Soil Genesis and Classification

Geography 418/518

Dr. James H. Speer

Spring 2002

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

With lecture material taken from

Buol S.W., Hole F.D., McCracken, R.J., and Southard, R.J.  Soil Genesis and Classification, Fourth Edition.  Iowa State University Press.  527 pp.

Brady N.C. and Weil, R.R. 2002. The Nature and Properties of Soils, Thirteenth Edition.  Prentice Hall. 960 pp.

Grissino-Mayer, H.D.  Class Notes.  Geology 3710: Introduction to Soil Science.  Valdosta State University.


Historical Development

Aristotle (384-322 B.C.)

Theophrastes (372-287 B.C.)

Considered the properties of soil in relation plant nutrition.

 

Roman writers who discussed differences among soils in relation to plant growth.

Cato the Elder (234-149 B.C.)

Varro (116-27 B.C.)

Virgil (70-19 B.C.)

Columella (about A.D. 45)

Pliny the Elder (A.D. 23-79)

 

In 1840, Justus von Liebig published Chemistry in its Application to Agriculture and Physiology in which he states plants assimilate mineral nutrients from the soil.

 

In the mid 1800s several German scientists including Ramann and Fallou developed agroecology that viewed soil as weathered, somewhat leached surficial rock.

 

In Russia, Lomonosov (1711-1765) wrote about soil as an evolutionary process.

 

In 1883, Dokuchaev (1846-1903) developed many of the early applications of soil science.  His dynamic research and teaching fueled extended studies of soil genesis in Russia.

 

Glinka (1867-1916) and Neustruyev (1874-1928) continued the work of Dokuchaev, stating that soil is a weathered crust that exhibits specific properties correlated with climatic zones.

 

Williams (1863-1939), another Russian soil scientist, emphasized the concept that soil genesis was a biological process.

 

Hilgard (1833-1916), one of the first U.S. soil scientists, wrote about the relationships between soils and climate.

 

Marbut (1863-1935) did much of the significant early work in soils in the United States emphasizing on the soil profile.

 

Smith (1907-1981) was the chief architect of Soil Taxonomy, which advanced and refined soil genesis in support of soil classification and soil survey.

 

Definitions

Soil – A dynamic natural body composed of mineral and organic solids, gases, liquids, and living organisms which can serve as a medium for plant growth and that has properties resulting from the integrated effects of climate and living organisms acting upon parent material, as conditioned by topography, over periods of time. 

 

Pedology – The science that deals with the origin, classification, distribution, and description of soil.

 

Edaphology – The study of soil as a medium for sustaining higher plants, emphasizing plant growth, fertility, and the differences in productivity among soils.

 

Ped – A unit of soil structure such as an aggregate, crumb, prism, block, or granule, formed by natural processes.

 

Pedon – A hexagonal column of soil measuring from 1 to 10m2 in top surface area.  A pedon is the basic sampling unit used in soil surveys.

 

Polypedon – An essential soil individual, comprising an identifiable series of soils in an area.  It is made up of multiple pedons and has distinctive characteristics that differentiate it from surrounding polypedons.

 

Soil Horizons – A unit of soil that is roughly parallel to the pedon surface and has characteristics distinctly different from horizons above and below it.

 

Soil Profiles – A vertical section of a soil that displays the succession of horizons from the surface to the parent material.

 

Classification – The process of sorting or arranging of objects into groups on the basis of one or more objectives and according to a system or set of principles.

 

Class – A group of individuals or other units similar in selected properties and distinguished from all other classes by differences in these properties.

 

Taxon – A class of a formal classification system at any level of generalization or abstraction. 

 

Category – A series or array of taxa produced by differentiation in the population being categorized at a given stated level of generalization. 

 

Differentiating Characterization – A property chosen as the basis for categorizing individuals into groups.

 

Taxonomy – A particular formalized system of classification developed for a specific purpose and categorized according to a set of prescribed differentiating characteristics.

 

Soil Taxonomy – This formal term refers to the system of classification developed by the USDA Soil Survey.

 


Categories in the System

The classification scheme begins with (1) 10 orders, (2) 57 suborders, (3) over 300 great groups, (4) subgroups, (5) families, and (6) series.

 

Orders: the properties used to differentiate among orders are those that reflect the type and degree of dominant soil-forming processes that took place. All orders end with "-sol," for example: Entisol

 

Suborders: based primarily on properties that influence pedogenesis, are important to plant growth, or are selected to represent the most important environmental variables within the order. For example, aquent, meaning an entisol influenced by water.

 

Great Group: grouping based on close similarities in kind, arrangement, and degree of expression of pedogenic horizons, soil moisture and temperature regimes, and base status. For example, haplaquent, meaning similar horizons found in Entisols influenced by water.

 

Subgroups: these define central concepts of the soils, such as typic ("typifies" the great group), lithic (related to rock stucture), aquic, (related to water), etc.

 

Families: based on similar physical and chemical properties that affect management. Consists of a series of adjectives, such as "fine-loamy, mixed, nonacid, mesic, Typic Haplaquents.

