Soil-forming factors: parent material, topography, climate, biological activity and time

Detail

1. The five factors: what they are and how they work together

• Soil-forming factors are the five controls that decide what kind of soil forms at a place: parent material, topography, climate, biological activity and time.
• They act together and affect one another. No single factor works alone. For example, climate decides the vegetation, and vegetation decides how much humus the soil gets.
• Active factors: climate and biological activity. They supply the energy, water and organisms that actually change rock into soil.
• Passive factors: parent material, topography and time. They do not drive the change themselves. They set the starting material, the setting and the length of time the active factors get.
• Hans Jenny (1941), a Swiss-American soil scientist, put these factors into one formula: s = f(cl, o, r, p, t). It means soil (s) depends on climate (cl), organisms (o), relief or topography (r), parent material (p) and time (t) [5].
• Among the five, climate has the greatest influence on soil formation [2].

Factor	Type	Main way it acts
Parent material	Passive	Gives the minerals, texture and chemistry to start with
Topography	Passive	Controls sunlight, drainage, erosion and soil depth
Time	Passive	Decides how mature the soil is and how well its layers have formed
Climate	Active	Moisture and temperature control weathering and leaching
Biological activity	Active	Adds organic matter, keeps moisture in, adds nitrogen, mixes the soil

2. Parent material (passive)

• Parent material is the rock or loose deposit that a soil forms from.
• Residual and transported soils:
• Residual soils form from rock that has weathered in situ (in its original place). Weathered debris builds up over its own bedrock. Example: soils formed over the Deccan basalt in Maharashtra.
• Transported soils form from material carried and dropped by rivers, wind, ice or waves. Indian example: the alluvial soils of the Indo-Gangetic plains (Punjab to West Bengal), laid down by the Indus, Ganga and Brahmaputra systems.

• Parent material can be solid rock or loose deposits such as river alluvium, lake and sea sediments, glacial till (unsorted debris dropped by ice), loess (wind-blown dust), volcanic ash and organic matter [2].

• What matters in the parent material:

• its texture (size of the debris particles) and structure (how the particles are arranged);
• its mineral and chemical composition;

• how fast it weathers, and how deep the mantle (the layer of weathered debris) is.

• Rock type changes soil chemistry. Soils formed from granite are more acidic than soils formed from basalt [2].

• Not a one-to-one link:
• Similar bedrock can carry different soils, and different bedrocks can carry similar soils. This happens when climate is strong enough to override the rock.
• Young soils show a strong link with the parent rock, because the active factors have not yet had time to change it much.
• Soils over some limestones also stay closely tied to the parent rock. Their weathering is very distinctive.

3. Topography (passive)

• Topography (also called relief) means the shape of the land: its height, slope and aspect.
• How it acts:
• Exposure to sunlight. A slope facing the sun is warmer and drier than one facing away.
• Surface and subsurface drainage. Slope decides whether water runs off the surface or sinks into the ground.

• Topography acts on soil formation through drainage and slope [2].

• Soil depth and slope:

Landform	Soil result	Reason
Steep slopes	Thin soils	Erosion removes debris as fast as it forms; water runs off instead of soaking in
Gentle slopes	Most favourable, well-developed soils	Erosion is slow and water percolates (seeps down) well
Flat uplands	Thick soils	Little erosion, so debris builds up
Flat lowlands	May form a thick, dark, clay-rich layer rich in organic matter	Poor drainage, so water and fine clay collect

4. Climate (active)

• Climate as a soil-forming factor works through two main variables:
• Moisture: how intense, how frequent and how long the rain falls; how much water evaporates; and the humidity of the air.

• Temperature: both seasonal (summer and winter) and diurnal (day and night) changes.

• High temperature and high rainfall increase weathering and so increase soil development [2].

4a. Eluviation and illuviation (moisture moving down)

• Eluviation is the washing down of soil parts (clay, iron, humus and salts) by water moving through the soil. It happens where there is too much water.

