Fertile soil has the following properties:

• It is rich in nutrients necessary for basic plant nutrition, including nitrogen, phosphorus and potassium.
• It contains sufficient minerals (trace elements) for plant nutrition, including boron, chlorine, cobalt, copper, iron, manganese, magnesium, molybdenum, sulfur, and zinc.
• It contains soil organic matter that improves soil structure and soil moisture retention.
• Soil pH is in the range 6.0 to 6.8 for most plants but some prefer acid or alkaline conditions.
• Good soil structure, creating well drained soil, but some soils are wetter (as for producing rice) or drier (as for producing plants susceptible to fungi or rot) such as agave.
• A range of microorganisms that support plant growth.
• It often contains large amounts of topsoil.

In lands used for agriculture and other human activities, fertile soil typically arises from the use of soil conservation practices. Basically the ability of a soil to supply plant nutrients.

Soil fertilization[link]

Bioavailable nitrogen is the element in soil that is most often lacking. Phosphorus and potassium are also needed in substantial amounts. For this reason these three elements are always identified on a commercial fertilizer analysis. For example a 10-10-15 fertilizer has 10 percent nitrogen, 10 percent (P₂O₅) available phosphorus and 15 percent (K₂O) water soluble potassium. Sulfur is the fourth element that may be identified in a commercial analysis - e.g. 21-0-0-24 which would contain 21% nitrogen and 24% sulfate.

Inorganic fertilizers are generally less expensive and have higher concentrations of nutrients than organic fertilizers. Some have criticized the use of inorganic fertilizers, claiming that the water-soluble nitrogen doesn't provide for the long-term needs of the plant and creates water pollution. Slow-release fertilizers may reduce leaching loss of nutrients and may make the nutrients that they provide available over a longer period of time.

Soil fertility is a complex process that involves the constant cycling of nutrients between organic and inorganic forms. As plant material and animal wastes decompose they release nutrients to the soil solution. Those nutrients may then undergo further transformations which may be aided or enabled by soil micro-organisms. Natural processes such as lightning strikes may fix atmospheric nitrogen by converting it to (NO₂). Denitrification may occur under anaerobic conditions (flooding) in the presence of denitrifying bacteria. The cations, primarily phosphate and potash, as well as many micronutrients are held in relatively strong bonds with the negatively charged portions of the soil in a process known as Cation Exchange Capacity

In 2008 the cost of phosphorus as fertilizer more than doubled, while the price of rock phosphate as base commodity rose eight-fold. Recently the term peak phosphorus has been coined, due to the limited occurrence of rock phosphate [1] in the world.

[edit] Light and CO₂ limitations

Photosynthesis This is the process whereby plants use light energy to drive chemical reactions which convert CO₂ into sugars. As such, all plants require access to both light and carbon dioxide to produce energy, grow and reproduce.

While typically limited by nitrogen, phosphorus and potassium, low levels of carbon dioxide can also act as a limiting factor on plant growth. Peer-reviewed and published scientific studies have shown that increasing CO₂ is highly effective at promoting plant growth up to levels over 300 ppm. Further increases in CO₂ can, to a very small degree, continue to increase net photosynthetic output (Chapin et al., 2002 - Principles of Terrestrial Ecosystem Ecology).

Since higher levels of CO₂ have only a minimal impact on photosynthetic output at present levels (presently around 380 ppm and increasing), we should not consider plant growth to be limited by carbon dioxide. Other biochemical limitations, such as soil organic content, nitrogen in the soil, phosphorus and potassium, are far more often in short supply. As such, neither commercial nor scientific communities look to air fertilization as an effective or economic method of increasing production in agriculture or natural ecosystems. Furthermore, since microbial decomposition occurs faster under warmer temperatures, higher levels of CO₂ (which is one of the causes of unusually fast climate change) should be expected to increase the rate at which nutrients are leached out of soils and may have a negative impact on soil fertility.

Soil depletion[link]

Soil depletion occurs when the components which contribute to fertility are removed and not replaced, and the conditions which support soil fertility are not maintained. This leads to poor crop yields. In agriculture, depletion can be due to excessively intense cultivation and inadequate soil management.

One of the most widespread occurrences of soil depletion as of 2008^[update] is in tropical zones where nutrient content of soils is low. The combined effects of growing population densities, large-scale industrial logging, slash-and-burn agriculture and ranching, and other factors, have in some places depleted soils through rapid and almost total nutrient removal.

Topsoil depletion occurs when the nutrient-rich organic topsoil, which takes hundreds to thousands of years to build up under natural conditions, is eroded or depleted of its original organic material.^[1] Historically, many past civilizations' collapses can be attributed to the depletion of the topsoil. Since the beginning of agricultural production in the Great Plains of North America in the 1880s, about one-half of its topsoil has disappeared.^[2]

Depletion may occur through a variety of other effects, including overtillage (which damages soil structure), underuse of nutrient inputs which leads to mining of the soil nutrient bank, and salinization of soil.

Global distribution[link]

Global distribution of soil types of the USDA soil taxonomy system. Mollisols, shown here in dark green, are a good (though not the only) indicator of high soil fertility. They coincide to a large extent with the world's major grain producing areas like the North American Prairie States, the Pampa and Gran Chaco of South America and the Ukraine-to-Central Asia Black Earth belt.

See also[link]

• Plaggen soil
• Shifting cultivation
• Soil contamination
• Soil life
• Terra preta
• Fertilizer

References[link]

Patrick Dunleavy (b. 1952) is Professor of Political Science and Public Policy within the Government Department of the London School of Economics (LSE) and the director of the Master of Public Administration (MPA) Programme (he is currently on sabbatical; the latter position is currently held by Timothy Besley).

He is a prominent political theorist specialising in the fields of public policy and government. His research has focused on the concepts of sectors and sectoral conflicts, rational choice theories of politics, the bureau-shaping model of bureaucracy, and the claimed contemporary public management paradigm of digital era governance.

Dunleavy is also the author of advice texts for humanities and social sciences students, most notably his 2003 book "Authoring a PhD: How to plan, draft, write and finish a doctoral dissertation or thesis".

Editorships of Journals[link]

• Global Policy or the External Page Global Policy

Publications[link]

• Theories of the democratic state, edited by Dryzek, John and Dunleavy, Patrick, Palgrave Macmillan, Basingstoke,2009 UK. (ISBN 9780230542860)
• Developments in British Politics" series (1-8), (co-edited), Palgrave/Macmillan 2006 (ISBN 9781403948434)
• Democracy, Bureaucracy and Public Choice, (London: Pearson Education, 1991)
• Theories of the State: The Politics of Liberal Democracy, (co-authored) Basingstoke: Macmillan, 1987 (ISBN 9780333386989)
• Digital Era Governance: IT Corporations, the State, and E-Government, Oxford University Press, Oxford, UK, 2006. (ISBN 978-0199296194)

External links[link]

• LSE Experts page
• LSE Government Department