Petrocalcic horizon

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Jornada G-IBPE soil pit horizons

Petrocalcic Horizon is a distinct layer in the soil profile characterized by the accumulation of secondary calcium carbonate (CaCO3) to the extent that the horizon becomes cemented or indurated. This horizon is typically found in arid or semi-arid regions where low precipitation rates limit the leaching of calcium carbonate from the soil profile. The formation of a petrocalcic horizon is a slow process that can span thousands of years, significantly affecting soil development and land use potential.

Formation

The formation of a petrocalcic horizon involves the precipitation of calcium carbonate from calcium-rich water as it moves upward through the soil profile by capillary action or downward following infrequent rain events. Over time, the accumulation of calcium carbonate cements soil particles together, creating a hard layer known as caliche, hardpan, or duripan, depending on regional terminologies. Factors influencing the formation of a petrocalcic horizon include soil texture, climate, vegetation, topography, and the presence of calcium sources.

Characteristics

Petrocalcic horizons are characterized by their hardness, which can range from soft and friable to extremely hard, depending on the degree of cementation. They are typically found beneath an accumulation zone of calcium carbonate, known as the calcic horizon. The presence of a petrocalcic horizon can be identified by its distinct physical properties, such as:

• High bulk density
• Low porosity
• Reduced water infiltration and permeability
• Layered or massive structure
• Whitish to light brown color due to the concentration of calcium carbonate

Classification and Nomenclature

In the Soil Taxonomy system, soils with a petrocalcic horizon are classified within the order Aridisol. The presence of a petrocalcic horizon is a diagnostic feature for certain subgroups within this order. The nomenclature and classification criteria for petrocalcic horizons are detailed in soil classification systems, including the World Reference Base for Soil Resources (WRB).

Environmental and Agricultural Implications

The presence of a petrocalcic horizon can have significant environmental and agricultural implications. Its impermeable nature can restrict root penetration and limit the availability of water and nutrients to plants, posing challenges for agriculture and natural vegetation. In some cases, the petrocalcic horizon can act as a natural water table, supporting unique ecosystems adapted to these conditions.

Management

Managing lands with a petrocalcic horizon requires strategies to mitigate its limitations. In agricultural settings, practices such as deep plowing, the application of soil amendments, and the use of deep-rooted crops can help to overcome the challenges posed by the petrocalcic horizon. In natural landscapes, conservation efforts focus on protecting the unique ecosystems that have adapted to these soil conditions.