Forestry & Forest Sciences · Foundations · also Forest Soils, Soil Science, Forest Soils and Nutrient Management
Soil formation under forest, the forest floor, soil classification and site quality, nutrient cycling, and conserving and reclaiming forest and degraded soils.
In the exams: IFoS Forestry Paper I Section B (forest soils, soil conservation and watershed management); ICAR AIEEA-PG Group 09 (forest soils, classification and conservation); ICAR AICE-JRF 59 Unit 8
Forest soils are the foundation of forest productivity and of many forest services. They store and supply water and nutrients, anchor trees, host the fungi and bacteria with which roots form partnerships, hold more carbon than the trees above them in many forests, and filter the water that leaves forested catchments. Forest soils differ from agricultural soils in important ways: they keep a continuous organic layer, the forest floor, on top of the mineral soil; they are rarely ploughed, so their horizons stay distinct; deep and long-lived roots and tree uprooting shape them over centuries; and many are acid and low in available nutrients, because forests have often been left on land that was too steep, too poor, too wet or too remote to farm.
This manual begins with how soils form under forest, through Jenny's five factors and the main pedogenic processes such as podzolisation, laterisation and gleying, and with the forest floor and its humus forms, mor, moder and mull. It then covers physical properties (texture, structure, bulk density and water holding) and chemical properties (pH, cation exchange and base saturation), followed by the two classification systems used worldwide, USDA Soil Taxonomy and the World Reference Base for Soil Resources. Later chapters treat nutrient cycling in forest ecosystems, soil fertility and the fertilisation of plantations, soil–site relationships and site quality, problem soils (saline, sodic, lateritic, acid and waterlogged), soil erosion under forest and after disturbance, and soil conservation measures on forest land.
Soil physics and chemistry are taught at greater depth in soil science courses, and watershed hydrology in the forest hydrology subject; this manual keeps to what foresters need to read a soil profile, judge a site, plan operations that protect soil, and manage plantations and restoration on difficult land.
Examinations ask about the soil-forming factors and processes, humus forms, horizon nomenclature, texture classes and bulk density, CEC and base saturation, the soil orders and reference groups found under the world's forests, nutrient cycles, site index, the definitions and reclamation of saline and sodic soils, and erosion control. Typical calculations include porosity from bulk density, soil organic carbon stock, available water, base saturation, exchangeable sodium percentage and sodium adsorption ratio, litter decomposition constants, fertiliser quantities and soil loss by the Universal Soil Loss Equation. Civil-service forest examinations, postgraduate and doctoral forestry examinations, and foresters' licensure and national forestry engineer examinations all include this material.
Hans Jenny expressed the classical view in 1941: a soil is a function of five state factors, climate, organisms, relief (topography), parent material and time, often written S = f(cl, o, r, p, t). Climate governs weathering and leaching through temperature and precipitation: warm, wet climates weather minerals quickly and leach bases; cold climates slow decomposition and favour organic accumulation; dry climates leave salts and carbonates. Organisms, above all the vegetation, add organic matter, cycle nutrients, acidify or enrich the surface and mix the soil. Under forest, roots penetrate deeply, mycorrhizal fungi weather minerals, earthworms mix litter into the soil in base-rich deciduous forests, and the uprooting of trees by wind creates pit-and-mound microrelief that overturns and mixes horizons over centuries. Conifer and heath litter is acid and slow to decompose, while litter of many broadleaves is richer in bases and decomposes faster, so vegetation change can change soils. Relief controls drainage, erosion and deposition: ridges and upper slopes have thinner, drier, more eroded soils; lower slopes and hollows receive water, sediment and nutrients and are deeper and moister, a sequence called a catena. Aspect affects temperature and moisture. Parent material, the rock or sediment in which the soil forms, determines texture and nutrient reserves: soils on granite and sandstone tend to be sandy and acid, those on basalt, limestone and shales finer and richer in bases, those on volcanic ash distinctive in their high water-holding and phosphorus fixation. Time determines how far processes have progressed: young soils on recent alluvium, glacial deposits or landslides show little horizon development, whereas old landscapes in stable tropical and subtropical regions carry deeply weathered soils with few weatherable minerals left. Humans are now often treated as a sixth factor.
Simonson described soil formation as the sum of four kinds of process: additions (organic matter, dust, deposition), losses (leaching, erosion, gases), transformations (weathering of minerals, humification) and translocations (movement of clay, iron, aluminium, organic matter or salts within the profile). Combinations of these produce characteristic processes. Podzolisation, typical of cool, humid climates with acid litter, sandy parent materials and coniferous or heath vegetation, involves organic acids from the forest floor that mobilise iron and aluminium as organic complexes and carry them downward; the upper mineral layer is left bleached and ash-grey (the eluvial E horizon), and the complexes accumulate below in a dark, reddish spodic B horizon. Ferralitisation or laterisation, in hot, humid tropics on old surfaces, removes silica and bases by intense weathering and leaching, leaving a residue of iron and aluminium oxides and kaolinite; the soils are deep, red or yellow, well structured but low in nutrients and in cation exchange capacity, and under fluctuating water tables iron can concentrate as plinthite, which hardens irreversibly to ironstone (laterite) on exposure and drying. Lessivage (clay illuviation) moves fine clay from upper horizons into a clay-enriched argillic B horizon, typical of many temperate and subtropical forest soils. Gleying under waterlogging reduces iron to the mobile ferrous form, giving grey or bluish colours and, where the water table fluctuates, rusty mottles. Calcification and salinisation occur in drier climates, melanisation (dark organic enrichment) under grasslands. Soil horizons are named with master letters: O (organic layers), A (mineral topsoil with humus), E (eluvial, leached), B (subsoil of accumulation or alteration), C (weathered parent material) and R (hard bedrock), with suffixes such as h (humus), s (sesquioxides), t (clay), g (gleying) and k (carbonates). Forest soils typically have distinct O horizons and, because they are not ploughed, sharper horizon boundaries than farmed soils.
