Agricultural Engineering · Foundations · also Principles of Soil Science, Soil Physics
The soil an engineer tills, irrigates, drains and builds on: its physical, water and chemical properties.
In the exams: ICAR AIEEA-PG Unit VII; ICAR AICE-JRF/SRF subjects 40 and 43; NCEES PE soil and water
Soil science for engineers is the part of soil science an agricultural engineer uses every working day: the soil's origin and layers, its texture, structure and density, how it holds and transmits water, how air and heat move in it, and the chemistry of pH, salts and exchangeable sodium that decides whether an irrigated field stays productive. Taught early in B.Tech and BSc agricultural engineering as Principles of Soil Science or Soil Physics, it underlies irrigation, drainage, conservation, machinery and land development design.
The manual serves students from Ludhiana and Los Baños to Davis, Nairobi and Wageningen. Where conventions differ between countries (particle-size limits, pH suspension ratios, salinity and sodicity limits, classification names) it gives the common variants. Worked numericals are computed, and common slips (wet versus dry basis, gravimetric versus volumetric water, molar mass versus equivalent weight) are flagged.
The subject is examined in the soil-water-plant part of GATE Agricultural Engineering, Unit VII of ICAR AIEEA-PG, the soil and water and irrigation and drainage specialisations of ICAR AICE-JRF/SRF and ASRB NET, the land and water part of the Philippine ABE licensure exam, and soil and water items of the NCEES PE Agricultural and Biological exam. Most marks come from short calculations. Soil mechanics (strength, consolidation, engineering compaction and classification) is a separate subject and is not repeated here.
Soil is a three-phase system of solids (mineral grains and organic matter), water carrying dissolved salts, and air; in a typical surface soil about half the volume is pore space shared between water and air. It forms from parent material by weathering. Physical weathering breaks rock without changing its chemistry: temperature cycling, freeze-thaw wedging, salt crystallisation, abrasion and root wedging. Chemical weathering changes the minerals: hydrolysis (the reaction of silicates such as feldspar with water, forming clay minerals and releasing bases), hydration, oxidation (notably of ferrous iron, giving red and yellow colours), carbonation and solution (of limestone by carbonic acid) and chelation by organic acids. It is fastest where it is warm and wet, so humid tropical soils are often deeply weathered. Hans Jenny (1941) wrote soil formation as a function of five factors, remembered as cl-o-r-p-t: climate, organisms, relief, parent material and time. Relief controls drainage and erosion, so soils on one parent material differ down a slope; such a sequence is a catena or toposequence. Parent material is residual (weathered in place) or transported: alluvial (rivers), colluvial (gravity), aeolian (wind, as in loess), glacial, lacustrine or marine. The great irrigated plains, from the Indo-Gangetic plain to the Nile valley, are on alluvium.
Pedogenic processes create layers called horizons through additions (organic matter, dust, salts), losses (leaching, erosion), transformations (mineral weathering, humification) and translocations. Eluviation is the washing out of clay, iron, aluminium or humus from an upper horizon; illuviation is its accumulation lower down. Named processes combine these: podzolisation (acid leaching of iron, aluminium and humus, typical of cool humid forests on sands), laterisation or ferralitisation (intense weathering and loss of silica leaving iron and aluminium oxides, in the humid tropics), calcification (carbonate accumulation in semi-arid subsoils), salinisation (soluble salts accumulating where evaporation exceeds leaching), alkalisation (accumulation of exchangeable sodium) and gleisation (reduction of iron under waterlogging, giving grey, mottled colours). The master horizons are O (organic litter and humus), A (mineral surface darkened by organic matter), E (pale eluvial horizon), B (subsoil of illuviation or of colour and structure change), C (little-altered parent material) and R (bedrock). Suffixes add detail: Bt clay accumulation, Bk carbonate, Bn sodium, Bss slickensides, Ap ploughed layer, g gleying; symbols differ slightly between systems. The vertical section through the horizons is the profile; the smallest three-dimensional unit showing them all (about 1 to 10 m²) is the pedon. In the field each horizon is described for depth, boundary, texture, structure, consistence, roots, mottles and colour. Colour is recorded in the Munsell system as hue, value (lightness) and chroma (strength). Dark colours mean organic matter, red and yellow mean oxidised iron and good aeration, and grey low-chroma colours and mottles mean periodic saturation, which shows the depth of seasonal waterlogging long before a piezometer record exists. The FAO Guidelines for Soil Description and the USDA Soil Survey Manual give the standard terms.
Classification groups soils so that knowledge from one place can be transferred to another with the same kind of soil. Modern systems classify on measurable properties of diagnostic horizons rather than on assumed origin. USDA Soil Taxonomy (first published 1975; the Keys to Soil Taxonomy are revised regularly) is hierarchical with six categories: order, suborder, great group, subgroup, family and series. There are 12 orders: Alfisols, Andisols, Aridisols, Entisols, Gelisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols and Vertisols. Each has a formative element that appears in lower category names, for example -ert in Usterts and Haplusterts for Vertisols, -oll for Mollisols, -alf for Alfisols and -id for Aridisols. Some orders worth knowing by their meaning: Entisols show little horizon development (young alluvium, sands, steep slopes); Inceptisols show weak development; Vertisols are clay-rich soils that shrink and crack when dry and swell when wet, with slickensides; Mollisols have a thick, dark, base-rich surface (the mollic epipedon) typical of grasslands; Alfisols have a clay-enriched subsoil and moderate to high base saturation; Ultisols are similar but strongly leached with low base saturation; Oxisols are the most weathered, dominated by iron and aluminium oxides and kaolinite; Spodosols have a subsoil of illuvial humus with aluminium and iron; Aridisols are soils of dry climates; Histosols are organic soils; Andisols form in volcanic ash; Gelisols have permafrost. The series is the lowest category and the unit most used in detailed farm and project mapping.
