Soil Science and Management

What to remember

• Soil is the natural body on the earth's surface formed from weathered rock and organic matter, and supports plant growth. Its parts are mineral matter (about 45%), organic matter (about 5%), water (about 25%) and air (about 25%) by volume in a good loam.
• Soil texture depends on the proportion of sand, silt and clay; soil pH shows acidity or alkalinity; organic matter is the key to soil health.
• Porosity = (1 − bulk density / particle density) × 100; particle density of mineral soil is about 2.65 g/cm³.

Soil formation and profile

Soil forms by weathering (physical, chemical and biological) of parent rock. Five factors control it: parent material, climate, organisms (biological activity), topography (relief) and time. Climate is the most active factor.

A soil profile is the vertical section showing layers called horizons:

The solum is the A and B horizons together.

Physical properties

Soil texture is the relative proportion of sand, silt and clay particles. Size limits (International system): sand 2.0–0.02 mm (coarse sand 2.0–0.2 mm, fine sand 0.2–0.02 mm), silt 0.02–0.002 mm, clay below 0.002 mm. (The USDA system sets sand at 2.0–0.05 mm and silt at 0.05–0.002 mm.) Loam has a balanced mix and is best for crops. Sandy soils drain fast but hold little water; clay soils hold water but drain slowly.

Structure is the arrangement of particles into aggregates: granular (good for crops, topsoil), platy, blocky, prismatic, columnar. Bulk density = mass of dry soil / total volume (typically 1.1–1.6 g/cm³); particle density = mass / volume of solids (about 2.65 g/cm³). Compaction raises bulk density and hinders roots.

Worked example: bulk density 1.325, particle density 2.65. Porosity = (1 − 1.325/2.65) × 100 = 50%.

Soil colour is described by the Munsell chart; dark colour shows organic matter, red shows iron oxide, grey or mottled shows poor drainage.

Soil water

Available water is highest in loams and clay loams, and lowest in sands. Infiltration is entry of water into soil; percolation is movement through the soil; permeability is the ease of movement.

Chemical properties

Soil colloids are very fine clay and humus particles with a negative charge that hold cations. Cation exchange capacity (CEC) is the total exchangeable cations a soil can hold; it is high in clay and humus and low in sand.

Soil pH: pH 7 neutral, below 6.5 acidic, above 7.5 alkaline. Most crops grow best near pH 6.5–7.5. Electrical conductivity (EC) measures soluble salts (dS/m). Exchangeable sodium percentage (ESP) shows sodium on exchange sites.

Reclamation: saline soils by leaching with good water and drainage; sodic soils by gypsum (calcium) followed by leaching; acid soils by liming; use of organic manure and salt-tolerant crops helps all.

Soil organic matter and manures

Soil organic matter improves structure, water holding capacity, CEC and microbial life. Humus is the stable, dark, decomposed part. C:N ratio of well-decomposed matter is about 10–12:1; wide C:N residues (straw) cause temporary N immobilisation.

Organic sources: Farm yard manure (FYM), compost, vermicompost (earthworm-made, rich in nutrients), green manure (sunhemp, dhaincha), green leaf manure, oilcakes and poultry manure. Biofertilizers (Rhizobium, Azospirillum, PSB) and crop residue recycling add to soil fertility. Bulky organic manures supply nutrients slowly and improve physical condition.

Essential nutrients and deficiency symptoms

There are 17 essential elements (C, H, O from air and water; N, P, K, Ca, Mg, S; Zn, Fe, Mn, Cu, B, Mo, Cl, Ni).

Mobile nutrients (N, P, K, Mg) show symptoms first in old leaves; immobile ones (Ca, Fe, S, B, Zn, Mn, Cu) show in young leaves. (Sulphur is partly mobile and is usually taught as showing on young leaves.)

Soils of Andhra Pradesh and their management

Soil testing, health card and conservation

Soil testing estimates available nutrients, pH, EC and organic carbon for fertilizer advice. A representative sample is drawn by dividing the field into uniform blocks, taking soil from 15–20 cm depth in V-shaped cuts at several spots, mixing, and taking the quarter-portion (quartering). Avoid bunds, manure heaps, and wet spots. The Soil Health Card scheme of the Government of India (2015) gives farmers a card with soil status and crop-wise fertilizer recommendation.

