• Latin root: Solum = floor/land surface; Humus = earth (enriched organic soil); Solum → Soil
• Standard definition: Soil = dynamic, living, three-dimensional natural body formed at Earth's surface by disintegration and decomposition of parent rock material under influence of climate, organisms, topography over time — capable of supporting plant life
• SSSA Definition: 'The unconsolidated mineral or organic material on the surface of the Earth that has been subjected to and shows effects of genetic and environmental factors of: parent material, climate (moisture and temperature), macro and micro organisms, and relief, acting over a period of time'
• Simple agronomic definition: Upper layer of Earth capable of supplying plants with: physical support (anchorage), water, nutrients, and aeration
• Non-renewable on human timescale: Formation ~2.5 cm per 100 years (tropical); loss by erosion = 1–2 cm/decade in intensive agriculture → loss >> formation
• Global food base: >95% of human food supply originates from soil — cereals, pulses, oilseeds, vegetables, fruits, livestock feed
• India's soil resource: Total geographical area = 328.7 million ha; net sown area = ~142 M ha (43%); net irrigated area = ~68 M ha
• Problem soils India: ~120 M ha degraded in some form; ~56 M ha actively problematic (saline, eroded, waterlogged, acidified)
• Annual soil erosion India: Estimated 5–7 billion tonnes/year (verify from ICAR); contributes to reservoir siltation, floods, desertification
• Economic value: 1 tonne of eroded topsoil = ~0.4 kg N + 0.1 kg P + 6 kg OM lost permanently; enormously valuable when aggregated over millions of ha
• Pedology: Study of soil as a natural body — formation, morphology, classification, distribution (basic science)
• Edaphology: Study of soil in relation to plant growth — practical, applied side; most relevant to agriculture
• Soil Physics: Physical properties — texture, structure, water movement, aeration, heat; tillage science
• Soil Chemistry: Chemical properties — pH, CEC, nutrient availability, OM chemistry, reactions with amendments
• Soil Biology / Microbiology: Living organisms — bacteria, fungi, earthworms, nematodes; nutrient cycling; biofertilizers
• Soil Mineralogy: Mineral composition, clay mineral types, weathering of primary and secondary minerals
• Soil Fertility & Plant Nutrition: Nutrient supply, uptake, deficiency symptoms, fertilizer management, INM
• Soil Survey & Classification: Systematic mapping and ordering of soils; land evaluation; land use planning
• Soil Conservation: Erosion prevention, watershed management, sustainable land use practices
• Soil Pedogenesis: Origin and formation processes — specifically pedological transformations
• Soil provides 4 basic needs for plant growth: physical support (anchorage), water, nutrients, and oxygen to roots
• India's crop production (2022-23): ~330 MT foodgrains + ~350 MT horticulture + ~45 MT oilseeds + ~360 MT sugarcane — ALL soil-dependent
• Soil health → yield: 10% improvement in soil organic carbon → ~15–25% increase in crop yield without additional inputs
• Soil depth importance: Every 10 cm of topsoil = ~300–500 kg/ha available nutrients + vast microbial life — irreplaceable in short term
• Agriculture's GDP contribution: ~17–18% of India's GDP; employs ~42% workforce — directly linked to soil productivity
• Soil degradation cost: Estimated economic loss from land degradation in India = Rs 2.54 lakh crore/year (verify from ICAR/NABARD)
• Green Revolution possible: Only due to deep, fertile alluvial soils of IGP — without this soil base, wheat + rice revolution would not have occurred
• Carbon storage: Global soil C stock ~1,500 Gt C — 2× more than atmosphere + all vegetation combined; soil management critical for climate
• Water purification: Soil filters percolating rainwater; removes pathogens, heavy metals, excess nutrients before reaching groundwater
• Biodiversity habitat: 1 teaspoon of fertile forest soil = 600–800 million bacteria + 5–6 km fungal hyphae + nematodes, protozoa, mites → most biodiverse ecosystem on Earth per unit volume
• Watershed function: Soil regulates floods (infiltration), drought (water storage), maintains stream flow — critical for water security
• GHG regulation: Soil is both SOURCE (CO2, N2O, CH4 from decomposition, nitrification, denitrification) and SINK (OM storage) of greenhouse gases
• Mineral matter: ~45% by volume (sand + silt + clay; provides structure, mineral nutrients, CEC surface)
• Organic matter: ~5% by volume (humus + living organisms + fresh residues; controls most biological + chemical processes)
• Soil water: ~25% by volume (hygroscopic + capillary + gravitational water; medium for nutrient movement)
• Soil air: ~25% by volume (N2, O2, CO2 — CO2 concentration 10–100× higher in soil than atmosphere due to respiration)
• Ideal ratio: 45:5:25:25 mineral:OM:water:air; in Indian cultivated soils, OM often <0.5% — severely below ideal
• Target for India: National Soil Health Mission aims to raise OM to >0.8% minimum; ideal >2% for tropical soils
• Pedon: Smallest unit of soil that includes all horizons — like a 3D column; area ~1–10 m2
• Polypedon: Group of similar pedons forming a mappable soil unit
• Solum: O + A + B horizons = 'true soil'; excludes parent material (C) and bedrock (R)
• Regolith: All loose material above bedrock — not necessarily soil; lacks profile development
• In-situ soil: Formed directly from underlying bedrock (residual soils — red, black cotton)
• Transported soil: Parent material moved by water/wind/ice before soil formation (alluvial, aeolian, glacial)
• Pedogenesis: The process of soil formation/development over time
• Soil horizon: Distinct layer in soil profile, roughly parallel to surface, differs from adjacent layers in properties
• Soil profile: Complete vertical cross-section showing all horizons from surface to parent rock
• Glomalin: Glycoprotein secreted by mycorrhizal fungi; most important biological aggregate-stabilizing agent
★ PYQ HOOK: Ch 1 content provides foundation for answering: What is soil? Why is soil a non-renewable resource? Define soil health. These appear as embedded sub-questions in 20M answers.