Encyclopedia
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 , or inorganic . They can be naturally-occurring compounds such as
peat or
mineral deposits, or manufactured through natural processes or chemical processes .
Fertilizers typically provide, in varying proportions, the three major plant nutrients , the secondary plant nutrients , and sometimes trace elements with a role in plant nutrition:
boron, chlorine,
manganese,
iron,
zinc,
copper and molybdenum.
Inorganic fertilizers
Macronutrients and micronutrientsFertilizers 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 by mass. Examples of micronutrients are as follows:
boron , calcium ,
copper ,
magnesium ,
iron , molybdenum , and
zinc .
Macronutrient fertilizersSynthesized materials are also called
artificial fertilizers, and may be described as
straight, where the product predominantly contains the three primary ingredients of
nitrogen ,
phosphorus and
potassium , 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 nitrogen is reported directly. However, phosphorus is reported as
diphosphorus pentoxide , the anhydride of
phosphoric acid, and potassium is reported as potash or
potassium oxide , which is the anhydride of
potassium hydroxide. Fertilizer composition is expressed in this fashion for historical reasons in the way it was analyzed ; this practice dates back to
Justus von Liebig . Consequently, an 18-51-20 fertilizer would have 18% nitrogen as N, 51% phosphorus as P
2O
5, and 20% potassium as K
2O. 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.
In general, the mass fraction of elemental phosphorus, [P] = 0.436 x [P
2O
5]
and the mass fraction of elemental potassium, [K] = 0.83 x [K
2O]
An 18-51-20 fertiliser therefore contains, by weight, 18% elemental nitrogen , 42% elemental phosphorus and 16% elemental potassium .
Agricultural versus Horticultural FertilizersIn general, agricultural fertilizers contain only one or two macronutrients. Agricultural fertilizers are intended to be applied infrequently and normally prior to or along side seeding. 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 or relatively insoluble . 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 LiebigChemist
Justus von Liebig 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 in the quantities of such minerals present in fruit and vegetables. One exception to this is in
Western Australia where deficiencies of
zinc,
copper,
manganese,
iron and molybdenum were identified 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 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
- Examples of naturally occurring organic fertilizers include manure, slurry, worm castings, urine, peat, seaweed and guano. Green manure crops are also grown to add nutrients to the soil. Naturally occurring minerals such as mine rock phosphate, sulfate of potash and limestone are also considered Organic Fertilizers.
- Benfits of organic fertilizer can be found at
- Examples of manufactured organic fertilizers include compost, dried blood, bone meal and seaweed extracts. Other examples are natural enzyme digested proteins, fish meal, and feather meal.
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:
- Mobilizing existing soil nutrients, so that good growth is achieved with lower nutrient densities while wasting less
- Releasing nutrients at a slower, more consistent rate, helping to avoid a boom-and-bust pattern
- Helping to retain soil moisture, reducing the stress due to temporary moisture stress
- Improving the soil structure
Organics also have the advantage of avoiding certain long-term problems associated with the regular heavy use of artificial fertilizers:
- the possibility of "burning" plants with the concentrated chemicals
- the progressive decrease of real or perceived "soil health", apparent in loss of structure, reduced ability to absorb precipitation, lightening of soil color, etc.
- the necessity of reapplying artificial fertilizers regularly to maintain fertility
- the cost and resulting lack of independence
Organic fertilizers also have their disadvantages:
- As acknowledged above, they are typically a dilute source of nutrients compared to inorganic fertilizers, and where significant amounts of nutrients are required for profitable yields, very large amounts of organic fertilisers must be applied. This results in prohibitive transportation and application costs, especially where the agriculture is practiced a long distance from the source of the organic fertilizer.
- The composition of organic fertilizers tends to be highly variable, so that accurate application of nutrients to match plant production is difficult. Hence, large-scale agriculture tends to rely on inorganic fertilizers while organic fertilizers are cost-effective on small-scale horticultural or domestic gardens.
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 or leaching into
groundwater . 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.
Excessive nitrogen fertilizer applications can lead to pest problems by increasing the birth rate, longevity and overall fitness of certain pests .
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 .
Ammonia gas may be emitted following application of inorganic fertilizers, or manure or slurry. Besides supplying nitrogen, ammonia can also increase soil acidity .
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 increases the contamination of soil with Cadmium, for example in New Zealand.
Uranium is another example for impurities of fertilizers
References
See also
- Plant nutrition
- Soil conditioner
- Ecological sanitation
External links
-
- Info, articles, and more on fertilizer.
- For conversions to elemental content see Part 1, para 7.
