Richard Melson

July 2006

Nitrogen Fertilizer

Nitrogen Fertilizer

spreading manure, an organic fertilizer


Fertilizers or fertilisers are compounds given to plants with the intention of promoting growth; they are usually applied either via the soil, for uptake by plant roots, or by foliar feeding, for uptake through leaves. Fertilizers can be organic (composed of organic matter, i.e. carbon based), or inorganic (containing simple, inorganic chemicals). They can be naturally-occurring compounds such as peat or mineral deposits, or manufactured through natural processes (such as composting) or chemical processes (such as the Haber process).

Fertilizers typically provide, in varying proportions, the three major plant nutrients (nitrogen, phosphorus, and potassium), the secondary plant nutrients (calcium, sulfur, magnesium), and sometimes trace elements (or micronutrients) with a role in plant nutrition: boron, manganese, iron, zinc, copper and molybdenum.


Inorganic fertilizers (Mineral Fertilizer)

Macronutrients and micronutrients

Fertilizers can be divided into macronutrients or micronutrients. There are three macronutrients: nitrogen, potassium, and phosphorus, which are consumed in high quantities and normally present as whole number percentages in plant tissues. There are many micronutrients, and their importance and occurrence differ from plant to plant. In general, most present from 5 to 100 parts per million (ppm) by mass. Examples of micronutrients are as follows: boron (B), calcium (Ca), copper (Cu), magnesium (Mg), iron (Fe), molybdenum (Mo), and zinc (Zn).

Macronutrient fertilizers

Synthesized materials are also called artificial fertilizers, and may be described as straight, where the product predominantly contains the three primary ingredients of nitrogen (N), phosphorus (P) and potassium (K), which are known as N-P-K fertilizers or compound fertilizers when elements are mixed intentionally. They are named or labeled according to the content of these three elements, which are macronutrients. The mass fraction (percent) nitrogen is reported directly. However, phosphorus is reported as diphosphorus pentoxide (P2O5), the anhydride of phosphoric acid, and potassium is reported as potash or potassium oxide (K2O), which is the anhydride of potassium hydroxide. Fertilizer composition is expressed in this fashion for historical reasons in the way it was analyzed (conversion to ash for P and K); this practice dates back to Justus von Liebig (see more below). Consequently, an 18-51-20 fertilizer would have 18% nitrogen as N, 51% phosphorus as P2O5, and 20% potassium as K2O. Although analyses are no longer carried out by ashing first, the naming convention remains. If nitrogen is the main element, they are often described as nitrogen fertilizers.

Agricultural versus Horticultural Fertilizers

In general, agricultural fertilizers contain only one or two macronutrients. Agricultural fertilizers are intended to be applied infrequently and normally prior to seeding or concurrently with it. Examples of agricultural fertilizers are granular triple superphosphate, potassium chloride, urea, and anhydrous ammonia. The commodity nature of fertilizer, combined with the high cost of shipping, leads to use of locally available materials or those from the closest/cheapest source, which may vary with factors affecting transportation by rail, ship, or truck. In other words, a particular nitrogen source may be very popular in one part of the country while another is very popular in another geographic region only due to factors unrelated to agronomic concerns.

Horticultural or specialty fertilizers, on the other hand, are formulated from many of the same compounds and some others to produce well-balanced fertilizers that also contain micronutrients. Some materials, such as ammonium nitrate, are used minimally in large scale production farming. The 18-51-20 example above is a horticultural fertilizer formulated with high phosphorus to promote bloom development in ornamental flowers. Horticultural fertilizers may be water-soluble (instant release) or relatively insoluble (controlled release). Controlled release fertilizers are also referred to as sustained release or timed release. Many controlled release fertilizers are intended to be applied approximately every 3-6 months, depending on watering, growth rates, and other conditions, whereas water-soluble fertilizers must be applied at least every 1-2 weeks and can be applied as often as every watering if sufficiently dilute. Unlike agricultural fertilizers, horticultural fertilizers are marketed directly to consumers and become part of retail product distribution lines.

