Sustainability & Environment (Agri-Food Systems) · Foundations · also Fundamentals of Soil Science, Soil Health Management, Pedology and Soil Fertility for Environmental Scientists
Soil as a living, finite resource: formation, physical, chemical and biological properties, organic matter, and how soil health is measured. The engineering properties of soil are in the Agricultural Engineering track; this subject is about soil function and its role in carbon, water and nutrient cycles.
In the exams: ICAR AIEEA-PG Physical Science Units III–IV (heavy); ICAR AICE subject 71 Unit 4 (soil pollution); GATE ES Section 2 (soil chemistry)
Soil is the thin, living skin of the land where rock, water, air and life meet. It anchors and feeds plants, stores and filters water, recycles nutrients and wastes, holds more organic carbon than the atmosphere and vegetation combined, and houses a large share of the planet's biodiversity. Soil forms very slowly -- often a few centimetres over centuries -- but can be lost in years by erosion, compaction, salinisation, sealing and contamination, so on human timescales it is a finite, non-renewable resource. FAO's Status of the World's Soil Resources report (2015) estimated that about a third of the world's soils are moderately to highly degraded.
This subject treats soil as a living, functioning system. It covers how soils form and how their profiles develop; their physical properties (texture, structure, bulk density, porosity) and water relations; colloids, clay minerals and cation exchange; soil reaction and problem soils (acid, saline, sodic and acid sulphate); the essential plant nutrients and their forms; soil organic matter and its decomposition; soil biology; classification, survey and land capability; soil health indicators and indices; soil testing and India's Soil Health Card; and the functions and ecosystem services soils provide.
Several neighbouring subjects build on it and are not repeated. Soil Organic Carbon and Carbon Sequestration treats soil carbon pools, saturation and measurement in depth; Nutrient Stewardship treats fertiliser management, nutrient budgets and 4R; Water Quality and Agricultural Non-point Source Pollution and Hydrology and Water Resources treat leaching, runoff and water movement at catchment scale; and Agricultural Water Conservation, Water Footprint and Governance treats irrigation water use. The engineering properties of soil (consolidation, shear strength, compaction for construction) are in the Agricultural Engineering track.
Soil science is examined heavily in ICAR AIEEA-PG (the soil science units of the Physical Science group), the ICAR PhD papers in Soil Science and Environmental Sciences (soil pollution), GATE Environmental Science (soil chemistry), UGC NET, state agricultural services and the Certified Professional Soil Scientist and Certified Crop Adviser credentials. Ratings, critical limits and classification details differ between national systems; values quoted are standard textbook reference points and should be checked against the system used in your country.
The Russian scientist V.V. Dokuchaev, in the late nineteenth century, recognised soils as natural bodies formed by interacting factors, and Hans Jenny (1941) expressed this as the state-factor equation S = f(cl, o, r, p, t): soil is a function of climate, organisms, relief (topography), parent material and time. Climate controls weathering and leaching through temperature and rainfall; organisms (vegetation, microbes, fauna, people) add organic matter and mix soil; relief controls drainage, erosion and deposition, producing sequences of related soils down a slope called catenas; parent material (weathered bedrock, alluvium, loess, glacial till, volcanic ash) sets texture and mineral reserves; and time determines how far processes have advanced, from young Entisols on recent alluvium to deeply weathered Oxisols on ancient tropical surfaces.
Weathering converts rock to soil material. Physical weathering (freeze-thaw, thermal expansion, root wedging) breaks rock into fragments; chemical weathering (hydrolysis, hydration, oxidation, carbonation, dissolution) transforms primary minerals such as feldspars and micas into secondary clay minerals and oxides and releases nutrients. Goldich's weathering series describes how minerals that crystallise at higher temperatures, such as olivine, weather faster than those that form at lower temperatures, such as quartz, which is why quartz dominates the sand fraction of old soils.
Simonson (1959) grouped soil-forming processes into four kinds: additions (organic matter, dust, salts), losses (leaching, erosion), transformations (weathering, humification) and translocations (movement of clay, organic matter, iron and salts within the profile). Specific combinations produce characteristic soils: podzolisation (acid leaching of iron, aluminium and organic matter from an eluvial E horizon to a spodic B horizon under coniferous forest in cool humid climates), laterisation or ferralitisation (intense weathering and silica loss leaving iron and aluminium oxides in hot humid tropics), calcification (accumulation of calcium carbonate in subsoils of semi-arid grasslands), gleisation (reduction of iron under waterlogging, giving grey and mottled colours), salinisation and alkalinisation (accumulation of salts and sodium in arid areas), and clay illuviation (lessivage).