Soil erosion is detachment and removal of soil by water or wind. Types of water erosion: splash, sheet, rill, gully, stream-bank. Conservation measures: contour cultivation, strip cropping, contour bunding, terracing, grassed waterways, check dams (for gullies), mulching, cover crops, and shelter belts for wind erosion. Land capability classes help to plan land use.

Soil organisms and fertility management

Soil is alive. Bacteria, fungi, actinomycetes, algae, protozoa and earthworms decompose residues and cycle nutrients. Nitrogen cycle steps: fixation (N₂ to ammonia by Rhizobium and free-living bacteria), mineralisation (organic N to ammonium), nitrification (ammonium to nitrite to nitrate by Nitrosomonas and Nitrobacter), denitrification (nitrate to N gas in waterlogged soil) and leaching losses. Urea is therefore best applied in splits and, in rice, placed in the soil rather than broadcast on standing water.

Soil fertility is the capacity of soil to supply nutrients; soil productivity is the capacity to give yield and also depends on climate and management. Good management practices are balanced fertilizer use based on soil tests, regular addition of organic matter, crop rotation with legumes, proper drainage, avoiding burning of residues and using gypsum or lime where needed. Mulching with crop residues reduces evaporation, controls weeds and keeps soil cooler.

Exam traps

• Field capacity is about −1/3 bar; PWP is about −15 bar.
• Sodic soils need gypsum; saline soils need leaching; acid soils need lime.
• Particle density (2.65) is not bulk density (1.1–1.6).
• Mobile nutrients show deficiency in older leaves; immobile in younger leaves.
• Humus is the stable fraction; FYM is a manure, not humus.
• Horizon A is topsoil; B is subsoil; C is parent material.
• Khaira disease in rice is zinc deficiency, not iron.
• Clay holds more water than sand but drains more slowly.

One-liners

• 1. Soil texture is determined by sand, silt and clay proportions.
• 2. Loam is the ideal soil texture for most crops.
• 3. Neutral pH is 7.
• 4. Saline soil has EC above 4 dS/m.
• 5. Sodic soil has pH above 8.5 and ESP above 15.
• 6. Gypsum reclaims sodic soils.
• 7. Lime corrects acidity.
• 8. Black cotton soil is best for cotton.
• 9. Red soils are poor in nitrogen and organic matter.
• 10. Porosity = (1 − BD/PD) × 100.
• 11. Vermicompost is made by earthworms.
• 12. Soil Health Card scheme began in 2015.

Practice questions

1. A soil has bulk density 1.325 g/cm³ and particle density 2.65 g/cm³. Its porosity is

   1. 50%
   2. 25%
   3. 40%
   4. 60%
   Answer

   A. 50%

   (1 - 1.325/2.65) x 100 = 50%.

2. Dry soil of mass 130 g occupies 100 cm³ in its natural state. Its bulk density is

   1. 1.3 g/cm³
   2. 2.6 g/cm³
   3. 0.77 g/cm³
   4. 1.0 g/cm³
   Answer

   A. 1.3 g/cm³

   130/100 = 1.3 g/cm³.

3. Field capacity is 30% and permanent wilting point is 15% (by weight). The available water is

   1. 30%
   2. 45%
   3. 15%
   4. 2%
   Answer

   C. 15%

   Available water = FC - PWP = 15%.

4. A 15 cm furrow slice over 1 ha with bulk density 1.2 g/cm³ weighs about

   1. 180 tonnes
   2. 1800 tonnes
   3. 1.2 tonnes
   4. 18,000 tonnes
   Answer

   B. 1800 tonnes

   10,000 m² x 0.15 m = 1500 m³; x 1.2 t/m³ = 1800 t.

5. A soil with porosity 40% and particle density 2.65 g/cm³ has bulk density of about

   1. 2.65 g/cm³
   2. 1.59 g/cm³
   3. 1.06 g/cm³
   4. 0.4 g/cm³
   Answer

   B. 1.59 g/cm³

   BD = PD x (1 - porosity) = 2.65 x 0.6 = 1.59.