The forest floor is the layer of organic matter lying on the mineral soil, made of fallen leaves, needles, twigs, bark, fruits, dead roots and their decomposition products. It is usually divided into three layers. The L layer (litter, Oi in USDA notation) contains recently fallen, little-altered material whose origin is easily recognised. The F layer (fermentation, Oe) contains partly decomposed material, fragmented and often matted with fungal hyphae, in which plant structures are still partly recognisable. The H layer (humus, Oa) contains well-decomposed, amorphous organic matter whose origin cannot be recognised. Decomposition proceeds through leaching of soluble compounds, fragmentation by soil animals, and chemical breakdown by fungi and bacteria. Its rate depends on climate (warmth and moisture speed it, waterlogging and cold slow it), on litter quality (litter with low carbon-to-nitrogen and lignin-to-nitrogen ratios and high base content decomposes fast; needles rich in lignin, resins and phenolics decompose slowly) and on the decomposer community. At steady state the forest floor mass reflects the balance of inputs and decomposition. Olson's decomposition constant k = annual litterfall / forest floor mass; with litterfall of 4 t ha⁻¹ yr⁻¹ and a forest floor of 20 t ha⁻¹, k = 0.2 per year, mean residence time 1/k = 5 years, half the material decomposes in about 0.693/k = 3.5 years and 95 percent in about 3/k = 15 years. Values of k exceed 1 in many lowland tropical rainforests, where litter rarely accumulates, and fall well below 0.1 in boreal coniferous forests, where thick forest floors build up. The forest floor protects mineral soil from raindrop impact and erosion, stores water and nutrients, insulates soil, provides a seedbed (favourable for some species, inhibitory for others) and is a large carbon store, which is lost by intense fire and by some site preparation practices.
The Danish forester P.E. Müller, in work published around 1880, distinguished forest humus forms by their morphology and biology, a classification still used and refined today. Mull is formed where decomposition is rapid and soil animals, especially earthworms, are active. Litter disappears within a year or two, there is no distinct F or H layer, and organic matter is intimately mixed with mineral soil into a dark, crumb-structured A horizon with a gradual lower boundary. Mull occurs under deciduous forests on base-rich, well-drained soils, with pH usually above about 5, and bacteria play a large role in decomposition. Mor (raw humus) is formed where decomposition is slow and dominated by fungi, under conifers or heath on acid, nutrient-poor parent materials in cool climates. It has thick, matted F and H layers, often several centimetres or more, sharply separated from the mineral soil, which is often a bleached podzolic E horizon; mor is strongly acid, with low pH and high C:N ratios, and nitrogen is released slowly, much of it in organic forms accessible to ectomycorrhizal fungi. Moder is intermediate: litter is fragmented and transformed largely by soil arthropods such as mites and springtails and by enchytraeid worms, whose faecal pellets make up much of the H layer; the transition to mineral soil is more gradual than in mor, and earthworms are scarce. Humus form is a useful field indicator of site quality and nutrient cycling: mull sites are usually more fertile, mor sites poorer and often regenerate less easily, as thick mor can be a poor seedbed for some species. Humus forms change with management: replacement of broadleaves with conifers can shift mull to moder or mor over decades, while liming, mixed planting and some site preparation can reverse it. Modern European humus-form classifications add intermediate forms (such as amphi forms with both mull and mor features) and forms for wet and peaty soils.
Soil texture is the relative proportion of mineral particles of different sizes. In the USDA system, sand is 2 to 0.05 mm, silt 0.05 to 0.002 mm and clay less than 0.002 mm; the international (Atterberg) system sets the sand–silt boundary at 0.02 mm. Particles larger than 2 mm are gravel, stones and boulders, and forest soils, especially on mountains and glacial deposits, often contain many coarse fragments that reduce the volume of fine earth available for water and nutrients. Texture classes are read from the textural triangle: a soil with about 40 percent sand, 40 percent silt and 20 percent clay is a loam; one dominated by sand is a sand or loamy sand; one with more than about 40 percent clay is a clay. In the field, texture is estimated by feel: sand is gritty, silt smooth and floury, clay sticky and plastic and able to form a long ribbon. Texture controls water-holding capacity, drainage, aeration, nutrient retention and workability: sandy soils drain fast and hold few nutrients, clays hold water and nutrients but can be poorly aerated, and loams are intermediate. Texture cannot be changed by management. Soil structure is the arrangement of particles into aggregates (peds), held together by clay, organic matter, iron oxides, fungal hyphae, roots and the glues produced by microbes. Types include granular or crumb (rounded aggregates typical of surface horizons rich in organic matter, as under mull), blocky (angular or subangular, common in B horizons), platy (horizontal plates, often from compaction), prismatic and columnar (vertical prisms, the latter with rounded tops typical of sodic soils), and structureless conditions, single-grain (loose sand) or massive (a coherent mass without aggregates). Good structure creates large pores for drainage and aeration and small pores for water storage, and is damaged by compaction and loss of organic matter.
