Leaching (agriculture)

In agriculture, leaching is the loss of water-soluble plant nutrients from the soil, due to rain and irrigation. Soil structure, crop planting, type and application rates of fertilizers, and other factors are taken into account to avoid excessive nutrient loss. Leaching may also refer to the practice of applying a small amount of excess irrigation where the water has a high salt content to avoid salts from building up in the soil (salinity control). Where this is practiced, drainage must also usually be employed, to carry away the excess water.

Leaching is a natural environment concern when it contributes to groundwater contamination. As water from rain, flooding, or other sources seeps into the ground, it can dissolve chemicals and carry them into the underground water supply. Of particular concern are hazardous waste dumps and landfills, and, in agriculture, excess fertilizer, improperly stored animal manure, and biocides (e.g. pesticides, fungicides, insecticides and herbicides).

Nitrogen leaching

Agriculture Production System
Nitrogen forms and pathways within an agricultural production system

Nitrogen is a common element in nature and an essential plant nutrient. Approximately 78% of Earth's atmosphere is nitrogen (N2). The strong bond between the atoms of N2 makes this gas quite inert and not directly usable by plants and animals. As nitrogen naturally cycles through the air, water and soil it undergoes various chemical and biological transformations. Nitrogen promotes plant growth. Livestock then eat the crops producing manure, which is returned to the soil, adding organic and mineral forms of nitrogen. The cycle is complete when the next crop uses the amended soil.[1] To increase food production, fertilizers, such as nitrate (NO3) and ammonium (NH4), which are easily absorbed by plants, are introduced to the plant root zone. However, soils do not absorb the excess NO3 ions, which then move downward freely with drainage water, and are leached into groundwater, streams and oceans.[2] The degree of leaching is affected by:

  • soil type and structure. For example, sandy soil holds little water while clay soils have high water-retention rates;
  • the amount of water used by the plants/crops;
  • how much nitrate is already present in the soil.[3]

The level of nitrous oxide (N2O) in the Earth's atmosphere is increasing at a rate of 0.2 to 0.3% annually. Anthropogenic sources of nitrogen are 50% greater than from natural sources, such as, soils and oceans. Leached agricultural inputs, i.e. fertilizers and manures, accounts for 75% of the anthropogenic source of nitrogen.[4] The Food and Agriculture Organization of the United Nations (FAO) estimates world demand for nitrogen fertilizers will increase by 1.7% annually between 2011 and 2015. An increase of 7.5 million tonnes. Regional increases of nitrogen fertilizer use are expected to be 67% by Asia, 18% by the Americas, 10% by Europe, 3% by Africa,and 1% by Oceania.[5]

Phosphorus leaching

Phosphorus (P) is a key nutrient regarding the eutrophication of surface waters and has been shown to limit algae growth in lake environments. Loss of P from agricultural fields has long been recognized as one of the major threats to surface water quality.[6] Leaching is an important transport pathway for P losses from agricultural fields in predominantly flat areas with sandy soils or soils prone to preferential flow.[7] As opposed to nitrogen phosphorus does interact with soil particles via adsorption and desorption. Important potential adsorption sites for P in soils are surfaces of iron and aluminum oxides or hydroxides such as gibbsite or ferrihydrite. Soils, especially those rich in such minerals do hence have a potential to store P added with fertilizer or manure. Adsorbed P stands in a complex equilibrium with P in the soil solution which is controlled by a multitude of different factors such as:

  • the availability of adsorption sites;
  • the concentration of phosphorus and other anions in the soil water solution;
  • soil pH;
  • soil redox potential.[7][8]

Phosphorus will leach when this equilibrium is shifted such that either previously adsorbed P is released into the soil solution or additional P cannot be adsorbed anymore. Many cultivated soils have been receiving fertilizer or manure P in amounts frequently exceeding the crop demand and this often over decades. Phosphorus added to such soils leaches simply because most of the potential adsorption sites are occupied by P input from the past, so called “legacy phosphorus”.[9][10] Leaching of P may also be caused by changing chemical conditions in the soil. A decrease in the soils redox potential due to prolonged water saturation may lead to reductive dissolution of ferric iron minerals that are important P sorption sites. Phosphorus adsorbed to these minerals is consequently released into the soil solution as well and may be leached. This process is of special concern in the restoration of natural wetlands that have previously been drained for agricultural production[11][12]

Health impacts

High levels of NO3 in water can adversely affect oxygen levels for both humans and aquatic systems. Human health issues include methemoglobinemia and anoxia, commonly referred to as blue baby syndrome. As a result of these toxic effects, regulatory agencies limit the amount of NO3 permissible in drinking water to 45–50 mg1-1. Eutrophication, a decline in oxygen content of water, of aquatic systems can cause the death of fish and other marine species. Finally, leaching of NO3 from acidic sources can increase the loss of calcium and other soil nutrients, thereby reducing an ecosystem's productivity.[2]

