Understanding the Composition and Characteristics of the C Horizon in Soil Profiles

The C horizon, a crucial component in soil profiles, is primarily composed of weathered rock and unconsolidated material, serving as the parent material from which the upper soil horizons A and B develop. This layer plays a significant role in the formation and stabilization of the soil system, acting as a repository for minerals and particulates that do not undergo extensive soil formation processes.

The C Horizon: A Key Component of Soil Profiles

The C horizon, or bedrock horizon, is described as a mineral layer that excludes strongly cemented and hard bedrock, and is minimally affected by pedogenic (soil-forming) processes (Soil Survey Staff, 2014). In the World Reference Base for Soil Resources (WRB), the C horizon also includes layers such as siliceous and calcareous formations like shells, corals, and diatomaceous earth (IUSS Working Group WRB, 2015). While many soils contain a C horizon overlying bedrock (designated an R layer in Soil Taxonomy), this is not always the case, especially in highly weathered soils like Ultisols and Oxisols (Table 3).

Depth and Composition Variations

Significantly, the bedrock at the base of the C horizon can be found at various depths, often below 200 cm. Soil profiles formed in unconsolidated geological deposits such as alluvium, colluvium, loess, or volcanic deposits may have bedrock at greater depths. Additionally, soils formed on loess and incrementally-accumulated thin tephra deposits typically have no C horizon, as all parts of such soils exhibit evidence of pedogenic alteration (Lowe, 2019; Lowe and Tonkin, 2010).

Properties and Variations

Several studies have investigated the C horizon variation across various soil types, including Alfisols, Gelisols, Inceptisols, Spodosols, and Ultisols. The variation in soil organic carbon (SOC) in the C horizon was found to be considerable, with a mean coefficient of variation (CV) of 92% and a range of 50-117%. In contrast, the bulk density showed small, similar variations with a mean CV of 7%, which is comparable to the A and B horizons. Textural properties, including sand, silt, and clay, exhibited high variations in the C horizon compared to the upper horizons.

Elemental Content in the C Horizon

There has been limited research on elemental content in the C horizon, but it was found to show lower variation compared to the A and B horizons. This indicates that the C horizon is generally stable with respect to elemental changes, consistent with its minimal influence from soil-forming processes.

Conclusion

The C horizon, a significant component of soil profiles, plays a critical role in the stability and nutrient provision of the soil system. Its composition and properties are crucial for understanding soil formation and the overall health of the soil. Further research is needed to better understand the variation and composition of the C horizon in different soil types and geologies.

References:

Lowe, K. D. (2019). Soil profiles, horizons, and characteristics. In Soil Formation and Classification (pp. 49-78). Lowe, K. D., Tonkin, P. J. (2010). Soil Formation and Classification. Springer. IUSS Working Group WRB (2015). World reference base for soil resources 2014, update 2015. World Soil Resources Reports, 106. Soil Survey Staff (2014). Soil Taxonomy.