These processes create horizons -- layers roughly parallel to the surface. The master horizons are O (organic litter), A (mineral topsoil enriched with humus), E (eluvial horizon depleted of clay, iron and aluminium, pale coloured), B (subsoil of accumulation or alteration, such as Bt with illuvial clay, Bk with carbonates, Bs with sesquioxides), C (weathered parent material little affected by soil formation) and R (hard bedrock). The sequence of horizons is the soil profile; describing it -- depth, colour (Munsell notation), texture, structure, consistence, roots, mottles, concretions -- is the basis of soil survey and classification. A pedon is the smallest three-dimensional unit of soil that shows the full profile.
Soil texture is the relative proportion of sand, silt and clay in the fine earth (particles smaller than 2 mm). In the USDA system, sand is 2-0.05 mm, silt 0.05-0.002 mm and clay smaller than 0.002 mm (2 micrometres); the International (Atterberg) system used in some countries sets the sand-silt boundary at 0.02 mm. The USDA textural triangle defines twelve classes, from sand and loamy sand through loam to clay; loam has moderate amounts of all three. Particle-size analysis disperses the soil (removing organic matter and cements, adding a dispersant) and separates sand by sieving and silt and clay by sedimentation, using pipette or hydrometer methods based on Stokes' law: settling velocity v = (2/9) r^2 g (rho_s - rho_w) / eta, so coarser particles settle faster. Texture controls water retention, drainage, aeration, nutrient-holding capacity, workability and erodibility and cannot be changed practically on a field scale. The feel method -- moistening a sample and rubbing it between the fingers to judge grittiness (sand), smoothness (silt) and stickiness and ribbon length (clay) -- lets trained surveyors estimate texture class in the field.
Soil structure is the arrangement of particles into aggregates (peds), bound by clay, organic matter, roots, fungal hyphae and microbial products such as polysaccharides and glomalin. Types include granular (typical of grassland topsoils, rich in organic matter), crumb, platy (horizontal plates, often from compaction), blocky (angular or subangular), prismatic and columnar (vertical prisms; columnar with rounded tops is typical of sodic subsoils). Good structure creates a range of pore sizes: large pores for drainage and aeration, small pores for water storage. Aggregate stability -- resistance of aggregates to breakdown by water -- is a key soil health indicator.
Particle density, the density of the solids, averages about 2.65 g/cm3 for mineral soils (quartz and many clays), lower for organic-rich soils. Bulk density is the dry mass of soil per unit total volume, including pores: typically about 1.0-1.6 g/cm3 for mineral soils, higher in compacted layers, much lower in organic soils. Total porosity = 1 - (bulk density / particle density); a soil with bulk density 1.33 g/cm3 has porosity 1 - 1.33/2.65 = 0.50, or 50%. Compaction by machinery and livestock raises bulk density, destroys large pores and restricts roots; root growth is commonly restricted above about 1.6 g/cm3 in clays and 1.8 g/cm3 in sands, though thresholds vary with texture.
Soil water content can be expressed gravimetrically (mass of water per mass of dry soil, theta_g) or volumetrically (volume of water per volume of soil, theta_v); theta_v = theta_g x bulk density / density of water. The depth of water held in a layer equals theta_v times the layer thickness: 0.30 m3/m3 in a 30 cm layer is 90 mm. Soil water is held against gravity by adsorption on surfaces and capillarity in small pores; its energy status is described by the soil water potential, the sum of matric (from adsorption and capillarity), osmotic (from solutes), gravitational and pressure components. Matric potential is negative and becomes more negative as soil dries; it is measured with tensiometers in moist soil and pressure plates in the laboratory, and the relationship between water content and matric potential is the soil water characteristic (retention) curve. Sandy soils release most of their water at small suctions, whereas clays retain water at high suctions, which is why the same water content can mean very different availability in different soils.