Bulk density (BD) is the mass of oven-dry soil divided by its total volume, including pores: a core of 100 cm³ holding 120 g of dry soil has a bulk density of 1.2 g cm⁻³. Particle density, the density of the solid particles alone, is about 2.65 g cm⁻³ for most mineral soils. Total porosity = 1 − BD/particle density, so a soil with BD 1.32 g cm⁻³ has about 50 percent pore space. Undisturbed forest topsoils rich in organic matter commonly have low bulk densities, often below 1.0 to 1.2 g cm⁻³, and organic forest floor layers much lower still, while compacted subsoils and skid trails can exceed 1.6 g cm⁻³, high enough to restrict root growth in many soils; the threshold depends on texture, being higher in sands than in clays. Heavy machinery used in harvesting and site preparation compacts soil, especially when wet, increases bulk density, reduces infiltration and aeration, and can depress tree growth for decades; ruts channel runoff and cause erosion. Bulk density is also needed to convert concentrations to stocks. Soil organic carbon stock (t ha⁻¹) = organic carbon (%) × bulk density (g cm⁻³) × depth (cm), corrected for coarse fragments; for 2 percent carbon, BD 1.3 and 30 cm depth, the stock is 78 t C ha⁻¹. The mass of soil in one hectare to 15 cm at BD 1.3 is about 2 million kilograms. Soil water is described by its energy status. Field capacity is the water held after free drainage, taken as the water content at a suction of about −10 to −33 kPa; the permanent wilting point is taken as −1500 kPa; the difference is available water capacity (AWC). Gravimetric water content (mass of water per mass of dry soil) is converted to volumetric content by multiplying by bulk density. For a soil with field capacity 25 percent and wilting point 12 percent by mass, BD 1.4 and rooting depth 50 cm, AWC = 0.13 × 1.4 × 500 mm, about 91 mm. Sandy soils hold the least available water, silt loams the most; organic matter increases it.
Soil pH is the negative logarithm of hydrogen ion activity in the soil solution, measured in a suspension of soil in water or in a dilute salt solution (calcium chloride or potassium chloride), which gives lower values; the method must be stated when comparing results. A difference of one pH unit is a tenfold difference in hydrogen ion activity. Many forest soils are acid, with pH commonly between about 4 and 5.5 in the mineral soil under conifers and on siliceous parent materials, and lower still in mor humus, while forest soils on limestone or base-rich volcanic rocks can be neutral or alkaline. Acidity arises from the leaching of base cations by rainfall, especially where precipitation exceeds evapotranspiration; from organic acids produced by litter decomposition; from carbonic acid from root and microbial respiration; from uptake of cations by trees in excess of anions, which releases hydrogen ions; from nitrification; and from atmospheric deposition of sulphuric and nitric acids, which acidified forest soils in parts of Europe, eastern North America and East Asia in the late twentieth century. Harvesting removes base cations stored in wood and especially in bark, branches and foliage, adding to acidification on poorly buffered soils. pH controls nutrient availability: below about pH 5 to 5.5, exchangeable aluminium rises and can be toxic to roots of sensitive species, phosphorus is fixed by iron and aluminium, and molybdenum becomes less available; above about pH 7.5, iron, manganese, zinc and phosphorus can become unavailable. Many forest trees are adapted to acid soils and grow well at pH values that would harm crops, and some, such as many pines and ericaceous plants, grow poorly on alkaline soils. Liming raises pH and supplies calcium and magnesium; it has been used to counter acidification in some forests but can disturb soil organisms and mobilise nitrogen, so its use is selective.
Clay minerals and organic matter carry negative charges on their surfaces that attract and hold cations in exchangeable form, protecting them from leaching while keeping them available to roots. Cation exchange capacity (CEC) is the total quantity of exchangeable cations a soil can hold, expressed in centimoles of charge per kilogram (cmolc kg⁻¹, numerically equal to the older milliequivalents per 100 g). It depends on the amount and type of clay and on organic matter. Typical values are roughly 3 to 15 cmolc kg⁻¹ for kaolinite, 10 to 40 for illite, 80 to 150 for smectites such as montmorillonite, 100 to 150 or more for vermiculite and 150 to 300 or more for humus, though values vary with method. Kaolinite and iron and aluminium oxides dominate highly weathered tropical soils, so their CEC is low and depends heavily on organic matter. Part of the charge of organic matter and oxides is pH-dependent, increasing as pH rises, so CEC measured at pH 7 overstates the exchange capacity of acid soils; the effective CEC, the sum of exchangeable cations at the soil's own pH, is more relevant for them. Exchangeable cations are divided into base cations (calcium, magnesium, potassium and sodium) and acid cations (hydrogen and aluminium). Base saturation is the percentage of CEC occupied by base cations: a soil with exchangeable Ca 6, Mg 2, K 0.5 and Na 0.5 cmolc kg⁻¹ and CEC 20 cmolc kg⁻¹ has a base saturation of 9/20 × 100 = 45 percent. Base saturation is used in soil classification, for example to separate Alfisols from Ultisols and Luvisols from Acrisols, and as an index of fertility and of sensitivity to acidification. Soils also have smaller anion exchange capacities, mainly in oxide-rich tropical soils, which retain sulphate and nitrate. Phosphate is held much more strongly by specific adsorption on iron and aluminium oxides and allophane, which is why phosphorus fixation limits plantations on many tropical and volcanic soils.
