Claypan is a dense, compact, slowly

permeable Permeability, permeable, and semipermeable may refer to: Chemistry *Semipermeable membrane, a membrane which will allow certain molecules or ions to pass through it by diffusion *Vascular permeability, the movement of fluids and molecules betwe ...

layer in the subsoil. It has a much higher

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

content than the overlying material, from which it is separated by a sharply defined boundary. The dense structure restricts root growth and water infiltration. Therefore, a

perched water table The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated. T ...

might form on top of the claypan. In the Canadian classification system, claypan is defined as a clay-enriched illuvial B (Bt) horizon.

Location

Claypan is present in a wide area of the central United States (about 4 million ha) across multiple states such as

Kansas Kansas () is a state in the Midwestern United States. Its capital is Topeka, and its largest city is Wichita. Kansas is a landlocked state bordered by Nebraska to the north; Missouri to the east; Oklahoma to the south; and Colorado to the ...

Oklahoma Oklahoma (; Choctaw language, Choctaw: ; chr, ᎣᎧᎳᎰᎹ, ''Okalahoma'' ) is a U.S. state, state in the South Central United States, South Central region of the United States, bordered by Texas on the south and west, Kansas on the nor ...

, and

Illinois Illinois ( ) is a U.S. state, state in the Midwestern United States, Midwestern United States. Its largest metropolitan areas include the Chicago metropolitan area, and the Metro East section, of Greater St. Louis. Other smaller metropolita ...

. It can also be found in

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

throughout the south-west

Queensland ) , nickname = Sunshine State , image_map = Queensland in Australia.svg , map_caption = Location of Queensland in Australia , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , established_ ...

Formation

Claypan is formed in different parent materials depending on geological locations, such as floodplains. The formation of the claypan relates to a lack of vegetation coverage, soil particle size distribution, and high rainfall. The lack of vegetation coverage makes soil become more susceptible to raindrop attacks. When the raindrops hit on bare soil with high energy, the fine sand, silt, and clay particles are re-arranged to plug all the pore spaces. When all the pores are filled, a packed layer is formed to limit the water infiltration.

Characteristics

The dominant material is the

montmorillonite Montmorillonite is a very soft phyllosilicate group of minerals that form when they precipitate from water solution as microscopic crystals, known as clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite gro ...

material which has a high swell and shrinks characteristics depending on the soil water content. In the dry season, evaporation moves water from the deep horizon toward the soil surface through capillary action. The water removal results in shrinkage of clay, and the soil becomes dry and hard. In the wet season, high precipitation leads to a swell of clay to absorb water. The high moisture content results in wet and sticky clay texture. When the clay swells, the low saturated hydraulic conductivity prevents the vertical water infiltration to the deeper soil horizon. It leads to water perches above the claypan layer. The water permeability is restricted in the claypan layer resulting in low soil aeration. The water-holding capability of the claypan is high. However, most of the stored water is not available for the plant since water evaporates frequently and soil pore size is tiny. Since the claypan is acidic and clay-rich, there is high sorption of Al, K and Fe oxides to clay minerals. Therefore, the claypan contains a cations-dominated zone that leads to a relatively high cation exchange capacity (CEC) to absorb and retain nutrients. The concentration of extractable potassium positively relates to clay content. There is a relatively high extractable potassium content in the claypan due to the accumulation of cations. The high content of aluminium oxidates, and iron oxidates attract phosphorus to clay particles which increases the phosphorus content in the soil.

Influences on plants

The major negative influences of claypan on plants are root restriction, available water limitation and nutrient limitation. The dense structure of the claypan restricts root development. Plants with shallow roots, might not withstand the soil contraction forces due to the shrinkage of clay in the dry season. The low water infiltration rate and hydraulic conductivity may lead to a perched water table form on top of the claypan layer. Water in the perched water table evaporates instead of uptake by plants, especially in the dry season. In the wet season with high precipitation, water can penetrate throughout the soil. However, the low aeration in saturated soil may result in root rots that reduce the stability of plants. The acidic, clay-rich characteristics of the claypan lead to phosphorus (P) sorption to clay minerals. Even though the total P content in the claypan is relatively high, they are strongly attracted by the clay particles that are not available for plant use. Therefore, a high amount of P fertilizer is required to increase the available P for plant absorption. Different from P, the high content of

potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...

(K) in the claypan is available for plant use which reduces the application of potassium fertilizer. Water erosion - geograph

Risk of soil erosion

Soil with a claypan layer is highly vulnerable to

soil erosion Soil erosion is the denudation or wearing away of the upper layer of soil. It is a form of soil degradation. This natural process is caused by the dynamic activity of erosive agents, that is, water, ice (glaciers), snow, air (wind), plants, and ...

. The low water infiltration rate and the perched water table form on top of the claypan layer largely increase the surface runoff during precipitation with a long duration or high intensity. The runoff water can remove the topsoil with mostly organic matter. It will further reduce the nutrient availability for plants.

References