Soil Science and Geology: Rocks, Soil Formation, Forest Soils and Fertilizers

What to remember

• Rocks are igneous, sedimentary or metamorphic. Weathering breaks rock into parent material. Soil forms from the parent material under the five factors: parent material, climate, organisms, relief and time.
• A soil profile has O, A, B, C horizons and R (bedrock). Texture comes from sand, silt and clay. Soil pH, organic matter and bulk density decide soil health.
• Fertilizers are named by N–P₂O₅–K₂O grade. Urea has about 46 per cent N. Lime raises pH of acid soils. Gypsum and organic matter help sodic soils.

1. Basic geology

The earth has the crust, mantle and core. The crust is made of rocks, which are mixtures of minerals. A mineral is a natural inorganic solid with a definite chemical make-up. Common rock-forming minerals are quartz, feldspar, mica, hornblende and calcite. Quartz (hardness 7) is hard. Talc (hardness 1) is the softest on Mohs scale.

Three classes of rocks

• Intrusive igneous rocks cool slowly inside the earth and have large crystals. Extrusive rocks cool quickly on the surface and have small crystals.
• The Deccan Traps are basalt. Granite and gneiss are widespread in the Eastern Ghats and the Rayalaseema region. Khondalite is an important rock of the Eastern Ghats. Sandstone and shale are found in the Cuddapah basin. These are general geological facts.

2. Weathering and soil formation

Weathering is the breakdown of rocks at the surface.

• Physical (mechanical): temperature changes, frost, wind, running water and plant roots.
• Chemical: water, oxygen and carbon dioxide act on minerals. Types are hydrolysis, hydration, oxidation, carbonation and solution.
• Biological: lichens, roots and microbes.

Soil is the loose upper layer of the earth's crust that supports plant life. Pedogenesis is the process of soil formation. Jenny's equation: S = f (cl, o, r, p, t). The five factors are climate, organisms, relief (topography), parent material and time. Climate and organisms are active factors. Parent material and relief are passive. Time is the amount of weathering done.

3. Soil profile

Eluviation means removal of materials from a layer. Illuviation means deposition of materials in a layer. Together A, E and B form the solum, the true soil.

4. Physical properties of soil

• Texture is the proportion of sand, silt and clay. Particle sizes (International system): sand 2.0 to 0.02 mm, silt 0.02 to 0.002 mm, clay below 0.002 mm.
• Loam is a balanced mixture of sand, silt and clay and is the best for most plants. Sandy soils drain fast and hold little water. Clayey soils hold water and nutrients but drain poorly.
• Structure is the arrangement of particles into aggregates: granular, blocky, platy, prismatic. Granular is best for growth.
• Bulk density = mass of dry soil ÷ total volume (g per cm³). Typical mineral soil is about 1.3 to 1.4 g per cm³. Particle density is about 2.65 g per cm³.
• Porosity (per cent) = (1 − bulk density ÷ particle density) × 100.

*Worked example:* bulk density 1.325 and particle density 2.65. Porosity = (1 − 0.5) × 100 = 50 per cent.

*Worked example:* a soil core of 200 cm³ has dry mass 260 g. Bulk density = 260 ÷ 200 = 1.3 g per cm³.

• Field capacity: water held after free drainage. Permanent wilting point: water level at which plants wilt permanently. Available water = field capacity − permanent wilting point.
• Infiltration is entry of water into soil. Permeability is the ease of water flow through it.

5. Chemical and biological properties

• Soil pH: below 7 is acidic, 7 is neutral, above 7 is alkaline. Most plants prefer 6 to 7.5. Forest soils in high-rainfall areas are acidic. Saline-alkaline soils occur in arid regions.
• Cation exchange capacity (CEC): the soil's ability to hold exchangeable positive ions. Clay and humus have high CEC.
• Soil organic matter: improves structure, water holding and nutrients. Humus is the stable dark organic matter.
• Electrical conductivity (EC) measures salts.
• Carbon to nitrogen ratio (C:N): of well-decomposed humus is about 10:1.
• Soil organisms: bacteria, fungi, actinomycetes, earthworms, termites.

