Claypan is a dense, compact, slowly

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

layer in the

subsoil Subsoil is the layer of soil under the topsoil on the surface of the ground. Like topsoil, it is composed of a variable mixture of small particles such as sand, silt and clay, but with a much lower percentage of organic matter and humus. The su ...

. It has a much higher

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

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 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 landlocked U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Nebraska to the north; Missouri to the east; Oklahoma to the south; and Colorado to the west. Kansas is named a ...

Oklahoma Oklahoma ( ; Choctaw language, Choctaw: , ) is a landlocked U.S. state, state in the South Central United States, South Central region of the United States. It borders Texas to the south and west, Kansas to the north, Missouri to the northea ...

, and

Illinois Illinois ( ) is a U.S. state, state in the Midwestern United States, Midwestern United States. It borders on Lake Michigan to its northeast, the Mississippi River to its west, and the Wabash River, Wabash and Ohio River, Ohio rivers to its ...

. It can also be found in

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

throughout the south-west

Queensland Queensland ( , commonly abbreviated as Qld) is a States and territories of Australia, state in northeastern Australia, and is the second-largest and third-most populous state in Australia. It is bordered by the Northern Territory, South Austr ...

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 grou ...

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 a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

(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 Topsoil, 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, Atmosphere of Ea ...

. The low water infiltration rate and the perched water table form on top of the claypan layer largely increase the

surface runoff Surface runoff (also known as overland flow or terrestrial runoff) is the unconfined flow of water over the ground surface, in contrast to ''channel runoff'' (or ''stream flow''). It occurs when excess rainwater, stormwater, meltwater, or other ...

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