Soil Fertility and Fertilizers

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 spraying, for uptake through leaves. Fertilisers 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

• Examples of naturally-occurring inorganic fertilizers include diatomaceous earth and limestone.
• Examples of manufactured or chemically-synthesized inorganic fertilisers include ammonium nitrate, potassium sulfate, and superphosphate, or triple super phosphate.

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), phosphorous (P) and potassium/potash (K), often described as NPK fertilizers. They are named or labelled according to the content of these three elements, thus a 5-10-5 fertilizer would have 10 percent phosphate in its ingredients. If nitrogen is the main element, they are often described as nitrogen fertilizers.

Alternatively they may be described as compound where there is a mix of nutrients.

Chemist Justus von Leibig (in the 19th century) contributed greatly to understanding the role of inorganic compounds in plant nutrition and devised the concept of Leibig'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. The Haber-Bosch process uses about one percent of the Earth's total energy supply (primarily in the form of natural gas) in order to provide half of the nitrogen needed in agriculture.

Inorganic fertilisers typically 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]

Organic fertilizers

• Examples of naturally occurring organic fertilizers include manure and slurry, urine, peat, seaweed and guano. Green manure crops are also grown to add nutrients to the soil.
• Examples of manufactured organic fertilizers include compost, dried blood, bone meal and seaweed extracts.

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.

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.

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 dryness
• 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 (and perhaps in increasing quantities) to maintain fertility
• the cost (substantial and rising in recent years) and resulting lack of independence

In practice a compromise between the use of artificial and organic fertilizers is not uncommon, typically in the form of chemical use, supplemented with the application of such organics as may be 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 fertilizers, or application at a time when the ground is waterlogged or the crop is not able to use the fertilizer, can lead to run-off in groundwater. This can enrich lakes and streams in a process called eutrophication and lead to algal blooms. It is possible to over-apply organic fertilizers as well, but their nutrient content, solubility, and release rate are typically lower. The problem is endemic, however, and is primarily associated with the use of artificial fertilizers, if only due to the massive quantities involved. Their high solubilities are also a factor.

Storage and application of fertilizers in particular weather or soil conditions can also cause emissions of the greenhouse gas nitrous oxide (N₂O). Ammonia gas (NH₃) may be emitted following application of manure or slurry or due to inorganic fertilizers (to a lesser extent unless ammonia itself is used directly). Besides suppling nitrogen, ammonia can 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 organic and inorganic fertiliser. This process is called nutrient budgeting. By doing this the farmer will avoid wasting fertiliser and also avoid the cost of avoiding or cleaning up pollution.

Application

Fertilizers can be buried around a tree's roots when it is planted, placed in bore holes near tree roots, spread onto soil, or sprayed by hand. Fertilization can also be achieved via aerial topdressing.

Sources

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

See also

• Soil amendments
• Soil conditioner