 

Series: consists of soils that formed in a particular kind of material and have horizons that are similar in differentiating characteristics and in arrangement in the soil profile. Among these characteristics are color, texture, structure, reaction, consistency, and mineral, and chemical composition. For example, Valdosta Series versus Tifton Series.

 

American Soil Taxonomy

Ten soil orders as originally defined by the Seventh Approximation and later by the book Soil Taxonomy:

 

Alfisols: soils with an argillic horizon and moderate to high base content. ("-alf")

Andisols: soils developed on/from volcanic materials. ("-and")

Aridisols: soils of desert and semidesert areas. ("-id")

Entisols: weakly developed soils. ("-ent")

Histosols: soils developed in organic materials. ("-ist")

Inceptisols: moderately developed soils. ("-ept")

Mollisols: soils with a dark A horizon and high base status. ("-oll")

Oxisols: soils with an oxic horizon or plinthite. ("-ox")

Spodosols: soils with a spodic horizon. ("-od")

Ultisols: soils with an argillic horizon and low base content. ("-utl")

Vertisols: cracking clay soils. ("-ert")

 

Suborders and great groups use suffixes to indicate a grouping based on close similarities in kind, arrangement, and degree of expression of pedogenic horizons, soil moisture and temperature regimes, and base status. Some common ones include:

 

"aq-" as in "aqualf" indicating an alfisol with features of gleying (aquaeous environment).

"cry-" as in "cryand" indicating an andisol developed in a very cold climate.

"sal-" as in "salid" indicating an aridisol with a salic horizon.

"fol-" as in "folist" indicating a histosol of fibric peat.

"hum-" as in "humult" indicating an ultisol with a humic A horizon.

"fluv-" as in "fluvent" indicating an entisol developed on alluvial deposits.

"alb-" as in "alboll" indicating a mollisol with an albic A horizon.

"xer-" as in "xerolt" indicating an ultisol developed in a semiarid climate.

"umb-" as in "umbrept" indicating an inceptisol with an umbric epipedon.

"ud-" as in "udert" indicating a vertisol developed in humid areas.


The American system became the standard for the development or evolution of soil taxonomic systems throughout the world.  European countries have developed their own classification schemes, but the American system has been adopted in many countries around the world.

 

An International Soil Taxonomy

Beginning in 1961, the FAO began the development of a world soil map that further added and improved upon the American system. It was based on over 800 soil surveys from around the world. However, the controversy is that this system is not a classification system in that it is based only on the legend of this world map, and it is not as sophisticated as the American system.

 

Beginning in 1990, a working group of the International Society of Soil Science has been working to develop a classification system, known as the World Reference Base for Soil Classification based on the FAO-Unesco Soil Map of the World. In 1994, this working group presented its initial document to members of the World Congress of Soil Science held in Mexico City.

 

The World Reference Base for Soil Classification resulted in 28 Major Soil Groupings. Allocation to one of these groupings is determined by the presence or absence of a limited number of diagnostic horizons, diagnostic properties, or diagnostic constituents.

 

Histosol: a soil with more than a defined amount of organic matter; an organic soil.

Anthrosol: a soil dominated by human activities.

Leptosol: a weakly developed shallow soil.

Vertisol: a clayey soil, which cracks widely when dry and swells when wet.

Fluvisol: a soil developed on river deposits showing alluvial stratification.

Solonchak: soil where salt accumulation is the dominant process.

Gleysol: waterlogged soils with poor drainage and anaerobic conditions.

Andosol: soils composed of volcanic materials, usually dark colored.

Arenosol: soils with sandy or loamy sand texture.

Regosol: weakly developed soil with texture finer than sandy loam.

Podzol: soil with bleached, light-colored horizon below surface, with spodic B horizon.

Plinthisol: soil with mottled appearance that harden on exposure to atmosphere.

Ferralsol: soil composed of kaolinite and quartz, enriched in Fe and Al oxides.

Planosol: soils in flat areas, with seasonal saturation caused by impermeable lower horizon.

Solonetz: soils dominated by sodium salts.

Greyzem: organic rich surface horizon with uncoated sand grains, typical of grass steppe/prairie.

Chernozem: dark colored, deep soils in organic matter, calcareous lower in profile, also typical of grass steppe/prairie.

Kastanozem: calcareous soils rich in organic matter, brown color, typical of semiarid climates with grasses.

Phaeozem: dark colored soils rich in organic matter, with deep leaching of carbonates, associated with forest steppe.

Podzoluvisol: soil with clay-enriched lower horizon into which an albic horizon is deeply tongued.

Gypsisol: presence of gypsum (calcium sulphate) in crystals or concretionary layers.

Calcisol: soils dominated by calcium carbonate as powdery lime or concretions.

Nitisol: soil with deep, clay-enriched lower horizon with shiny ped surfaces.

Alisol: acid soil with clay-enriched lower horizon, high CEC, but low saturation of bases.

Acrisol: acid soil with clay-enriched lower horizon, low CEC, and low saturation of bases.

