What is soil made of, how do its horizons form, and why does soil matter as a resource?
Describe the components of soil and the soil horizons, explain the factors that control soil formation, and evaluate soil as a resource that can be conserved or lost to erosion (Virginia 2018 Earth Science SOL ES.6 surface processes).
A SOL-level answer on soil for the Virginia Earth Science EOC: the components of soil (weathered rock, humus, water, air), the O, A, B and C horizons, the factors that control soil formation (climate, parent material, time, organisms, slope), residual versus transported soil, and why soil conservation matters, with worked exam questions.
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What this topic is asking
Virginia Earth Science SOL standard ES.6 (surface processes) includes soil: what it is made of, how its horizons develop, the factors that control its formation, and why it is a valuable resource worth conserving. The EOC tests this with horizon-identification items, factor-reasoning items (why soil is thicker or thinner in a place), and resource items about soil erosion and conservation. Soil is the bridge between the weathering you just studied and the living world.
What soil is made of
The soil horizons
Soil develops into distinct layers, or horizons, that you read from the top down:
- O horizon: the surface layer of organic litter (leaves, twigs) in various stages of decay.
- A horizon (topsoil): dark, rich in humus mixed with mineral particles; most plant roots and soil life are here.
- B horizon (subsoil): lighter, where clays and minerals leached (washed) down from above accumulate.
- C horizon: partly weathered parent rock, the transition to solid bedrock.
- Bedrock: the unweathered solid rock beneath.
A soil profile is a vertical slice through all the horizons. The deeper, more developed the profile, generally the older the soil and the more time weathering has had to work.
The factors that control soil formation
Soil that forms in place from the bedrock below it is residual soil; soil whose material was carried in by water, wind or ice from elsewhere is transported soil (a river floodplain has transported soil).
Soil as a resource
Topsoil is essential for agriculture and forms very slowly, often taking hundreds to thousands of years to build a few centimeters. That makes it effectively non-renewable on a human timescale and worth protecting. Soil erosion (loss of topsoil to wind and water, especially on bare, sloping land) reduces fertility and clogs streams with sediment. Conservation methods include planting cover crops and vegetation, contour plowing and terracing across slopes, windbreaks, and reduced tillage. In Virginia, soil and sediment runoff into the Chesapeake Bay watershed is a notable environmental concern.
Try this
Q1. Name the four main components of soil. [2]
- Cue. Weathered rock (mineral particles), humus (organic matter), water, and air.
Q2. Explain why soil is usually thin on a steep slope and thicker on flat ground. [2]
- Cue. Steep slopes lose soil to erosion (water carries it downhill), so little accumulates; flat ground holds water and soil, so a thicker, more developed soil forms over time.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
VA Earth Science SOL 2023 (style)1 marksWhich soil horizon is the topsoil, richest in humus (decayed organic matter)? (A) the A horizon. (B) the B horizon. (C) the C horizon. (D) the bedrock.Show worked answer →
A 1-point multiple-choice item on soil horizons.
The correct answer is A. The A horizon is the topsoil, dark and rich in humus (decayed organic matter) mixed with mineral particles, where most plant roots grow. The B horizon (B) is the subsoil where minerals accumulate, the C horizon (C) is partly weathered parent rock, and bedrock (D) is the unweathered rock below.
The test rewards knowing the order O, A, B, C downward and that the A horizon (topsoil) holds the most humus among these choices.
VA Earth Science SOL 2024 (style)2 marksA farmer notices that bare, sloping fields are losing topsoil after heavy rain. (a) Explain why topsoil loss is a problem. (b) Describe one method the farmer could use to reduce soil erosion.Show worked answer →
A 2-point item on soil as a resource.
(a) 1 point: topsoil is the fertile, humus-rich layer that plants need to grow, and it forms very slowly (over hundreds to thousands of years), so losing it reduces the land's productivity and is hard to replace.
(b) 1 point for any one method: planting cover crops or vegetation to hold the soil, contour plowing or terracing across the slope, building windbreaks, or reducing tillage.
Markers reward explaining that topsoil is fertile and slow to form in (a) and a genuine conservation method in (b).
Related dot points
- Distinguish mechanical and chemical weathering, identify the agents of erosion and deposition, and explain how particle size, sorting and water velocity control where sediment is deposited (Virginia 2018 Earth Science SOL ES.6 surface processes).
A SOL-level answer on surface processes for the Virginia Earth Science EOC: mechanical versus chemical weathering and what speeds each, the agents of erosion (water, wind, ice, gravity), how water velocity controls the size of sediment carried and deposited, sediment sorting and rounding, and landforms like deltas and moraines, with worked exam questions.
- Read a topographic map: interpret contour lines and the contour interval, find elevation and relief, judge slope steepness from contour spacing, and use the rule of Vs to find stream direction (Virginia 2018 Earth Science SOL ES.1 and ES.6).
A SOL-level answer on topographic maps for the Virginia Earth Science EOC: what contour lines and the contour interval mean, how to find elevation and total relief, how contour spacing shows slope steepness, how the rule of Vs gives stream direction, and how to calculate gradient, with worked exam questions.
- Distinguish renewable and non-renewable resources, describe the major energy sources and Virginia's mineral and energy resources, and evaluate the environmental impacts and conservation of resource use (Virginia 2018 Earth Science SOL ES.3 and ES.4).
A SOL-level answer on Earth's resources for the Virginia Earth Science EOC: renewable versus non-renewable, how fossil fuels form, nuclear and the alternatives (solar, wind, hydroelectric, geothermal), Virginia's coal, limestone, sand and gravel, and the environmental trade-offs and conservation of resource use, with worked exam questions.
- Explain the processes of the water cycle (evaporation, transpiration, condensation, precipitation, runoff, infiltration) and describe watersheds, groundwater and the water table (Virginia 2018 Earth Science SOL ES.9 and ES.10).
A SOL-level answer on the water cycle for the Virginia Earth Science EOC: the processes that move water (evaporation, transpiration, condensation, precipitation, runoff, infiltration), the energy that drives it, what a watershed and divide are, groundwater and the water table, and porosity and permeability, with worked exam questions.
- Describe estuaries and the Chesapeake Bay, including brackish water and its role as a nursery, and explain how watershed land use, runoff and sea-level rise affect coastal Virginia (Virginia 2018 Earth Science SOL ES.10 and human impact).
A SOL-level answer on the Chesapeake Bay for the Virginia Earth Science EOC: what an estuary is and why the Bay's brackish water makes it a nursery, the threats from nutrient runoff and eutrophication, the role of the watershed, sea-level rise and coastal flooding, and conservation, with worked exam questions.
Sources & how we know this
- 2018 Science Standards of Learning (Earth Science) — Virginia Department of Education (2018)
- SOL Practice Items (All Subjects) — Virginia Department of Education (2024)