Soil formation and erosion
Soil starts as parent material: rock broken by weathering.
- Physical weathering: freeze–thaw, heating and cooling, roots wedging rocks apart.
- Chemical weathering: water and acids dissolve or change minerals.
- Biological: lichens, roots, worms and microbes break rock and add humus.
Five factors control soil: climate, organisms, relief (slope), parent material and time. Making 2–3 cm of topsoil can take hundreds of years.
Soil horizons
O (organic litter), A (topsoil, humus-rich), E (eluviation: nutrients washed down), B (subsoil: minerals and clay collect), C (weathered parent rock), R (bedrock).
Erosion
Erosion is the removal of soil by water or wind. It is worst on steep, bare, ploughed or overgrazed land. Results: less fertile fields, muddy rivers, silted dams. Prevention: plant cover, terracing, contour ploughing, windbreaks, no-till farming, cover crops.
Soil composition and properties
A healthy soil is roughly 45% minerals, 5% organic matter, 25% water and 25% air.
Texture is the mix of particle sizes: sand (0.05–2 mm), silt (0.002–0.05 mm), clay (< 0.002 mm). A soil texture triangle names soils from their percentages; loam (about 40% sand, 40% silt, 20% clay) is good for most crops.
- Porosity: share of space between particles.
- Permeability: how fast water passes. Sand: high. Clay: low.
- Water-holding capacity: clay and humus hold water; sand drains fast.
- Fertility: nutrients (N, P, K), pH and organic matter. Clay and humus hold nutrient ions.
Watersheds
A watershed (drainage basin, catchment) is all the land that drains into one river, lake or outlet. High ground called the divide separates one watershed from the next.
Its behaviour depends on area, slope, soil, vegetation and land use. Forests slow runoff and filter water; paved cities and farms send water, sediment, fertiliser and pollution quickly downstream. Large watersheds like the Ganga or Amazon contain many smaller ones. Protecting the upstream land protects drinking water downstream.
Solar radiation and seasons
Insolation is the solar energy reaching a surface, in W/m². It is highest where the Sun is high in the sky (near the equator) because direct rays spread over a smaller area; low-angle rays spread out and pass through more atmosphere.
Earth's axis is tilted 23.5° and points the same way all year. As Earth orbits:
- June solstice: Sun overhead at 23.5°N (Tropic of Cancer); long days in the north.
- December solstice: Sun overhead at 23.5°S; summer in the south.
- March and September equinoxes: Sun overhead at the equator; day and night about equal everywhere.
Noon Sun angle = 90° − |latitude − Sun's overhead latitude|. Seasons come from tilt, not distance: Earth is closest to the Sun in early January.
Try it: a jar soil test and a torch season
Fill a jar one-third with garden soil, add water and a pinch of salt, shake and leave it a day. Sand settles first, then silt, then clay: measure each layer to find your soil's texture. Then shine a torch straight down on paper and at a slant: the slanted patch is bigger and dimmer. That is why low winter sun warms less.
Key formulas and definitions
- Soil factors: climate, organisms, relief, parent material, time
- Horizons from top: O, A, E, B, C, R
- Sand 0.05–2 mm > silt > clay < 0.002 mm
- Noon Sun angle = 90° − |latitude − subsolar latitude|
- Insolation ∝ sin(Sun angle) for the same beam
- Seasons: 23.5° tilt + orbit, not distance
Worked examples
1. A soil drains water very fast and dries out quickly. Is it mostly sand or clay? What would help it hold water?
Mostly sand: large pores give high permeability and low water-holding. Adding compost (humus) helps it hold water and nutrients.
2. Which horizon would you expect to be darkest and richest in humus, and why?
The A horizon (topsoil) just under the O layer, because decaying plant matter mixes into it.
3. List three ways a hill farmer can reduce soil erosion.
Terracing, contour ploughing, and keeping plant cover (cover crops, grass strips, trees as windbreaks).
4. Find the noon Sun angle at 30°N on the June and December solstices.
June: 90 − |30 − 23.5| = 83.5°. December: 90 − |30 − (−23.5)| = 36.5°.
5. Why does a city built on a watershed's upstream slopes raise flood risk downstream?
Roads and roofs stop water soaking in, so more rain runs off, and faster, into streams, raising river levels downstream.
6. Why are days longer in summer?
The hemisphere tilted towards the Sun spends more of each rotation in sunlight, so it gets more hours of insolation as well as more direct rays.
Common mistakes
- Saying seasons happen because Earth is closer to the Sun in summer. It is the tilt; the distance change is small.
- Thinking clay drains well because the particles are fine. Tiny pores make clay slow to drain.
- Confusing weathering (breaking rock in place) with erosion (moving soil away).
- Thinking a watershed is a building or tank. It is an area of land bounded by a divide.