Luvisol: soil with clay-enriched lower horizon, high CEC, and high saturation of bases.

Lixisol: soil with clay-enriched lower horizon, low CEC, and high saturation of bases.

Cambisol: moderately developed soils with lower horizons having color or structure changes from the parent material which permit the identification of a cambic B horizon.

 


Other changes were the refinement or addition of diagnostic horizons:

 

fimic A horizon: a man-made layer 50 cm or more in thickness, produced by long-continued manuring with earthy admixtures.

argic B horizon: shortened form of "argillic."

ferrallic B horizon: formerly an oxic horizon.

 

Within each of these Major Soil Groupings, between three and nine different varieties of Soil Units are distinguished at a second level of classification. This is accomplished by the addition of an adjectival word before the soil name, such as a Humic Cambisol or a Calcic Luvisol. The most common adjectives describe the:

 

a.  topsoil character (such as humic, umbric)

b.  fertility property (such as eutric or dystric)

c.  a property of the B horizon (such as calcic, gypsic, gleyic, or luvic).

 

Concepts of Soil Genesis

 

Concept 1: Pedogenic processes active in soils today have been operating over time and have varying degrees of expression over space. (Uniformatarianism)

 

Concept 2: Many soil-forming processes proceed simultaneously in a soil, and the resulting profile reflects the balance of these processes, present and past.

 

Concept 3: Distinctive regimes or combinations of processes produce distinctive soils.

 

Concept 4: Five external factors of soil formation drive the internal pedogenic processes within the soil – climate, organisms, relief, parent material, and time (clorpt).

 

Concept 5: Present day soils may carry the imprint of a combination of pedogenic processes that was active in the geologic past.

 

Concept 6: A succession of different soils may have taken place at a particular site as soil genetic factors changed, and soil erosion or deposition of soil material proceeded over time, with the soil surface lowered or raised.

 

Concept 7: There are very few old soils (in a geologic sense) because they are either destroyed or buried by geological events or modified by shifts in climate through their vulnerable position as the skin of our dynamic earth.

 

Concept 8: Complexity of soil genesis is much commoner than simplicity.

 

Concept 9: Soils are natural clay factories.

 

Concept 10: An understanding of, and knowledge about, the genesis of a soil is useful in its classification and in mapping, but scientific classification systems cannot be based entirely on genesis.

 

Concept 11: Knowledge of soil genesis is basic to soil use and management.

 

 


Introduction to soil horizons

 

A soil horizon is a layer of soil, revealed in a soil profile, lying approximately parallel to the earth's surface, and possessing relatively homogeneous physical, chemical, and biological properties. Often, these are readily identified in the field, but only in extended vertical sections (road cuts and trenches) can the true picture of lateral variability be appreciated.

 

Each soil horizon is associated with a letter according to a system adopted by the Food and Agricultural Organization (FAO) of the United Nations, which we will use in this class. First used by Dokuchaev, each horizon is associated with a capital letter representing the master horizons. These horizon designations are of great use when describing, interpreting, discussing, and classifying soils.

 

The master horizons are:

 

O: organic horizon at the soil surface, usually unconsolidated organic material (leaf litter, roots, leaves, etc.), not saturated with water. Occasionally, one will see the O and H horizons replaced by the letters L for "litter," F for "fermentation," and H for "humus."

 

H: organic horizon at the soil surface normally saturated with water, characteristic of peaty deposits.

 

A: mineral horizon formed at or near the surface where humified organic matter is associated with mineral materials. Humus is defined as the stable, dark-colored organic material that accumulates as a by-product of decomposition of plant and animal residues added to soil.

 


E: mineral horizon just below the soil surface that has lost its silicate clay, organic matter, aluminum, or iron by downward movement, leaving a concentration of resistant sand and silt particles. "E" stands for "eluvial horizon," a soil layer formed by the removal of constituents such as clay or iron. Eluviation describes the process whereby constituents of soil are removed in suspension.

 

B: a subsurface mineral horizon resulting from (1) the change in situ of soil material, i.e., the obliteration of the original rock structure, or (2) the washing in of material from overlying horizons, i.e., the accumulation of silicate clay, organic matter, aluminum, or iron. Illuviation describes the process of accumulation of materials from overlying horizons.

 

C: unconsolidated or weakly consolidated mineral horizon that retains evidence of rock structure, but lacks diagnostic properties of the overlying A, E, and B horizons. This horizon is little affected by pedogenic (i.e. soil forming) processes. Examples include beach sand, windblown silt (or loess), alluvium deposited by rivers, and glacial till deposited by glacial ice.

 

R: continuous (consolidated) hard or very hard bedrock.

 


Subhorizons occur in the A, E, B, and C horizons:

Ah: uncultivated A horizon, or an A horizon with accumulation of humus

Ap: cultivated (plowed) A horizon

Ag: poorly drained A horizon

Eg: poorly drained E horizon

Bg: poorly drained B horizon

Bh: B horizon with accumulation of humus

Bs: B horizon with accumulation of iron and aluminum

Bt: B horizon with accumulation of clay

Bw: B horizon with changes of color or structure

Bx: B horizon with fragipan, a compact, slowly permeable subsurface horizon that is brittle when moist and hard when dry.

