Chapter 11 — Soils, land capability and degradation

Exam fit: soil formation, regional patterns, agriculture and conservation. Level: foundation to intermediate.

Soil is a living, changing layer formed through interaction among parent material, climate, organisms, relief and time, with people increasingly modifying the process. It contains mineral particles, organic matter, water, air and organisms. The familiar Indian soil groups in school geography are useful national-scale categories, but they simplify local variation. Soil maps are not substitutes for field tests or parcel-level land capability assessment.

11.1 How soil forms

Parent material supplies minerals and influences texture. Climate affects weathering and leaching through temperature and water. Organisms add organic residues and help mix the soil. Relief influences drainage, erosion and accumulation: steep slopes often lose material, while depressions may collect it. Time allows horizons and chemical changes to develop. Human land use can improve or degrade soil through vegetation cover, tillage, irrigation, fertiliser, compaction, mining and construction.

A soil profile may show layers called horizons. The surface organic-rich layer, topsoil, subsoil and weathered parent material differ in organic matter, minerals and structure. Not every soil profile has a complete textbook sequence. Texture refers to the relative proportions of sand, silt and clay; structure describes how particles are arranged into aggregates. These affect drainage, water-holding capacity, aeration and root growth.

11.2 Major soil groups in India

Alluvial soils occur widely in river plains and deltas, with texture and fertility varying by sediment source, age, drainage and management. Newer floodplain deposits can differ from older terraces. “Alluvial” does not mean identical nutrients or guaranteed high yields.

Black soils are strongly associated with basaltic parent material over parts of the Deccan. Many have high clay content, swell when wet and shrink/crack as they dry. Their water-holding property can be useful, but drainage, workability and depth vary. They are often linked with cotton in traditional crop-geography examples; that does not mean cotton is grown only there or that every black soil field suits the crop.

Red and yellow soils acquire their colour largely from iron compounds under oxidation; the colour is not itself a direct measure of fertility. These soils are widespread in parts of the Peninsular Plateau and can vary in texture, depth and nutrient status. Management, organic matter and water availability matter.

Laterite soils form under conditions of intense leaching in warm, wet environments. Soluble bases and silica may be removed while iron and aluminium compounds become relatively concentrated. They occur in parts of high-rainfall regions and plateaus. The term refers to a broad soil formation process and material; local profiles and land uses differ.

Arid soils develop under limited moisture and may be sandy, low in organic matter and locally saline. Wind action and irrigation practices influence their condition. Forest and mountain soils vary sharply with altitude, slope, vegetation and parent material; thin soils on steep slopes are vulnerable to erosion. Peaty or marshy soils occur in waterlogged settings with substantial organic matter accumulation. These labels are broad and should not be read as a complete local survey classification.

11.3 Soil properties and crop decisions

Crop suitability depends on soil depth, texture, drainage, pH, nutrients, salinity, water supply, climate, slope, seed and farm practices. Sandy soil drains quickly; clay-rich soil can retain more water but may drain slowly. Loam is a useful balanced texture category, not a universal ideal. Soil testing helps guide nutrient decisions and avoid applying inputs without evidence.

Irrigation can raise yields, but poorly managed irrigation in arid or poorly drained conditions can cause waterlogging and salinity. Excess fertiliser can pollute water and disturb nutrient balance. Organic matter improves structure and biological activity; cover crops and residue retention can reduce erosion in suitable systems. Conservation choices should match local crop, water and labour conditions.

11.4 Erosion, salinity and degradation

Water erosion removes soil through splash, sheet, rill and gully processes. Wind erosion lifts and moves exposed dry particles. Deforestation, overgrazing, inappropriate tillage, construction and mining can accelerate loss. Soil compaction reduces pore space and infiltration. Salinisation occurs when salts accumulate in the root zone, often in dry climates where evaporation is high and drainage inadequate; it can also have other causes. Waterlogging raises the water table and restricts roots.

Land degradation is broader than erosion. It includes declining soil quality and productivity from physical, chemical or biological changes. Its causes are context-specific, and restoration may involve vegetation, contour farming, terracing, drainage, balanced nutrients, reduced disturbance, shelterbelts or watershed treatment. No single practice works everywhere.

11.5 Land capability and slope

Land capability classifies land by constraints for uses such as cultivation, forestry or grazing. Slope, erosion risk, soil depth, drainage and climate affect the level of limitation. Steep slopes may be better suited to protective vegetation than intensive ploughing. Capability is not the same as current land use: land can be farmed unsustainably or left underused due to economic and institutional conditions.