After a soil is saturated and allowed to drain freely for a day or two, it reaches field capacity, conventionally taken as the water content at a matric potential of about -33 kPa (-1/3 bar) for most soils (-10 kPa for sandy soils in some practice). As plants extract water, the soil reaches the permanent wilting point, conventionally -1,500 kPa (-15 bar), at which most crops can no longer extract water and wilt irreversibly. The water held between them is the plant-available water capacity: AWC = theta_FC - theta_PWP. Loams and silt loams hold the most available water; sands hold little because their pores drain easily; clays hold much water but much of it is held too tightly. Example: a loam with theta_FC = 0.32 and theta_PWP = 0.14 holds 0.18 m3/m3 of available water, or 108 mm in a 60 cm root zone; irrigation is often scheduled when about half of this is used. The hygroscopic coefficient (about -3,100 kPa) marks water held so tightly it is essentially unavailable.
Water moves in saturated soil according to Darcy's law, with saturated hydraulic conductivity far higher in sands and well-structured soils than in compact clays; in unsaturated soil, conductivity falls steeply as soil dries, and flow is described by the Richards equation. Infiltration, redistribution, drainage, capillary rise from shallow water tables (which can carry salts to the surface) and preferential flow through macropores such as root channels and earthworm burrows all affect crop water supply and the leaching of nutrients and pollutants (see Hydrology and Water Resources and Water Quality and Agricultural Non-point Source Pollution).
Soil colloids -- particles smaller than about 1 micrometre with very large surface area -- dominate soil chemistry. Most clay minerals are layer silicates built from sheets of silica tetrahedra (T) and alumina or magnesia octahedra (O). In 1:1 minerals such as kaolinite, one tetrahedral and one octahedral sheet form a layer; layers are held tightly by hydrogen bonds, so kaolinite does not swell and has low surface area and low cation exchange capacity (CEC). In 2:1 minerals, an octahedral sheet lies between two tetrahedral sheets. Smectites such as montmorillonite have weakly held interlayers that admit water and cations, so they swell and shrink strongly and have high CEC; vermiculite has even higher CEC with limited swelling; illite (fine-grained mica) has potassium fixed between layers, little swelling and moderate CEC. Chlorite is a 2:1:1 mineral with a hydroxide interlayer. Highly weathered tropical soils are dominated by kaolinite and iron and aluminium oxides (goethite, haematite, gibbsite), which have low and pH-dependent charge. Allophane and imogolite occur in volcanic soils. Humus has the highest CEC of all, per unit mass.
Negative charge arises from isomorphous substitution -- replacement of Si4+ by Al3+ in tetrahedral sheets, or Al3+ by Mg2+ or Fe2+ in octahedral sheets during crystallisation -- which creates permanent charge, and from dissociation of hydrogen from edge hydroxyl groups and organic functional groups, which creates pH-dependent (variable) charge that increases as pH rises.
Cations held on negatively charged colloid surfaces can be exchanged for cations in the soil solution; this reversible, stoichiometric process supplies plants with calcium, magnesium and potassium and buffers the soil against change. Cation exchange capacity is expressed in centimoles of charge per kilogram, cmol(+)/kg, numerically equal to milliequivalents per 100 g. Typical values are roughly 3-15 cmol(+)/kg for kaolinite, 15-40 for illite, 80-120 for montmorillonite, 100-150 for vermiculite and 150-300 or more for humus; a soil's CEC depends on how much and which clay and organic matter it contains. Example: a soil with 25% montmorillonite (100 cmol/kg) and 2% humus (200 cmol/kg) has an approximate CEC of 0.25 x 100 + 0.02 x 200 = 29 cmol(+)/kg, ignoring other components.
Base saturation is the percentage of CEC occupied by base cations (Ca2+, Mg2+, K+, Na+); the remainder is held by acidic cations (H+ and Al3+). The strength with which cations are held, and hence their ability to replace others, generally follows the lyotropic series Al3+ > Ca2+ > Mg2+ > K+ = NH4+ > Na+ (with H+ behaving like a strongly held cation), so trivalent and divalent cations displace monovalent ones, and highly hydrated small ions like Na+ are held weakly. Soils also have a smaller anion exchange capacity, important in oxide-rich tropical soils, and specific adsorption of phosphate on iron and aluminium oxides (phosphate fixation).
Soil reaction (pH) affects nutrient availability, toxicities and biological activity. Most nutrients are most available between about pH 6 and 7.5. Acid soils develop where leaching removes base cations faster than weathering supplies them, under high rainfall, acid parent materials, and the acidifying effects of ammonium fertilisers, legume nitrogen fixation and removal of produce. Below about pH 5.5, exchangeable and soluble aluminium increase and become toxic to roots; manganese toxicity and deficiencies of calcium, magnesium, phosphorus and molybdenum are common. Liming with calcium or magnesium carbonate raises pH; the lime requirement depends on the target pH and the soil's buffer capacity (which rises with clay and organic matter) and is determined with buffer methods. Acid soils cover large areas of the humid tropics and, in India, parts of the north-east, the Western Ghats and eastern plateau. Alkaline and calcareous soils (pH above about 7.5-8) occur in drier regions; there, phosphorus and micronutrient availability is the main concern, and lime is never applied.
