Environmental Geosciences II: Weathering, Erosion, Transport and Deposition, Soil Formation, Clay Minerals, Soil Physical and Chemical Properties, Soil Types and Cation Exchange Capacity, the Geochemical Classification and Abundance of Elements, Partitioning and Recycling, and Palaeoclimate

Hans Jenny (1941) expressed soil as a function of five state factors, s = f(cl, o, r, p, t): climate, organisms, relief, parent material and time, the "clorpt" equation. Climate is the most important over large areas, because temperature and rainfall set the rates of weathering, leaching and organic-matter turnover. Soils develop horizons: an O horizon of organic litter, an A horizon of mixed humus and mineral matter, an E horizon from which clay, iron and aluminium are leached (eluviated), a B horizon where they accumulate (illuviated), a C horizon of weathered parent material and the R layer of bedrock.

The Indian Council of Agricultural Research recognises eight major soil groups: alluvial soils of the Indo-Gangetic plain and deltas, the most extensive and agriculturally important; black soils (regur) formed from the Deccan basalts, rich in smectite, highly retentive of moisture, swelling when wet and cracking deeply when dry, and suited to cotton; red soils on the crystalline rocks of the peninsula, coloured by ferric oxides; laterite soils of the high-rainfall Western Ghats, Odisha and the north-east; arid (desert) soils of Rajasthan; forest and mountain soils; saline and alkaline soils; and peaty and marshy soils of Kerala's kari lands and the coastal wetlands.

Clay minerals are layer silicates built of sheets of silica tetrahedra (T) and aluminium or magnesium octahedra (O). Isomorphous substitution, such as Al³⁺ for Si⁴⁺ in the tetrahedral sheet or Mg²⁺ for Al³⁺ in the octahedral sheet, leaves a permanent negative charge that is balanced by exchangeable cations. They are identified chiefly by X-ray diffraction, from the basal spacing between layers and how it changes when the sample is saturated with glycol or heated, supported by differential thermal analysis, infrared spectroscopy and electron microscopy.

The physical properties of soil begin with texture, the proportions of sand (2 to 0.05 mm on the USDA scale), silt (0.05 to 0.002 mm) and clay (below 0.002 mm), read from the textural triangle, and structure, the arrangement of particles into aggregates. Bulk density is the dry mass per unit of total volume, usually 1.1 to 1.6 g/cm³, rising with compaction; particle density, the density of the solids alone, is close to 2.65 g/cm³ for most mineral soils; and porosity = 1 − (bulk density ÷ particle density), so a soil of bulk density 1.325 g/cm³ has a porosity of 1 − 0.5 = 50%. Water is held between field capacity, after free drainage, and the permanent wilting point, and the difference is the water available to plants, greatest in loams.

The chemical properties are pH, electrical conductivity (a measure of soluble salts), organic carbon and, above all, the cation exchange capacity: the total quantity of exchangeable cations a soil can hold, expressed in centimoles of positive charge per kilogram (numerically equal to milliequivalents per 100 g). CEC comes from the permanent charge of clays and the pH-dependent charge of humus and oxides, so it is controlled by mineralogy and organic matter. Humus has the highest CEC, around 200 cmol(+)/kg or more, smectite around 100 and kaolinite around 10. A soil with 30% smectite, 10% kaolinite and 2% humus therefore has a CEC of about 0.30 × 100 + 0.10 × 10 + 0.02 × 200 = 30 + 1 + 4 = 35 cmol(+)/kg. Base saturation is the percentage of the CEC occupied by the basic cations Ca²⁺, Mg²⁺, K⁺ and Na⁺, falling as soils acidify. CEC governs fertility, the retention of heavy metals and pesticides, and a soil's capacity to buffer acid rain.

Abundances differ sharply between reservoirs. By mass the bulk Earth is dominated by iron (about 32%), oxygen (about 30%), silicon (about 15%) and magnesium (about 14%), iron being so abundant because of the core. The crust, by contrast, is led by oxygen, silicon and aluminium. Sea water, apart from hydrogen and oxygen, is dominated by chloride (about 19.4 g/kg) and sodium (about 10.8 g/kg), followed by sulphate, magnesium, calcium and potassium, for a total salinity of about 35 g/kg. The biosphere is built mainly of the light elements: the human body is about 65% oxygen, 18% carbon, 10% hydrogen and 3% nitrogen by mass, with calcium and phosphorus next.

How an element is partitioned during weathering and sedimentation depends largely on its ionic potential, the ratio of its charge to its ionic radius. Ions of low ionic potential, such as Na⁺, K⁺, Ca²⁺ and Mg²⁺, stay in solution as cations and are carried to the sea; those of intermediate ionic potential, such as Al³⁺, Fe³⁺ and Ti⁴⁺, hydrolyse and precipitate as insoluble hydroxides (the hydrolysates) and stay behind in soils and laterites; and those of high ionic potential, such as C, N, S and P, form soluble oxyanions such as carbonate, nitrate, sulphate and phosphate. Redox conditions add a second control, iron and manganese being mobile when reduced and immobile when oxidised. Elements are recycled geochemically by the rock cycle: weathering releases them, rivers carry them to the ocean, sedimentation and burial lock them into rock, and subduction, metamorphism and volcanism return them to the surface over tens to hundreds of millions of years.

Palaeoclimate is reconstructed from proxies, natural archives that record past conditions. Ice cores preserve both the isotopic composition of the snow, whose δ¹⁸O and δD become more negative in colder periods, and bubbles of ancient air, giving past carbon dioxide and methane directly; the EPICA Dome C core from Antarctica extends about 800,000 years and shows carbon dioxide varying between roughly 180 ppm in glacial periods and 280 ppm in interglacials, far below today's levels. In marine sediments the δ¹⁸O of foraminiferal shells rises when the climate is cold and ice sheets lock up light ¹⁶O. Tree rings, corals, pollen in lake and bog sediments, cave speleothems and annually layered lake varves fill in finer detail.

Over the last few million years the glacial-interglacial cycles have been paced by the Milankovitch cycles, periodic changes in the Earth's orbit that redistribute sunlight by season and latitude: eccentricity, the shape of the orbit, with periods of about 100,000 and 400,000 years; obliquity, the tilt of the axis between about 22.1° and 24.5°, with a period of about 41,000 years; and precession, the wobble of the axis, with periods of about 19,000 to 23,000 years. The orbital changes are small, and feedbacks from greenhouse gases and ice albedo amplify them. The Last Glacial Maximum was about 21,000 years ago, when sea level was more than 100 m lower, and the present interglacial, the Holocene, began about 11,700 years ago. The palaeoclimate record matters because it shows how sensitive the climate is to carbon dioxide and how fast it has changed before.