By: B horizon with accumulation of gypsum.

Bz: B horizon with accumulation of salts more soluble than gypsum.

Cg: poorly drained C horizon

Ck: C horizon enriched with CaCO3

Cm: C horizon with cemented material

Cx: C horizon with fragipan

Cy: C horizon with accumulation of gypsum.

Cz: C horizon with accumulation of salts more soluble than gypsum.

 


Soil Taxonomy

 

Soil classification begins by first recognizing a limited number of diagnostic horizons based on morphology and chemical characteristics. Two types of diagnostic horizons emerged based on location on top of or within the profile: diagnostic subsurface horizons and diagnostic surface horizons are known as epipedons (“on top of”).

 

Epipedons include:

mollic: dark colored, thick surface horizon, typical of steppe regions, with over 50% of the exchange capacity dominated by the base cations.

 

umbric: similar to mollic, except that the base saturation is less than 50%.

 

histic: a peaty surface horizon, saturated with water part or all of the year, having a large amount of organic carbon.

 

anthropic: similar to a mollic epipedon, but man-made with a large amount of phosphate accumulated by long-continued farming.

 

plaggen: a man-made epipedon more than 50 cm thick raised above the original soil surface with properties that depend on the original soil.

 

mellanic: a black, thick epipedon occurring in soils developed in volcanic ash. Usually has low bulk density.

 

ochric: epipedons that are too light in color, too low in organic carbon, or too thin to belong to mollic, umbric, anthropic, plaggen, or histic epipedons. The most common form of epipedon.

 


Diagnostic subsurface horizons include:

agric (= ager = field): a compact horizon formed immediately below the plow layer by cultivation, and contains significant amounts of illuvial silt, clay, and humus.

 

albic (= albus = white): a bleached, light colored horizon from which the clay and free iron oxides have been removed.

 

argillic (= argilla = white clay): an illuvial horizon enriched with clay to a significant extent.

 

calcic (= calcic = lime): a horizon enriched with calcium carbonate or calcium and magnesium carbonate in the form of powdery lime or secondary concretions, more than 15 cm thick.

 

cambic (= cambiare = to change): an altered horizon in which the parent material has been changed into soil by formation of soil structure, liberation of iron oxides, clay formation and obliteration of the original rock structure. “The definition of a cambic horizon is complex because the soil material must show evidence of change by pedogenesis, but not so much change that an argillic, spodic,”... or other horizon is evident. Does not occur in extremely sandy soils, must be in the very fine sands or finer.

 

gypsic (= gypsum): a horizon enriched with calcium sulphate, more than 15 cm thick.

 

kandic: new term, derived from the word kandite, used to describe kaolinitic clay minerals. A low-activity clay subsurface horizon similar to an oxic horizon but with more clay in the overlying surface horizon and an abrupt change of texture between the surface and lower horizons.

 

natric (= natrum = sodium): a clay-enriched illuvial horizon, with the cation exchange complex dominated by a high sodium content. Similar in all respects to an argillic horizon.

 

oxic (= oxides): a horizon with a very low content of weatherable minerals (meaning it is the most highly weathered), in which clay is composed largely of kaolinite, contains accessory highly insoluble minerals such as quartz sand, low exchange capacity, and poorly dispersed clays.

 

placic (= plax = flat): a thin, black or reddish-brown brittle pan, cemented with iron, iron and manganese, or an iron-organic complex. Forms a barrier to roots. A thin iron pan.

 

salic (= sal = salt): a horizon enriched with salts more soluble than gypsum, more than 15 cm thick.

 

sombric (= sombre = dark): a freely drained, dark subsurface horizon containing illuvial humus with a low CEC and low base saturation. Sometimes mistaken for a buried A horizon.

 

spodic (= spodos = wood ash): an illuvial horizon enriched with organic matter, iron, and aluminum.

 

sulfuric (= sulfur): a mineral or organic horizon more than 15 cm thick which has a pH of 3.5 or less and contains the mineral jarosite or more than 0.05% water-soluble sulphate.

 


Other diagnostic horizons:

glossic: a horizon more than 5 cm thick in which an upper E horizon penetrates (tongues) down into a lower argillic, natric, or kandic horizon.

 

duripan (= durus = hard + pan): a subsurface horizon cemented by silica or aluminum silicate to the degree that fragments from the air-dry horizon do not slake during prolonged shaking.

 

fragipan (=fragillis = fragile + pan): a compact slowly permeable loamy subsurface horizon with a high bulk density, brittle when moist, but hard when dry. Slakes or fractures when placed in water.

 

petrogypsic: a cemented gypsic horizon.

 

petrocalcic: a cemented calcic horizon.

 

andic: soil horizon composed of volcanic glass.

 

permafrost: a soil horizon where temperature is constantly below 0°C, with permanent ice.

 

plinthite: material found in tropical regions, arising due to ferralitizaion (laterization) soil formation processes. Vesicular, porous first then hardens into iron crusts (ironstone).