6. 100 kg of DAP (18-46-0) contains nitrogen of

   1. 18 kg
   2. 46 kg
   3. 28 kg
   4. 64 kg
   Answer

   A. 18 kg

   18% of 100 kg = 18 kg N.

7. 10 tonnes of FYM containing 0.5% N supplies nitrogen of

   1. 500 kg
   2. 5 kg
   3. 0.5 kg
   4. 50 kg
   Answer

   D. 50 kg

   10,000 kg x 0.005 = 50 kg.

8. A soil has EC 6 dS/m, pH 8.0 and ESP 10. It is

   1. normal
   2. saline-sodic
   3. saline
   4. sodic
   Answer

   C. saline

   EC above 4 with ESP below 15 and pH below 8.5 is saline.

9. A soil has pH 9.2, EC 2 dS/m and ESP 20. It is

   1. acidic
   2. sodic (alkali)
   3. normal
   4. saline
   Answer

   B. sodic (alkali)

   pH above 8.5 and ESP above 15 mean sodic.

10. The most active factor in soil formation is

    1. time
    2. topography
    3. climate
    4. parent material
    Answer

    C. climate

    Climate is called the most active factor.

11. The horizon known as topsoil is

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

    A. A

    The A horizon is topsoil with humus.

12. The horizon of weathered parent material is

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

    D. C

    C is the parent material horizon.

13. Soil of the best texture for most crops is

    1. gravel
    2. heavy clay
    3. pure sand
    4. loam
    Answer

    D. loam

    Loam has balanced sand, silt and clay.

14. Clay particles are smaller than

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

    B. 0.002 mm

    Clay is below 0.002 mm.

15. Water held in soil after free drainage is

    1. permanent wilting point
    2. hygroscopic water
    3. field capacity
    4. gravitational water
    Answer

    C. field capacity

    Field capacity is water held after drainage.

16. At permanent wilting point, plants

    1. cannot recover even if water is added
    2. grow fastest
    3. show no stress
    4. have saturated roots
    Answer

    A. cannot recover even if water is added

    Wilting is permanent at PWP.

17. Cation exchange capacity is highest in soil rich in

    1. coarse sand
    2. gravel
    3. quartz
    4. clay and humus
    Answer

    D. clay and humus

    Negatively charged colloids hold cations.

18. Gypsum is applied for reclamation of

    1. saline soils by leaching alone
    2. acid soils
    3. sodic soils
    4. sandy soils
    Answer

    C. sodic soils

    Calcium from gypsum replaces sodium.

19. Lime is applied to correct

    1. sodicity
    2. acidity
    3. salinity
    4. erosion
    Answer

    B. acidity

    Liming raises the pH of acid soils.

20. Khaira disease in rice is caused by deficiency of

    1. zinc
    2. iron
    3. boron
    4. manganese
    Answer

    A. zinc

    Khaira is zinc deficiency.

21. Yellowing of older leaves first is a symptom of deficiency of

    1. calcium
    2. boron
    3. iron
    4. nitrogen
    Answer

    D. nitrogen

    N is mobile and moves from old leaves.

22. Interveinal chlorosis in young leaves is typical of deficiency of

    1. iron
    2. potassium
    3. nitrogen
    4. phosphorus
    Answer

    A. iron

    Fe is immobile, so young leaves show chlorosis.

23. Which nutrient deficiency causes purplish leaves and stunted growth?

    1. Zinc
    2. Phosphorus
    3. Sulphur
    4. Potassium
    Answer

    B. Phosphorus

    P deficiency gives purple pigmentation.

24. Which soils of Andhra Pradesh are best known for cotton?

    1. Coastal sands
    2. Laterite soils
    3. Black cotton (regur) soils
    4. Red sandy soils
    Answer

    C. Black cotton (regur) soils

    Regur soils hold water well.

25. Red soils are generally poor in

    1. sand
    2. nitrogen and organic matter
    3. iron
    4. drainage
    Answer

    B. nitrogen and organic matter

    Red soils are light-textured and low in N and organic matter.

26. Fertile delta soils deposited by rivers are

    1. red soils
    2. desert soils
    3. laterite soils
    4. alluvial soils
    Answer

    D. alluvial soils

    Alluvial soils are river-borne and fertile.

