User:BigBigGoose/sandbox

In geology, claypan is a subsoil layer. It is typically found 15 to 60 cm below the soil surface. Compared with the layer above, the clay content of the claypan layer suddenly increases by a high amount. The dense structure restricts root growth and water infiltration. Therefore, a perched water table might form on top of the claypan. Claypan presents in a wide area of the central US (about 4 million ha) across multiple provinces such as Kansas, Oklahoma, and Illinois.

Formation
Claypan is formed in different parent materials depending on geological locations, such as floodplains. The exchangeable Na in some parent materials can be carried by water into the soil profile. The exchangeable Na breaks aggregates formed by clay and silt particles. The dispersed particles and organic fragments move downward by water to plug the small pores. It restricts water infiltration. Positive charges of exchangeable Na cations form a shell around clay particles. Besides Na, exchangeable divalent or trivalent cations such as Ca tightly bond with clay particles. The cation shells around clay particles are weakened and stop repelling each other. Clay particles can aggregate and form a stable claypan layer.

Characteristics
The dominant material is the clay which has the swell and shrinks characteristic depending on the soil water content. The claypan is hard under low soil moisture, and sticky under high soil moisture. 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. Claypan has a relatively high cation exchange capacity (CEC) to absorb and retain nutrients. It contains a cations-dominated zone includes such as Al, K, and Fe. The concentration of extractable K positively relates to clay content. There is a relatively high extractable K content in the claypan due to the accumulation of cations. The high content of Al oxidates, and Fe oxidates attract P to clay particles that increase P 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. Even though the 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 improve soil productivity. Different from P, the high content of K in the claypan is available for plant use which reduces the application of K fertilizer.

Risk of soil erosion
Soil with a claypan layer is highly vulnerable to soil erosion. 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.

Soil with a claypan layer is highly vulnerable to soil erosion. 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.