Justus von Liebig

Chemist Justus von Liebig (in the 19th century) contributed greatly to understanding the role of inorganic compounds in plant nutrition and devised the concept of Liebig's barrel to illustrate the significance of inadequate concentrations of essential nutrients. At the same time he deemphasized the role of humus. This theory was influential in the great expansion in use of artificial fertilizers in the 20th century.

Nitrogen fertilizer is often synthesized using the Haber-Bosch process, which produces ammonia. This ammonia is applied directly to the soil or used to produce other compounds, notably ammonium nitrate, a dry, concentrated product. It can also be used in the Odda Process to produce compound fertilizers such as 15-15-15.

Inorganic fertilizers sometimes do not replace trace mineral elements in the soil which become gradually depleted by crops grown there. This has been linked to studies which have shown a marked fall (up to 75%) in the quantities of such minerals present in fruit and vegetables.[1] One exception to this is in Western Australia where deficiencies of zinc, copper, manganese, iron and molybdenum were identifed as limiting the growth of crops and pastures in the 1940's and 1950's. Soils in Western Australia are very old, highly weathered and deficient in many of the major nutrients and trace elements. Since this time these trace elements are routinely added to inorganic fertilizers used in Agriculture in this state.

In many countries there is the public perception that inorganic fertilizers "poison the soil" and result in "low quality" produce. However, there is very little (if any) scientific evidence to support these views. When used appropriately, inorganic fertilizers enhance plant growth, the accumulation of organic matter and the biological activity of the soil, while reducing the risk of water run-off, overgrazing and soil erosion. The nutritional value of plants for human and animal consumption is typically improved when inorganic fertilizers are used appropriately.

Organic fertilizers

The decomposing crop residue from prior years is another source of fertility. Though not strictly considered "fertilizer", the distinction seems more a matter of words than reality.

Some ambiguity in the usage of the term 'organic' exists because some of synthetic fertilizers, such as urea and urea formaldehyde, are fully organic in the sense of organic chemistry. In fact, it would be difficult to chemically distinguish between urea of biological origin and that produced synthetically. On the other hand, some fertilizer materials commonly approved for organic agriculture, such as powdered limestone, mined "rock phosphate" and Chilean saltpeter, are inorganic in the use of the term by chemistry.

Although the density of nutrients in organic material is comparatively modest, they have some advantages. For one thing organic growers typically produce some or all of their fertilizer on-site, thus lowering operating costs considerably. Then there is the matter of how effective they are at promoting plant growth, chemical soil test results aside. The answers are encouraging. Since the majority of nitrogen supplying organic fertilizers contain insoluble nitrogen and are slow release fertilizers their effectiveness can be greater than conventional nitrogen fertilzers.

Implicit in modern theories of organic agriculture is the idea that the pendulum has swung the other way to some extent in thinking about plant nutrition. While admitting the obvious success of Leibig's theory, they stress that there are serious limitations to the current methods of implementing it via chemical fertilization. They re-emphasize the role of humus and other organic components of soil, which are believed to play several important roles:

Organics also have the advantage of avoiding certain long-term problems associated with the regular heavy use of artificial fertilizers:

Organic fertilizers also have their disadvantages:

In practice a compromise between the use of artificial and organic fertilizers is common, typically by using inorganic fertilizers supplemented with the application of organics that are readily available such as the return of crop residues or the application of manure.

It is important to differentiate between what we mean by organic fertilizers and fertilizers approved for use in organic farming and organic gardening by organizations and authorities who provide organic certification services. Some approved fertilizers may be inorganic, naturally occurring chemical compounds, e.g. minerals..