Salt-affected soils occur in arid and semi-arid regions and in irrigated land with poor drainage, where evaporation concentrates salts. The US Salinity Laboratory classification (1954) uses the electrical conductivity of the saturation extract (ECe), the exchangeable sodium percentage (ESP = 100 x exchangeable Na / CEC) and pH. Saline soils have ECe above 4 dS/m, ESP below 15 and pH usually below 8.5; their salts reduce water uptake by osmotic effects, and a white crust often forms. Sodic (alkali) soils have ESP above 15, ECe below 4 dS/m and pH usually above 8.5; sodium disperses clay, destroying structure and making soils hard, poorly permeable and dark from dispersed organic matter ('black alkali'). Saline-sodic soils have both ECe above 4 and ESP above 15. Saline soils are reclaimed by leaching with good-quality water and drainage; sodic soils need a calcium source -- most commonly gypsum, sometimes pressmud or acids where carbonates are present -- to replace exchangeable sodium, followed by leaching. The gypsum requirement is calculated from the reduction in ESP needed, the CEC and the soil mass treated. A worked example: to lower ESP from 30 to 10 in a soil with CEC 20 cmol(+)/kg, 4 cmol(+)/kg of sodium must be replaced. The top 15 cm of a hectare at bulk density 1.5 g/cm3 weighs 2,250,000 kg, so 0.04 equivalents per kg x 2,250,000 kg = 90,000 equivalents of calcium are needed; with gypsum (CaSO4.2H2O, equivalent weight 86 g), that is about 7.7 t of pure gypsum per hectare, increased in practice for gypsum purity and exchange efficiency. Salt-tolerant crops and varieties, such as barley, cotton and salt-tolerant rice and wheat varieties developed by India's Central Soil Salinity Research Institute, help production during reclamation.
Acid sulphate soils form where sediments containing pyrite (FeS2), typically coastal mangrove and marsh deposits, are drained or excavated: pyrite oxidises to sulphuric acid, lowering pH below about 4, releasing aluminium and iron, and producing yellow jarosite mottles. They are hazardous to crops, fish and infrastructure; management keeps them waterlogged or applies lime. Examples include parts of Kerala's Kuttanad and coastal areas of South-East Asia and Australia.
Arnon and Stout (1939) set the criteria of essentiality: without the element the plant cannot complete its life cycle; the element's role cannot be replaced by another; and it is directly involved in plant metabolism. Seventeen elements are generally accepted as essential for higher plants. Carbon, hydrogen and oxygen come from air and water. The primary macronutrients are nitrogen, phosphorus and potassium; the secondary macronutrients are calcium, magnesium and sulphur; and the micronutrients are iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel (the last added in the 1980s). Beneficial elements such as sodium, silicon, cobalt (for nitrogen fixation in legumes) and selenium benefit some plants. Liebig's law of the minimum states that growth is limited by the nutrient in shortest supply relative to need. Nutrients are also classed by the amounts needed: macronutrients are required in kilograms per hectare and are typically present in plant dry matter at 0.1-5%, micronutrients in grams per hectare and at parts-per-million concentrations, but each is equally essential.
Plants take up nutrients mainly as ions from the soil solution. Nitrogen is absorbed as nitrate (NO3-) and ammonium (NH4+); phosphorus as orthophosphate (H2PO4- and HPO4 2-, depending on pH); potassium as K+; calcium and magnesium as Ca2+ and Mg2+; sulphur as sulphate (SO4 2-); iron, manganese, zinc and copper as divalent cations or chelates; boron as boric acid (H3BO3); molybdenum as molybdate (MoO4 2-); chlorine as Cl-; and nickel as Ni2+. Nutrients reach roots by mass flow with transpired water (important for nitrate, calcium, sulphate), diffusion (important for phosphorus and potassium, which move slowly) and root interception. Availability depends on pH: phosphorus is fixed by iron and aluminium in acid soils and by calcium in calcareous soils; iron, manganese, zinc and copper become less available at high pH; molybdenum becomes more available as pH rises.