Soil classification groups soils with similar properties so that knowledge and experience can be transferred between places. Most countries have national systems, but two systems are used worldwide and are named in international work: USDA Soil Taxonomy and the World Reference Base for Soil Resources (WRB). Soil Taxonomy, developed by the United States Department of Agriculture and first published in 1975, is hierarchical with six categories: order, suborder, great group, subgroup, family and series. It classifies soils by measurable properties of diagnostic horizons and soil moisture and temperature regimes. Diagnostic surface horizons (epipedons) include the mollic (thick, dark, base-rich), umbric (dark but base-poor), ochric (light-coloured or thin) and histic (organic). Diagnostic subsurface horizons include the argillic (illuvial clay accumulation), spodic (illuvial organic matter with aluminium and iron), oxic (highly weathered, low-activity clays), cambic (weakly altered), natric (argillic with high sodium) and others. There are twelve orders. Entisols show little horizon development; Inceptisols weak development; Histosols are organic soils (peats); Gelisols have permafrost; Andisols form in volcanic material with andic properties; Spodosols have spodic horizons and are typical of coniferous and heath forests on sandy materials in cool humid climates; Alfisols have argillic horizons and moderate to high base saturation and are common under temperate and subtropical deciduous forests; Ultisols have argillic horizons with low base saturation (less than 35 percent at depth) and are typical of warm, humid forested regions such as the south-eastern United States and parts of the tropics; Oxisols are the deeply weathered soils of humid tropical forests and savannas, with oxic horizons; Mollisols are grassland soils with mollic epipedons; Aridisols are dry soils; and Vertisols are shrink–swell clays that crack in the dry season. Names are formed from syllables: an Udult is an Ultisol of a humid (udic) moisture regime.
The World Reference Base for Soil Resources, developed under the International Union of Soil Sciences with FAO and now in its fourth edition (2022), succeeded the legend of the FAO–UNESCO Soil Map of the World and serves as the international language for soil correlation. It has two levels: 32 Reference Soil Groups (RSGs), defined by diagnostic horizons, properties and materials, and a second level formed by adding principal and supplementary qualifiers. Reference Soil Groups important under forests include Podzols (equivalent in concept to Spodosols), Ferralsols (broadly Oxisols), Acrisols and Alisols (low-base-status clay-illuviation soils, broadly corresponding to Ultisols), Luvisols and Lixisols (higher base status, broadly Alfisols), Cambisols (weakly developed, broadly Inceptisols), Andosols (volcanic, Andisols), Histosols (organic), Cryosols (permafrost, Gelisols), Gleysols (groundwater-saturated) and Stagnosols (perched water), Leptosols (shallow over rock), Arenosols (sands), Nitisols (deep, well-structured tropical clays), Plinthosols (with plinthite or ironstone), Umbrisols and Retisols. Soils of drylands and special conditions include Solonchaks (saline), Solonetz (sodic), Calcisols, Gypsisols, Durisols, Vertisols, Chernozems, Kastanozems and Phaeozems (the last three grassland soils), Fluvisols (alluvial), Regosols, Planosols, Anthrosols and Technosols (human-made). Correlation between systems is approximate because they define classes differently, so equivalences should be given as 'broadly corresponds to'. Broad patterns are useful: boreal forests are dominated by Podzols, Cryosols, Histosols and Cambisols; temperate forests by Cambisols, Luvisols, Podzols and Acrisols; humid tropical forests by Ferralsols, Acrisols and Nitisols; and volcanic regions such as parts of the Andes, Central America, East Africa, Indonesia, the Philippines, Japan and New Zealand by Andosols. Foresters use soil classification to read soil maps, predict productivity and limitations, and transfer silvicultural knowledge between similar soils in different countries.
Nutrient cycling in forests is usually described as three linked cycles. The geochemical cycle covers exchanges between the ecosystem and the outside world: inputs from atmospheric deposition (wet deposition in rain and snow, dry deposition of particles and gases, and cloud water captured by canopies), from the weathering of minerals in soil and rock, and from biological nitrogen fixation; and outputs through leaching to groundwater and streams, gaseous losses (denitrification, volatilisation, fire), erosion and the removal of harvested products. The biogeochemical cycle is the circulation between soil and vegetation: uptake by roots, return to the soil in litterfall, throughfall and stemflow (rain enriched in nutrients washed from leaves and bark), root turnover and dead wood, and release by decomposition and mineralisation. The biochemical or internal cycle is the redistribution within the tree, including resorption of nutrients from senescing leaves before they fall and their reuse in new growth; in mature trees this internal recycling can supply a large share of the annual requirement for nitrogen and phosphorus. As forests age, uptake for new biomass declines relative to recycling, and mature forests tend towards a balance in which inputs roughly equal outputs. The famous Hubbard Brook Ecosystem Study in the north-eastern United States showed this with small catchments: when a catchment was clear-cut and regrowth suppressed with herbicides, stream nitrate concentrations rose many-fold because uptake stopped while mineralisation and nitrification continued, demonstrating the role of vegetation in retaining nutrients. Distribution of nutrient stocks differs between biomes. In boreal and cool temperate forests a large share of nitrogen and other nutrients is in the forest floor and soil organic matter, released slowly. In lowland tropical rainforests on highly weathered soils, a large share of some nutrients, especially calcium, magnesium and potassium, is held in the living biomass, and cycling through litter is rapid and tight, with surface root mats and mycorrhizae capturing nutrients as they are released.