• It happens in the upper layer. The A horizon is called the eluvial horizon, the "zone of maximum leaching" [2].

• Illuviation is the deposition of those washed-down parts in a lower layer.

• The B horizon is the illuvial horizon. Clay, iron and aluminium oxides and carbonates collect there [2].

• Chain: heavy rain → water seeps down through the soil → it carries fine material from the top layer (eluviation) → it drops this material in the lower layer (illuviation) → the soil develops clear layers.

4b. Desilication: laterite formation (wet equatorial climate)

• Desilication is the removal of silica from the soil by intense leaching.
• Chain:
• Cause: heavy rain all year and high temperature in wet equatorial climates.
• Process: calcium, sodium, magnesium and potassium, and a major part of the silica, dissolve and are washed away.

• Result: iron and aluminium oxides are left behind. This gives the red, brick-like laterite soil.

• Where: in India, on the Western Ghats (Kerala, Karnataka, Goa), the Eastern Ghats in Odisha and the Meghalaya plateau. In the world, the Amazon and Congo basins and South-East Asia.

4c. Hardpan (dry climates)

• Hardpan is a hard crust of salts that forms in or on the soil.
• Chain:
• Cause: in dry climates, evaporation is greater than precipitation.
• Process: capillary action (water rising through tiny spaces between soil grains) pulls groundwater up to the surface. The water then evaporates.

• Result: the salts dissolved in the water are left behind and form a crust, the hardpan.

• Where: in India, western Rajasthan and the Rann of Kutch (Gujarat). In the world, the Sahara, the Thar and the dry interior of Australia.

4d. Kanker (intermediate rainfall)

• Kanker (kankar) is nodules of calcium carbonate (lime) in the soil.
• It forms in tropical areas with intermediate rainfall. There is too little rain to wash the lime out completely, but enough to move it and let it collect as lumps.
• Indian example: the older alluvium (bhangar) of the Ganga plains contains kankar.

4e. Temperature

• Higher temperature speeds up chemical activity. Warm, wet tropical areas therefore have deep soil profiles.
• Exception: carbonation. Carbon dioxide dissolves better in cold water, so carbonation (rock dissolving in weak carbonic acid) is stronger in cold climates.
• Freezing stops chemical activity. Tundra soils are therefore made mostly of mechanically broken material, with little chemical change.

Climate	Main process	Soil result	Indian example
Wet equatorial / humid tropical	Leaching, desilication	Laterite (Fe, Al oxides left)	Western Ghats, Meghalaya
Dry / arid	Capillary rise + evaporation	Salt crust (hardpan)	Western Rajasthan, Kutch
Tropical, intermediate rain	Partial leaching of lime	Kanker nodules	Bhangar of the Ganga plains
Cold / tundra	Freezing, little chemical change	Thin, mechanically broken soil	Higher Himalaya (world: Siberia, northern Canada)

5. Biological activity (active)

• Biological activity means the work of plants, animals and micro-organisms in the soil. It:
• adds organic matter;
• helps the soil hold moisture;

• adds nitrogen.

• Vegetation type matters. Grasslands produce soils very different from those under forests [2].

5a. Humus

• Humus is finely divided, dark organic matter that forms when dead plants decay.
• During humification (the change of dead plant matter into humus), organic acids form. They help break down the minerals of the parent material.

5b. Bacteria: cold soils compared with warm soils

• How active the bacteria are decides how much humus a soil keeps.

Climate	Bacterial activity	Result
Cold	Slow	Humus builds up
Sub-arctic and tundra	Very slow	Half-rotted matter collects as peat layers
Humid tropics	Very fast	Dead vegetation oxidises (breaks down with oxygen) quickly, so little humus is left

• Peat is partly decayed plant matter that builds up where cold and waterlogging slow the bacteria.
• It is the first stage in coal formation (peat → lignite → bituminous → anthracite).
• Where: in the world, sub-arctic and tundra areas of Siberia, Canada and Scandinavia. In India, peaty soils occur in waterlogged areas such as Kuttanad (Kerala) and the Sundarbans (West Bengal).

