Soil Survey, Classification, and Land Use Interpretation
Core concepts
Understand the USDA Soil Taxonomy hierarchy (Order, Suborder, Great Group, Subgroup, Family, Series) and the diagnostic horizons and characteristics used to classify soils. Know the 12 soil orders and their general settings-for example, Alfisols (moderately leached, high base saturation), Ultisols (highly weathered, low base saturation), Mollisols (dark, fertile grassland soils), Aridisols (dry), Histosols (organic), and Andisols (volcanic). Grasp soil mapping units, map scale, soil series, phases, and the concept of land capability classification (Classes I–VIII) and its limitations. Understand how to read a soil survey report and interpret it for a specific use such as cropping, engineering, or waste disposal.
Traps in questions
Do not confuse soil order with soil series; the series is the most detailed taxonomic unit and is the basic mapping unit. Remember that land capability classes I–IV are suited to cultivation with increasing limitations, while V–VIII are generally not suited to sustained cultivation. Watch for questions that treat map scale as unrelated to mapping unit purity-larger-scale maps (e.g., 1:12,000) show more detail than small-scale maps (e.g., 1:250,000).
Worked example
A soil has a mollic epipedon, high base saturation, and is found under grassland. Identify the order: Mollisol. Rule of thumb: match the diagnostic horizon to the order-mollic (Mollisols), argillic with low base saturation (Ultisols), oxic (Oxisols), and so on.
Rule of thumb
Classify from the top down: identify the diagnostic horizon and moisture/temperature regime, then move through the hierarchy to the series.