The World Reference Base for Soil Resources (WRB), published by the International Union of Soil Sciences with FAO, is the international correlation system and the successor to the FAO-UNESCO Soil Map of the World legend. It has two levels: a Reference Soil Group (32 groups in the current editions) and qualifiers (principal and supplementary) added to the group name. Names that an irrigation engineer meets often include Vertisols (swelling clays), Fluvisols (young alluvial soils), Solonchaks (soils with high soluble salts), Solonetz (soils with a dense sodium-rich subsoil horizon), Gleysols (groundwater-affected soils), Calcisols and Gypsisols (accumulations of carbonate and gypsum), Ferralsols (deeply weathered tropical soils), Podzols, Chernozems and Kastanozems (dark grassland soils), Histosols (peat) and Anthrosols (soils strongly modified by long human use, including long-term paddy soils). Many countries keep their own systems as well: Australia, Canada, China, France, Brazil and others have national classifications, and India uses Soil Taxonomy for survey together with familiar descriptive names such as black soils (largely Vertisols and associated soils), red soils, laterite soils and alluvial soils. Correlation between systems is approximate, never one to one, so quote the system with any name you use.
A soil survey maps the kinds of soil in an area using base maps, aerial and satellite images, elevation models, auger borings, profile pits and laboratory analysis. Reconnaissance surveys at small scales (for example 1:50,000) support regional planning; detailed surveys at large scales (for example 1:10,000 or larger; the break varies by agency) support farm and command-area design. A map unit is named after its dominant soil but always contains inclusions; a phase subdivides a series by a use-related feature such as slope, erosion, stoniness or salinity. Survey data are then interpreted. The USDA Land Capability Classification (Klingebiel and Montgomery, 1961) has eight classes by degree of limitation for sustained cultivation. Classes I to IV are suited to cultivation, with limitations increasing from I to IV; classes V to VIII are generally not, and are used for pasture, range, woodland or wildlife, class VIII only for recreation, wildlife, water supply or aesthetics. Subclasses name the limitation: e erosion, w wetness, s root-zone soil limitations (shallow, stony, droughty, saline) and c climate. Class V has little erosion hazard but limitations such as frequent flooding that cannot practically be removed. For irrigation, the US Bureau of Reclamation system has six classes: 1 to 3 arable, 4 limited or special use, 5 needing further study, 6 non-arable. The FAO Framework for Land Evaluation (1976) rates suitability for a stated use: order S with classes S1, S2, S3 (highly, moderately, marginally suitable) and order N with N1 (currently) and N2 (permanently not suitable). A class describes limitations under stated management and changes if drainage, terracing or reclamation removes a limitation.
Texture is the relative proportion by mass of the mineral particle-size fractions, called separates, in the fine earth (the fraction finer than 2 mm). Gravel and stones above 2 mm are reported separately as coarse fragments. In the USDA system sand is 2 to 0.05 mm, silt is 0.05 to 0.002 mm and clay is finer than 0.002 mm (2 micrometres). The International Society of Soil Science (Atterberg) scale, used in many countries including India for agricultural work, puts sand at 2 to 0.02 mm (coarse sand 2 to 0.2 mm and fine sand 0.2 to 0.02 mm), silt at 0.02 to 0.002 mm and clay below 0.002 mm. Engineering classification systems used in soil mechanics have different limits again (for example a sand-silt boundary at 0.075 mm, the No. 200 sieve), so always state which scale applies. Sand feels gritty and has little surface area; silt feels smooth and floury. Clay particles are mostly secondary layer-silicate minerals and oxides, plate-shaped, and have an enormous specific surface, from roughly 10 to more than 700 square metres per gram depending on mineral, which is why clay controls water retention, cation exchange, swelling and stickiness. Unlike structure, texture does not change with ordinary management.
Sand, silt and clay percentages are converted into one of twelve USDA textural classes with the textural triangle: sand, loamy sand, sandy loam, loam, silt loam, silt, sandy clay loam, clay loam, silty clay loam, sandy clay, silty clay and clay. Because the three percentages sum to 100, two fix the point. Loam is 7 to 27 percent clay, 28 to 50 percent silt and less than 52 percent sand, so 40 percent sand, 36 silt and 24 clay is a loam. Other national systems use other triangles. In the field, texture is estimated by feel: the length of ribbon moist soil forms between thumb and finger, with grittiness and smoothness, separates sandy, loamy, silty and clayey soils. Texture predicts much before any measurement. Sands drain fast, infiltrate rapidly, hold little available water and are easy to till but erode by wind. Clays hold much water, much of it too tightly for plants, infiltrate slowly once cracks close, and have a narrow moisture range for tillage. Silty soils crust and are the most erodible by water. Texture also sets drip emitter spacing, because water spreads laterally much further in clays than in sands. Pedotransfer functions estimate hydraulic properties from texture, organic matter and bulk density; they serve planning but carry errors that matter in final design.
In the laboratory, organic matter is removed (commonly with hydrogen peroxide), cementing carbonates may be removed with acid, and the soil is dispersed with a chemical dispersant such as sodium hexametaphosphate plus mechanical shaking. Sand is separated by sieving. Silt and clay are measured by sedimentation, which rests on Stokes' law: a small sphere falling in a fluid reaches a terminal velocity v = g d² (ρs − ρw) / (18 η), where d is particle diameter, ρs and ρw are particle and water densities and η is the viscosity of water. Velocity rises with the square of diameter, so a particle ten times larger falls one hundred times faster. Worked example: at 20 °C (η = 1.002 × 10⁻³ Pa s), with ρs − ρw = 1650 kg/m³, a 0.05 mm silt particle settles at 2.24 mm/s and passes 10 cm in about 45 s, while a 2 µm clay particle settles at 3.59 µm/s and needs about 7.7 hours to fall the same 10 cm. This is why a hydrometer (Bouyoucos) reading taken at about 40 seconds gives silt plus clay still in suspension, and a reading after a couple of hours gives clay alone; the pipette method samples the suspension at a fixed depth and time instead. Readings are corrected for temperature and dispersant density, and results are equivalent spherical diameters because platy clays are not the rigid spheres Stokes' law assumes. Laser diffraction results are not directly interchangeable with sedimentation results, particularly for clay.