Nitrogen enters forests mainly by deposition and biological fixation, since rocks contain little. It cycles through mineralisation of organic nitrogen to ammonium by microbes, nitrification of ammonium to nitrate by specialised bacteria and archaea, uptake by roots and mycorrhizae (which can also take up organic nitrogen), immobilisation in microbial biomass, and losses through nitrate leaching and denitrification. In many temperate and boreal forests nitrogen limits growth, while chronic deposition of nitrogen from fuel combustion and agriculture has led to 'nitrogen saturation' in some forests of Europe and North America, with increased nitrate leaching, soil acidification and nutrient imbalances. Phosphorus comes mainly from weathering of apatite and other primary minerals; as soils age over thousands of years, primary phosphorus is depleted and the remainder becomes occluded in iron and aluminium oxides or bound in organic forms, so very old, highly weathered soils of the tropics and of ancient landscapes such as much of Australia and southern Africa are phosphorus-limited. Harvesting removes nutrients in the exported biomass. Because foliage, twigs and bark have much higher nutrient concentrations than stemwood, whole-tree harvesting, which removes crowns as well as stems, can remove two to three times more nitrogen, phosphorus, potassium and calcium than stem-only harvesting for a modest gain in biomass. Short rotations remove nutrients more often. Site preparation by burning or by windrowing slash and topsoil can remove further nutrients and organic matter. On poor soils, repeated short-rotation harvests can therefore deplete nutrient capital unless inputs replace it, which is why retaining harvest residues on site, avoiding topsoil displacement, longer rotations, fertilisation and inclusion of nitrogen-fixing species are recommended in plantation management. Nutrient budgets, comparing inputs with outputs over a rotation, are used to judge the sustainability of harvesting regimes, including the harvesting of residues for bioenergy.
Forest soil fertility is the capacity of the soil to supply nutrients and water for tree growth. In natural forests it is rarely managed directly, but in intensively managed plantations, especially fast-growing pines, eucalypts, acacias, poplars and Douglas-fir, fertilisation is routine where it pays. Diagnosis combines several lines of evidence. Visual symptoms, such as yellowing (chlorosis) of older needles in nitrogen deficiency, purpling in phosphorus deficiency, or dieback of leaders in boron deficiency, are useful but appear only when deficiency is severe and can be confused with other stresses. Foliar analysis compares nutrient concentrations in a standard sample (a specific foliage age and crown position, collected in a specified season) with critical or optimum ranges established for the species and region, and is the most widely used diagnostic method for plantations; ratios between nutrients, such as N:P, help identify imbalances. Soil analysis measures available nutrients by standard extractions, but relations between soil tests and tree response are weaker for trees than for crops because trees root deeply, rely on mycorrhizae and recycle nutrients internally. Field fertiliser trials are the definitive test and underlie the fertiliser recommendations used in major plantation regions. Site classification by soil type, parent material and landform, combined with trial results, allows prescriptions to be applied across estates. Growth responses depend on stand age and condition: young plantations often respond to phosphorus at establishment on deficient soils, while older stands of some conifers respond to nitrogen in mid-rotation after crown closure, once internal and soil supplies no longer meet demand. Competing vegetation reduces responses, so weed control and fertilisation are often combined.
Well-documented examples of plantation fertilisation are found on several continents. Phosphorus at planting has transformed productivity of pine plantations on the phosphorus-deficient soils of the south-eastern United States coastal plain, of radiata pine on poor soils in Australia and New Zealand and of eucalypt plantations in Brazil and elsewhere in South America and Africa; nitrogen in mid-rotation is widely used in Douglas-fir plantations of the north-western United States and in conifer forests of Scandinavia, where single applications commonly increase volume growth for several years; boron is applied on soils where leader dieback occurs. Fertiliser rates are always expressed as elemental nutrient or oxide per hectare and converted to product. Urea contains 46 percent nitrogen, so 200 kg N ha⁻¹ requires 200 / 0.46, about 435 kg of urea. Phosphorus is often quoted as P₂O₅, which contains 43.6 percent P, so 100 kg of P₂O₅ supplies 43.6 kg of P; triple superphosphate containing 46 percent P₂O₅ supplies about 20 percent P, so 50 kg P ha⁻¹ requires about 250 kg of product. Diammonium phosphate (18-46-0) supplies both N and P. Application methods include spot or band placement at planting, broadcast application by ground machinery and aerial application in established stands. Fertilisation has environmental risks: nitrogen and phosphorus can be lost to streams if applied near water, before heavy rain or at rates beyond uptake, and nitrous oxide emissions add to greenhouse-gas budgets; best practice keeps buffer zones along streams and applies only proven rates. Alternatives and complements include retaining harvest residues, including nitrogen-fixing species in mixtures (such as acacias with eucalypts), applying wood ash to return base cations, and using organic residues such as biosolids from wastewater treatment where regulations permit, with attention to contaminants. Fertiliser decisions should rest on economic analysis of the response, since fertiliser is costly and responses vary widely by site.
