A tropical rainforest soil has a thin A horizon, a deep reddish B horizon enriched in iron and aluminum oxides, and very little clay or organic carbon at depth. A semi-arid grassland soil has a thick, dark A horizon and calcium carbonate accumulation at mid-depth. Which factor most directly explains this difference?
AParent material: the tropical and grassland soils formed from chemically different rock types
BClimate: intense rainfall in the tropics causes extreme leaching that removes everything except iron and aluminum; lower rainfall in the semi-arid region limits downward movement, allowing carbonates to accumulate
CTopography: the grassland soil formed on a flatter surface, which encourages accumulation
DTime: the tropical soil is much younger, having not yet developed a deep B horizon
Climate is often the dominant CLORPT factor. Heavy tropical rainfall drives intense leaching that strips soluble minerals, silica, and clay — leaving behind only the least soluble residues, iron and aluminum oxides (producing Oxisols/laterites). In semi-arid climates, there is enough water to move calcium carbonate downward but not enough to flush it out entirely, creating a calcic horizon (Aridisols). The same parent rock under these two climates produces fundamentally different soils, demonstrating climate's primacy.
Question 2 Multiple Choice
The E horizon in a soil profile is characterized by:
ADark coloration from organic matter mixed intimately with mineral particles through bioturbation
BPale, bleached coloration due to removal of iron oxides, clay, and organic matter by downward-percolating water (eluviation)
CAccumulation of clay, iron oxides, and carbonates that have been translocated from horizons above
DPartially weathered parent material that retains the original rock structure but has lost some soluble minerals
E stands for eluviation — the washing-out of material. Iron oxides give soil its red and yellow colors; removing them leaves a pale, often grayish or whitish layer. Clay and organic matter are also leached from the E horizon. The material removed from the E horizon doesn't disappear — it moves down and accumulates in the B horizon (illuviation). The E horizon is the most visually striking evidence of vertical material transfer in a soil profile.
Question 3 True / False
The B horizon in a soil profile is enriched in materials that have been leached from the A and E horizons above and deposited downward through the profile.
TTrue
FFalse
Answer: True
This is the process of illuviation — downward deposition of dissolved or suspended material. The B horizon accumulates clay minerals, iron oxides, aluminum hydroxides, carbonates, or organic-metal complexes depending on the soil type and climate. The E-B horizon pair is essentially a redistribution system: the E loses what the B gains. The characteristic reddish or yellowish coloration of many B horizons comes from iron oxide coatings on soil particles deposited from above.
Question 4 True / False
Parent material is the dominant factor controlling soil type because it determines the starting chemistry from which most soil properties derive.
TTrue
FFalse
Answer: False
Parent material is one of the five CLORPT factors and sets the starting chemistry, but it is often not the dominant factor. Climate frequently overrides parent material effects — given enough time and rainfall, soils developing on different rock types under the same climate can converge to similar weathered profiles. Conversely, the same parent material under different climates produces dramatically different soils (as the tropical vs. semi-arid example shows). Parent material is most influential in young soils (Entisols) where climate hasn't had time to overprint it.
Question 5 Short Answer
Explain how reading a soil profile from top to bottom tells a story about the dominant processes occurring in that environment.
Think about your answer, then reveal below.
Model answer: Each horizon records a dominant pedogenic process. The O horizon shows organic matter accumulation from vegetation. The A horizon shows mixing of organic and mineral material through biological activity. The E horizon (when present) shows eluviation — downward washing of iron, clay, and organics. The B horizon shows illuviation — accumulation of what was removed above. The C horizon shows the starting parent material before pedogenesis significantly altered it. The thickness, color, and chemistry of each horizon reveal climate (depth of leaching), biology (thickness of A horizon), hydrology (presence of E), and time (overall profile development). A thin A over unaltered C signals a young or cold/dry soil; a deep, strongly differentiated profile signals old age and high rainfall.
This reading-a-profile skill is the practical application of understanding CLORPT. A soil scientist can estimate past vegetation, drainage, climate, and landscape position from a single pit observation. The profile is a vertical record of environmental history — analogous to a sedimentary stratigraphic section but recording process rather than deposition events.