Structure is the arrangement of primary particles into aggregates (peds) and the pore spaces between them. Soils without peds are structureless: single-grained (loose sand) or massive (a coherent mass with no planes of weakness). Among structured soils, the USDA and FAO descriptions recognise types by shape: granular (small, roughly spherical, porous peds typical of grassland and well-managed surface horizons; very porous ones are called crumb), platy (thin horizontal plates, typical of compacted layers, crusts and some E horizons), blocky (angular blocky with sharp edges, subangular blocky with rounded edges, common in B horizons), prismatic (vertical columns with flat tops) and columnar (vertical columns with rounded, domed tops, characteristic of sodium-affected subsoils such as natric horizons). Each type is further described by class (size, from very fine to very coarse, with limits that differ by type) and grade (weak, moderate or strong, according to how distinct and durable the peds are). Structure produces the larger pores between aggregates (inter-aggregate or structural pores) that carry drainage water and air, while the pores inside aggregates (textural pores) store water. A clay soil with good structure can therefore drain and aerate far better than its texture alone suggests.
Aggregation starts with flocculation of clay: particles held together when the soil solution has enough electrolyte and enough divalent cations such as calcium. Flocculation alone does not make stable aggregates; they need binding agents. These are organic (polysaccharide gums from microbial decomposition, fungal hyphae, fine roots and root exudates, and humified organic matter), inorganic (iron and aluminium oxides, calcium carbonate) and physical (wetting-drying and freezing-thawing cycles, which crack and rearrange soil, and the compressing and drying action of roots). Earthworms and termites build aggregates in their casts. Aggregate stability, the ability of aggregates to survive wetting and mechanical stress, is measured by wet sieving (the Yoder method, reported as mean weight diameter or percentage of water-stable aggregates) or by slaking tests. Structure is destroyed by tillage that pulverises soil or works it too wet, by wheel and hoof traffic that compacts it, by raindrop impact that detaches particles and forms surface seals, by loss of organic matter under continuous cultivation, and by sodium. Exchangeable sodium causes clay to disperse when the soil is wetted with low-salinity water, so aggregates collapse, pores block, and the soil becomes dense, slowly permeable and hard on drying. Puddling for transplanted rice deliberately destroys structure to reduce percolation; the resulting dense layer and hardpan must be managed when a dryland crop follows.
Consistence describes resistance to deformation at different moisture contents: soft or hard when dry, friable or firm when moist, sticky and plastic when wet. Friable soil, crumbling under gentle pressure, is in the right state for tillage; clay worked plastic smears and clods, and worked dry needs large draft. Crusts form when raindrop or sprinkler drop impact breaks surface aggregates and fines fill the pores; dry crusts block seedlings and cut infiltration. Machinery compaction removes large pores first, raising bulk density and penetration resistance and lowering conductivity and aeration; damage is worst when heavy loads cross wet soil, and deep subsoil compaction from high axle loads persists for years. It is assessed for crop production by bulk density, cone index from a penetrometer and air permeability, and reduced by controlled traffic, lower tyre pressures and working only when the soil is friable. Proctor compaction for earthworks belongs to soil mechanics, and wheel-soil mechanics to traction and soil dynamics.
Take a soil sample with total volume Vt made up of solids Vs, water Vw and air Va, with solid mass Ms and water mass Mw. Particle density ρs = Ms/Vs is the density of the solid grains alone. For most mineral soils it lies between about 2.60 and 2.75 g/cm³ because quartz, feldspars and clay minerals are near 2.65; 2.65 g/cm³ is the conventional default when it has not been measured. Organic matter (about 1.2 to 1.5 g/cm³) lowers it, and iron oxides (above 5 g/cm³ for hematite and magnetite) raise it. It is measured with a pycnometer (density bottle). Bulk density ρb = Ms/Vt is the oven-dry mass divided by the total volume including pores. It depends on texture, structure, organic matter and compaction. Porosity n = (Va + Vw)/Vt is the pore fraction of the total volume, and from the definitions n = 1 − ρb/ρs. Void ratio e = (Va + Vw)/Vs = n/(1 − n) is preferred in soil mechanics because the solids volume stays constant when the soil compresses. Gravimetric water content w = Mw/Ms is on an oven-dry mass basis; volumetric water content θ = Vw/Vt = w ρb/ρw. Air-filled porosity is εa = n − θ, and degree of saturation is θ/n. Keep the bases straight: the commonest error in this topic is dividing the water mass by the wet mass instead of the dry mass.
The core method drives a thin-walled metal cylinder of known volume into the soil, trims both ends flush, dries the core at 105 °C to constant mass and divides by the cylinder volume. Worked example: a core 5.0 cm in diameter and 5.0 cm long has volume π × 2.5² × 5 = 98.2 cm³. If its oven-dry soil weighs 135 g, ρb = 135/98.2 = 1.375 g/cm³, and with ρs = 2.65 the porosity is 1 − 1.375/2.65 = 0.481, about 48 percent. Using the wet mass instead of the dry mass, or forgetting to subtract the core's own tare, are the usual slips. Stony soils and loose surface soils are measured by the excavation method (a hole dug, the soil dried and weighed, the hole volume found with sand, water in a membrane or a scanner), and clods are measured by coating them in paraffin wax or resin and weighing in air and water. Field gamma-ray gauges measure density non-destructively but need radiation licensing. Bulk density changes with water content in swelling soils, so for Vertisols the water content at sampling must be reported.
Well-aggregated, organic-rich surface soils can be below 1.0 g/cm³; cultivated loams commonly 1.1 to 1.5; sands 1.3 to 1.7; compacted pans can exceed 1.8; volcanic-ash soils and peats fall well below 1.0. Porosity of most mineral soils is about 30 to 60 percent, and clays usually have higher total porosity than sands because of their many tiny pores. Drainage and aeration depend on pore-size distribution, not total porosity: a sand at 40 percent porosity drains freely while a clay at 50 percent may stay wet. Root growth is restricted at bulk densities that commonly cited guidelines put at roughly 1.4 to 1.5 g/cm³ in clays and 1.7 to 1.8 in sands, and more so as the soil dries and hardens. Bulk density converts nearly every field quantity: gravimetric to volumetric water, and per-kilogram requirements to per-hectare amounts (a hectare 15 cm deep at 1.5 g/cm³ holds 2.25 × 10⁶ kg of soil).
