: Involved in osmosis (movement of water or solutes in cells),
ionic balance necessary to take up mineral elements and photosynthesis.
Def.: wilting, stubby roots, yellowing, bronzing. Scents in some plants may
be decreased. Leached by watering.
(Co)-Cobalt: Required by nitrogen fixing bacteria. Def.: may result in
nitrogen deficiency symptoms.
(Ni)-Nickel: Recently recognized as an essential element. Required for
the urease enzyme to break down urea into usable nitrogen and for iron
absorption. Seeds need nickel to germinate.
(Na)-Sodium: Involved in osmotic (water movement) and ionic balance in plants.
(Si)-Silicon: Component of cell walls; creates mechanical barrier to piercing-
sucking insects and fungi. Foliar sprays reduce population of aphids on some
plants. Enhances leaf presentation; improves heat and drought tolerance,
and reduce transpiration. Def.: wilting, poor fruit and flower set, increase
susceptibility to insects and disease.