Soil Erosion, Conservation and Reclamation

What to remember

• Erosion is detachment plus transport of soil by water or wind. Water erosion goes in stages: splash, sheet, rill, gully. Wind erosion moves particles by saltation, suspension and surface creep.
• Soil loss is predicted by the Universal Soil Loss Equation: A = R × K × L × S × C × P. Vegetation cover (C) and support practices (P) are the two factors a forester can change most.
• Salt-affected soils need different cures. Saline soil is leached with good water and drained. Sodic (alkali) soil needs gypsum, then leaching. Tree planting helps both after the soil is treated.

Types and causes of soil erosion

Natural (geological) erosion is slow and balanced by soil formation. Accelerated erosion is caused by people: deforestation, overgrazing, shifting cultivation, up-and-down slope ploughing, forest fires and road building on slopes.

Water erosion has these forms:

• Splash erosion: raindrops hit bare soil, break aggregates and throw particles. It is the first step.
• Sheet erosion: thin, uniform removal of the top layer by overland flow. It is hard to notice, but it removes the most fertile soil.
• Rill erosion: small channels (finger-sized) that can be removed by normal ploughing.
• Gully erosion: deep channels that cannot be crossed by normal tillage. Gullies grow headward, sideways and downward.
• Stream-bank erosion: cutting of banks by flowing water.
• Landslides and slips: mass movement on steep, wet slopes (Eastern Ghats hills are prone in heavy rain).
• Ravines: a network of deep gullies, common along rivers on alluvial soils.

Gully classification by depth (common scheme): small gully less than about 3 m deep; medium gully about 3 to 9 m; large (deep) gully more than about 9 m.

Wind erosion happens in dry, bare, loose soil with strong wind and little vegetation.

• Saltation: particles hop along the surface in short jumps. It is the largest share of movement. Particles about 0.1 to 0.5 mm are the most easily moved.
• Suspension: very fine dust is lifted and carried far.
• Surface creep: large grains are pushed along the ground by the impact of hopping grains.

Factors and the Universal Soil Loss Equation

Erosion depends on rainfall energy, soil, slope, cover and management.

Worked example: R = 200, K = 0.2, LS = 1.5 × 2 = 3, C = 0.5, P = 1. A = 200 × 0.2 × 3 × 0.5 × 1 = 60 t/ha/year. If bunding and contouring bring P down to 0.5, A becomes 30 t/ha/year.

Why forest cover protects soil: the canopy breaks the fall of raindrops; the litter layer absorbs energy and slows runoff; roots bind soil and open channels for infiltration; humus builds soil structure.

Soil and water conservation measures

Agronomic and biological measures: contour cultivation, strip cropping, cover crops, mulching, grass strips (vetiver, Cenchrus), afforestation of steep slopes, rotational grazing, social fencing.

Engineering (mechanical) measures:

• Contour bunds: earth ridges along contours on gentle slopes with low rainfall; they hold water in place.
• Graded bunds: laid on a slight grade to carry excess water safely to a waterway; used in high rainfall or heavy soils.
• Terraces: bench terraces on steep slopes (they convert a long slope into level steps); broad-based terraces on gentler slopes.
• Contour trenches: continuous or staggered trenches on hill slopes in forest land; they catch runoff, and seedlings are planted on the bund below the trench.
• Gully control: treat from the top downward. Check dams (brushwood, loose boulder, gabion, earthen) reduce flow speed and trap silt. Gully plugs are small structures in the gully head. Vegetative cover (grass, shrubs, trees) holds the silt after trapping.
• Waterways: grassed channels to take excess runoff down a slope without cutting.

Horizontal interval: HI = VI × 100 / S, where VI is the vertical interval between bunds (m) and S the land slope (%). Example: VI = 1.5 m, S = 5% gives HI = 30 m.

Shelterbelts and windbreaks

A windbreak is a single or double row of trees and shrubs. A shelterbelt is wider, of several rows, with trees in the centre and shrubs on the edges.

