SOIL SCIENCE: COMPLETE REVISION NOTES
Unit 1: Basics & Rocks
1. Introduction to Soil
Etymology: Derived from the Latin word "Solum", meaning 'Floor'.
Definition: The uppermost layer of earth; a natural body developed by natural forces acting on parent material.
Dimensions: It is a three-dimensional body having length, breadth, and depth (Land is two-dimensional),.
Fathers of the Field:
Soil Science/Modern Pedology: V.V. Dokuchaev (Russian Scientist).
Soil Science in India & Agricultural Chemistry: J.W. Leather.
2. Branches of Soil Science (Confusion Hack)
Pedology: Study of soil genesis, classification, and description. Considers soil as a natural body.
Edaphology: Study of soil in relation to the growth of higher plants. Considers soil as a medium for plant growth.
3. Rocks & Minerals (Petrology)
A. Rock Cycle & Types
Igneous Rocks (Primary): Formed by cooling of magma. E.g., Granite, Basalt. They make up 95% of the earth's crust.
Sedimentary Rocks (Secondary): Formed by accumulation/cementation of sediments. E.g., Sandstone, Limestone, Shale. They cover 74% of the earth's surface (visible layer).
Metamorphic Rocks: Formed by heat/pressure.
Sandstone > Quartzite
Limestone > Marble
Granite > Gneiss
Coal > Graphite.
B. Important Minerals
Quartz: Most resistant to weathering,.
Feldspar: Most dominant mineral in the earth's crust (48%).
Mica:
Biotite: Black mica (rich in Mg/Fe).
Muscovite: White mica (rich in K/Al).
Unit 2: Soil Formation (Pedogenesis)
1. Weathering
Chemical Weathering: The most important process for soil formation. Key types:
Hydrolysis: Reaction with water; most important chemical weathering process.
Carbonation: Important in minerals containing Ca, Mg, Na, K.
Time: Approx 800-1000 years are required to form 1 inch of soil.
2. Soil Forming Factors (Jenny’s Equation)
Dokuchaev (1889): S = f(P, Cl, O).
Jenny (1941): Added time and relief. S = f(Cl, O, R, P, T).
Active Factors: Climate and Organisms (Biosphere).
Passive Factors: Parent material and Relief (Topography).
Neutral: Time.
3. Soil Profile (Vertical Section)
O Horizon: Organic horizon, absent in cultivated/arable soils.
A Horizon: Zone of Eluviation (Washing Out).
B Horizon: Zone of Illuviation (Washing In/Accumulation). Also called sub-soil.
Solum: A + B Horizons.
Regolith: A + B + C Horizons (Loose material above bedrock).
Unit 3: Physical Properties
1. Soil Texture
Definition: Relative proportions of sand, silt, and clay. It is a basic/permanent property and cannot be changed by tillage.
Analysis: Mechanical analysis uses Stoke's Law.
Best Agricultural Soil: Loam (Sand, silt, clay in balanced mixture).
Particle Size (ISSS/USDA):
Gravel: > 2 mm
Sand: 0.02 – 2.0 mm
Silt: 0.002 – 0.02 mm
Clay: < 0.002 mm
2. Soil Structure
Granular/Crumb: Spheroidal, porous. Best for agriculture.
Platy: Horizontal axis developed; hinders drainage.
Blocky: Cube-like; common in B horizon.
3. Density & Porosity (Numerical Hack)
Particle Density (PD): True density. Solid particles only. Average = 2.65 g/cm³. Not affected by tillage.
Bulk Density (BD): Soil solids + Pore space. Average = 1.33 g/cm³. Clay has lower BD than sand because it has more pore space,.
Porosity Formula: Porosity (%) =(1 - {Bulk Density/Particle Density} x 100
Example: If BD=1.33 and PD=2.65, Porosity \approx 50%.
4. Soil Air
CO_2 in soil air (0.25 - 0.3%) is 8-10 times higher than atmospheric air (0.03%),.
5. Soil Color (Munsell Chart)
Hue: Spectral color (Red/Yellow).
Value: Lightness or darkness (0=Black, 10=White).
Chroma: Purity or strength of color.
Red Soil: Due to Hematite (Un-hydrated Fe oxide).
Yellow Soil: Due to Limonite (Hydrated Fe oxide).
Unit 4: Soil Water
1. Classification (Physical)
Gravitational: Held at < 0.33 bar. Drains away. Not available to plants.
Capillary: Held between 0.33 and 31 bar. The main source of water for plants.
Hygroscopic: Held at > 31 bar. Bound tightly by adhesion. Non-liquid.
2. Soil Moisture Constants (Critical for Exams)
Available Water: Water between FC and PWP.
pF Concept: Introduced by Schofield (1935). Defined as logarithm of soil moisture tension in cm of water column.
3. Measurement Instruments
Tensiometer: For moist soils (up to 0.85 bar). Best for Sandy soils.
Neutron Moisture Meter: For continuous measurement without disturbing soil.
Pressure Plate: Most accurate lab method (0 to -15 bars).
Unit 5: Chemical Properties
1. Soil Reaction (pH)
Term: Coined by Sorensen (1909). "Puissance of Hydrogen".
Relationship: Inverse to H+ ions. 1 unit change in pH = 10-fold change in acidity.
Buffering Capacity: Ability of soil to resist change in pH. Highest in Clay/Organic Matter.
2. Soil Colloids & Clays
Kaolinite: 1:1 Type. Non-expanding. Low CEC (3-15). Found in Red/Laterite soils.
Montmorillonite: 2:1 Type. Expanding. High CEC (80-150). Found in Black Cotton Soils (Vertisols).
