In agriculture, leaching refers to 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 an environmental 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 1
- Health impacts 2
- See also 3
- References 4
- External links 5
- International Panel on Climate Change (IPCC) On line : 
- R.J.Oosterbaan, Water and salt balances in agricultural hydrology. Lecture notes, International Course on Land Drainage, ILRI, Wageningen, The Netherlands. On line : 
- 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 : 
- Ontario Ministry of Agriculture, Food and Rural Affairs. Environmental Impacts of Nitrogen Use in Agriculture
- FAO, Current world fertilizer trends and outlook to 2015
- Leaching (pedology)
- Leaching model (soil)
- Soil salinity control (leaching requirement, leaching efficiency)
- SahysMod (spatial agro-hydro salinity, leaching and groundwater model)
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.
The level of N2O in 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.
- 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.
The degree of leaching is affected by:  ions, which then move downward freely with drainage water, and are leached into groundwater, streams and oceans.3), which are easily absorbed by plants, are introduced to the plant root zone. However, soils do not absorb the excess NO4 (NHammonium) and 3 (NOnitrate To increase food production, fertilizers, such as