• Purpose: reduce wind speed, control wind erosion, cut evaporation, protect crops, shelter cattle, and stop sand movement.
• Layout: set at right angles to the prevailing wind.
• Protected distance: wind speed is reduced on the leeward side for a distance of roughly 10 to 20 times the height of the trees (H); the strongest effect is within about 10 H.
• Porosity: a medium-porous belt (about 40 to 50% open) protects over a longer distance than a dense belt. A very dense belt makes eddies and protects only a short distance.
• Species: tall, fast-growing, wind-firm, deep-rooted, evergreen or semi-evergreen, with little shade harm to crops. Casuarina equisetifolia is the classic coastal shelterbelt species in Andhra Pradesh. Others used include Eucalyptus, Prosopis, Acacia, Azadirachta indica, Dalbergia sissoo and Pongamia pinnata.

Sand dune stabilization

Moving dunes bury fields, roads and settlements. The usual steps are:

• 1. Protect the area from grazing and cutting.
• 2. Mechanical stabilization: erect brushwood or palm-leaf fences or clay barriers across the wind to cut wind speed at ground level.
• 3. Biological stabilization: plant grasses and creepers first (for example Ipomoea pes-caprae on coasts, Lasiurus sindicus in deserts), then shrubs and trees.
• 4. Tree belts on the windward side and over the dune. Species: Prosopis juliflora, Acacia tortilis, Calligonum polygonoides (desert), Casuarina equisetifolia and cashew on the coast, Ziziphus, Cassia auriculata.
• 5. Aftercare: continued protection, gap filling and controlled grazing.

On the Andhra Pradesh coast, Casuarina plantations along the shore serve as shelterbelts against cyclone winds and sea-borne sand, and also give a pulpwood crop.

Saline, sodic and waterlogged lands

(EC is electrical conductivity of the saturation extract; ESP is exchangeable sodium percentage.)

Reclamation of saline soil: improve drainage, apply good-quality water to leach salts below the root zone, level the land, use mulch, and grow salt-tolerant crops and trees.

Reclamation of sodic soil: apply an amendment that supplies calcium to replace sodium on the clay surface. Gypsum (CaSO4·2H2O) is the usual choice. Others are pyrites, sulphur, and organic manure or green manure. Then leach the displaced sodium out with water. Gypsum requirement depends on the ESP to be lowered, the soil depth and the soil's cation exchange capacity.

Biological reclamation: salt-tolerant trees such as Prosopis juliflora, Acacia nilotica, Tamarix, Casuarina equisetifolia, Eucalyptus tereticornis, Pongamia pinnata, Terminalia arjuna, Azadirachta indica and Salvadora persica. Pits are filled with good soil and gypsum before planting.

Coastal and mangrove areas: mangroves protect shores and trap silt; they are a natural defence against salt-laden wind and storm waves. The Godavari and Krishna delta mangroves are important in Andhra Pradesh.

Waterlogging raises the water table, brings salts to the surface and kills roots. Remedies: surface and sub-surface drainage, bio-drainage by planting high-water-use trees such as Eucalyptus, and lining of canals.

Exam traps

• Splash vs sheet erosion: splash is detachment by raindrops; sheet is removal in a thin uniform layer by runoff.
• Rill vs gully: a rill can be levelled by normal tillage; a gully cannot.
• Saltation vs suspension: saltation is hopping of sand-size grains; suspension is airborne fine dust.
• Contour bund vs graded bund: contour bunds hold water (low rainfall); graded bunds drain it safely (high rainfall).
• Saline vs sodic: saline has high EC with low pH; sodic has high pH and high ESP. Gypsum is for sodic soil, not for saline soil without sodium problem.
• Windbreak vs shelterbelt: windbreak is narrow; shelterbelt is wide with several rows.
• Dense vs medium-porous belt: medium porosity protects the longer distance.
• USLE factors: C is cover; P is practice. Low C means good cover.