5c. Nitrogen fixation

• Nitrogen fixation is the change of nitrogen gas from the air into forms that plants can use. Some bacteria do this.
• Rhizobium is a bacterium that lives in the root nodules of legumes (pulses such as gram, pea and moong). It fixes nitrogen for its host plant.
• This is why farmers rotate pulses with cereals to make the soil richer.

5d. Soil animals

• Ants, termites, earthworms and rodents turn the soil over and mix it. This is mechanical reworking.
• Earthworms feed on soil, so they also change its texture and chemistry.
• Humans also act as an organism factor, for example through farming [2].

6. Time (passive)

• Time decides how mature a soil is and how well its profile (the set of layers, or horizons, seen in a vertical cut) has developed.
• Young soils on fresh alluvium (such as the khadar of the Ganga plains, renewed by floods every year) or on glacial till show no horizons or poorly developed horizons.
• Older soils show clearer differences between their horizons [2].
• No absolute time can be fixed for a soil to mature. It depends on how fast the active factors work.
• As a general measure, it can take up to 1,000 years to form just 2–3 cm of soil (FAO, 2015 Global Symposium on Soil Erosion) [3][4].

• For this reason soil is treated as a finite resource: lost soil cannot be rebuilt within a human lifetime [3].

• Human pressure is much faster than this:

• Erosion on farmed and grazed land runs 100–1,000 times faster than natural erosion [3].
• About 33% of the Earth's soils are already degraded. Over 90% of topsoil could be at risk by 2050 (FAO, 2022) [3][4].
• About 95% of our food comes from the soil [3].

Prelims Hooks

• Active factors: climate and biological activity. Passive factors: parent material, topography and time. A common trap is to call topography or time "active".
• Eluviation = washing down (A horizon); illuviation = deposition below (B horizon).
• Desilication (removal of silica plus Ca, Na, Mg and K) happens in wet equatorial climates → laterite.
• Hardpan = salt crust in dry climates, formed by capillary rise and evaporation. Kanker = calcium carbonate nodules in tropical, intermediate-rainfall areas.
• Higher temperature speeds up all chemical weathering except carbonation. Freezing stops chemical activity, so tundra soils are mechanically broken.
• Humus builds up in cold climates (slow bacteria) and stays low in humid tropics (fast oxidation). This is often reversed in MCQ options.
• Peat forms in sub-arctic and tundra areas. It is the first stage of coal.
• Rhizobium lives in the root nodules of legumes and fixes nitrogen.
• Alluvial soils of the Indo-Gangetic plains = transported soils. Soils formed in situ over bedrock = residual soils.
• Hans Jenny (1941): s = f(cl, o, r, p, t). Climate has the greatest influence [2][5]. It takes up to 1,000 years to form 2–3 cm of soil (FAO) [3].

Mains Points

• Why India's soils differ from region to region (GS-I).
• Climate overrides parent material over large areas: heavy-rain areas get laterite (Western Ghats, Meghalaya), dry areas get saline crusts (Rajasthan, Kutch), and intermediate-rain plains get kankar.

• Parent material dominates where soils are young or residual: Deccan basalt soils, and transported alluvium in the Indo-Gangetic plains.

• Topography and soil management.

• Thin soils on steep Himalayan and Western Ghats slopes erode easily when forest cover is removed.

• Responses include terrace farming, contour bunding and afforestation. Gentle slopes and flat uplands are the best soil reserves.

• Soil as a non-renewable resource (GS-III).

• Soil forms slowly (up to 1,000 years for 2–3 cm) but erodes 100–1,000 times faster on farmed land [3].

• With about 33% of soils already degraded, conserving soil is central to food security, since about 95% of food comes from soil [3][4].

• Biological factor in sustainable farming.

• Legume rotation (Rhizobium nitrogen fixation), keeping humus through organic manure, and protecting earthworm activity all improve fertility.
• In humid tropical India, where humus oxidises fast, these practices cut the need for chemical fertilisers.