The reference method is gravimetric: weigh a sample, dry it at 105 °C to constant mass (commonly 24 hours), reweigh, and express the water lost as a fraction of the dry mass. A moist sample of 150 g that dries to 125 g has w = 25/125 = 20 percent; dividing by the wet mass would give the wrong 16.7 percent. Multiply by ρb/ρw for volumetric content, and by depth for the equivalent depth of water: θ = 0.25 over 60 cm holds 15 cm of water. The neutron probe counts neutrons slowed by hydrogen; it is accurate over a large soil volume but needs radiation licensing. Time-domain reflectometry (TDR) measures the travel time of a pulse along rods, set by dielectric permittivity; water's permittivity (about 80) far exceeds that of solids (about 3 to 5) and air (1), so permittivity tracks water content, and the Topp equation is the common general calibration. Capacitance sensors use the same principle at lower cost but are more sensitive to salinity, temperature and installation gaps and usually need soil-specific calibration.
Water is held in soil by adsorption and capillarity. Its energy state is its potential, the work per unit quantity needed to move it to a reference pool of pure free water at the same elevation and temperature. Total potential sums gravitational potential (elevation above a datum), matric potential (adsorption and capillarity; zero or negative in unsaturated soil), osmotic potential (solutes; it acts across root membranes but does not drive liquid flow within the soil) and pressure potential (positive below a water table). Per unit weight, potential is a head in metres or centimetres of water. Conversions are often tested: 1 bar = 100 kPa = 0.1 MPa ≈ 1,020 cm of water, and pF is the base-10 logarithm of suction in cm. Field capacity at −33 kPa is about 337 cm, pF ≈ 2.5; wilting point at −1,500 kPa is about 15,300 cm, pF ≈ 4.2. Capillary rise in a tube of radius r is h = 2γ cos α/(ρw g r), about 0.15/r with h and r in cm for water at 20 °C, so a pore of 0.01 mm radius lifts water about 1.5 m. That is the physics of capillary rise from shallow saline water tables, which drives secondary salinisation.
The soil water retention curve (soil water characteristic) plots water content against matric suction. At zero suction the soil is saturated. As suction increases, the largest pores empty first; the suction at which air first enters a saturated soil is the air-entry value, low in sands and higher in fine-textured soils. Sands release most of their water over a narrow, low range of suction and then hold very little; clays release water gradually over the whole range and still hold substantial water at −1,500 kPa. Structure affects the wet end of the curve (large structural pores), texture and mineralogy the dry end. The curve is measured with hanging water columns and tension tables at low suctions and pressure-plate or pressure-membrane extractors (Richards apparatus) at higher suctions, on undisturbed cores for the wet range. Curves differ on wetting and drying (hysteresis): at a given suction a soil holds more water when drying than when wetting, because of the ink-bottle effect of irregular pores, contact-angle differences and entrapped air. Empirical equations fit the curve for modelling; the van Genuchten (1980) and Brooks-Corey (1964) forms are the most used, with parameters for residual and saturated water content and pore-size distribution.
After thorough wetting, when free drainage has largely stopped (usually one to three days in well-drained soils), the water remaining is field capacity (FC). It is a field concept, not a true equilibrium; drainage continues slowly, especially in clays and layered soils. In the laboratory, water content at −33 kPa is customarily used for medium and fine soils and −10 kPa for sands; some agencies use other values. Permanent wilting point (PWP) is the water content at which plants wilt and do not recover overnight in humid air, conventionally taken at −1,500 kPa; it varies somewhat with crop. The hygroscopic coefficient, near −3.1 MPa, is unavailable water held as thin films. Older texts divide soil water into gravitational (drains above FC), capillary (FC to hygroscopic coefficient) and hygroscopic water. Available water capacity (AWC) is FC minus PWP. Loams and silt loams usually have the highest AWC and sands the lowest; clays hold most water at FC but also most at PWP, so their AWC is often no more than that of loams. Organic matter raises AWC, especially in coarse soils. In compacted soils even this range overstates availability, because aeration can limit roots near FC and mechanical resistance can limit them well before PWP; the least limiting water range (da Silva, Kay and Perfect, 1994) narrows the range by those two limits.
Available water in the root zone, as a depth, is d = (θFC − θPWP) × D = (wFC − wPWP) × (ρb/ρw) × D, where D is root depth. Worked example: FC 28 percent and PWP 12 percent by mass, ρb 1.40 g/cm³, root depth 1.0 m. Volumetric available water is (0.28 − 0.12) × 1.40 = 0.224, so the root zone holds 0.224 × 1,000 mm = 224 mm. Forgetting the bulk density gives 160 mm, which is the gravimetric number wrongly used as volumetric. Irrigation is not delayed until all available water is used; it is applied when a chosen fraction has been depleted, called management allowed depletion (MAD) or, in FAO-56, the readily available water fraction p, typically 0.3 to 0.7 depending on crop sensitivity and evaporative demand. With p = 0.5 the net depth per irrigation is about 112 mm; with crop evapotranspiration of 6 mm/day the interval is about 19 days. Gross depth divides the net depth by application efficiency. For layered soils, sum the available water of each layer within the root zone. Remember that roots rarely extract water uniformly: a common rule of thumb for many crops is that about 40, 30, 20 and 10 percent of uptake comes from successive quarters of the root zone, though it varies with crop and wetting pattern. Field water balance and scheduling are developed further in irrigation engineering.