One-liners

• 1. Splash erosion is the first stage of water erosion.
• 2. Sheet erosion removes soil in a thin, even layer.
• 3. Gullies are too deep to be crossed by normal tillage.
• 4. Saltation is the main process of wind erosion.
• 5. USLE: A = R × K × L × S × C × P.
• 6. Gully control starts at the head (top) and works downward.
• 7. Gypsum (calcium sulphate) is the standard amendment for sodic soil.
• 8. Saline soil has EC above 4 dS/m and pH below 8.5.
• 9. Sodic soil has pH above 8.5 and ESP above 15.
• 10. Shelterbelts are placed at right angles to the prevailing wind.
• 11. Casuarina equisetifolia is the main coastal shelterbelt tree in Andhra Pradesh.
• 12. Horizontal interval of bunds = VI × 100 / slope (%).

Practice questions

1. What is the first stage of water erosion?

   1. Sheet erosion
   2. Splash erosion
   3. Gully erosion
   4. Rill erosion
   Answer

   B. Splash erosion

   Raindrop impact detaches soil first, before runoff moves it.

2. Small channels that normal ploughing can remove are called:

   1. Gullies
   2. Ravines
   3. Rills
   4. Stream banks
   Answer

   C. Rills

   Rills are shallow; gullies cannot be crossed by tillage.

3. The main way soil particles move in wind erosion is:

   1. Suspension
   2. Saltation
   3. Sedimentation
   4. Leaching
   Answer

   B. Saltation

   Saltation (hopping) carries the largest share of moved soil.

4. The standard chemical amendment for reclaiming sodic soil is:

   1. Gypsum
   2. Common salt
   3. Urea
   4. Ammonium chloride
   Answer

   A. Gypsum

   Calcium from gypsum replaces sodium on clay, which is then leached.

5. Which tree is the main coastal shelterbelt species on the Andhra Pradesh coast?

   1. Pterocarpus santalinus
   2. Santalum album
   3. Tectona grandis
   4. Casuarina equisetifolia
   Answer

   D. Casuarina equisetifolia

   Casuarina grows on coastal sand and resists wind.

6. Gully control structures should first be built at:

   1. The lowest end of the gully
   2. The outlet of the watershed
   3. The field below the gully
   4. The upper end (head) of the gully
   Answer

   D. The upper end (head) of the gully

   Treatment goes from the head downward to stop headward cutting.

7. In the USLE, the letter C represents:

   1. Crop yield factor
   2. Cover and management factor
   3. Climate factor
   4. Compaction factor
   Answer

   B. Cover and management factor

   C is lower for dense vegetation and 1 for bare soil.

8. In the USLE, the letter K represents:

   1. Soil erodibility
   2. Slope length
   3. Runoff volume
   4. Rainfall energy
   Answer

   A. Soil erodibility

   K shows how easily a soil is detached and carried.

9. A soil is sodic when its exchangeable sodium percentage exceeds:

   1. 25
   2. 40
   3. 15
   4. 5
   Answer

   C. 15

   ESP above 15 with pH above 8.5 defines sodic soil.

10. A soil is saline when the electrical conductivity of its saturation extract is above:

    1. 4 dS/m
    2. 1 dS/m
    3. 2 dS/m
    4. 8 dS/m
    Answer

    A. 4 dS/m

    The usual limit is EC above 4 dS/m.

11. Which creeper is commonly used to stabilise coastal sand dunes?

    1. Eichhornia crassipes
    2. Ipomoea pes-caprae
    3. Nelumbo nucifera
    4. Typha angustata
    Answer

    B. Ipomoea pes-caprae

    It spreads over sand and binds it before shrubs are planted.

12. The term 'white alkali' refers to:

    1. Acid peat soil
    2. Sodic soil with dark patches
    3. Laterite soil
    4. Saline soil with a surface salt crust
    Answer

    D. Saline soil with a surface salt crust

    Neutral salts form a white crust; sodic soil is 'black alkali'.

13. A windbreak should be laid out:

    1. At 45 degrees to the wind and slope
    2. Parallel to the prevailing wind
    3. At right angles to the prevailing wind
    4. In a random pattern
    Answer

    C. At right angles to the prevailing wind

    Right-angle placement gives the greatest wind reduction.