27. Vermicompost is prepared with the help of

    1. Azolla
    2. termites
    3. earthworms
    4. Rhizobium
    Answer

    C. earthworms

    Earthworms convert organic waste to vermicompost.

28. The Soil Health Card scheme provides

    1. soil nutrient status and fertilizer recommendation
    2. free seeds
    3. crop insurance
    4. crop loans
    Answer

    A. soil nutrient status and fertilizer recommendation

    The card lists soil status and advice.

29. A representative soil sample is generally taken from a depth of about

    1. 15-20 cm
    2. 50 cm
    3. 1 m
    4. 2 cm
    Answer

    A. 15-20 cm

    Plough layer depth is 15-20 cm.

30. The reduction of sample size by mixing and dividing into four parts is called

    1. tillage
    2. quartering
    3. leaching
    4. percolation
    Answer

    B. quartering

    Quartering reduces the composite sample.

31. Detachment of a thin uniform layer of soil by water is

    1. gully erosion
    2. rill erosion
    3. sheet erosion
    4. splash only
    Answer

    C. sheet erosion

    Sheet erosion removes a thin layer.

32. Which measure best checks gully erosion?

    1. Deep ploughing across slope only
    2. Check dams
    3. Burning stubble
    4. Flood irrigation
    Answer

    B. Check dams

    Check dams slow flow in gullies.

33. Shelter belts are mainly used against

    1. acidity
    2. salinity
    3. water logging
    4. wind erosion
    Answer

    D. wind erosion

    Tree barriers reduce wind speed.

34. Consider the statements. 1. Sodic soils have ESP above 15. 2. Sodic soils are reclaimed with gypsum. Which is/are correct?

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

    A. Both 1 and 2

    Both are correct.

35. Consider the statements. 1. Bulk density is usually lower than particle density. 2. Compaction increases bulk density. Which is/are correct?

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

    D. Both 1 and 2

    Pore space makes bulk density lower; compaction removes it.

36. Consider the statements. 1. Acid soils are treated with gypsum only. 2. Saline soils are reclaimed by leaching with good water and drainage. Which is/are correct?

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

    C. 2 only

    Acid soils need lime.

37. Consider the statements. 1. Nitrogen is a mobile nutrient in plants. 2. Calcium deficiency appears first in older leaves. Which is/are correct?

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

    C. 1 only

    Calcium is immobile and shows in young tissues.

38. Consider the statements. 1. Clay soils have high water holding capacity. 2. Sandy soils have high water holding capacity. Which is/are correct?

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

    B. 1 only

    Sands drain fast and hold little water.

39. Match the pair correctly.

    1. Khaira - zinc deficiency
    2. Hollow stem - nitrogen deficiency
    3. Little leaf - iron deficiency
    4. Whip tail - zinc deficiency
    Answer

    A. Khaira - zinc deficiency

    Khaira in rice results from zinc deficiency.

40. Contour bunding is practised to

    1. increase salinity
    2. raise soil pH
    3. add nitrogen
    4. reduce runoff and soil loss on slopes
    Answer

    D. reduce runoff and soil loss on slopes

    Bunds along contours slow runoff.

41. The stable, dark, well-decomposed organic fraction of soil is

    1. humus
    2. green manure
    3. straw
    4. FYM
    Answer

    A. humus

    Humus is the stable organic fraction.

42. A wide C:N ratio residue like paddy straw, when added fresh, can cause

    1. quick release of N
    2. nitrogen fixation
    3. soil salinity
    4. temporary nitrogen immobilisation
    Answer

    D. temporary nitrogen immobilisation

    Microbes consume soil N to decompose it.

43. Which nutrient is in the secondary group of macronutrients?

    1. Iron
    2. Potassium
    3. Magnesium
    4. Molybdenum
    Answer

    C. Magnesium

    Ca, Mg and S are secondary nutrients.

44. Water that is drawn out of soil by gravity and not held is

    1. hygroscopic water
    2. gravitational water
    3. capillary water
    4. bound water
    Answer

    B. gravitational water

    Gravitational water drains from macropores.