Henry Darcy (1856), working on sand filters in Dijon, found that the volume flow rate Q through a saturated porous column is proportional to the cross-sectional area A and the hydraulic head difference Δh, and inversely proportional to the length L: Q = K A Δh/L, or as a flux q = Q/A = −K dH/dz. The proportionality constant K is the hydraulic conductivity, with units of velocity (m/s, cm/h or m/day). Hydraulic gradient i = Δh/L is dimensionless. The Darcy flux q is a flux per unit total area, not the speed of water in the pores; the average pore-water (seepage) velocity is q divided by the effective porosity, so with q = 0.5 m/day and effective porosity 0.25 the water actually moves at 2 m/day. This distinction matters for travel time of contaminants and for leaching. Darcy's law holds for laminar flow, which covers almost all flow in soils and aquifers; it fails at high velocities in coarse gravels (Reynolds number above roughly 1 to 10) and may deviate at very low gradients in some clays. Worked example from a constant-head permeameter: a column 15 cm long and 10 cm in diameter (area 78.5 cm²) under a constant head difference of 20 cm passes 120 cm³ in 30 minutes. Then K = V L/(A Δh t) = 120 × 15/(78.5 × 20 × 1,800) = 6.37e-04 cm/s, or 2.29 cm/h.
Hydraulic conductivity combines medium and fluid: K = k ρ g/μ, where k is intrinsic permeability (m², set only by pore geometry), ρ fluid density, g gravity and μ dynamic viscosity. Water viscosity falls as it warms, so K is commonly corrected to 20 °C. Saturated K (Ks) depends on the largest continuous pores, so structure, cracks, root channels and earthworm burrows matter even more than texture. Values span orders of magnitude, from more than 10 cm/h in coarse sands to well below 0.1 cm/h in dense or sodic clays, and vary widely within a field; a log-normal distribution is commonly assumed. Sodicity with low-salinity water can cut Ks drastically by dispersing clay. In unsaturated soil the large pores are empty, so unsaturated conductivity K(θ) falls steeply, often by several orders of magnitude, as soil dries; at high suction a sand can conduct worse than a clay. Darcy's law combined with conservation of mass gives the Richards equation, the basis of most numerical models of infiltration, drainage and root uptake.
Real soils are layered. For flow perpendicular to the layers (vertical flow through horizontal layers) the layers act in series and the effective conductivity is the thickness-weighted harmonic mean, Kv = Σd/Σ(d/K). For flow parallel to the layers they act in parallel and the effective value is the thickness-weighted arithmetic mean, Kh = Σ(K d)/Σd. Worked example: a 0.3 m layer with K = 2 cm/h over a 0.2 m layer with K = 0.5 cm/h gives Kv = 0.5/(0.3/2 + 0.2/0.5) = 0.909 cm/h but Kh = (0.3 × 2 + 0.2 × 0.5)/0.5 = 1.40 cm/h. The least permeable layer controls vertical flow, which is why a thin plough pan or clay band can perch water above it; Kh is always at least as large as Kv in a layered profile. A related, often misunderstood effect is that a fine layer over a coarse layer also holds water up: water does not enter the coarse layer until the fine layer above it is nearly saturated, because the coarse pores cannot fill at the suction the fine layer holds. This capillary barrier increases water storage above it and is used deliberately in landfill covers.
Laboratory methods use undisturbed cores: the constant-head permeameter for permeable soils and the falling-head permeameter for slowly permeable ones, where K = (a L/(A t)) ln(h1/h2), a being the standpipe area and A the sample area. Small cores often miss the cracks and channels that dominate field flow, so field methods are preferred for design. Below a water table the auger-hole method is standard for drainage design: water is bailed from a hole and its rate of rise converted to K. Above a water table the inversed auger-hole (Porchet) method, constant-head well permeameters and tension infiltrometers are used; tension infiltrometers apply a small suction so that the largest pores are excluded and matrix flow can be separated from macropore flow. Pumping tests give large-scale aquifer transmissivity and belong to groundwater engineering. Because K varies so much in space, often by a factor of ten within one field, many measurements and a geometric mean are normally needed for one design value.
Infiltration is the entry of water through the soil surface. Infiltration rate is volume per unit area per unit time (mm/h); infiltration capacity is the maximum rate under ponding; cumulative infiltration is the total depth entered. Percolation is the later downward movement within the profile. On a dry soil the rate starts high, because the suction gradient at the wetting front is steep and cracks fill, and declines towards a nearly constant value as the wetted zone deepens and the gradient approaches unity. The steady value, the basic or final infiltration rate, approaches the saturated conductivity of the surface layer but is often lower because of entrapped air. A working definition used in Indian irrigation texts takes the basic rate as the rate when its change in one hour is 10 percent or less of its value. Indicative basic rates in FAO irrigation training material are about 1 to 5 mm/h for clays, 5 to 10 for clay loams, 10 to 20 for loams, 20 to 30 for sandy loams and over 30 for sands; structure, surface condition and water quality shift them a lot. Rates fall with crusting, compaction, initial wetness, sodicity and water repellency, and rise with stable structure, residue cover, cracks and biological pores.
Several equations describe the curve; constants are fitted to field data and differ between soils and seasons. Kostiakov (1932): cumulative I = a t^b with 0 < b < 1, and rate i = a b t^(b−1). It fits short irrigation times well but predicts a rate tending to zero, so the modified Kostiakov (Kostiakov-Lewis) equation adds a steady term, I = a t^b + f0 t. Horton (1940): f = fc + (f0 − fc) e^(−kt), with initial rate f0, final rate fc and decay constant k; integrating gives F = fc t + ((f0 − fc)/k)(1 − e^(−kt)). Worked example: f0 = 8 cm/h, fc = 1 cm/h, k = 2 h⁻¹; after 1 hour the rate is 1 + 7e⁻² = 1.95 cm/h and cumulative infiltration is 1 + 3.5(1 − e⁻²) = 4.03 cm. Philip (1957): I = S t^0.5 + A t, where S is sorptivity (the early, suction-driven uptake) and A a constant related to Ks, so the rate is S/(2√t) + A. Green and Ampt (1911), physically based on a sharp wetting front: f = K (1 + ψf Δθ/F), where ψf is the suction at the wetting front, Δθ the change in water content and F cumulative infiltration; as F grows the rate tends to K. Hydrologists also use the curve-number method of the US Soil Conservation Service (now NRCS) for storm runoff, and the NRCS intake families for surface irrigation design; those belong to hydrology and irrigation subjects.