14. Bench terraces are suited to:

    1. Coastal sand dunes
    2. Level plains
    3. Saline flats
    4. Steep slopes
    Answer

    D. Steep slopes

    They turn a steep slope into level steps.

15. A large (deep) gully is generally one with depth of:

    1. More than about 9 m
    2. About 3 to 9 m
    3. Less than 3 m
    4. Less than 1 m
    Answer

    A. More than about 9 m

    Common scheme: small below 3 m, medium 3 to 9 m, large above 9 m.

16. Which tree is commonly planted for bio-drainage of waterlogged land?

    1. Pterocarpus santalinus
    2. Eucalyptus
    3. Cassia fistula
    4. Santalum album
    Answer

    B. Eucalyptus

    Eucalyptus uses much water and lowers the water table.

17. Which coastal ecosystem traps silt and protects shores from storm waves?

    1. Dry savanna
    2. Alpine meadow
    3. Mangroves
    4. Desert scrub
    Answer

    C. Mangroves

    Mangrove roots slow water and hold sediment.

18. Forest litter reduces erosion mainly because it:

    1. Increases soil salinity
    2. Blocks all infiltration
    3. Absorbs raindrop energy and slows runoff
    4. Raises soil pH sharply
    Answer

    C. Absorbs raindrop energy and slows runoff

    Litter cushions rain impact and improves infiltration.

19. Sodic soils have poor infiltration mainly because:

    1. Sodium disperses clay particles and seals pores
    2. High organic matter clogs pores
    3. Soluble salts form big crumbs
    4. Calcium makes soil loose
    Answer

    A. Sodium disperses clay particles and seals pores

    Dispersed clay forms a hard, sealed surface.

20. After applying gypsum to a sodic soil, the essential next step is:

    1. Deep burning of the field
    2. Leaching with good-quality water
    3. Dry ploughing only
    4. Adding more sodium salts
    Answer

    B. Leaching with good-quality water

    Displaced sodium must be washed below the root zone.

21. For heavy soils in high-rainfall areas, which bund is preferred?

    1. Contour bund
    2. Graded bund
    3. No bund
    4. Level bench only
    Answer

    B. Graded bund

    Graded bunds drain excess water safely instead of ponding it.

22. A very dense shelterbelt protects only a short distance because it:

    1. Is too short for wind
    2. Lets wind pass freely
    3. Absorbs all the rain
    4. Creates strong eddies just behind the belt
    Answer

    D. Creates strong eddies just behind the belt

    Medium porosity (40 to 50%) gives protection over a longer distance.

23. Which practice lowers the P value in the USLE?

    1. Contour farming
    2. Down-slope ploughing
    3. Heavy grazing
    4. Burning crop residue
    Answer

    A. Contour farming

    Support practices such as contouring and terracing reduce P.

24. A brushwood fence across a moving dune helps by:

    1. Raising soil pH
    2. Increasing evaporation
    3. Adding nitrogen
    4. Reducing wind speed at ground level
    Answer

    D. Reducing wind speed at ground level

    Lower wind speed makes sand drop and settle.

25. Gullies grow headward mainly because:

    1. Salt crust expands
    2. Roots push the soil upward
    3. Runoff falling over the head cuts it back
    4. Wind blows the head away
    Answer

    C. Runoff falling over the head cuts it back

    Plunging water undercuts the gully head.

26. Which soil is generally most erodible by water?

    1. Stony gravel
    2. Clay rich in organic matter
    3. Well-aggregated loam with humus
    4. Silt and fine sand
    Answer

    D. Silt and fine sand

    Silt and fine sand have weak binding and detach easily.

27. In USLE: R = 120, K = 0.25, LS = 2, C = 0.5, P = 1. Annual soil loss A is:

    1. 60 t/ha
    2. 15 t/ha
    3. 30 t/ha
    4. 120 t/ha
    Answer

    C. 30 t/ha

    120 x 0.25 x 2 x 0.5 x 1 = 30 t/ha/yr.