11.6 Worked soil-data interpretation

Suppose two fields have the same parent material but different slope and drainage. Field A sits on a gentle lower slope with deeper soil; Field B is steep and shallow. Both may share a broad soil-group label, yet their erosion risk, water storage and crop options differ. If Field A receives heavy irrigation with poor drainage, salinity or waterlogging can develop. The map category begins the enquiry; a soil profile, testing and management history complete it.

Worked map workshop

Draw a soil profile with organic surface, topsoil, subsoil and weathered material, while noting that real profiles do not always show every horizon. Label texture, structure, drainage and depth. Use two adjacent slope positions: a shallow eroded upper slope and a deeper lower slope where material accumulates. The contrast demonstrates that a broad soil-group name does not determine local crop suitability.

Compare black soil and alluvium through properties rather than stereotypes. Many clay-rich black soils swell when wet and crack on drying. Alluvial soils vary by source and deposition history. Both can support agriculture under suitable conditions, but nutrients, water, drainage, salinity and management determine productivity. Soil testing provides a local basis for nutrient application.

Make a degradation decision tree: exposed soil plus runoff may cause sheet/rill/gully erosion; high evaporation and poor drainage may contribute to salinity; compaction reduces infiltration; saturation can cause waterlogging. Choose conservation by diagnosis—cover, contouring, drainage, residue retention, watershed treatment or land-use change. No single intervention fits every slope and crop.

Chapter recall

• Soil forms through parent material, climate, organisms, relief and time, modified by people.
• Alluvial, black, red/yellow, laterite, arid, forest/mountain and peaty/marshy are broad groups with local variation.
• Texture affects water movement; depth, drainage, nutrients and management also matter.
• Erosion, salinity, waterlogging and compaction are distinct degradation processes.
• Land capability considers limitations and sustainable use; it is not a crop guarantee.

Soil formation links parent material, climate, organisms, relief and time.

Chapter practice MCQs

1. Which set contains major soil-forming factors?

A. Parent material, climate, organisms, relief and time

B. Longitude, map font and page number

C. Roads, ports and currency only

D. River names only

Answer: A. These interacting factors shape soil development; human activity increasingly modifies them.

2. Soil texture refers mainly to the relative proportions of:

A. forests, farms and cities

B. sand, silt and clay

C. rivers, lakes and seas

D. minerals, ports and roads

Answer: B. Texture is the particle-size composition, which influences drainage, water retention and aeration.

3. Black soils in parts of the Deccan are commonly associated with:

A. coral reefs

B. glacial till only

C. basaltic parent material

D. recent sea ice

Answer: C. Basalt weathering contributes to many black-soil regions, though soil properties vary locally.

4. Many clay-rich black soils show which wet–dry behaviour?

A. Turning into glaciers

B. Permanent loss of all water

C. No change in volume ever

D. Swelling when wet and cracking as they dry

Answer: D. Shrink–swell behaviour is characteristic of many clay-rich black soils, but not identical everywhere.

5. Red and yellow soil colour is commonly linked with:

A. iron compounds and oxidation

B. sea salt alone

C. coal dust in every case

D. glacier ice

Answer: A. Iron oxides contribute colour; colour alone does not establish fertility or nutrient status.

6. Laterisation is associated broadly with:

A. permanent freezing

B. intense leaching in warm, wet conditions

C. no chemical weathering

D. tidal deposition only

Answer: B. Warm, wet conditions can leach soluble bases and silica, leaving iron/aluminium compounds relatively concentrated.

7. Which statement about alluvial soil is safest?

A. It has identical fertility across India.

B. It is always newer floodplain sediment.

C. It varies in texture, age, nutrients and drainage by location.

D. It cannot occur in deltas.

Answer: C. Alluvium varies by source, deposition history, profile and management.

8. A steep slope is often more vulnerable to:

A. tidal flooding only

B. saline intrusion in every case

C. no soil loss

D. soil erosion

Answer: D. Gravity and runoff can remove soil more readily on steep terrain, especially where cover is reduced.

9. Salinisation is the accumulation of:

A. soluble salts in the root zone

B. snow at a mountain summit

C. sand in a river delta only

D. organic matter in a forest

Answer: A. Salt build-up can harm crops and is influenced by climate, irrigation and drainage.

10. Waterlogging can damage crops because it:

A. increases oxygen in every soil

B. restricts soil aeration and root function

C. removes all salts instantly

D. prevents groundwater from rising

Answer: B. Saturated pore spaces reduce oxygen availability and can stress roots.

11. Which is a form of water erosion?

A. Plate collision

B. Cyclone eye formation

C. Gully erosion

D. Deltaic tide

Answer: C. Concentrated runoff can cut gullies; sheet and rill erosion are other water-erosion forms.