Environmental effects of fertilizer use

Over-application of chemical fertilizers, or application of chemical fertilizers at a time when the ground is waterlogged or the crop is not able to use the chemicals, can lead to surface runoff (particularly phosphorus) or leaching into groundwater (particularly nitrates). One of the adverse effects of excess fertilizer in lacustrine systems are algal blooms, which can lead to excessive mortality rates for fish and other aquatic organisms. When prolonged alae blooms occur over many years, the effect is a process called eutrophication. Worldwide the issues of nutrient fate are analyzed using hydrology transport models.

It is also possible to over-apply organic fertilizers. However: their nutrient content, their solubility, and their release rates are typically much lower than chemical fertilizers, partially because by their nature, most organic fertilizers also provide increased physical and biological storage mechanisms to soils.

The problem of over-fertilization is primarily associated with the use of artificial fertilizers, because of the massive quantities applied and the destructive nature of chemical fertilizers on soil nutrient holding structures. The high solubilities of chemical fertilizers also exacerbate their tendency to degrade ecosystems.

Storage and application of some fertilizers in some weather or soil conditions can cause emissions of the greenhouse gas nitrous oxide (N2O). Ammonia gas (NH3) may be emitted following application of inorganic fertilizers, or manure or slurry. Besides supplying nitrogen, ammonia can also increase soil acidity (lower pH, or "souring").

For these reasons, it is recommended that knowledge of the nutrient content of the soil and nutrient requirements of the crop are carefully balanced with application of nutrients in inorganic fertiliser especially. This process is called nutrient budgeting. By careful monitoring of soil conditions, farmers can avoid wasting expensive fertilizers, and also avoid the potential costs of cleaning up any pollution created as a byproduct of their farming.

The concentration of up to 100 mg/kg of Cadmium in phosphate minerals (for example Nauru[2] and the Christmas islands [3]) increases the contamination of soil with Cadmium, for example in New Zealand.[4] Uranium is another example for impurities of fertilizers


Organic fertilizer

  1. Lawrence, Felicity (2004). "214", Kate Barker Not on the Label. Penguin. ISBN 0-141-01566-7, 213.

  2. Syers JK, Mackay AD, Brown MW, Currie CD (1986). "Chemical and physical characteristics of phosphate rock materials of varying reactivity". J Sci Food Agric 37: 1057-1064.

  3. Trueman NA (1965). "The phosphate, volcanic and carbonate rocks of Christmas Island (Indian Ocean)". J Geol Soc Aust 12: 261-286.

  4. Taylor MD (1997). "Accumulation of Cadmium derived from fertilisers in New Zealand soils". Science of Total Environment 208: 123-126.

Plant nutrition is the study of the chemical elements that are necessary for plant growth. There are several principles that apply to plant nutrition.

Some elements are essential, meaning that the absence of a given mineral element will cause the plant to fail to complete its life cycle; that the element cannot be replaced by the presence of another element; and that the element is directly involved in plant metabolism (Arnon and Stout, 1939). However, this principle does not leave any room for the so-called beneficial elements, whose presence, while not required, has clear positive effects on plant growth.

Plants require specific elements for growth and, in some cases, for reproduction.

The mnemonic for the elements essential to plant growth is: "CHOPKN'S CaFe Mg MoB CuMnZn" or "C-Hopkins Cafe. Mighty Good. Mob comes in."

Major nutrients include:

Minor Nutrients:

These nutrients are further divided into the mobile and immoblile nutrients. A plant will always supply more nutrients to its younger leaves than its older ones, so when nutrients are mobile, the lack of nutrients is first visible on older leaves. When a nutrient is less mobile, the younger leaves suffer because the nutrient does not move up to them but stays lower in the older leaves. Nitrogen, phosphorus, and potassium are mobile nutrients, while the others have varying degrees of mobility. Concentration of ppm (parts per million) represents the dry weight of a representative plant.

Plant uses for essential nutrients

Each of these nutrients are used in a different place for a different essential function.