 


Composition of Soils

 

Soil consists of four main constituents: (1) mineral matter, (2) organic matter, (3) air, and (4) water.

 

Mineral matter consists of two groups: (1) primary minerals, resistant coarse-grained minerals weathered from rocks, and (2) secondary minerals, formed in the soil by recombination of substances, usually fine-grained.

 

Organic matter is derived mostly from decaying plant matter, but also consists of decaying animal matter. Organic matter composed of cellulose, starch, and lignin in various states of decomposition.

 

In soils that have structure, the mineral and organic components are aggregated into discrete structural units called peds, which are surrounded by open spaces.

 

Air and water occupy these spaces. In soils that are saturated, most air is removed. In freely drained soils, water adheres to the mineral particles.

 

I. Mineral Matter

 

The mineral portion comes primarily from in situ weathering of the geological substrate. Occasionally, however, minerals are transported in, as well as blown in from eolian wind activity.

 

Particles range in size from very small clay particles measured in microns up to sand-size particles that can be measured in millimeters. This fraction of the soil is called fine earth, and usually consists of particles less than 2 mm in size. It is upon this fraction that soil texture is determined.

 


The fine earth fraction can be further broken down into three classes:

 

a.sand: diameters between 0.02 to 2.0 mm

b.silt: diameters between 0.02 to 0.002 mm

c.clay: diameters less than 0.002 mm

 

To determine texture, a soil sample is wetted, and the sand, silt, clay fractions determined and related to a particular texture class. Twelve different classes constitute the textural triangle, which has 100% sand on its left point, 100% clay at its top point, and 100% silt on its right point.

 

Primary minerals are derived from parent material and have usually been through one cycle of weathering, so that only the most resistant ones remain. For example, the sand fraction may largely be composed of quartz grains, along with feldspar and mica. Quartz grains often constitute 90-95% of all sand and silt particles in soils derived from sedimentary rocks.

 

II. Clay minerals:

 

*  consist of fine (< 0.002 micron), platy-shaped mineral grains that can only be identified indirectly,

*  all clay minerals are formed in the soil itself and are therefore secondary minerals,

*  chemically they are hydrous silicates of aluminum,

*  three main members of this group: (1) kaolinite, (2) smectite, and (3) hydrous micas,

*  all clay minerals are constructed from layers of silica and aluminum

*  if soil has solutions rich in aluminum and silicon, kaolinite can form - Al4Si4O10(OH)8,

*  if soils are base-rich, montmorillonite can form = NaMgAl5Si12O10(OH)6 = smectite group.

 

Where oxidation and reduction are prevalent, iron and manganese oxides precipitate as scattered concentrations that grow by the addition of concentric layers of iron and manganese compounds.

 

oxidation = loss of electrons while reduction is a gain of electrons. An electron donor (reductant) gives electrons to an electron acceptor (oxidant). Oxygen (O2) is the most abundant oxidizing agent in natural earth processes. Oxidation/reduction processes = ("redox").

 

acceptor + donor = acceptor + e- + donor - e-

 

acceptor + donor = acceptor (reduced) + donor (oxidized)

 

Oxidation processes are important because they tend to mobilize some nutrients, such as elemental sulphur, and make them available to plants. Oxidation processes also lower the soil pH, making it more acidic. Hence, when you add nitrogen fertilizers, the oxidation of ammonia causes long-term soil acidification in agriculture. This may not be desirable depending on the plants being grown. Oxidation can only occur in soils if it is aerated.

 

Clay minerals are so small that a colloidal state occurs. A colloidal state occurs when particles less than one micron in size are dispersed evenly throughout another medium. In the case of clay minerals, minute electrical forces of the molecules at the surface of the clay become dominant, causing the colloidal state. The colloidal state is important because it affects plasticity, cohesion, shrinkage, swelling, and dispersion of soil materials.

 


III. Organic Matter

 

Four basic types of organic matter identified by pedologists: (1) peat, (2) mor, (3) moder, and (4) mull.

 

Peat: organic accumulations composed of fibrous, semi-fibrous, or amorphous materials, developed in saturated conditions.

*  Plant remains often still recognizable. If plants fibers can be seen when the peat is rubbed, the peat is referred to as fibric.

*  An intermediate degree of decomposed organic matter indicates the peat is hemic.

*  If the organic matter is almost completely decomposed, the peat is referred to as terric.

*  If the organic matter is completely decomposed, the peat is referred to as sapric.

 

Mor: layer of organic matter that develops beneath conifer forest communities and is associated with Bly acidic soils.

*    Plant litter is highly acidic which restricts the presence of soil fauna.

*    Breakdown of litter is retarded such that three distinct layers are present in the O horizon: (1) litter (L), fermentation (F), and humus (H) layers. The H layer is distinct and rests abruptly on the A horizon.

*    Organic breakdown occurs mostly from fungi as earthworms are not present.

 

Moder: layer of organic matter that forms in woodland areas that is not as acidic as mor. Decomposition is more rapid, resulting in only a litter (L) and fermentation (F) layer in equal thicknesses.