6. Major soils of India and Andhra Pradesh

7. Forest soils

• Forest soils have a surface layer of litter and humus. Nutrients are quickly recycled by decomposition.
• Soils under teak need well-drained, deep loam. Sal grows best on well-drained deep soils with a reasonable moisture. Casuarina and Eucalyptus tolerate poor soils.
• Soil erosion is removal of topsoil by water or wind. Forest cover reduces it. Types of water erosion: splash, sheet, rill, gully. Conservation: contour bunds, trenches, check dams, afforestation and cover crops.
• Reclamation of saline-alkali soils: gypsum (for sodic soil), leaching with good water, drainage, organic matter and salt-tolerant tree species.
• Reclamation of acid soils: application of lime.

8. Fertilizers

Fertilizer grade shows percentage of N, P₂O₅ and K₂O. For example, 10:26:26 means 10 per cent N, 26 per cent P₂O₅ and 26 per cent K₂O.

• Straight fertilizers give one nutrient. Complex fertilizers give two or more.
• Organic manures: farmyard manure, compost, green manure, vermicompost. They improve soil structure.
• Biofertilizers: *Rhizobium*, *Azotobacter*, *Azospirillum*, blue-green algae, *Azolla* and phosphate-solubilising bacteria.

*Worked example:* nitrogen needed is 60 kg per hectare from urea. Urea required = 60 ÷ 0.46 ≈ 130 kg.

*Worked example:* 50 kg of DAP contains 50 × 0.46 = 23 kg of P₂O₅ and 50 × 0.18 = 9 kg of N.

Exam traps

• 1. Granite is igneous. Marble is metamorphic. Limestone is sedimentary.
• 2. Eluviation is removal. Illuviation is accumulation.
• 3. Horizon A is topsoil. Horizon B is subsoil. Horizon C is parent material.
• 4. Lime is for acid soils. Gypsum is for sodic soils.
• 5. Black soil is formed from basalt. Red soil is formed from granite and gneiss.
• 6. Bulk density is lower than particle density.
• 7. Available water = field capacity − wilting point.
• 8. A grade of 46 per cent in urea refers to N, while 46 per cent in DAP refers to P₂O₅.

One-liners

• 1. Quartz has a hardness of 7 on Mohs scale.
• 2. Basalt is an extrusive igneous rock.
• 3. Slate is metamorphosed shale.
• 4. Pedogenesis means soil formation.
• 5. The five soil forming factors are climate, organisms, relief, parent material and time.
• 6. Clay particles are smaller than 0.002 mm.
• 7. Loam is the best soil texture for most crops.
• 8. Porosity = (1 − bulk density ÷ particle density) × 100.
• 9. Black soil is rich in clay and cracks in summer.
• 10. Urea has 46 per cent nitrogen.
• 11. DAP has 18 per cent N and 46 per cent P₂O₅.
• 12. Gypsum helps reclaim sodic soils.

Practice questions

1. Granite is an example of:

   1. Sedimentary rock
   2. Metamorphic rock
   3. Extrusive igneous rock
   4. Intrusive igneous rock
   Answer

   D. Intrusive igneous rock

   Granite cools slowly inside the earth and is a coarse-grained igneous rock.

2. Marble is formed by metamorphism of:

   1. Limestone
   2. Sandstone
   3. Shale
   4. Granite
   Answer

   A. Limestone

   Limestone changes into marble under heat and pressure.

3. Slate is formed from:

   1. Basalt
   2. Limestone
   3. Sandstone
   4. Shale
   Answer

   D. Shale

   Shale metamorphoses into slate.

4. Which of these is a sedimentary rock?

   1. Quartzite
   2. Sandstone
   3. Basalt
   4. Gneiss
   Answer

   B. Sandstone

   Sandstone forms from deposited and compacted sand.

5. Fossils are most commonly found in:

   1. Gneiss
   2. Granite
   3. Sedimentary rocks
   4. Basalt
   Answer

   C. Sedimentary rocks

   Layered sedimentary rocks preserve fossils.

6. Which mineral is hardest on Mohs scale among these?

   1. Quartz
   2. Feldspar
   3. Calcite
   4. Talc
   Answer

   A. Quartz

   Hardness: talc 1, calcite 3, feldspar 6, quartz 7.

7. The Deccan Traps are made of:

   1. Granite
   2. Marble
   3. Basalt
   4. Limestone
   Answer

   C. Basalt

   The Deccan Traps are basaltic lava flows.