Carbon is what most of the plant is made of. It forms the backbone of many plant biomolecules, including starches and cellulose. Carbon is fixed through photosynthesis from the carbon dioxide in the air and is a part of the carbohydrates that store energy in the plant.
Hydrogen also is necessary for building sugars and building the plant. It is obtained from air and liquid water.
Oxygen is necessary for cellular respiration. Cellular respiration is the process of generating energy-rich adenosine triphosphate (ATP) via the consumption of sugars made in photosynthesis. It is obtained from the air.
Phosphorus is important in plant bioenergetics. As a component of ATP, phosphorus is needed for the conversion of light energy to chemical energy (ATP) during photosynthesis. Phosphorus can also be used to modify the activity of various enzymes by phosphorylation, and can be used for cell signalling. Since ATP can be used for the biosynthesis of many plant biomolecules, phosphorus is important for plant growth and flower/seed formation.
Potassium regulates the opening and closing of the stoma by a potassium ion pump. Since stomata are important in water regulation, potassium reduces water loss from the leaves and increases drought tolerance. Potassium deficiency may cause necrosis or interveinal chlorosis.
Nitrogen is an essential component of all proteins, and as a part of DNA, it is essential for growth and reproduction as well. Nitrogen deficiency most often results in stunting.
Sulphur is another important component of amino acids and proteins, and is therefore important in plant growth.
Calcium a part of cell walls. It also regulates transport of other nutrients into the plant. Calcium deficiency results in stunting.
Magnesium is an important part of chlorophyll, a critical plant pigment important in photosynthesis. It is important in the production of ATP through its role as an enzyme cofactor. There are many other biological roles for magnesium-- see Magnesium in biological systems for more information. Magnesium deficiency can result in interveinal chlorosis.
Iron is necessary for photosynthesis and is present as an enzyme cofactor in plants. Iron deficiency can result in interveinal chlorosis and necrosis.
Molybdenum is a cofactor to enzymes important in building amino acids.
Boron is important in sugar transport, cell division, and synthesizing certain enzymes. Boron deficiency causes necrosis in young leaves and stunting.
Copper is important for photosynthesis. Symptoms for copper deficiency include chlorosis.
Manganese is necessary for building the chloroplasts. Manganese deficiency may result in coloration abnormalities, such as discolored spots on the foliage.
Zinc is required in a large number enzymes and plays an essential role in DNA transcription. A typical symptom of zinc deficiency is the stunted growth of leaves, commonly known as "little leaf" and is caused by the oxidative degredation of the growth hormone auxin

Additional elements include nickel and silicon, whose requirements are vague for all but a very few select plants. Cobalt has proven to be beneficial to at least some plants, but is essential in others, such as legumes where it is required for nitrogen fixation. Vanadium may be required by some plants, but at very low concentrations. It may also be substituting for molybdenum. Selenium and sodium may also be beneficial. Sodium can replace potassium's regulation of stomatal opening and closing.

Plant nutrition is a difficult subject to understand completely, partially because of the variation between different plants and even between different species or individuals of a given clone. Elements present at low levels may demonstrate deficiency, and toxicity is possible at levels that are too high. Further, deficiency of one element may present as symptoms of toxicity from another element, and vice-versa.

Carbon and oxygen are absorbed from the air, while other nutrients are absorbed from the soil. Green plants obtain their carbohydrate supply from the carbon dioxide in the air by the process of photosynthesis.

See also:

Related Journals Journal of Plant Nutrition

Retrieved from ""

Soil conditioner

Soil conditioners, also called soil amendments, are materials added to soil to improve plant growth and health. The type of conditioner added depends on the current soil composition, climate and the type of plant. Some soils lack nutrients necessary for proper plant growth and others hold too much or too little water. A conditioner or a combination of conditioners corrects the soil's deficiencies. Lime is used to make soil less acidic. Fertilizers, such as peat, manure or compost, add depleted plant nutrients. Materials such as clay, vermiculite, hydrogel and shredded bark will make soil hold more water. Gypsum releases nutrients and improves soil structure. Sometimes a soil inoculant is added for legumes.

External Links


July 13, 2006