 


Mull: layer of organic matter that forms in freely drained, base-rich soils with good aeration.

*  Excellent conditions for plant-life, abundant nutrients, rich soil fauna, many earthworms.

*  Organic debris completely broken down by soil fauna, with humified remains incorporated rapidly into the A horizon.

*  Hence, only a small, shallow L layer is associated with a mull.

 

IV. The Clay-Humus Complex: In certain soils, the humus can exist in a colloidal state and become associated with the clay minerals to form the clay-humus complex.

 

Soil chemistry is largely concerned with the chemical and physical properties of the particles that make up the clay-humus complex.

 

Acids, alkalis (very basic as opposed to acidic), and their salts can dissociate to form ions with positive (cations) and negative (anions) charges, i.e., NaNO3 = Na+

+ NO3-.

 

Both clay and organic particles have net negative charges, such that a clay-humus particle acts as a large highly charged anion, known in chemistry as a micelle.

 

Attracted to the negative charge are numerous cations in a diffuse cloud about the micelle. This cloud is known as the Gouy layer.

 

The electrical charges enable clays to attract, hold, and exchange cations, known as the cation exchange capacity, or CEC, defined as the total amount of exchangeable cations that can be held by a given mass of soil. The CEC is important because the exchangeable ions (those held in the exchange complex) are (1) available to plants, supplementing the small quantity of nutrients held in solution, and (2) retained in soils and not lost with leaching water. Cations are positively charged while anions are negatively charged. Cations include Al3+, Ca2+, Mg2+, Na+, and K+.


Lecture 4

Climate, Organism, Relief, Parent Material, Time

 

Soil formation occurs due to many environmental processes, first postulated by Dokuchaev in the late 1800s. Hans Jenny, an American, further developed a model of soil formation processes, which is widely cited today, mainly because of its simplicity:

s = f ( c, o, r, p, t, ...)

 

This approach is useful because we can next consider each factor in turn and discuss its contribution to soil formation.

 

Climate:

Note the difference between weather and climate: Weather: the physical state of the atmosphere at any given time and place. When averaged over longer periods, we describe the climate, defined as the characterization of average weather conditions and the extremes for any given region over longer time scales.

Weather = local observation, short-term, actual measurement; Climate = broad region, long-term, average and extremes.

 

Temperature:

*  In general, temperature does not appear to play an important role in soil formation on the short-term, but does have significant implications over the long-term.

*  Sets the speed of any chemical reaction in the soil. Higher temperatures indicate faster decomposition rates of organic matter.

*  For example, temperature will affect the rate of dissociation of water into hydrogen (H+) and hydroxyl (OH-) ions.

*  The rate of this dissociation can be taken as an index of chemical activity. Multiplied by the length of the weathering period (related to length of growing season) gives the weathering factor.

*  Weathering: the chemical or physical breakdown of rocks and minerals exposed to air and water to their basic constituents. This is a geological process that contributes to soil formation.

*  Temperate regions = 3 times amount of weathering as arctic regions, while tropical regions = 10 times the amount of weathering.

*  In most cases, weathering has been persistent for much longer in tropical regions because it has not been interrupted by glacial episodes. Therefore, tropical soils will be deeper (up to 50 meters) of regolith may be weathered compared to only one meter or less in temperate regions.

 


Precipitation:

*  Rainfall percolating through the soil acts as a medium in which chemical reactions take place.

*  Total annual rainfall is not very adequate when assessing the type of soil found in a region.

*  In addition, you must consider the distribution of the annual total (through the seasons), and whether it occurs predominantly in the cool or warm season.  Furthermore, you must also consider the intensity of the rainfall as the effects of intense short-term downpours will have different effects on soil formation and other properties as does a long-term, less intense rainfall.

*  Amount of rainfall that infiltrates into the soil is less than actual total because of runoff and evaporation from soil and vegetation surfaces.

*  Water that percolates through the soil can (1) be taken up by plant roots, (2) become capillary water bonded to organic and mineral particles, (3) become gravitational water and seep through to the groundwater, or emerge in springs and seepages.

*  Occasionally, a link can be seen between precipitation and soil type. For example, the greater the amount of rainfall, the deeper the horizon containing calcium carbonate in the soil, until eventually all the CaCO3 is leached out.

 

Soil classification does not easily conform to patterns of climate on the earth's surface, especially when using the Kφppen or Thornthwaite climate classification schemes. Instead, by international agreement, eight major Agro-ecological Zones have been defined based on the length of the growing season, which is used similarly to the weathering factor: (1) polar, (2) boreal, (3) cold (upper midlatitude), (4) temperate (lower midlatitude), (5) seasonal dry subtropical, (6) subtropical (7) humid tropical, (8) equatorial.

 

 

 

 


Organisms:

Can be divided into three general groups: (1) microorganisms, (2) macrofauna, and (3) plants. Numbers can be huge! 28-54 million springtails (Collembolae) can be found in the top 22 cm of soil in one acre. Actual numbers may be a poor indicator of the number of organisms due to dormancy. Rather, the amount of carbon dioxide (by-product of respiration of fauna) is a better indirect measure of the number of living organisms.

