Identifying the Organic Soil Type: Histosols Explained

The question asks us to identify which of the given options represents an organic soil. Organic soils are distinct soil types characterized by a very high content of organic matter.

What Makes a Soil Organic?

Soils are typically composed of mineral particles, organic matter, water, and air. Mineral soils are those where the mineral component dominates. Organic soils, on the other hand, have accumulated large amounts of partially decayed plant and animal material. This accumulation usually happens in environments where decomposition is slow, such as waterlogged conditions. For a soil to be classified as organic, it generally needs to meet specific criteria regarding the thickness and organic carbon content of its organic horizons.

Analyzing the Soil Types

Let's look at the options provided:

• Aridisols: These are soils that form in arid and semi-arid climates. They are characterized by limited leaching and often have accumulations of calcium carbonate, gypsum, or soluble salts. Due to the low amount of plant growth and rapid decomposition in dry conditions when moisture is available, Aridisols typically have low organic matter content.
• Histosols: These soils are primarily composed of organic materials. They form in saturated or near-saturated conditions, like wetlands, swamps, or bogs. The waterlogged environment slows down the decomposition of organic matter, leading to its accumulation over time. Histosols have a thick layer of organic material, often more than 40 cm, depending on the type of organic material and the underlying mineral layer.
• Oxisols: These are highly weathered soils common in tropical and subtropical regions with high rainfall. They are characterized by a subsurface horizon dominated by oxides of iron and aluminum and have low base saturation. While surface horizons may have some organic matter, the overall profile is dominated by mineral weathering products, not accumulated organic material.
• Vertisols: These are clay-rich soils that contain a high proportion of swelling clays. They are known for their deep cracks that form during dry periods and the churning or inversion of the soil profile as the clay swells and shrinks. While they can have significant clay content and unique physical properties, they are not primarily defined by a high organic matter content accumulated under saturated conditions.

Conclusion

Based on the characteristics of each soil type, Histosols are the ones defined by their high organic matter content resulting from accumulation in wet environments. Therefore, Histosols are classified as organic soils.

The correct option is Histosols.

Revision Table: Soil Orders and Characteristics

Additional Information on Organic Matter in Soils

Organic matter in soil, often referred to as soil organic carbon (SOC), is derived from dead plants, animals, and microorganisms. It plays a crucial role in soil fertility, structure, water holding capacity, and nutrient cycling. The decomposition rate of organic matter is influenced by factors like temperature, moisture, aeration, and the activity of soil organisms.

In most mineral soils, organic matter makes up a relatively small percentage of the total soil volume or mass, typically between 1% and 6% in the topsoil. However, in organic soils like Histosols, organic matter can constitute more than 50% of the soil volume and is the dominant component that determines the soil's properties.

The classification of soils, like the orders mentioned in the options, is part of soil taxonomy, a system used globally to classify soils based on their properties and horizons.