Forms and Function of Essential Plant Nutrients - Valley Crops Home

Forms and Functions of Essential Plant Nutrients
Brian Jones, Extension Agent Augusta County
There are 16 elements required by plants. Three of these: carbon (C),
oxygen (O 2 ) and hydrogen (H) are taken from either atmospheric carbon dioxide
or water. The other 13 nutrients are taken up by plants from the soil or by foliar
applications. Of these 13, three are called primary nutrients: nitrogen (N),
phosphorus (P) and potassium (K), three are secondary: calcium (Ca),
magnesium (Mg) and sulfur (S) and seven are micronutrients: boron (B), chlorine
(Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn).
Primary nutrients are commonly supplied to the plant either as blended
fertilizers or as organic forms (manure or poultry litter). These nutrients are
utilized in the largest amounts by crops, and thus are typically applied at higher
rates than secondary nutrients and micronutrients. Secondary nutrients are still
required by the plant, but in smaller amounts than primary nutrients. These
nutrients are available in many forms, such as fertilizers or manures, but may
also be available through applications of lime or gypsum. Micronutrients are
required in even smaller amounts than secondary nutrients and are available in
various forms depending on the application.
The following paragraphs will discuss the function and forms of the three
primary nutrients, and will also touch on basic deficiency symptoms in the plant.
Nitrogen
Function
Of the three primary nutrients, plants require nitrogen in the largest amounts.
Plants typically contain between 1 and 5% N by weight. It is absorbed by plants
as either nitrate (NO 3 - ), ammonium (NH 4 + ) or urea. Once taken up by the plant,
the NO 3 - form must be reduced to either NH 4 + or ammonia (NH 3 ). The NH 3 is
then used to create numerous amino acids that are incorporated into proteins
and nucleic acids (DNA, RNA). These proteins become the framework for
chloroplasts, mitochondria and other structures in which most biochemical
reactions occur. Nitrogen is also an integral part of chlorophyll, the primary
absorber of light energy needed for photosynthesis. Photosynthesis is the
process through which plants use light energy to convert carbon dioxide into
carbohydrates (sugars) which provide energy required for plant growth and
development.
Nitrogen:
1. Promotes vigorous vegetative growth
2. increases leaf size and quality
3. hastens crop maturity
4. promotes fruit and seed development
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Forms
Nitrogen fertilizer is available to the plant from several different sources, including
fertilizers, manures and legumes. The amount of nitrogen from organic forms
varies depending on the origin, how it was handled and stored and the
decomposition rate of the material. Common inorganic fertilizer forms of N
available in Virginia include:
- Ammonium nitrate (33-0-0)
- Ammonium sulfate (21-0-0-24S)
- Diammonium phosphate or DAP (18-46-0)
- Urea (46-0-0)
- Urea ammonium nitrate or UAN (liquid form containing 30% N by
weight)
Nitrogen Deficiency
Plants that are deficient in N typically show stunted growth and their foliage is
pale green. Symptoms generally appear on the bottom leaves first, and lower
leaves will sometimes have a “fired” appearance on the tips, turn brown and fall
off. In crops such as vegetables, forage and pasture, low N results in low yield
and quality. With grain crops, such as corn and small grains, deficiency results in
yellow leaf tips, stunted growth with spindly stalks and low yields of poor quality
grain. Too much N causes excessive vegetative growth, delays maturity,
increases lodging risk and promotes plant diseases.
Phosphorus
Function
Plant growth will not occur without phosphorus (P). Concentration of P in most
plants is between 0.1 and 0.4%, lower than either N or potassium. Plants absorb
P either as H 2 PO 4 - or HPO 4 2- orthophosphate ions, depending on the pH. Plants
may also absorb organic phosphate forms, made available as soil organic matter
is decomposed. The most essential function of P in plants is in energy storage
and transfer. Nucleic acids, phospholipids, coenzymes DNA and NADP, and
most importantly ATP rely on P for the energy required to function. ATP is the
source of energy that powers practically every energy-requiring biological
process in plants. Phosphorus is also required for other essential metabolic
processes, such as photosynthesis, glycolysis, respiration, and fatty acid
synthesis.
Phosphorus:
1. Enhances seed germination and early growth
2. Is associated with increased root growth
3. Stimulates blooming and bud set
4. Improves straw strength
5. Increases disease resistance
6. Aids in seed formation
7. Hastens plant maturity
8. Provides winter hardiness
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Forms
Phosphorus is made available to the plant through fertilizer or organic forms.
Some common forms of P fertilizer in Virginia include:
- Monoammonium phosphate, or MAP (11-48-0)
- Diammonium phosphate, or DAP (18-46-0)
- Triple super phosphate (0-46-0)
- Various liquid forms, including 10-34-0 or 9-18-9
Phosphorus Deficiency
Plants deficient in P typically have stunted growth. Leaves in some cases may be
dark green with a leathery texture, with reddish purple leaf tips and margins. In
corn, reddish purple leaf margins are characteristic of P deficiency. Deficiency
symptoms may appear even when soil P levels are adequate, particularly if soil
temperatures are less than 60°F. When soil is cool, less P is made available for
plant uptake. These symptoms will generally disappear as soils warm.
Potassium
Function
Potassium has many functions in plant growth, but primarily acts as a regulator
for the many metabolic processes required for growth, fruit and seed
development. Potassium (K) is actively taken up from the soil solution by plant
roots. Potassium concentrations in most plants range from 1 to 4% by weight.
Plants require a good deal of K to properly function. Unlike the other primary
nutrients, K forms no other compounds in the plant, but remains a lone ion.
Potassium is also vital for animal and human nutrition, and thus healthy fruits,
vegetables and grains must have adequate levels of K.
Potassium:
1. Adds stalk and stem stiffness
2. Increases disease resistance
3. Increases drought tolerance
4. Gives plumpness to grain and seed (increasing
kernel and test weight)
5. Improves firmness, texture, size and color of fruit
crops
6. Increases oil content of oil crops
7. Regulates opening and closing of cell stomata,
increasing water use efficiency
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Forms
Potassium can be made available to crop plants either as commercial inorganic
fertilizers or through manure. Only a few inorganic forms of potash are typically
available in Virginia, including:
- Potassium chloride, or muriate of potash (0-0-60)
- Potassium sulfate (0-0-50-16S)
- Potassium magnesium sulfate (0-0-22-23S-11Mg)
Potassium Deficiency
Plants deficient in K generally exhibit chlorosis (a loss of green color) along the
leaf margins or tips starting with the bottom leaves and moving up the plant. In
severe cases, whole plant yellowing may occur and the lower leaves may fall off.
Grain crops will have weak stalks, small grain size and low yields. Forage crop
yield, vigor and quality will be low. Often however serious K deficiencies can
occur without the appearance of deficiency symptoms.
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