Nutrient mobility within the plant determines where deficiency appears first. Mobile nutrients (N, P, K, Mg) move from old to young leaves, so deficiency shows first on older leaves (for example, yellowing of older leaves in N deficiency, marginal scorch in K deficiency, interveinal chlorosis of older leaves in Mg deficiency). Immobile nutrients (Ca, S, Fe, Mn, Zn, Cu, B) show deficiency first on young leaves and growing points (interveinal chlorosis of young leaves in Fe deficiency; death of growing points in Ca and B deficiency). Zinc deficiency is widespread in India's intensively cropped soils; in rice it causes 'khaira' disease. Soil and plant tests confirm visual diagnoses (chapter 11; nutrient management is in Nutrient Stewardship).
Soil organic matter (SOM) comprises plant and animal residues at all stages of decomposition, living microbial biomass, and stabilised organic compounds; it is roughly 58% carbon (hence SOM is approximately soil organic carbon x 1.724). Residues added to soil are decomposed by microorganisms, which use their carbon for energy and growth, releasing CO2, and incorporate part into new biomass and stable compounds. Decomposition rate depends on residue quality (sugars and proteins decompose fast; cellulose slower; lignin slowest), temperature, moisture, aeration, pH and soil contact.
The carbon-to-nitrogen (C:N) ratio of residues determines whether decomposition releases or ties up mineral nitrogen. Soil microbes have C:N ratios of roughly 5-10:1, and stable soil organic matter around 10-12:1. Residues with low C:N, such as legume residues and manures (below about 20-25:1), supply more nitrogen than decomposers need, and the surplus is mineralised to ammonium and nitrate. Residues with high C:N, such as cereal straw (often 60-100:1) and sawdust (several hundred to one), contain too little nitrogen, so microbes take mineral nitrogen from the soil (immobilisation), which can cause temporary nitrogen deficiency in crops; as decomposition proceeds and carbon is lost as CO2, the ratio narrows and net mineralisation resumes. Adding some nitrogen with high-C:N residues speeds decomposition and avoids crop deficiency.
Humification forms humus, the dark, relatively stable fraction of organic matter. Classical chemical fractionation by solubility in alkali and acid divides it into fulvic acid (soluble in both), humic acid (soluble in alkali, precipitated by acid) and humin (insoluble); current understanding emphasises that persistence depends largely on mineral association and physical protection rather than on special 'humic' molecules (see Soil Organic Carbon and Carbon Sequestration). Mineralisation releases nutrients (N, P, S) from organic forms to inorganic ions; immobilisation is the reverse.
Organic matter has outsized effects relative to its usually small share of soil mass (often 1-5% in mineral topsoils): it binds aggregates and improves structure and infiltration, raises water-holding capacity, supplies and stores nutrients, contributes much of the CEC and buffering capacity, chelates micronutrients, feeds soil organisms, and reduces erosion. Indian soils under intensive tillage and residue burning in hot climates are often low in organic carbon (below 0.5% by the Walkley-Black method in many areas). Practices that maintain organic matter -- residue retention, manures and composts, legumes and cover crops, reduced tillage, integrated nutrient management -- are central to soil health.
Soils are among the most biologically diverse habitats on Earth: a teaspoon of fertile soil can contain hundreds of millions of bacteria and many metres of fungal hyphae. The soil food web links primary producers (plants, algae) and the organic matter they supply with decomposers (bacteria and fungi), their grazers (protozoa, bacterial- and fungal-feeding nematodes, microarthropods such as mites and springtails), predators (predatory nematodes, mites, centipedes, beetles), and ecosystem engineers (earthworms, termites, ants) that physically reshape soil. Bacteria dominate decomposition of easily degradable residues and are especially important in tilled and flooded soils; fungi dominate decomposition of lignin and cellulose, are favoured by reduced tillage and perennial cover, and their hyphae bind aggregates. When grazers consume microbes, they release surplus nitrogen as ammonium, an important pathway of nutrient mineralisation.
Microbes carry out key transformations: nitrogen fixation (Rhizobium in legume nodules, free-living Azotobacter, cyanobacteria), nitrification (ammonia-oxidising bacteria and archaea, nitrite oxidisers), denitrification, sulphur oxidation, phosphate solubilisation, methanogenesis and methane oxidation, and degradation of pesticides and pollutants. Microbial biomass, respiration and enzyme activities are used as indicators of biological activity.