8. Which factor is NOT one of the five soil forming factors?

   1. Atmospheric pressure of the moon
   2. Parent material
   3. Climate
   4. Time
   Answer

   A. Atmospheric pressure of the moon

   The factors are climate, organisms, relief, parent material and time.

9. The breakdown of rock by temperature change and frost is:

   1. Chemical weathering
   2. Physical weathering
   3. Biological weathering only
   4. Pedogenesis
   Answer

   B. Physical weathering

   Temperature and frost cause physical weathering.

10. The layer of organic litter and humus in forest soil is the:

    1. C horizon
    2. R horizon
    3. B horizon
    4. O horizon
    Answer

    D. O horizon

    The O horizon is the organic layer.

11. The horizon of accumulation of clay and iron is the:

    1. C
    2. A
    3. B
    4. O
    Answer

    C. B

    The B horizon is the zone of illuviation.

12. Horizon C represents the:

    1. Parent material
    2. Topsoil
    3. Organic layer
    4. Subsoil
    Answer

    A. Parent material

    The C horizon is partly weathered parent material.

13. Clay particles are smaller than:

    1. 0.02 mm
    2. 0.002 mm
    3. 2 mm
    4. 0.2 mm
    Answer

    B. 0.002 mm

    Clay is below 0.002 mm in the international system.

14. The best soil texture for most plants is:

    1. Gravel
    2. Sand
    3. Clay
    4. Loam
    Answer

    D. Loam

    Loam has a balanced mix and good drainage.

15. A soil core of 250 cm3 has dry mass of 325 g. Bulk density is:

    1. 1.5 g per cm3
    2. 1.3 g per cm3
    3. 1.0 g per cm3
    4. 2.0 g per cm3
    Answer

    B. 1.3 g per cm3

    325 / 250 = 1.3 g per cm3.

16. Bulk density is 1.325 g per cm3 and particle density 2.65. Porosity is:

    1. 50 per cent
    2. 60 per cent
    3. 25 per cent
    4. 40 per cent
    Answer

    A. 50 per cent

    Porosity = (1 - 1.325/2.65) x 100 = 50 per cent.

17. Field capacity is 28 per cent and permanent wilting point is 12 per cent. Available water is:

    1. 12 per cent
    2. 40 per cent
    3. 16 per cent
    4. 28 per cent
    Answer

    C. 16 per cent

    28 - 12 = 16 per cent.

18. Urea contains about:

    1. 60 per cent N
    2. 16 per cent N
    3. 20 per cent N
    4. 46 per cent N
    Answer

    D. 46 per cent N

    Urea has about 46 per cent nitrogen.

19. To apply 92 kg N per hectare using urea (46 per cent N), urea needed is:

    1. 100 kg
    2. 150 kg
    3. 250 kg
    4. 200 kg
    Answer

    D. 200 kg

    92 / 0.46 = 200 kg of urea.

20. Single super phosphate holds 16 per cent P2O5. P2O5 in 100 kg is:

    1. 46 kg
    2. 16 kg
    3. 8 kg
    4. 32 kg
    Answer

    B. 16 kg

    100 x 0.16 = 16 kg.

21. 50 kg of DAP (18 per cent N and 46 per cent P2O5) gives nitrogen of:

    1. 9 kg
    2. 18 kg
    3. 5 kg
    4. 23 kg
    Answer

    A. 9 kg

    50 x 0.18 = 9 kg of N.

22. A fertilizer grade 10:26:26 contains 10 per cent N. In 200 kg of it, N is:

    1. 26 kg
    2. 10 kg
    3. 20 kg
    4. 52 kg
    Answer

    C. 20 kg

    200 x 0.10 = 20 kg N.

23. Muriate of potash supplies about 60 per cent:

    1. N
    2. K2O
    3. P2O5
    4. CaO
    Answer

    B. K2O

    MOP has about 60 per cent K2O.

24. Lime is applied to:

    1. Acid soils
    2. Saline soils
    3. Sodic soils
    4. Waterlogged soils only
    Answer

    A. Acid soils

    Lime raises pH of acid soils.