28. For VI = 1.2 m and land slope 4%, the horizontal interval of bunds is:

    1. 20 m
    2. 30 m
    3. 48 m
    4. 60 m
    Answer

    B. 30 m

    HI = VI x 100 / S = 1.2 x 100 / 4 = 30 m.

29. For VI = 2 m and land slope 8%, the horizontal interval is:

    1. 25 m
    2. 40 m
    3. 50 m
    4. 16 m
    Answer

    A. 25 m

    HI = 2 x 100 / 8 = 25 m.

30. Soil loss is 50 t/ha/yr with P = 1. If contouring reduces P to 0.4 and other factors stay the same, the new loss is:

    1. 20 t/ha/yr
    2. 40 t/ha/yr
    3. 25 t/ha/yr
    4. 10 t/ha/yr
    Answer

    A. 20 t/ha/yr

    50 x 0.4 = 20 t/ha/yr.

31. A watershed loses soil at 25 t/ha/yr. Total annual loss from 10 ha is:

    1. 2,500 t
    2. 250 t
    3. 100 t
    4. 25 t
    Answer

    B. 250 t

    25 x 10 = 250 t.

32. Soil of bulk density 1.3 t/m3 is lost to a depth of 1 mm from 1 ha. The mass lost is:

    1. 10 t
    2. 130 t
    3. 1.3 t
    4. 13 t
    Answer

    D. 13 t

    1 mm over 1 ha is 10 m3; 10 x 1.3 = 13 t.

33. A windbreak of tree height 12 m gives strongest protection up to about 10 H, which is:

    1. 12 m
    2. 60 m
    3. 120 m
    4. 240 m
    Answer

    C. 120 m

    10 x 12 m = 120 m.

34. If bunds are 25 m apart on a slope, the length of bund required per hectare is:

    1. 400 m
    2. 100 m
    3. 250 m
    4. 1,000 m
    Answer

    A. 400 m

    Bund length per ha = 10,000 / 25 = 400 m.

35. Consider the statements: 1. Rills can be removed by normal ploughing. 2. Gullies cannot be crossed by normal tillage. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are true: rills are shallow, gullies are deep.

36. Consider the statements: 1. Gypsum is the cure for saline soil with no sodium problem. 2. Sodic soil has pH above 8.5. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Gypsum is for sodic soil; statement 1 is false, 2 is true.

37. Consider the statements: 1. Saltation is the main process of wind erosion. 2. Suspension moves large pebbles. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Suspension carries only fine dust, so only 1 is true.

38. Consider the statements: 1. Shelterbelts are laid parallel to the prevailing wind. 2. A medium-porous belt protects over a longer distance. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Belts are at right angles to wind; statement 2 is true.

39. Consider the statements: 1. Contour bunds hold water in low-rainfall areas. 2. Graded bunds carry excess water safely off the field. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both statements describe the bunds correctly.

40. Consider the statements: 1. Mangroves trap silt and protect shores. 2. Mangroves occur only in deserts. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Mangroves grow in tidal coasts, not deserts.

41. Consider the statements: 1. In the USLE, C is lower for forest cover than for bare soil. 2. Contouring lowers the P factor. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct USLE principles.

42. Consider the statements: 1. Saline soil has EC above 4 dS/m. 2. Saline soil always has ESP above 15 and pH above 8.5. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Saline soil usually has ESP below 15 and pH below 8.5.

43. Consider the statements: 1. Casuarina fixes nitrogen with the help of Frankia. 2. Casuarina cannot grow on coastal sand. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Casuarina is a classic coastal sand species.

44. Consider the statements: 1. Sheet erosion forms deep channels that cross tillage lines. 2. Surface creep is airborne fine dust. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    D. Neither 1 nor 2

    Sheet erosion is thin uniform loss; creep is rolling of large grains.

45. Consider the statements: 1. Gully control should begin at the lowest end only. 2. Check dams trap silt in gullies. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Work starts from the head; check dams do trap silt.