Earthworms are grouped by ecology: epigeic species live in litter and surface organic matter and do not burrow much (Eisenia fetida and Eudrilus eugeniae are used in vermicomposting); endogeic species live and feed in the mineral topsoil, making horizontal burrows; and anecic species, such as Lumbricus terrestris, make deep vertical burrows and pull surface litter down. Their casts are rich in available nutrients, and their burrows increase infiltration and aeration. Earthworm numbers fall with intensive tillage, compaction, low organic inputs and some pesticides, and are a widely used, easily observed soil health indicator.
Mycorrhizae are symbiotic associations between fungi and plant roots in which the plant supplies sugars and the fungus extends the root system's reach for water and immobile nutrients, especially phosphorus and zinc. Arbuscular mycorrhizal fungi (phylum Glomeromycota) penetrate root cortical cells, forming arbuscules; they associate with most crop plants (notable exceptions include brassicas and chenopods) and produce glomalin-related soil protein associated with aggregate stability. Ectomycorrhizal fungi form a sheath around roots of many forest trees, such as pines, oaks and eucalypts. Mycorrhizal colonisation is reduced by high soil phosphorus, long fallows and tillage, and is supported by continuous living roots and diverse rotations. Commercial mycorrhizal inoculants give variable results in field soils that already contain native fungi.
Soil classification groups soils with similar properties so that knowledge can be transferred. USDA Soil Taxonomy, used widely including in India, is hierarchical (order, suborder, great group, subgroup, family, series) and based on measurable diagnostic horizons and properties. It has twelve orders: Alfisols (moderately leached, clay-enriched subsoil, high base saturation), Andisols (volcanic ash soils), Aridisols (dry-region soils), Entisols (little profile development), Gelisols (permafrost), Histosols (organic soils, peat), Inceptisols (weakly developed), Mollisols (dark, base-rich grassland topsoils), Oxisols (highly weathered tropical soils with oxide-rich horizons), Spodosols (podzols with spodic horizons), Ultisols (strongly leached, low base saturation) and Vertisols (high smectite clay, deep cracking, self-mulching). The World Reference Base for Soil Resources (WRB), the international system of the International Union of Soil Sciences, uses reference soil groups (32 in the 2022 edition) with qualifiers.
India's major soil groups, in traditional terms, are alluvial soils (Indo-Gangetic plains, mostly Entisols, Inceptisols and Alfisols), black soils (Deccan plateau, Vertisols and vertic intergrades, 'regur'), red soils (peninsula, Alfisols), laterite soils (Western Ghats and eastern plateau, Ultisols and Oxisols in strict terms), desert soils (Aridisols and Entisols in Rajasthan), and mountain and forest soils. The National Bureau of Soil Survey and Land Use Planning (NBSS&LUP) maps and classifies India's soils.
Soil survey maps the distribution of soils in a landscape: surveyors describe profiles in pits and auger holes, relate soils to landforms, geology and vegetation (catenas), delineate map units on base maps and aerial or satellite imagery, and characterise representative soils in the laboratory. Survey intensity ranges from reconnaissance (small scales, for regional planning) to detailed surveys (large scales, for farm planning). Digital soil mapping uses statistical models relating soil observations to environmental covariates to predict soil properties continuously.
Land capability classification (USDA, Klingebiel and Montgomery, 1961) groups land into eight classes according to its limitations for sustained agricultural use without degradation. Classes I to IV are suited to cultivation with progressively more severe limitations and need for conservation practices: Class I has few limitations; Class IV has very severe limitations and needs very careful management. Classes V to VIII are generally not suited to cultivation: V (little erosion hazard but other limitations, such as wetness or stones, limiting use to pasture or forest), VI and VII (severe and very severe limitations, suited to grazing, forestry or wildlife), and VIII (suitable only for recreation, wildlife, water supply or aesthetic purposes). Subclasses indicate the main limitation: e (erosion), w (wetness), s (soil limitations in the root zone) and c (climate). India uses land capability classification in watershed planning. FAO's Land Evaluation framework (1976) assesses land suitability for specific uses.
Soil quality has been defined as the capacity of a specific kind of soil to function, within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation (Karlen and colleagues, 1997, building on Doran and Parkin, 1994). Soil health is used almost synonymously but emphasises soil as a living system. Soil health is assessed with indicators that respond to management and relate to soil functions. Physical indicators include aggregate stability, infiltration rate, bulk density and penetration resistance, available water capacity and depth to restrictive layers. Chemical indicators include pH, electrical conductivity, CEC, base saturation and available nutrients. Biological indicators include soil organic carbon, active (permanganate-oxidisable) carbon, microbial biomass, soil respiration (CO2 burst), potentially mineralisable nitrogen, soil protein, enzyme activities and earthworm counts.