12. Wind erosion is most likely to affect:

A. deep ocean trenches

B. saturated wetlands only

C. snow-covered peaks only

D. exposed dry soil with limited protective cover

Answer: D. Dry, exposed fine material can be lifted and transported by wind.

13. Which method can reduce erosion on suitable sloping farmland?

A. Contour cultivation and maintaining cover

B. Ploughing directly downslope in every case

C. Removing all vegetation

D. Compaction of topsoil

Answer: A. Contour practices slow runoff and cover protects soil; suitability depends on local conditions.

14. Which statement about soil colour is correct?

A. Red soil is always fertile.

B. It can offer clues but cannot alone prove fertility.

C. Black colour guarantees cotton yield.

D. Soil colour has no relation to minerals.

Answer: B. Colour reflects some constituents and processes but does not replace testing for nutrients and other properties.

15. Which property describes soil particle arrangement into aggregates?

A. Longitude

B. Drainage basin

C. Structure

D. Relief region

Answer: C. Soil structure describes aggregation; texture describes particle-size proportions.

16. A soil test is useful because it can:

A. guarantee a crop's market price

B. replace rainfall forecasts

C. prove a river's annual flow

D. guide nutrient management based on measured conditions

Answer: D. Testing informs input decisions; it does not guarantee yields or market outcomes.

17. Which condition can promote salinity in an irrigated dry region?

A. High evaporation combined with inadequate drainage

B. Permanent forest cover alone

C. High elevation alone

D. A map projection

Answer: A. Evaporation leaves salts behind, and poor drainage can allow them to accumulate in the root zone.

18. Land capability is primarily concerned with:

A. district population alone

B. land limitations and appropriate sustainable use

C. current crop price alone

D. river names

Answer: B. Capability considers constraints such as slope, erosion, depth and drainage for different uses.

19. Which statement distinguishes soil texture and structure?

A. Texture is rainfall; structure is wind.

B. They are identical terms.

C. Texture is particle-size mix; structure is particle arrangement.

D. Texture is land use; structure is a political boundary.

Answer: C. Both influence soil behaviour but describe different properties.

20. Which practice may improve organic matter and protect soil in suitable systems?

A. Burning all residues automatically

B. Removing all vegetation

C. Repeated deep tillage on every slope

D. Retaining crop residues or using cover crops

Answer: D. Residue and cover can support soil structure and reduce erosion, subject to local farming needs.

21. Which statement about crop suitability is correct?

A. It depends on soil, climate, water, slope, management and market conditions.

B. Soil group alone determines every crop.

C. Soil testing is never useful.

D. Every field can grow every crop equally.

Answer: A. Crop decisions reflect interacting physical and human factors.

22. Compaction generally reduces:

A. longitude

B. pore space and infiltration

C. rainfall normal

D. river basin size

Answer: B. Compacted soil has reduced pore space, affecting water movement, aeration and root growth.

23. Which description best fits mountain/forest soils?

A. They are identical at every height.

B. They contain no organic matter.

C. They can vary sharply with elevation, slope, vegetation and parent material.

D. They never erode.

Answer: C. Relief and climate change over short distances, creating varied soil profiles.

24. Peaty or marshy soils are broadly associated with:

A. high dunes only

B. fresh basalt flows

C. steep rocky peaks only

D. waterlogged settings and organic matter accumulation

Answer: D. Poor drainage can allow organic material to accumulate in some wetland soils.

25. Which is the best reason a national soil map cannot replace a local field survey?

A. National categories generalise; soil depth and properties vary locally.

B. Soils do not exist locally.

C. All parcels have identical soil.

D. Maps cannot use a legend.

Answer: A. Generalised maps support regional understanding but not site-specific engineering or farm recommendations.

26. Which process describes soil being detached and moved by water from bare land?

A. Weather forecast

B. Erosion

C. Condensation

D. Tectonic collision

Answer: B. Erosion includes detachment and transport; weathering breaks material down in place.

27. Why can a lower slope have deeper soil than a nearby upper slope?

A. Gravity transports soil only uphill.

B. Soil depth is fixed by latitude.

C. Material can be eroded from above and accumulate downslope.

D. Rivers cannot move sediment.

Answer: C. Relief controls both loss and accumulation; local parent material and drainage also matter.

28. Which statement about land degradation is most accurate?

A. It means only a landslide.

B. It is identical to drought.

C. It cannot be influenced by land use.

D. It includes physical, chemical and biological declines in land quality.

Answer: D. Degradation is broad and can involve erosion, salinity, compaction, nutrient loss and ecosystem change.