Soil fertility - wikipedia Soil fertility Soil fertility refers to the ability of a soil to sustain agricultural plant growth, i.e. to provide plant habitat and result in sustained and consistent yields of high quality. A fertile soil has the following properties : The ability to supply essential plant nutrients and water in adequate amounts and proportions for plant growth and reproduction ; and The absence of toxic substances which may inhibit plant growth. The following properties contribute to soil fertility in most situations : Sufficient soil depth for adequate root growth and water retention ; Good internal drainage, allowing sufficient aeration for optimal root growth ( although some plants, such as rice, tolerate waterlogging ) ; Topsoil with sufficient soil organic matter for healthy soil structure and soil moisture retention ; Soil pH in the range 5.5 to 7.0 ( suitable for most plants but some prefer or tolerate more acid or alkaline conditions ) ; Adequate concentrations of essential plant nutrients in plant - available forms ; Presence of a range of microorganisms that support plant growth. In lands used for agriculture and other human activities, maintenance of soil fertility typically requires the use of soil conservation practices. This is because soil erosion and other forms of soil degradation generally result in a decline in quality with respect to one or more of the aspects indicated above. Soil scientists use the capital letters O, A, B, C, and E to identify the master horizons, and lowercase letters for distinctions of these horizons. Most soils have three major horizons -- the surface horizon ( A ), the subsoil ( B ), and the substratum ( C ). Some soils have an organic horizon ( O ) on the surface, but this horizon can also be buried. The master horizon, E, is used for subsurface horizons that have a significant loss of minerals ( eluviation ). Hard bedrock, which is not soil, uses the letter R. Contents 1 Soil fertilization 2 Light and CO limitations 3 Soil depletion 4 Irrigation water effects 5 Global distribution 6 See also 7 References Soil fertilization Main article : Fertilizer Bioavailable phosphorus is the element in soil that is most often lacking. Nitrogen 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 ( P O ) available phosphorus and 15 percent ( K O ) 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. Also, since nitrogen, phosphorus and potassium generally must be in the inorganic forms to be taken up by plants, inorganic fertilizers are generally immediately bioavailable to plants without modification. However, some have criticized the use of inorganic fertilizers, claiming that the water - soluble nitrogen does n'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 are decomposed by micro-organisms, they release inorganic nutrients to the soil solution, a process referred to as mineralization. Those nutrients may then undergo further transformations which may be aided or enabled by soil micro-organisms. Like plants, many micro-organisms require or preferentially use inorganic forms of nitrogen, phosphorus or potassium and will compete with plants for these nutrients, tying up the nutrients in microbial biomass, a process often called immobilization. The balance between immobilization and mineralization processes depends on the balance and availability of major nutrients and organic carbon to soil microorganisms. Natural processes such as lightning strikes may fix atmospheric nitrogen by converting it to ( NO ). Denitrification may occur under anaerobic conditions ( flooding ) in the presence of denitrifying bacteria. Nutrient cations, including potassium and many micronutrients, are held in relatively strong bonds with the negatively charged portions of the soil in a process known as cation exchange. 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 in the world. Nathai Light and CO limitations Photosynthesis is the process whereby plants use light energy to drive chemical reactions which convert CO 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 CO is highly effective at promoting plant growth up to levels over 300 ppm. Further increases in CO can, to a very small degree, continue to increase net photosynthetic output. Soil depletion Soil depletion occurs when the components which contribute to fertility are removed and not replaced, and the conditions which support soil 's fertility are not maintained. This leads to poor crop yields. In agriculture, depletion can be due to excessively intense cultivation and inadequate soil management. Soil fertility can be severely challenged when land use changes rapidly. For example, in Colonial New England, colonists made a number of decisions that depleted the soils, including : allowing herd animals to wander freely, not replenishing soils with manure, and a sequence of events that led to erosion. William Cronon wrote that ``... the long - term effect was to put those soils in jeopardy. The removal of the forest, the increase in destructive floods, the soil compaction and close - cropping wrought by grazing animals, plowing -- all served to increase erosion. '' Karl Marx wrote of the role of capitalism in soil depletion. In Capital, Volume I, he wrote :... all progress in capitalistic agriculture is a progress in the art, not only of robbing the labourer, but of robbing the soil ; all progress in increasing the fertility of the soil for a given time, is a progress towards ruining the lasting sources of that fertility. The more a country starts its development on the foundation of modern industry, like the United States, for example, the more rapid is this process of destruction. Capitalist production, therefore, develops technology, and the combining together of various processes into a social whole, only by sapping the original sources of all wealth -- the soil and the labourer. One of the most widespread occurrences of soil depletion as of 2008 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. 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. 