The USDA Natural Resources Conservation Service promotes five soil health principles: minimise soil disturbance, maximise soil cover, maximise biodiversity (crop and soil life), maximise the presence of living roots throughout the year, and integrate livestock where appropriate.
Soil quality indices combine indicators into a single score. The usual steps are: select a minimum data set of indicators relevant to the soil functions of interest (often by expert judgement or statistical methods such as principal component analysis); transform each indicator to a 0-1 score using scoring functions -- 'more is better' (organic carbon, aggregate stability), 'less is better' (bulk density, salinity) or 'optimum' (pH); weight the scores; and combine them, often as a weighted sum. Frameworks include the Soil Management Assessment Framework (SMAF) and the Cornell Comprehensive Assessment of Soil Health, which reports scored indicators and an overall rating with management suggestions; India's Soil Health Card reports chemical parameters but few biological indicators.
Interpreting indicators requires context: values differ naturally with texture, climate and land use, so a sandy soil's organic carbon should be compared with similar soils, not with a clay. Indicators vary seasonally and with sampling depth and method, so time series should use consistent protocols. Soil health assessment is most useful for tracking change on the same field over years and for comparing management systems on similar soils.
Soil testing estimates the nutrient-supplying capacity of a soil to guide fertiliser and amendment use. Its value depends first on sampling: a composite sample of many cores (commonly 10-20) taken in a zig-zag pattern across a uniform field or management zone, from the plough layer (commonly 0-15 cm for arable crops in India; deeper for some purposes), avoiding unusual spots such as manure heaps, bunds and fertiliser bands, air-dried, sieved to 2 mm and labelled. Laboratory methods extract the 'available' fraction of each nutrient with standard reagents that correlate with crop uptake: in India, commonly organic carbon by Walkley-Black, available nitrogen by the alkaline permanganate method (Subbiah and Asija), available phosphorus by Olsen's bicarbonate method in neutral and alkaline soils and Bray's method in acid soils, available potassium by neutral normal ammonium acetate, and micronutrients by DTPA extraction (Lindsay and Norvell). pH and EC are measured in soil-water suspensions.
Results are interpreted against critical levels or rating classes derived from field calibration experiments. Common Indian ratings classify soils as low, medium or high; for example, organic carbon below 0.5% is commonly rated low, 0.5-0.75% medium and above 0.75% high, and available nitrogen below 280 kg/ha low, 280-560 medium and above 560 high. Ratings for phosphorus, potassium and micronutrients vary somewhat between states and laboratories. Recommendations follow from ratings (adjusting general doses up or down) or from more precise approaches such as Soil Test Crop Response (STCR) targeted-yield equations that relate fertiliser doses to soil test values and yield targets.
India's Soil Health Card scheme, launched in 2015, aims to provide every farm holding with a card reporting its soil status and fertiliser recommendations at regular intervals. The card reports twelve parameters: macronutrients N, P and K; secondary nutrient S; micronutrients Zn, Fe, Cu, Mn and B; and pH, EC and organic carbon. It gives crop-wise recommendations for fertilisers and amendments. Hundreds of millions of cards have been distributed. The scheme has raised awareness and, in evaluations, been associated with more balanced fertiliser use in some areas, but limitations include grid-based rather than field-specific sampling in many places, laboratory capacity and quality, delays between sampling and recommendations, weak extension follow-up, and limited coverage of physical and biological soil health.
Internationally, soil testing services, accreditation of laboratories, proficiency testing and calibration networks (such as FAO's Global Soil Laboratory Network, GLOSOLAN) aim to improve reliability. New tools -- spectroscopy, portable sensors and digital soil maps -- are expanding access, but need local calibration against standard methods.