See also


  1. ^ Ontario Ministry of Agriculture, Food and Rural Affairs. Environmental Impacts of Nitrogen Use in Agriculture
  2. ^ a b Lin, BL; Sakoda, A; Shibasaki, R; Suzuki, M (2001). "A modelling approach to global nitrate leaching caused by anthropogenic fertilisation". Water Research. 35 (8): 1961–8. doi:10.1016/s0043-1354(00)00484-x. PMID 11337842.
  3. ^ "WQ262 Nitrogen in the Environment: Leaching | University of Missouri Extension". Extension.missouri.edu. Retrieved 2013-03-08.
  4. ^ Mosier, A. R.; Duxbury, J. M.; Freney, J. R.; Heinemeyer, O.; Minami, K. (1996). "Nitrous oxide emissions from agricultural fields: Assessment, measurement and mitigation" (PDF). Plant and Soil. 181: 95. doi:10.1007/BF00011296.
  5. ^ FAO, Current world fertilizer trends and outlook to 2015
  6. ^ Carpenter S. R., Caraco N. F., Correll D. L., Howarth R. W., Sharpley A. N., Smith V. H., 1998. Nonpoint pollution of surface waters with phosphorus and nitrogen. Ecological Applications 8, 559–568. https://doi.org/10.1890/1051-0761(1998)008[0559:NPOSWW]2.0.CO;2
  7. ^ a b Börling, Katarina (2003). Phosphorus sorption, accumulation and leaching. Diss. Sveriges lantbruksuniv., Acta Universitatis agriculturae Sueciae. Agraria, 1401-6249 ; 428
  8. ^ Schoumans, O. (2015). Phosphorus leaching from soils: process description, risk assessment and mitigation. Diss. Wageningen University and Research Centre.
  9. ^ Jarvie, H.P., Sharpley, A.N., Spears, B., Buda, A.R., May, L., Kleinman, P.J.A., 2013. Water Quality Remediation Faces Unprecedented Challenges from “Legacy Phosphorus.” Environ. Sci. Technol. 47, 8997–8998. https://doi.org/10.1021/es403160a
  10. ^ Schmieder, F., Bergström, L., Riddle, M., Gustafsson, J.-P., Klysubun, W., Zehetner, F., Condron, L., Kirchmann, H., 2018. Phosphorus speciation in a long-term manure-amended soil profile – Evidence from wet chemical extraction, 31P-NMR and P K-edge XANES spectroscopy. Geoderma 322, 19–27. https://doi.org/10.1016/j.geoderma.2018.01.026
  11. ^ Shenker M., Seitelbach S., Brand S., Haim A., Litaor M. I., 2004. Redox reactions and phosphorus release in re‐flooded soils of an altered wetland. European Journal of Soil Science 56, 515–525. https://doi.org/10.1111/j.1365-2389.2004.00692.x
  12. ^ Zak, D., Gelbrecht, J., 2007. The mobilisation of phosphorus, organic carbon and ammonium in the initial stage of fen rewetting (a case study from NE Germany). Biogeochemistry 85, 141–151. https://doi.org/10.1007/s10533-007-9122-2

External links

  • International Panel on Climate Change (IPCC) On line : [1]
  • R.J.Oosterbaan, Water and salt balances in agricultural hydrology. Lecture notes, International Course on Land Drainage, ILRI, Wageningen, The Netherlands. On line : [2]
  • R.J.Oosterbaan, 1997. "SaltMod: A tool for interweaving of irrigation and drainage for salinity control". In: W.B.Snellen (ed.), Towards integration of irrigation, and drainage management. ILRI Special report, p. 41–43. On line : [3]
Atmosphere of Earth

The atmosphere of Earth is the layer of gases, commonly known as air, that surrounds the planet Earth and is retained by Earth's gravity. The atmosphere of Earth protects life on Earth by creating pressure allowing for liquid water to exist on the Earth's surface, absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation).

By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases.

Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere. Air content and atmospheric pressure vary at different layers, and air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in Earth's troposphere and in artificial atmospheres.

The atmosphere has a mass of about 5.15×1018 kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The Kármán line, at 100 km (62 mi), or 1.57% of Earth's radius, is often used as the border between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km (75 mi). Several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition.

The study of Earth's atmosphere and its processes is called atmospheric science (aerology). Early pioneers in the field include Léon Teisserenc de Bort and Richard Assmann.

Index of soil-related articles

This is an index of articles relating to soil.


Leaching is the loss or extraction of certain materials from a carrier into a liquid (usually, but not always a solvent). and may refer to:

Leaching (agriculture), the loss of water-soluble plant nutrients from the soil; or applying a small amount of excess irrigation to avoid soil salinity

Leaching (chemistry), the process of extracting substances from a solid by dissolving them in a liquid

Leaching (metallurgy), a widely used extractive metallurgy technique which converts metals into soluble salts in aqueous media

Dump leaching, an industrial process to extract metals from ore taken directly from the mine and stacked on the leach pad without crushing

Heap leaching, an industrial process to extract metals from ore which has been crushed into small chunks

Tank leaching, a hydro metallurgical method of extracting valuable material from ore

In-situ leaching, a process of recovering minerals such as copper and uranium through boreholes drilled into the deposit

Leaching (pedology), the loss of mineral and organic solutes due to percolation from soil

Bioleaching, the extraction of specific metals from their ores through the use of bacteria and fungi

Leaching (pedology)

In pedology, leaching is the removal of soluble materials from one zone in soil to another via water movement in the profile. It is a mechanism of soil formation distinct from the soil forming process of eluviation, which is the loss of mineral and organic colloids. Leached and elluviated materials tend to be lost from topsoil and deposited in subsoil. A soil horizon accumulating leached and eluviated materials is referred to as a zone of illuviation.

Laterite soil, which develops in regions with high temperature and heavy rainfall, is an example of this process in action.


Lessivage is a kind of leaching from clay particles being carried down in suspension.

The process can lead to the breakdown of peds (the particles that give the soil its characteristic structure).

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