Site quality is the inherent capacity of a site to grow trees of a given species, the sum of climatic, soil, topographic and biotic factors. It is measured directly from trees or indirectly from site factors. The most widely used direct measure is site index: the average height of dominant (and often codominant) trees at a standard base age, such as 50 years for many temperate species or 10, 20 or 25 years for fast-growing plantations. Height of dominant trees is used because it is little affected by stand density and thinning, unlike diameter or volume, and is closely related to volume production. Site index curves (families of height–age curves) allow site index to be estimated from the height and age of a stand at any age. A stand whose dominant height is 24 m at base age 50 has site index 24 m (base age 50). Site index cannot be measured directly on bare land, in uneven-aged or mixed stands, or where past suppression or damage distorted height growth, and it is specific to a species. Indirect methods estimate site quality from features of the site. Indicator plants of the ground vegetation are used in ecological site classification systems such as the Finnish forest site types developed by Cajander in the early twentieth century, the habitat types of western North America and forest ecosystem classifications in Canada and Europe. Soil–site studies relate site index statistically to soil and topographic variables to predict productivity where trees are absent or to explain differences among stands; integrated ecological land classification combines climate, landform, soils and vegetation into hierarchical units. Process-based productivity models, driven by climate and soil water and nutrient supply, offer a further indirect approach and can project site quality under future climates, when historical site index may no longer apply.
Soil–site studies across many regions and species have repeatedly found that soil properties controlling the supply of water and oxygen to roots matter most, followed by nutrient supply. Effective rooting depth, the depth of soil roots can exploit before meeting bedrock, a hardpan, a compacted layer, a permanent water table or coarse gravel, is one of the strongest predictors; shallow soils limit water storage and anchorage. Available water capacity, determined by texture, coarse fragments, organic matter and depth, combined with climate determines how often trees experience drought. Drainage and aeration matter in both directions: poorly drained soils with gley features restrict rooting of species intolerant of waterlogging, while excessively drained sands and gravels are droughty. Topographic position and aspect act through soil depth and water: lower slopes and hollows usually have deeper, moister soils and higher site index than ridges and upper slopes, and in mid-latitudes slopes facing away from the equator are cooler and moister, often more productive for moisture-limited species. Thickness of the A horizon and its organic matter content, depth to mottling, surface soil texture, and nutrient indicators such as total nitrogen, C:N ratio and available phosphorus also enter many models. The importance of each factor depends on the limiting conditions of the region: on deep loams in a humid climate, nutrients may dominate; in a seasonally dry climate, water storage may dominate. Soil–site relations are therefore regional, and equations developed in one region should not be applied elsewhere without testing. Site quality is not fixed: erosion, compaction, loss of topsoil, nutrient removal and changes in drainage can degrade it, while fertilisation, drainage, mounding and restoration of organic matter can improve it. Climate change is shifting site quality as temperature and water balance change, which is a reason to combine soil information with climate projections when selecting species.
Salt-affected soils occur on every continent, mainly in arid and semi-arid regions where evaporation exceeds rainfall, in coastal lowlands, and in irrigated lands where poor drainage has raised saline water tables. They are classified by the electrical conductivity of the saturation extract (ECe), the exchangeable sodium percentage (ESP) and pH, following the classic scheme of the US Salinity Laboratory (1954). Saline soils have ECe greater than 4 dS m⁻¹, ESP less than 15 and pH usually below 8.5; their excess soluble salts (chlorides and sulphates of sodium, calcium and magnesium) cause osmotic stress and ion toxicity, and they often show white surface crusts. Sodic (alkali) soils have ESP greater than 15, ECe less than 4 dS m⁻¹ and pH often above 8.5; excess exchangeable sodium disperses clay, destroying structure, so the soil becomes sticky when wet, hard when dry and almost impermeable, often with a dark surface from dispersed organic matter and a columnar subsoil. Saline–sodic soils have both high ECe and high ESP. ESP = exchangeable Na / CEC × 100, so 3 cmolc kg⁻¹ of exchangeable sodium in a soil of CEC 12 cmolc kg⁻¹ gives ESP 25. The sodium hazard of irrigation water is expressed by the sodium adsorption ratio, SAR = Na / √((Ca + Mg)/2), with concentrations in mmolc L⁻¹. Reclamation of saline soils needs leaching with good-quality water and drainage; reclamation of sodic soils needs a calcium source, usually gypsum, to replace exchangeable sodium, followed by leaching. Tree planting on salt-affected land, sometimes called biodrainage where trees lower water tables, uses salt-tolerant species and provenances such as Prosopis, Tamarix, Casuarina, salt-tolerant eucalypts such as Eucalyptus camaldulensis and some acacias, often planted on ridges or with amendments in pits; species choice must match salinity and sodicity levels, and invasive species such as some Prosopis require caution.