The double-ring infiltrometer is the standard field method for ponded infiltration. Two concentric rings, typically about 30 and 60 cm in diameter (sizes vary by standard), are driven a few centimetres into the soil and both are kept flooded. The outer ring absorbs the lateral spread, so the fall of level in the inner ring, read at increasing intervals until steady, approximates vertical one-dimensional infiltration; a fall of 1.5 cm in 15 minutes is 6 cm/h. On fine soils readings often need two hours or more. Rings disturb the soil, miss raindrop crusting and give point values, so replicates are needed. Sprinkler infiltrometers and rainfall simulators reproduce drop impact and suit sprinkler design; the inflow-outflow method measures furrow infiltration directly as the difference between inflow and outflow over a measured reach of a real furrow. The design rule is that a sprinkler application rate must not exceed the infiltration rate late in the irrigation, or water ponds and runs off.
Roots and microbes consume oxygen and release carbon dioxide, so soil air is nearly saturated with water vapour, holds from a few tenths of a percent to several percent CO₂ (against about 0.04 percent in the atmosphere) and correspondingly less O₂ than the 21 percent of free air. Exchange with the atmosphere is mainly by diffusion (Fick's law); mass flow from pressure changes, wind and wetting is minor. Gases diffuse about ten thousand times more slowly in water than in air, so aeration depends on continuous air-filled pores. An air-filled porosity of about 10 percent is widely used as a rough lower limit for most crops, though the true critical value varies. When oxygen is exhausted, soil is reduced in a known sequence: nitrate (denitrification), then manganese and iron oxides (grey gleyed colours), then sulphate (hydrogen sulphide), and finally methane forms, as in flooded rice soils. Redox potential and oxygen diffusion rate, measured with platinum electrodes, are the usual measures. The engineer's aeration problems are waterlogging (drainage), compaction (lighter loads, controlled traffic, subsoiling) and crusts.
Soil temperature controls germination, root growth, microbial activity and the timing of field operations. The surface gains heat by solar and atmospheric radiation and loses it by long-wave radiation, evaporation and convection; heat moves within the soil mainly by conduction. Three properties govern it. Volumetric heat capacity C (J m⁻³ K⁻¹) is the heat needed to warm a unit volume by one degree; following de Vries it is often estimated as C ≈ 1.92 xm + 2.51 xo + 4.18 θ MJ m⁻³ K⁻¹, where xm, xo and θ are the volume fractions of minerals, organic matter and water (air contributes almost nothing). Thermal conductivity λ (W m⁻¹ K⁻¹) is the rate of heat flow per unit gradient; it rises steeply as a dry soil is first wetted, because water bridges the contacts between grains, and it is higher in quartz-rich sands than in clays and much lower in organic soils. Thermal diffusivity κ = λ/C (m²/s) controls how quickly a temperature change penetrates. Because wetting raises C as well as λ, diffusivity first rises with water content and then levels off or falls. Wet, poorly drained soils warm slowly in spring because of their high heat capacity and evaporative cooling, one of the less obvious benefits of drainage.
Surface temperature varies roughly sinusoidally over the day and the year. In a uniform soil the amplitude decays exponentially with depth, A(z) = A0 e^(−z/D), and the wave is delayed by a time z/(Dω), where ω is the angular frequency and the damping depth D = √(2κ/ω). Worked example: with κ = 5 × 10⁻⁷ m²/s and the daily cycle (ω = 2π/86,400 s = 7.27 × 10⁻⁵ s⁻¹), D = 0.117 m. A daily surface amplitude of 10 °C shrinks to 0.77 °C at 30 cm depth, and the maximum arrives about 9.8 hours after the surface maximum. Because D scales with the square root of the period, the annual damping depth is √365 ≈ 19 times the daily one, which is why the temperature a few metres down is nearly constant through the year. Soil temperature is managed with mulches (organic mulches cool and buffer the surface; clear plastic warms it most; black plastic warms less but suppresses weeds), with residue and tillage choices (residue-covered no-till soils warm more slowly in spring), with drainage and with irrigation timing. Soil solarisation, covering moist soil with clear plastic in the hottest season, raises temperatures enough to reduce some soil-borne pathogens and weed seeds.
The chemically active fraction is the colloids, particles below about 1 µm: clay minerals, oxides and humus. Kaolinite is a 1:1 layer silicate (one silica tetrahedral sheet to one alumina octahedral sheet) held by hydrogen bonds; it does not swell and has low CEC, often quoted at about 3 to 15 cmol(+)/kg. Smectites such as montmorillonite are 2:1 with expanding interlayers; they shrink and swell strongly and have CEC around 80 to 120. Illite is 2:1 but potassium holds its layers shut, so it does not expand and its CEC is intermediate. Vermiculite is 2:1, limited-expanding, with very high CEC. Humus has the highest CEC per unit mass, commonly 150 to 300 or more, mostly pH-dependent. Ranges differ between textbooks; learn the order. Permanent negative charge comes from isomorphous substitution (Al³⁺ for Si⁴⁺, or Mg²⁺ for Al³⁺, during crystal formation); pH-dependent charge comes from broken edges, oxides and humus. Exchangeable cations are displaced by others in chemically equivalent amounts. CEC is reported in cmol(+)/kg, equal to the older meq/100 g. Replacing power follows the lyotropic series Al³⁺ > Ca²⁺ > Mg²⁺ > K⁺ ≈ NH₄⁺ > Na⁺. Base saturation is the share of CEC held by Ca²⁺, Mg²⁺, K⁺ and Na⁺: 12 cmol(+)/kg of bases on a CEC of 20 is 60 percent.
pH is the negative base-10 logarithm of hydrogen-ion activity, so pH 5 has ten times the H⁺ activity of pH 6. It is measured with a glass electrode in a suspension whose ratio and solution vary: 1:1 soil:water is common in the USA, 1:2.5 in India and many other countries, 1:5 in Australia, and 0.01 M CaCl₂ or 1 M KCl in parts of Europe and Australia. Salt suspensions usually read lower than water, often by about half a unit, so report the method. Active acidity is H⁺ in solution; exchangeable acidity is H⁺ and Al³⁺ on exchange sites; reserve acidity adds the non-exchangeable, pH-dependent part. The reserve is many times the active acidity, so soils resist pH change (buffering) and lime requirement depends on CEC and organic matter, not pH alone. Soils acidify through leaching of bases, acid-forming nitrogen fertilisers, removal of bases in crops and oxidation of sulphides. Calcareous soils sit up to about pH 8.3 (calcium carbonate with atmospheric CO₂); sodic soils exceed 8.5 and can reach 10 as sodium carbonate hydrolyses. Most nutrients are near best availability for most crops between about pH 6 and 7.5. Phosphorus is fixed by iron and aluminium in acid soils and by calcium in calcareous soils; iron, manganese, zinc and copper become less available as pH rises; molybdenum less available as it falls; and aluminium and manganese toxicity appear in strongly acid soils.