 

Microorganisms:

*  Members include: viruses, bacteria, actinomycetes, protozoa, fungi, and algae.

*  Bacteria can amount to 1.5 tons per acre, living in thin water films around soil particles. Both aerobic (requiring oxygen) and anaerobic (not requiring oxygen) bacteria are present in soils.

*  Large numbers of aerobic bacteria can deplete a soil of oxygen, thus causing reducing conditions.

*  Soil bacteria obtain their energy from organic matter, and produce humus.

*  Some bacteria are important for fixing nitrogen (Rhizobacter = symbiotic nodular bacteria that exist in soil near and on plant roots) from the air, which is an important plant nutrient.

*  Bacteria are fed upon by protozoans (i.e., amoebae).

*  Mushrooms and toadstools are the above-ground evidence of fungal hyphae that exists virtually throughout decomposable substances within the soil.

*  Fungi are very important decomposers in acidic soils where soil fauna are limited.

 

Macrofauna:

*  Small animals live within the pore space between peds, while other larger animals create burrows.

*  Arthropodae are the most numerous, especially the springtails, living within the top 5 cm of soil. Springtails are responsible for breaking down organic matter into small fragments.

*  Earthworms play an important role in the upper soil by mixing organic matter with mineral matter. Estimated to be 10 tons of earthworms per acre per year.  In 50 years, it's estimated that the entire volume of soil down to 9 inches would have been brought to the surface by earthworms.

*  Earthworms ingest soil and organic matter, resulting in a wormcast that is a close mixture of soil and humus, richer in plant nutrients than surrounding soil.

*  Casting by earthworms creates a deep, organic layer (O horizon) on the A horizon which is gradually thickened. Commonly responsible for the mull type of O horizon.

*  Cultivation kills about 25% of all earthworms. Cultivation also lowers plant litter, which further reduces earthworm populations.

*  Other small critters include: nematodes (bad, kill crops, reduce yields, transmit diseases), centipedes (predators of soil fauna), millipedes (feed on decaying plant matter), fly larvae and beetle larvae (feed on plant roots).

*  Ants and termites create large underground colonies that alter the structure, density, composition, and chemical make-up of soil. Termites bring in plant matter to their nests to create "fungus gardens."

*  Slugs and snails feed mainly on organic debris, which they break down and make it ready for colonization by bacteria.

*  Small mammals: foxes, gophers, badgers, moles, shrews, rabbits, and even birds serve to loosen and mix the soil through their burrowing activities.

*  Large mammals: herbivores provide huge quantities of dung to the soil organic content.

 

Plants:

*  Different vegetation types can directly and indirectly contribute to the type of soil found in a location. For example, an oak woodland will have an alkaline soil.  If a conifer forest then takes over, soils will become more acidic, thus changing its properties.

*  Littlerfall from plants and decaying roots provide the soil with organic matter and many of the soil fauna their nutrient supply.

*  Amount of litter can range from 2 tons per acre per year in temperate regions to as high as 10 tons per acre per year in the tropical rainforest.

*  Microbial activity will be seasonally dependent, increasing in numbers until the first frost occurs or some other event that limits their activity.

*  An important zone of interaction is the rhizosphere, which consists of the soil environment that surrounds the roots. The rhizosphere organisms include the nitrogen-fixers. Roots provide conditions favorable for microbial activity.

*  Mycorrhizae are symbiotic associations between fungi and plant roots that enable many plants to gain access to nutrients. Mycorrhizae are beneficial to plants, and crops can benefit from the injection of mycorrhizae cultures into the soil.

*  Mycorrhizae therefore act to extend the root system of plants. The fungus infects the root, getting organic matter from the root. But the fungus then extends its hyphae further into the soil, from which they absorb nutrient ions and deliver them to the host plant.

 


Relief

Elevation

*  An increase in elevation causes lower temperatures and higher amounts of rainfall. These changes in climate can create a sequence of soils along a mountainside as one increases elevation.

*  Higher elevations tend to be more humid and therefore more cloudy, causing a decrease of solar warming, less evapotranspiration, and colder, wetter soil conditions. These colder, wetter conditions, combined with less oxygen at higher elevations, reduce the speed of plant decomposition and favor development of thick O horizons.

 

Aspect

*  The compass direction in which a slope faces is called aspect.

*  The aspect of a slope determines the amount of solar warming received. For example, a south facing slope in the northern hemisphere will always be the warmer, drier slope, while a north-facing slope will always be a cooler, wetter slope. These differences in microclimatic properties cause differences in vegetation properties, and therefore differences in the soil type and properties.

 

Slope

*  The angle at which a slope resides can also affect soil properties. Slope affects the movement of water on the land surface and through its soils.

*  Sites on crests and ridge tops and steep slopes shed water into the neighboring low-lying areas. Therefore, lower slopes will remain wetter longer than upper-elevation slopes. Lower slopes will therefore experience greater leaching of soil constituents.