25. Sodic (alkali) soils are reclaimed by:

    1. Urea
    2. Lime
    3. Gypsum
    4. Sand
    Answer

    C. Gypsum

    Gypsum supplies calcium to replace sodium.

26. Black soil is mainly formed from:

    1. Marble
    2. Sandstone
    3. Granite
    4. Basalt
    Answer

    D. Basalt

    Black (regur) soil forms from basalt.

27. Red soils owe their colour to:

    1. Lime
    2. Salts
    3. Carbon
    4. Iron oxides
    Answer

    D. Iron oxides

    Iron compounds give the red colour.

28. Which soil is formed under high rainfall with alternate wet and dry seasons and is rich in iron and aluminium?

    1. Black
    2. Alluvial
    3. Laterite
    4. Desert
    Answer

    C. Laterite

    Laterite is formed by intense leaching.

29. Which soil is typical of the deltas of the Godavari and Krishna?

    1. Alluvial
    2. Laterite
    3. Mountain
    4. Desert
    Answer

    A. Alluvial

    Rivers deposit alluvium in deltas.

30. Which is a biofertilizer?

    1. DAP
    2. Rhizobium
    3. Lime
    4. Urea
    Answer

    B. Rhizobium

    Rhizobium culture fixes nitrogen for legumes.

31. Casuarina grows well in coastal sandy soils because it:

    1. Fixes nitrogen and tolerates poor soil
    2. Needs flooding
    3. Needs clay
    4. Needs high humus
    Answer

    A. Fixes nitrogen and tolerates poor soil

    Casuarina has nitrogen-fixing nodules and tolerates sandy soil.

32. Consider these statements. 1. Eluviation is removal of materials from a horizon. 2. Illuviation is accumulation of materials in a horizon. Which is 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 define these terms correctly.

33. Consider these statements. 1. Intrusive rocks cool slowly. 2. Extrusive rocks have large crystals. Which is correct?

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

    A. 1 only

    Extrusive rocks cool fast and have small crystals.

34. Consider these statements. 1. Bulk density is usually lower than particle density. 2. Clay and humus have high cation exchange capacity. Which is 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.

35. Consider these statements. 1. Gypsum is used for acid soils. 2. Lime is used for acid soils. Which is 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, lime is for acid soil.

36. Consider these statements. 1. Forest cover reduces soil erosion. 2. Gully erosion is a stage following sheet and rill erosion. Which is 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 standard statements.

37. Consider these statements. 1. Urea has 46 per cent N. 2. DAP is a straight fertilizer. Which is correct?

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

    A. 1 only

    DAP supplies N and P so it is a complex fertilizer.

38. Consider these statements. 1. Sandy soils hold more water than clay soils. 2. Granular structure is good for plant growth. Which is correct?

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

    B. 2 only

    Clay holds more water than sand.

39. Consider these statements. 1. Soil pH below 7 is acidic. 2. Soil pH above 7 is alkaline. Which is 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 standard definitions.

40. Match the pair: Gneiss is formed from:

    1. Sandstone
    2. Limestone
    3. Shale
    4. Granite
    Answer

    D. Granite

    Gneiss is metamorphosed granite (or similar rocks).

41. Match the pair: Quartzite is formed from:

    1. Limestone
    2. Granite
    3. Shale
    4. Sandstone
    Answer

    D. Sandstone

    Quartzite is metamorphosed sandstone.

42. The true soil (solum) is made up of horizons:

    1. A, E and B
    2. O and R
    3. C and R
    4. O only
    Answer

    A. A, E and B

    The solum includes A, E and B horizons.

43. Which erosion removes a thin uniform layer of topsoil?

    1. Gully erosion
    2. Sheet erosion
    3. Stream bank erosion
    4. Wind blow-out only
    Answer

    B. Sheet erosion

    Sheet erosion is a uniform removal of the top layer.

44. Organic matter in soil improves:

    1. Hardness of rocks
    2. Only salinity
    3. Only erosion
    4. Structure and water holding
    Answer

    D. Structure and water holding

    Humus improves structure and holds water and nutrients.

45. Which soil crack deeply on drying and swell on wetting?

    1. Sandy soil
    2. Red soil
    3. Laterite
    4. Black soil
    Answer

    D. Black soil

    Black soils rich in clay shrink and crack when dry.