Salinity is measured as the electrical conductivity (EC) of the soil solution, because conductance rises almost in proportion to dissolved salt concentration at the levels that matter for crops. EC is reported in deci-siemens per metre (dS/m), numerically equal to the older millimho per centimetre (mmho/cm), and is standardised to 25 °C because conductivity rises roughly 2 percent per degree. The reference measurement is ECe, the EC of the extract of a saturated soil paste, because the saturation water content is related to field water contents in a roughly constant way across textures. Routine laboratories often measure EC1:2 or EC1:5 extracts instead; conversion to ECe depends on texture and salt type and must be calibrated locally. Rough relations, which vary with the salt mixture, are total dissolved solids (mg/L) ≈ 640 × EC (dS/m) (factors from about 550 to 700 are in use), total cations (meq/L) ≈ 10 × EC, and osmotic potential (kPa) ≈ −36 × EC. In the field, salinity is mapped with electromagnetic induction (EM) sensors and four-electrode probes and calibrated against laboratory ECe. Crop response to salinity is commonly described by the Maas-Hoffman threshold-slope model, relative yield = 100 − b (ECe − a) for ECe above the threshold a, with slope b in percent per dS/m; the crop parameters are tabulated in FAO Irrigation and Drainage Paper 29 and similar sources.
Fertility is a soil's capacity to supply nutrients in adequate amounts and balance; productivity is its capacity to produce a crop under given management, including water, structure and climate. Seventeen elements are generally accepted as essential: C, H and O from air and water; primary macronutrients N, P and K; secondary nutrients Ca, Mg and S; and micronutrients Fe, Mn, Zn, Cu, B, Mo, Cl and Ni (older lists give sixteen, before nickel was added). Liebig's law of the minimum says growth is controlled by the scarcest factor. Nutrients reach roots by mass flow (important for nitrate, calcium, sulphate), diffusion (phosphate and potassium, which move only millimetres) and root interception. Soil tests (for example Olsen bicarbonate phosphorus for neutral to alkaline soils, Bray for acid soils, ammonium acetate potassium; methods and critical levels vary by country) guide fertiliser rates. The engineer meets fertility in fertigation design, placement accuracy of drills and spreaders, variable-rate application and nutrient losses in drainage water.
Sodicity is enough exchangeable sodium to damage structure. On the soil it is the exchangeable sodium percentage, ESP = (exchangeable Na⁺/CEC) × 100, both in cmol(+)/kg; 3.6 cmol(+)/kg of sodium on a CEC of 20 gives ESP 18. In water and extracts it is the sodium adsorption ratio, SAR = Na⁺/√((Ca²⁺ + Mg²⁺)/2) with concentrations in meq/L; its units, (mmol/L)^0.5, are normally not written. Convert mg/L to meq/L by dividing by equivalent weight: 23 for Na⁺, 20.04 for Ca²⁺ and 12.15 for Mg²⁺. Example: Na⁺ 20, Ca²⁺ 4 and Mg²⁺ 2 meq/L give SAR = 20/√3 = 11.55; forgetting the division by 2 gives the wrong 8.16. The US Salinity Laboratory (Handbook 60) relation between solution SAR and the equilibrium ESP is ESR = −0.0126 + 0.01475 SAR, where ESR = ESP/(100 − ESP), so ESP = 100 ESR/(1 + ESR), about 13.6 percent for SAR 11.55; it is empirical and soil-specific. Residual sodium carbonate, RSC = (CO₃²⁻ + HCO₃⁻) − (Ca²⁺ + Mg²⁺) in meq/L, flags waters that precipitate calcium and magnesium as carbonates and leave sodium dominant; USSL guidance rates RSC above 2.5 meq/L unsuitable, 1.25 to 2.5 marginal and below 1.25 probably safe. Adjusted SAR and SAR° allow for calcium precipitation or dissolution. Damage depends on salinity too: saline water keeps clay flocculated, while the same SAR in very pure water disperses it, so FAO Paper 29 judges infiltration hazard by SAR and ECw together.
The classic USSL (Handbook 60) classification, still the basis of most textbooks, uses ECe, ESP and pH. Saline soils have ECe above 4 dS/m, ESP below 15 and pH usually below 8.5; they often show a white salt crust ('white alkali' in old terms, solonchak), are well flocculated and permeable, and crops suffer from osmotic stress and specific ion toxicity. Sodic (alkali) soils have ECe below 4 dS/m, ESP above 15 and pH commonly above 8.5, up to 10; dispersed organic matter can give a dark surface ('black alkali', solonetz in some usage), and the soils are dense, slowly permeable, crusting and hard, with columnar subsoil structure. Saline-sodic soils have ECe above 4 and ESP above 15, with pH usually below 8.5; they behave like saline soils while the salts remain, but if they are leached with good water and no calcium is supplied they turn sodic. The limits are conventions. Some authorities use SAR 13 in place of ESP 15; Australian practice treats ESP above 6 as sodic; and the 4 dS/m limit is often lowered (2 dS/m is used by some authorities) because sensitive crops suffer well below it. State the classification you use. Salt sources include weathering, saline groundwater rising by capillarity when the water table is shallow, irrigation water itself, and seawater intrusion. Secondary salinisation in canal commands is most often caused by a rising water table from seepage and over-irrigation without drainage.