Illite: 2:1 Type. Non-expanding. Rich in Potassium (K).
Cation Exchange Capacity (CEC):
Humus (150-300) > Vermiculite > Montmorillonite > Illite > Kaolinite > Sand,.
Lyotropic Series (Binding Strength): Al{3+} > Ca{2+} > Mg{2+} > K+ > Na+.
Unit 6: Organic Matter & Biology
1. Composition
Organic Carbon (OC): Organic Matter = OC x 1.724 (Bemmelen Factor).
Humus: Lignin-protein complex. Fractions:
Fulvic Acid: Soluble in acid & base.
Humic Acid: Soluble in base, insoluble in acid.
Humin: Insoluble in both.
2. C:N Ratios (Exam Favorite)
Soil Microorganisms: 4:1 to 9:1.
Normal Soil/Humus: 10:1.
FYM: 20-30:1.
Sawdust: 400:1 (Widest).
Legumes: 20:1 to 30:1.
Unit 7: Soil Fertility & Plant Nutrients
1. Essentiality Criteria
Proposed by Arnon and Stout (1939), redefined by Arnon (1954).
Total Essential Elements: 17 (Ni added last in 1987),.
2. Classification & Functions
Structural: C, H, O (96% of plant weight).
Primary (Macro): N, P, K.
Secondary: Ca, Mg, S.
Micronutrients: Fe, Mn, Zn, Cu, B, Mo, Cl, Ni.
Beneficial Elements: Na, Si, Co, Vanadium.
"Ballast Elements": Al and Si.
3. Specific Nutrient Facts
Nitrogen (N): Essential for chlorophyll. Deficiency: V-shaped chlorosis on lower (old) leaves,.
Phosphorus (P): "Key of Life," Energy currency (ATP). Deficiency: Purple coloration on older leaves,.
Potassium (K): "Traffic Policeman" (Stomata regulation). Hidden Hunger. Deficiency: Scorch/Burn on margins of old leaves,.
Calcium (Ca): Cell wall constituent. Deficiency: Terminal buds hook/die.
Magnesium (Mg): Central atom of Chlorophyll. Deficiency: Interveinal chlorosis on old leaves (Sand drown in tobacco),.
Sulfur (S): Oil synthesis. Deficiency: Interveinal chlorosis on new (young) leaves,.
Zinc (Zn): Auxin synthesis. Deficiency: Khaira disease (Rice), White bud (Maize),.
Boron (B): Pollen germination. Deficiency: Heart rot (Sugar beet), Fruit cracking,.
Molybdenum (Mo): Nitrate reductase enzyme. Deficiency: Whiptail in cauliflower,.
Unit 8: Fertilizers
1. Nitrogenous
Urea: 46% N. Amide form. Cheapest. Hygroscopic.
Ammonium Sulphate: 20.6% N, 24% S. Good for Tea/Rice.
CAN (Kisan Khad): 25% N. Neutral fertilizer.
Anhydrous Ammonia: Highest N (82%).
2. Phosphatic
SSP (Single Super Phosphate): First fertilizer made in India (1906). 16% P_2O_5 + 12% S,.
DAP: 18% N, 46% P_2O_5,.
3. Potassic
MOP (KCl): 60% K_2O. Most common. Avoid in Solanaceous crops (Tobacco/Potato) due to Chloride.
SOP (K_2SO_4): 50% K_2O. Suitable for Tobacco.
4. Biofertilizers
Rhizobium: Symbiotic N-fixation in legumes.
Azotobacter: Free-living, for cereals (Wheat, Rice).
Azospirillum: Associative, for Sorghum, Maize.
Azolla: Water fern for Rice (Companion with Anabaena).
VAM: Mobilizes Phosphorus.
Unit 9: Soil Erosion & Conservation
1. Water Erosion Stages (Sequence Trick: S-S-R-G)
Splash: Raindrop impact.
Sheet: Removal of thin uniform layer. Most dangerous ("Farmer's Death") as it goes unnoticed.
Rill: Finger-like channels. Can be removed by tillage.
Gully: Advanced rills. Cannot be smoothed by tillage.
2. Wind Erosion
Saltation: Bouncing (0.1-0.5 mm particles). 50-75% of erosion.
Suspension: Floating (<0.1 mm). 3-4%.
Surface Creep: Rolling (>0.5 mm). 5-25%.
3. Conservation Equation (USLE)
A = RKLSCP (Universal Soil Loss Equation by Wischmeier & Smith).
Unit 10: Problematic Soils (Comparison Table)
Acid Soils: pH < 7. Reclaimed using Lime (CaCO_3 or Dolomite).
Unit 11: Classification & Survey
1. Soil Orders (USDA Taxonomy)
Vertisols: Black soils. Expanding clay. Self-ploughing (cracks). Gilgai micro-relief.
Entisols: Recent soils (Alluvial). No profile development (A-C horizon only). Largest order in India,.
Inceptisols: Young soils with minimal horizon development. Largest order in the World,.
Oxisols: Highly weathered. Laterite soils. Rich in Fe/Al oxides.
2. Land Capability Classification (LCC)
Total Classes: 8.
Cultivable: Class I to IV.
Not Cultivable (Pasture/Forest): Class V to VIII.
Unit 12: Soil Testing
Father of Soil Testing: Troug.
Sampling: V-shaped cut used. 15 cm depth for field crops.
Nutrient Estimation Methods:
Organic Carbon: Walkley and Black method.
Available P: Olsen’s method (Alkaline soil), Bray’s (Acidic soil).
Available K: Ammonium Acetate method.