Soils perform functions that sustain ecosystems and societies. Commonly listed functions include: biomass production (food, feed, fibre, timber, energy); storing, filtering and transforming nutrients, contaminants and water (soils purify water as it percolates and degrade many pollutants); regulating the water cycle (infiltration, storage and release reduce floods and sustain baseflow); storing carbon and regulating greenhouse gas fluxes (soils are a large carbon store and both source and sink of CO2, CH4 and N2O); providing habitat and a gene pool (a large share of terrestrial species live in soil, and soil microbes are a source of antibiotics); providing a physical platform for buildings and infrastructure; supplying raw materials (clay, sand, peat); and preserving cultural and archaeological heritage. These functions are interdependent and often compete: sealing soil under buildings removes most other functions, and intensive production can reduce filtering, habitat and carbon storage. Assessing a land-use decision therefore means asking which functions a soil currently provides, which the proposed use will enhance or degrade, and whether those losses are reversible on human timescales.
In the Millennium Ecosystem Assessment (2005) framework, soils contribute to provisioning services (food, fibre, water, fuel), regulating services (climate regulation, flood control, water purification, erosion control, pest and disease regulation), supporting services (nutrient cycling, soil formation, primary production) and cultural services (heritage, recreation, aesthetic value). Valuation of soil services is difficult but shows that losses through erosion, compaction, salinisation, contamination and sealing carry large hidden costs (see Natural Resource and Environmental Economics for valuation methods).
Policy attention has grown. The United Nations declared 2015 the International Year of Soils, and 5 December is World Soil Day. FAO's Global Soil Partnership coordinates efforts and adopted the revised World Soil Charter (2015) and Voluntary Guidelines for Sustainable Soil Management (2017). The European Union proposed a soil monitoring law in 2023 to assess soil health across member states. India's policies on soil include the Soil Health Card scheme, watershed development, promotion of organic and natural farming, and programmes on soil and water conservation. Sustainable soil management maintains or enhances the supporting, provisioning, regulating and cultural services of soils without significantly impairing soil functions or biodiversity -- a definition that links this subject to the wider sustainability goals of carbon sequestration, clean water and food security.
Learn the core facts and numbers exactly: Jenny's factors, horizon designations, USDA particle sizes and Stokes' law, porosity from bulk density, field capacity and wilting point potentials, clay mineral structures and CEC ranges, the lyotropic series, saline and sodic thresholds, the 17 essential elements and their forms, C:N thresholds, earthworm groups, the 12 Soil Taxonomy orders, land capability classes, Indian soil-test methods and ratings, and the 12 Soil Health Card parameters. Practise calculations: porosity, water depth, AWC, CEC from components, ESP and gypsum requirement.
As a soil scientist, extension officer, crop adviser or environmental consultant you sample and describe soils, interpret laboratory results and soil health cards, diagnose problems such as acidity, salinity, sodicity, compaction and nutrient deficiencies, recommend amendments and practices, map soils and land capability for farm and watershed plans, and assess contamination for environmental reports. You explain to farmers why a practice will help their particular soil, and you keep careful records so that changes in soil health can be tracked over the years.
As a head of a soil testing network, research station or land-use planning agency you set standards for sampling, laboratory quality and recommendations, design soil health and reclamation programmes, decide where land should and should not be cultivated or built on, and advise on policy for soil degradation, salinity, sealing and contamination. You balance productivity goals with long-term soil conservation and explain soil's value to decision-makers, financiers and the public, including its role in national climate, water and food security goals.
As a researcher you work on soil processes -- weathering, organic matter dynamics, microbial ecology, nutrient cycling, water and solute transport, salinity and sodicity chemistry -- and on new tools such as spectroscopy, sensors and digital soil mapping. You develop soil health indicators and indices calibrated to function, maintain long-term experiments and soil archives, contribute to national soil information systems, policy reviews and global assessments, and train the next generation of soil scientists in field, laboratory, statistical and modelling skills.
Rating classes for available phosphorus, potassium and micronutrients are not tabulated because they vary between Indian states, laboratories and countries; only commonly cited organic carbon and nitrogen ratings are given. CEC and other ranges are typical values. The manual introduces Soil Taxonomy orders and WRB but not the lower categories or diagnostic horizon criteria in detail. Soil physics (Richards' equation, solute transport), soil mineralogy beyond the main clays, soil pollution and remediation (see Soil Contamination and Remediation), engineering properties, and soil carbon dynamics in depth (see Soil Organic Carbon and Carbon Sequestration) are covered only briefly. The EU soil monitoring law is described as proposed; check its current status.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions. Every numerical answer was computed by script; Ratings and constants are stated in each stem because they differ between national systems; every numerical answer was computed by script.
These notes are free to read here, with no account.
What the Laisha app adds:
Free for ten days, then a subscription. We would rather write that here than have you find it later.