Lateritic soils, in the forester's usage, are highly weathered tropical soils rich in iron and aluminium oxides, including those with plinthite or hardened ironstone (Plinthosols and some Ferralsols and Acrisols). Exposed plinthite hardens irreversibly into laterite crusts that restrict rooting and water storage, especially after forest clearing and erosion of the topsoil. These soils are low in available nutrients and CEC, fix phosphorus strongly and are often acid. Afforestation uses hardy species tolerant of shallow, infertile soils, such as cashew, Acacia auriculiformis, some eucalypts and native dry-forest species, with pit planting that breaks the crust, organic matter and phosphorus at planting. Strongly acid soils, with pH below about 5, limit growth of sensitive species through aluminium and manganese toxicity and deficiencies of calcium, magnesium, phosphorus and molybdenum. Many forest trees tolerate them; where amelioration is needed, lime, dolomite or wood ash raises pH and supplies bases, and acid-tolerant species such as many pines and Acacia mangium are planted. Acid sulphate soils are a special case: coastal sediments rich in pyrite, common under mangroves and in deltas, are harmless while waterlogged, but if drained the pyrite oxidises to sulphuric acid, pH falls below 4, and aluminium, iron and acid drain into waterways; the best management is to keep them waterlogged. Waterlogged soils, with perched or high water tables, show gley colours and mottles; lack of oxygen restricts rooting depth, makes trees shallow-rooted and prone to windthrow, and favours anaerobic processes. Management options include drainage (with care, since drainage of peatlands releases large amounts of carbon), planting on mounds or ridges, and choosing tolerant species such as alders, willows, some ashes, Melaleuca, bald cypress and native swamp-forest species. Peat soils are a further category: tropical and boreal peatland forests hold enormous carbon stocks, and drainage for plantations has caused subsidence, fires and major emissions, so restoring water tables is now a priority in many regions.
Soil erosion is the detachment and transport of soil by water, wind, gravity or ice. Water erosion begins with raindrop impact (splash erosion), which detaches particles and seals the surface, followed by sheet erosion (a thin, even removal by overland flow), rill erosion (small channels) and gully erosion (deep channels that cannot be removed by normal tillage or vegetation). Mass wasting includes landslides, debris flows and soil creep on steep slopes. Wind erosion is important on dry, bare, sandy and silty soils. Undisturbed forests usually have among the lowest erosion rates of any land use. The canopy and especially the forest floor intercept raindrops, the litter layer protects the surface and slows overland flow, high organic matter and root channels keep infiltration rates high so that overland flow is rare, and roots bind soil and reinforce slopes. The forest floor is more important than the canopy: tall canopies can produce large drops that reach the ground with considerable energy, and plantations where the litter is removed, burnt or prevented from forming, for example by raking or by grazing, can erode significantly despite tree cover. The Universal Soil Loss Equation, A = R × K × LS × C × P, estimates average annual sheet and rill erosion from rainfall erosivity (R), soil erodibility (K), slope length and steepness (LS), cover and management (C) and support practices (P). The C factor for undisturbed forest with full litter cover is very small, often in the range of about 0.0001 to 0.003, whereas bare or recently disturbed soil has values one or more orders of magnitude higher. With R = 300, K = 0.3, LS = 2 and P = 1, a forest with C = 0.003 loses about 0.54 t ha⁻¹ per year (in the units of the factors used), while the same site with C = 0.05 after disturbance loses about 9 t ha⁻¹. The equation does not estimate gully erosion, mass movement or sediment delivery to streams.
Disturbance changes erosion sharply. Forest roads, landings and skid trails are usually the largest sources of sediment from managed forests, often exceeding erosion from the harvested area itself: they remove vegetation and litter, compact the surface so water runs off, concentrate flow in ditches and wheel ruts, cut into slopes, and deliver sediment directly to streams at crossings. Poorly located and drained roads also trigger landslides. Clear-cutting itself, if the forest floor is left largely intact, usually causes modest increases in surface erosion, but mechanical site preparation and broadcast burning that expose mineral soil can raise erosion considerably. On steep slopes, the loss of root reinforcement after harvesting increases landslide risk: the roots of cut trees decay over several years while those of the new crop are still small, so slope stability is commonly lowest roughly three to ten years after cutting, and landslide frequency rises during this window in many mountain regions. Wildfire, especially high-severity fire, removes the forest floor and canopy and can create water-repellent (hydrophobic) layers in the soil, as waxy compounds volatilised during burning condense on cooler soil particles below the surface; infiltration drops, overland flow and erosion rise dramatically, and debris flows can follow intense rain on burned slopes in the first few years. Grazing in forests, shifting cultivation with short fallows, mining and the collection of litter for fuel or fertiliser also expose soil. Wind erosion threatens cleared dry forests and dunes. Erosion has both on-site costs (loss of topsoil, nutrients and organic matter, reduced rooting depth and site productivity) and off-site costs (sedimentation of streams, reservoirs and irrigation systems, turbidity harming aquatic life and water supply, and transport of attached phosphorus and pollutants). Because forest operations account for a large share of the erosion and sediment from forested catchments, their planning is central to soil and water conservation.