Saline soils are reclaimed by leaching with water in excess of crop need, which requires natural or artificial drainage to carry the leachate away and keep the water table deep. Intermittent ponding or sprinkling removes more salt per unit water than continuous ponding. Sodic soils cannot be reclaimed by leaching alone: calcium must replace exchangeable sodium, and the displaced sodium must then be leached. Gypsum (CaSO₄·2H₂O) is the commonest amendment; calcium chloride is faster but costlier. In calcareous sodic soils, acid-forming amendments (elemental sulphur, sulphuric acid, iron sulphate, pyrite) dissolve native calcium carbonate to release calcium; they do not work in non-calcareous soils. Organic manures and press mud help structure and release CO₂ that dissolves carbonates. Gypsum requirement: GR (cmol(+)/kg) = (ESPinitial − ESPfinal)/100 × CEC. Example: CEC 20, ESP lowered from 30 to 10, GR = 0.20 × 20 = 4 cmol(+)/kg = 0.04 eq/kg. Gypsum dihydrate's equivalent weight is 172.17/2 = 86.09 g/eq, so pure gypsum needed is 3.44 g/kg of soil. For 15 cm at ρb 1.5 g/cm³ (2.25 × 10⁶ kg/ha) that is about 7.75 t/ha pure, or 9.69 t/ha at 80 percent purity. The usual slips are using the molar mass 172, which doubles the answer, and ignoring purity. Practitioners often add an efficiency factor (about 1.1 to 1.3) because exchange is incomplete, and some programmes apply only part of the GR at first; follow local recommendations. One tonne of elemental sulphur (equivalent weight 16) is equivalent to about 5.38 t of gypsum dihydrate.
Strongly acid soils limit crops through aluminium and manganese toxicity and deficiencies of calcium, magnesium, phosphorus and molybdenum. They are corrected by liming. Lime requirement is estimated by buffer methods (for example the SMP buffer, Shoemaker-McLean-Pratt, widely used in North America) or, in highly weathered tropical soils, from exchangeable aluminium, often applying lime equivalent to 1.5 to 2 times the exchangeable Al (Kamprath's approach). Worked example: to neutralise 2 cmol(+)/kg of exchangeable acidity in the top 15 cm at ρb 1.3 g/cm³ (1.95 × 10⁶ kg/ha), CaCO₃ needed is 0.02 eq/kg × 50 g/eq = 1.0 g/kg, or 1.95 t/ha. Liming materials are compared by calcium carbonate equivalent (CCE): pure CaCO₃ is 100, dolomite CaMg(CO₃)₂ about 109, calcium hydroxide about 135, and calcium oxide about 178, because their equivalent weights are smaller. Effectiveness also depends on fineness, so many standards combine CCE with a fineness factor. Gypsum is not a liming material: it supplies calcium and can lessen subsoil aluminium toxicity, but it does not neutralise acidity. Acid sulphate soils form where sulphide (mainly pyrite) in waterlogged coastal and estuarine sediments is exposed to air by drainage or excavation and oxidises to sulphuric acid, driving pH below about 4 and releasing aluminium, iron and heavy metals into drainage water. They are best managed by keeping the sulphidic layers wet (maintaining a high water table), avoiding deep drainage and excavation, and liming where disturbance is unavoidable. This is a case where routine drainage design can cause serious environmental damage.
Learn the mass-volume relations until they are automatic: ρb, ρs, porosity, gravimetric and volumetric water, and the conversion of a per-kilogram quantity to tonnes per hectare through soil mass. Practise the available-water, Darcy, Horton and gypsum-requirement calculations by hand, writing units on every line. Memorise the USDA particle-size limits, the classic salinity and sodicity limits (ECe 4 dS/m, ESP 15, pH 8.5) with their variants, the SAR formula in meq/L and the 12 Soil Taxonomy orders. In the laboratory, run one hydrometer analysis, one pressure-plate curve and one double-ring test yourself; numbers you have measured stay with you.
A field engineer five years in uses this subject to design and troubleshoot. Typical tasks: reading a soil survey report to set root-zone available water and irrigation interval for a drip or sprinkler design; running double-ring infiltrometer tests to choose a sprinkler application rate; sampling bulk density and penetration resistance to show a farmer where traffic has compacted a field; interpreting EC and SAR of a tubewell water and deciding whether gypsum, blending or a different crop is needed; and computing gypsum or lime quantities for a reclamation subsidy. The key decision is how many measurements are enough, because K, infiltration and salinity vary by a factor of ten across one field.
With 25 years' experience the engineer manages programmes rather than fields: salinity and waterlogging control across a canal command, reclamation of sodic land at district scale, or land-capability-based planning for a watershed project. Decisions include where subsurface drainage pays and where conjunctive use or crop change is better, how to phase gypsum supply and leaching water, how to set up long-term monitoring of water tables and ECe, and how to stop secondary salinisation from returning after the project ends. This person also judges contractors' soil reports and knows when a pedotransfer estimate is good enough and when field measurement is needed.
A researcher with long field experience works on the open questions: scaling hydraulic properties measured on cores and rings to fields and catchments; preferential flow through macropores and its effect on leaching and contaminant travel times; sodicity and dispersion under low-salinity or treated wastewater irrigation; structure dynamics under controlled traffic and conservation agriculture; digital soil mapping and proximal sensing (electromagnetic induction, gamma, visible and near-infrared spectroscopy) for precision management; and coupled water, heat and salt transport models. The task is to design experiments and models whose uncertainty is stated honestly and whose results change practice.
This manual does not yet cover soil mineralogy beyond the main clay groups, soil biology and organic-matter dynamics in depth, nutrient cycling and fertiliser recommendation systems (it gives fertility basics only), solute transport theory (convection-dispersion equation, breakthrough curves), the auger-hole and Guelph permeameter equations, detailed pedotransfer functions, or the national soil classification systems of Australia, Canada, China, Brazil and others. Crop salt-tolerance thresholds and slopes, texture-specific default water constants and lime-requirement buffer tables are referred to but must be taken from the named sources. Soil strength, consolidation, Proctor compaction and engineering classification are in the separate soil mechanics subject, and leaching requirement and drainage design in drainage engineering.
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