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. Irrigation water effects The quality of irrigation water is very important to maintain soil fertility and tilth, and for using more soil depth by the plants. When soil is irrigated with high alkaline water, unwanted sodium salts build up in the soil which would make soil draining capacity very poor. So plant roots can not penetrate deep into the soil for optimum growth in Alkali soils. When soil is irrigated with low pH / acidic water, the useful salts ( Ca, Mg, K, P, S, etc. ) are removed by draining water from the acidic soil and in addition unwanted aluminium and manganese salts to the plants are dissolved from the soil impeding plant growth. When soil is irrigated with high salinity water or sufficient water is not draining out from the irrigated soil, the soil would convert into saline soil or lose its fertility. Saline water enhance the turgor pressure or osmotic pressure requirement which impedes the off take of water and nutrients by the plant roots. Top soil loss takes place in alkali soils due to erosion by rain water surface flows or drainage as they form colloids ( fine mud ) in contact with water. Plants absorb water - soluble inorganic salts only from the soil for their growth. Soil as such does not lose fertility just by growing crops but it lose its fertility due to accumulation of unwanted and depletion of wanted inorganic salts from the soil by improper irrigation and acid rain water ( quantity and quality of water ). The fertility of many soils which are not suitable for plant growth can be enhanced many times gradually by providing adequate irrigation water of suitable quality and good drainage from the soil. Global distribution 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 Arable land Fertilizer Plaggen soil Shifting cultivation Soil contamination Soil life Terra preta References Jump up ^ Bodenfruchtbarkeit, Retrieved on 2015 - 11 - 09. Jump up ^ `` Soil Fertility ''. www.fao.org. Retrieved 18 June 2016. Jump up ^ Brady N., Weil R. 2002 Nitrogen and sulfur economy of soils. pp. 543 - 571 in Helba ( ed. ), The Nature and properties of soils. Pearson Education, NJ. Jump up ^ Sims, G.K., and M.M. Wander. 2002. Proteolytic activity under nitrogen or sulfur limitation. Appl. Soil Ecol. 568 : 1 - 5. Jump up ^ Sims, G.K. 2006. Nitrogen Starvation Promotes Biodegradation of N - Heterocyclic Compounds in Soil. Soil Biology & Biochemistry 38 : 2478 - 2480. Jump up ^ F. Stuart Chapin III ; Pamela A. Matson ; Harold A. Moon ( 2002 ). Principles of Terrestrial Ecosystem Ecology. Springer. ISBN 0387954392. ^ Jump up to : Cronon, William, Changes in the Land : Indians, Colonists, and the Ecology of New England, NY : Hill & Wang, 1983, p. 145 - 152. Jump up ^ Marx, Karl. 1867. `` Modern Industry and Agriculture, '' in Capital, Volume I, pp. 244 - 5. http://www.marxists.org/archive/marx/works/1867-c1/ch15.htm#S10 Jump up ^ Bjonnes, R., 1997, Food vs Feed, People 's News Agency ; Frederiksberg C, Denmark Jump up ^ Kötke, William H. ( 1993 ). The Final Empire : The Collapse of Civilization and the Seed of the Future. Arrow Point Press. ISBN 0963378457. Jump up ^ Managing Soil Tilth ; Colorado state university garden notes, Retrieved on 2014 - 10 - 04. Jump up ^ Managing irrigation water quality, Oregon State University, USA, Retrieved on 2012 - 10 - 04. Plant nutrition / Fertilizer Imbalances Boron deficiency Calcium deficiency Iron deficiency Magnesium deficiency Manganese deficiency Molybdenum deficiency Nitrogen deficiency Phosphorus deficiency Potassium deficiency Zinc deficiency Micronutrient deficiency Chlorosis Fertilizer burn Assimilation Nitrogen assimilation Phosphorus assimilation Sulfur assimilation Microbial assistance Photorespiration Methods Fertigation Fertilizer tree Green manure Hoagland solution Hydroponic dosers Living mulch Nutrient budgeting Nutrient management Organic fertilizer Plant tissue test Miscellaneous Soil fertility Nutrient pollution Soil pH Agrobiology Related concepts Algal nutrient solutions Natural resources Air Pollution / quality Ambient standards ( USA ) Index Indoor developing nations Law Clean Air Act ( USA ) Ozone depletion Emissions Airshed Trading Deforestation ( REDD ) Energy Law Resources Fossil fuels ( peak oil ) Geothermal Nuclear Solar sunlight shade Tidal Wave Wind Land Arable peak farmland Degradation Law property Management habitat conservation Minerals mining law sand peak rights Soil conservation fertility health resilience Use planning reserve Life Biodiversity Bioprospecting Biosphere Bushfood Bushmeat Fisheries law management Food Forests genetic resources law management Game law Gene bank Herbalist plants Marine conservation Non-timber forest products Rangeland Seed bank Wildlife conservation management Wood Water Types / location Aquifer storage and recovery Drinking Fresh Groundwater pollution recharge remediation Hydrosphere Ice bergs glacial polar Irrigation Rain harvesting Stormwater Surface water Wastewater reclaimed Aspects Desalination Floods Law Leaching Sanitation Conflict Conservation Peak water Pollution Privatization Quality Right Resources management policy Related Commons enclosure global land tragedy of Economics ecological land Ecosystem services Exploitation overexploitation Earth Overshoot Day Management adaptive Natural capital accounting Nature reserve Systems ecology Urban ecology Wilderness Resource Common - pool Conflict ( perpetuation ) Curse Depletion Extraction Nationalism Renewable / Non-renewable Portals Agriculture and agronomy Energy Environment Fishing Forestry Mining Water Wetlands agencies law management ministries organizations Colleges Natural resources Retrieved from `` https://en.wikipedia.org/w/index.php?title=Soil_fertility&oldid=830235391 '' Categories : Soil Soil improvers Fertilizers Horticulture and gardening Hidden categories : Articles containing potentially dated statements from 2008 All articles containing potentially dated statements Talk Contents About Wikipedia Azərbaycanca Беларуская Català Čeština Dansk Deutsch Esperanto Français 한국어 Հայերեն മലയാളം Bahasa Melayu Nederlands नेपाली 日本 語 ਪੰਜਾਬੀ پنجابی Polski Runa Simi Русский Türkçe Українська Winaray 中文 16 more Edit links This page was last edited on 13 March 2018, at 16 : 12. Text is available under the Creative Commons Attribution - ShareAlike License ; additional terms may apply. By using this site, you agree to the Terms of Use and Privacy Policy. Wikipedia ® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization. About Wikipedia

Question: which layer of soil is the most fertile
Answer: Topsoil