*  Soil material will always be drawn downslope due to gravity, whether in a solution or as a solid. Soil creep is the slow imperceptible movement of soil downslope due to wetting-drying and freeze-thaw processes.

*  The long-term effect of slope is to develop a sequence of soils that are linked by a progression of pedogeomorphic activities, each activity being unique to slope position. This sequence of soils is called a catena.

*  As a general rule, footslope areas will have deeper soils due to accumulation of weathered material and eroded soil known as colluvium. Note difference with alluvium.

 

Parent material

*  Note that parent material is not simply the “material that lies underneath the true soil horizons,” because many soils have developed from composite parent materials of differing origins. Classic examples are the presence of glacial till and wind-blown loess under the soil horizons, overlying the bedrock beneath. In this case, the soils can be a composite of glacial till, loess, and bedrock.

*  Parent material can be described as the consolidated or unconsolidated material, little affected by the present weathering cycle, from which the soil has developed. The key phrase here is that the parent material is not currently prone to weathering, but has been in the past over the long-term.

*  Remember that weathering represents geological processes acting upon the mantle to produce regolith. Soil formation represents pedological processes that act upon the material produced by weathering.

*  Weathering and soil formation processes can act together or separately. In old deep soils, weathering can occur well below the soil horizons that form nearer to the surface. In younger soils, both processes may occur within the top few centimeters.

*  In general, different rock types weather to produce distinct types of regolith which, in turn, create different soil types, depending on the mineral composition of the parent material.

 
Siliceous Crystalline Rocks – Felsic rocks like Granite and rhyolite or Mafic rocks like Gabbro, Bassalt, and Greenstone.  Both sets have a high concentration of magnesium but low contents of aliminum, calcium, potassium, and sodium.  Coarse grained rocks weather more quickly than fine grained rocks.

 

Felsic Rocks – Granite and Granite Gneiss have 25% quartz and 65% orthoclase (potassium feldspar), and small amounts of muscovite and hornblende.  Form soils that are coarse-loamy in texture.  The soils are friable, permeable, and acidic.  Soil colors are yellow to yellow-brown as a result of low iron content.  Kaolinite clay is often formed.

 

Mafic Rocks – Andesite, diorite, basalt, and hornblende gneiss are rich in iron and magnesium as well as plagioclase (calcic feldspars).  These rocks weather quickly and yielding a lot of clay and free iron.  Absence of quartz accounts for a low sand content in the soils so that textures are usually loams or clay loams.  Kaolinite, Halloysite, and Montmorillonite are common clays.

 


Sedimentary Rocks

Glacial Till – Physically ground to small particles creating a soil with a loamy texture.  Montmorillonite clay is the most common.

Loess – Windblown silts from glacial deposits.  85% silt and 15% clay with an abundance of weatherable minerals.  Montmorillonite clay is again dominant.  These soils have large nutrient reserves and good physical conditions for plant growth.

Limestone and Dolomite – Composed of more than 50% carbonates with remainder being silt, clay, quartz sand, and iron.  Soils are often red and clay rich.

Sandstone – Contains 50% sand-size particles with the rest consisting of impurities such as feldspar, mica, and cement of silica, iron or carbonates.  Most of the soils from these rocks are coarse in texture, acid, deep, and low in nutrients.

 
Time

*  Certain processes can provide situations where soil development must begin from the very initial stages: (1) volcanic eruptions (2), wind-blown loess and dune movement, (3) open-pit mining, (4) draining of lakes, and (5) glacial retreat, basically any process that exposes a new surface. These would be examples of primary succession in ecology. The rapidity of soil formation will depend largely on climate, parent material, and human intervention.

*  Examples: (1) eruption of Krakatoa in 1883 and El Malpais 9,980 year ago, (2) the retreat of Swiss glaciers during the last 100 years since the end of the Little Ice Age or the retreat of the glaciers in glacier bay Alaska over the past 250 years, (3) the draining of the polders in The Netherlands to create more arable land, (4) the movement of dunes into and out of an area, such as Great Sand Dunes national Monument.

*  Soils developed from sandy parent materials will rapidly achieve a mature soil profile because they contain few soluble materials.

*  Clay soils are slower draining and are therefore slower to develop a fully mature soil profile. From this, we can surmise that the development of mature soil profiles is heavily dependent on the ability of water to move through the soil profiles.

*  Fluvial action tends to sort alluvium into layers of different particle sizes. Different stream velocities will redeposit material of differing sizes.

*  Can soils be used to deduce climate change? Yes. Different climate regimes can lead to soils with differing properties and development of paleosols.

 


Catenas – When soils are developed on the same parent material and the soils only differ on the basis of drainage due to variations in relief.

 

Chronosequence – A sequence of related soils that differ in certain properties primarily as a result of time as a soil-forming process.

 

Lithosequence – A group of related soils that differ as a result of parent material.

 

Climosequence – A sequence of soils that differ as a result of changes in climatic regimes (temperature and precipitation).

 

Biosequence – A group of related soils that differ primarily due to variation in kinds and numbers of plants and soil organisms.