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In soil science, agriculture and gardening, hardpan or soil pan is a dense layer of soil, usually found below the uppermost topsoil layer. There are different types of hardpan, all sharing the general characteristic of being a distinct soil layer that is largely impervious to water. Some hardpans are formed by deposits in the soil that fuse and bind the soil particles. These deposits can range from dissolved silica to matrices formed from and calcium carbonate. Others are man-made, such as hardpan formed by compaction from repeated , particularly with , or by heavy traffic or pollution.
Another major determinant is the soil particle size. Clay particles are some of the smallest particles commonly found in soils. Due to their structure the space between individual clay particles is small and already restricts the passage of water, reducing infiltrationC.Michael Hogan. 2010. Abiotic factor. Encyclopedia of Earth. eds Emily Monosson and C. Cleveland. National Council for Science and the Environment. Washington DC and hence drainage. Soils with a high clay content are also easily compacted and affected by man-made discharges. Clay particles have a strong negative electrostatic charge and will readily bond to positively charged ions dissolved in the soil-water matrix. Common salts such as sodium ions contained in wastewater can fulfil this role and lead to a localized hardpan in some soil types. This is a common cause of septic system failure due to the prevention of proper drainage in field.
The use of Soil conditioner can also be employed to alter the soil structure and promote the dissolution of the hard pan. It has been observed that increasing the amount of soil organic matter through the working-in of manure, compost or peat can both improve local drainage and promote the proliferation of Earthworm that can, over time, break relatively thin hardpan layers.
More difficult hardpans may be further improved through the action of both adjusting the soil pH with lime if the soil is acidic, and with the addition of gypsum. This combination can help loosen clay particles bound into a hardpan by the actions of hard salts such as iron, calcium carbonate and sodium, by promoting their mobility through a higher pH while proving a suitable source of exchanging minerals (the gypsum). This works because gypsum salts, although not "soft", are still water permeable and have a larger, more open structure, the results of which do not promote as hard a matrix as was replaced. However, unlike when employing mechanical means, breaking a hardpan through the use of amendments may require action over the course of years, and even then one is by no means assured success. The results are primarily determined by how extensive and / or intractable the hardpan is.
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