Soil conservation on forest land aims to keep soil in place, maintain its productive capacity and protect water quality. Most forest jurisdictions and certification standards set out best management practices (BMPs) for operations, which share common principles. Planning comes first: soil and slope maps identify erosion-prone, steep, wet and unstable areas to avoid or treat with special care; operations are scheduled for dry or frozen seasons where possible; and harvesting systems are matched to terrain, with cable or aerial systems on steep slopes instead of ground-based machines. Roads cause most sediment, so they are located on stable ground away from streams, kept to the minimum length and gradient needed, and drained frequently so that water leaves the road surface before it gathers erosive force: through outsloping, rolling dips, water bars and cross-drain culverts at spacings that decrease as road grade increases. Stream crossings are kept few, made at right angles, and sized for floods, and road surfaces near streams are gravelled. Streamside management zones, buffer strips of forest along streams where harvesting and machine traffic are restricted, trap sediment, shade water and stabilise banks; their width varies by stream size, slope and jurisdiction. Skid trails are planned, spaced and kept off steep slopes; machines travel on mats of slash to reduce compaction and rutting; and harvest residues are left spread on the site to protect and nourish soil. After operations, trails and landings are closed with water bars, ripped where compacted and revegetated, and temporary roads are decommissioned. Prescribed burning is done at low intensity to retain the forest floor. Monitoring of BMP implementation and effectiveness, through audits after operations and stream turbidity measurements, shows that properly applied BMPs reduce sediment delivery greatly, while failures at a few stream crossings or roads can produce most of the sediment from a catchment.
On degraded land, steep farmland and burned areas, soil conservation combines vegetative, mechanical and management measures. Vegetative measures are the foundation: establishing and maintaining a continuous ground cover of litter, grasses, shrubs and trees through afforestation, assisted natural regeneration, protection from fire and grazing, mulching and cover crops; planting contour hedgerows of shrubs or grasses, such as vetiver grass, along slopes so that sediment accumulates behind them and forms natural terraces; and sloping agricultural land technology, in which hedgerows of nitrogen-fixing shrubs alternate with crop strips, developed in Southeast Asia. Mechanical measures include contour trenches and contour bunds that intercept runoff and store water for planted trees, especially in semi-arid regions; half-moon and micro-catchment structures around planting pits; terraces on steep slopes; and gully control through brushwood, loose-stone or gabion check dams that slow flow, trap sediment and allow vegetation to establish in the gully bed, from the head of the gully downward, together with diversion of water away from the gully head. Stabilisation of landslides and road cuts uses drainage, bioengineering with live cuttings of willows, poplars and other rooting species, and retaining structures. Sand dunes are fixed with fences, grasses and trees, an approach with long histories on coasts of Europe and in deserts of China, North Africa and the Sahel. After severe wildfires, emergency stabilisation teams assess burn severity and soil water repellency and apply treatments such as straw or wood mulch, seeding, log erosion barriers and road drainage improvements on the highest-risk slopes, though the effectiveness of some treatments, particularly seeding, has proved variable. Successful programmes treat the whole watershed, combine conservation with livelihoods so that communities maintain the works, protect regeneration from grazing and fire, and monitor results over years, since soil formation is so slow that soil lost is, on human time scales, lost for good.
A student uses this subject to describe a soil profile in the field, name its horizons and humus form, estimate texture by feel, and relate what is seen to tree growth. Calculations to master include porosity, organic carbon stock, available water, base saturation, ESP and SAR, Olson's k and fertiliser product quantities. Common errors are mixing gravimetric and volumetric water, forgetting bulk density in stock calculations, confusing saline with sodic soils, and assuming soil tests predict tree response as well as they do crop response.
A forester five years into practice uses soils daily: reading soil maps and site classifications to choose species, prescribing phosphorus at planting or nitrogen in mid-rotation from foliar analysis, planning roads, skid trails and stream buffers to meet best management practices, deciding whether to retain harvest residues, and choosing species, mounding or pit planting for saline, waterlogged or lateritic land.
A senior professional with twenty-five years of experience sets soil policy for estates and agencies: rotation-length and residue-harvesting rules based on nutrient budgets, fertiliser programmes justified by long-term trials, road standards and audits, peatland and acid-sulphate soil management, and responses to landslides and post-fire erosion. Judgements weigh productivity against water quality, carbon and community needs.
A researcher with a PhD and long experience studies soil carbon dynamics, nutrient limitation and deposition, mycorrhizal effects on weathering, long-term productivity experiments after harvesting and compaction, and soil responses to climate change. Such researchers maintain long-term site networks, contribute to soil classification and global soil carbon assessments, supervise doctoral students and set examination questions, so they must know both classical pedology and current debates on soil organic matter formation.
This manual gives general ranges, not regional standards: critical foliar concentrations, fertiliser rates, bulk density thresholds, buffer widths and water-bar spacings vary by species, soil and jurisdiction and must be taken from local guidelines and trials. It does not cover laboratory methods in detail, soil microbiology beyond mycorrhizae and decomposition, peatland hydrology, contaminated soils, or national soil classification systems other than USDA Soil Taxonomy and WRB. Correlations between classification systems are approximate.
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.
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