The highest category of soil classification system is

Correct option is D

The correct answer is (D) Order

Explanation:
• Soil classification, specifically the USDA Soil Taxonomy, is a hierarchical system similar to biological classification.
• The system has six categories, from broadest to most specific: 1. Order, 2. Suborder, 3. Great Group, 4. Subgroup, 5. Family, 6. Series.
• Order is the highest and broadest category. All soils on Earth are grouped into just 12 orders based on major soil-forming processes and diagnostic horizons (e.g., Entisols, Inceptisols, Vertisols).
• As you move down the hierarchy, the criteria become more specific and the number of classes increases significantly.

Information Booster:
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• There are 12 Soil Orders recognized globally: Alfisols, Andisols, Aridisols, Entisols, Gelisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols, and Vertisols.
• The 'Series' is the lowest (most specific) category and is the most common unit used in soil mapping at the local level.

Additional Knowledge:
• Series (Option A): The most detailed level; soils in a series have similar profiles and properties.
• Great Groups (Option C): Third level of classification, based on the presence of diagnostic horizons and moisture/temperature regimes.
• Type (Option B): An older term formerly used for the specific textural class of the surface soil within a series.