Which one of the following with regard to Aridisol, one of the soil orders, is not correct?
High organic matter
Understanding Aridisol Soil Order
Aridisols are a major order in soil classification (Soil Taxonomy). They are soils characteristic of arid and semi-arid environments. These regions receive very low amounts of rainfall, leading to specific soil properties.
Key Characteristics of Aridisols
Aridisols develop under conditions where water is scarce for extended periods. This scarcity of water influences several soil formation processes, resulting in distinct features. Let's examine the characteristics mentioned in the options:
- Lack of water for plants during most part of the year: This is a defining characteristic of arid and semi-arid climates where Aridisols are found. Plant growth is severely limited by water availability for much of the year. This statement is correct regarding Aridisols.
- High organic matter: Organic matter in soil comes from decomposed plant and animal remains. In arid environments, vegetation is sparse, and the low moisture content limits microbial activity, which is essential for decomposition and incorporation of organic matter into the soil. Therefore, Aridisols typically have low organic matter content, especially in the surface horizons. This statement is NOT correct regarding Aridisols.
- Large accumulation of carbonates at depth: In arid climates, the limited rainfall does not provide enough water to leach soluble salts and carbonates completely out of the soil profile. Instead, these substances are dissolved near the surface and then moved downwards with the limited infiltrating water, accumulating at some depth. Accumulation of calcium carbonate, often forming a calcic or petrocalcic horizon, is common in Aridisols. This statement is correct regarding Aridisols.
- Absence of deep wide cracks: Deep, wide cracks that open and close with changes in moisture are characteristic of soils high in shrink-swell clays, specifically Vertisols. While some cracking can occur in Aridisols depending on clay content, deep wide cracks are not a distinguishing feature of the Aridisol order. Their absence is generally expected compared to orders like Vertisols. This statement is correct regarding Aridisols.
Based on the analysis of typical Aridisol properties, the statement "High organic matter" is inconsistent with the characteristics of soils formed in arid environments.
Detailed Analysis of Statements
| Statement | Relevance to Aridisols | Correctness |
|---|---|---|
| Lack of water for plants during most part of the year | Directly related to the arid/semi-arid climate where Aridisols form. | Correct |
| High organic matter | Arid environments have limited vegetation and slow decomposition, resulting in low organic matter. | Incorrect |
| Large accumulation of carbonates at depth | Limited leaching in arid climates leads to accumulation of carbonates (like calcium carbonate) below the surface. | Correct |
| Absence of deep wide cracks | Deep cracks are typical of shrink-swell clays (Vertisols), not a primary feature of Aridisols. | Correct |
Therefore, the statement that is NOT correct with regard to Aridisol is "High organic matter".
Revision Table: Aridisol Characteristics
| Typical Characteristic | Description |
|---|---|
| Dry Conditions | Lack of available water for plants for long periods. |
| Low Organic Matter | Limited vegetation and slow decomposition in surface horizons. |
| Presence of Diagnostic Horizons | Often have an ochric epipedon (light-colored surface) and diagnostic subsurface horizons like argillic (clay accumulation), calcic (carbonate accumulation), gypsic (gypsum accumulation), or salic (salt accumulation). |
| Accumulation of Salts/Carbonates | Limited leaching leads to accumulation of soluble salts, gypsum, or calcium carbonate within the profile. |
Additional Information on Soil Orders
Soil Taxonomy is a classification system developed by the United States Department of Agriculture (USDA). It classifies soils into 12 orders based on soil properties that reflect major factors of soil formation.
- Soil Orders: The 12 soil orders are Alfisols, Andisols, Aridisols, Entisols, Gelisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols, and Vertisols.
- Aridisols: Represent about 12% of the Earth's ice-free land area. They are common in deserts and semi-arid regions worldwide.
- Contrast with other Orders: Comparing Aridisols to orders like Mollisols (grassland soils, high organic matter) or Vertisols (cracking clays) helps highlight their unique characteristics.
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