9) Soil Morphology

9.1) Introduction

9.2) Horizons

9.3) Boundary

9.4) Soil Color

9.5) Soil Texture

9.6) Soil Structure

9.7) Consistence

9.8) Roots

9.9) PH and Effervescence

9.10) Special Features

Further Reading:

Fanning D.S., M.C. Balluff Fanning. 1989. Soil: Morphology, Genesis, and Classification. New York, Wiley.

Schoeneberger P.J., D.A. Wysocki, E.C. Benham, and W.D. Broderson. 1998. Field Book for Describing and Sampling Soils. National Soil Survey Service - U.S. Department of Agriculture, Lincoln, Nebraska.

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9.1) Introduction

Soil morphology deals with the form and arrangement of soil features. Micromorphology is using micromorphological techniques (e.g. thin sections) and measurements in the laboratory. Field morphology is the study of soil morphological features in the field by thorough observation, description and interpretation. Observations may be refined with the aid of a hand lens. Simple tests are also used in the field to record salient chemical properties (e.g., pH, presence of carbonates). In addition, field observations and measurements may be refined through a range of laboratory analytical procedures that include more sophisticated evaluation of chemical, biological and physical attributes. However, the quality of field description and sampling ultimately defines the utility of any subsequent laboratory analyses. A keen eye that can discern specific features and their relationship to adjoining features coupled with well-calibrated fingers that can distinguish among relative differences in physical properties of soil material are essential and can only be acquired and maintained through practice. In this course we will focus on field morphology.

Field morphology starts with an in situ examination of a soil profile. Field descriptions are organized by subdividing a vertical exposure of the soil (soil profile) into reasonably distinct layers or horizons that differ appreciably from the horizons immediately above and below in one or more of the soil features listed below. The delineation of horizons is necessarily a somewhat subjective processes because changes in soil attributes are often gradational rather than abrupt. Thus, obvious boundaries between horizons are not always apparent and their assignment may require integrated assessment of changes in several attributes before a sensible and defensible delineation can be made. Knowledge of similar soils and a well-defined rationale for the purpose of the description helps considerably in development of systematic criteria for defining and delineating horizons.

The following information is collected for assembling standard profile descriptions:

Depth intervals of horizons or layers (measured from the top of the mineral horizon)

Horizon boundary characteristics

Color

Texture

Structure, pores

Consistence

Roots

pH, effervescence

Special features such as coatings, nodules, and concretions

Differences between horizons generally reflect the type and intensity of processes that have caused changes in the soil. Ideally, we should always be striving in our descriptions to maintain a link between process and morphology. In many soils, these differences are expressed by horizonation that lies approximately parallel to the land surface, which in turn reflects vertical partitioning in the type and intensity of the various processes that influence soil development. However, there are many exceptions to this preferred horizontal organization.

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9.2) Horizons

The epipedons (uppermost soil horizons) are described in chapter 8.4.2: Diagnostic Surface Horizons: Epipedons

9.2.1) Master Horizons 

Master horizons (major horizons) are designated by capital letters, such as O, A, E, B, C, and R.

O horizons: They are dominated by organic material. Some O layers consist of undecomposed or partially decomposed litter, such as leaves, twigs, moss, and lichens, that has been decomposed on the surface; they may be on the top of either mineral or organic soils. Other O layers, are organic materials that were deposited in saturated environments and have undergone decomposition. The mineral fraction of these layers is small and generally less than half the weight of the total mass. In the case of organic soils (peat, muck) they may compose the entire soil profile. Organic rich horizons which are formed by the translocation of organic matter within the mineral material are not designated as O horizons.

A horizons: Mineral horizons that formed at the surface or below an O layer, that exhibit obliteration of all or much of the original rock or depositional structure (in the case of transported materials). A horizons show one or more of the following:

An accumulation of humified organic matter intimately mixed with the mineral fraction and not dominated by characteristic properties of the E or B horizons or,

Properties resulting from cultivation, pasturing or other similar kinds of disturbance.

E horizons: Mineral horizons in which the main feature is loss of silicate clay, iron, aluminum, or some combination of these, leaving a concentration of sand and silt particles and lighter colors. The horizons exhibit obliteration of all of much of the original rock structure.

B horizons: Horizons in which the dominant feature(s) is one or more of the following:

An illuvial concentration of silicate clay, iron, aluminium, carbonates, gypsum, or humus

Removal of carbonates

A residual concentration of sesquioxides or silicate clays, alone or mixed, that has formed by means other than solution and removal of carbonates or more soluble salts

Coatings of sesquioxides adequate to give darker, stronger, or redder colors than overlying and underlying horizons but without apparent illuviation of iron

An alteration of material from its original condition that obliterates original rock structure, that form silicate clay, liberates oxides, or both, and that forms a granular, blocky, or prismatic structure

Any combination of these.

C horizons: Mineral horizons that are little altered by soil forming processes. They lack properties of O, A, E, or B horizons. The designation C is also used for saprolite, sediments, or bedrock not hard enough to qualify for R. The material designated as C may be like or unlike the material form the A, E, and B horizons are thought to have formed.

R Layers: Consolidated bedrock (hard bedrock), such as granite, basalt, quarzite, sandstone, or limestone. Small cracks, partially or totally filled with soil material and occupied by roots, are frequently present in the R layers.

9.2.2) Transitional Horizons

Transitional horizons are layers of the soil between two master horizons. There are two types of transitional horizons:

Horizons dominated by properties of one master horizon that also have subordinate properties of an adjacent master horizon. The designation is by two master horizon capital letters:

The first letter indicates the dominant master horizon characteristics

The second letter indicated the subordinate characteristics

For example, an AB horizon indicates a transitional horizon between the A and B horizon, but one that is more like the A horizon than the B horizon. An AB or BA designation can be used as a surface horizon if the master A horizon is believed to have been removed by erosion.

Separate components of two master horizons are recognizable in the horizon and at least one of the component materials is surrounded by the others. The designation is by two capital letters with a slash inbetween. The first letter designates the material of greatest volume in the transitional horizon. For example A/B, B/A, E/B or B/E.

9.2.3) Subordinate Distinctions Within Master Horizons

Lower case letters are used to designate specific features within master horizons. They are listed in alphabetical order below:

a: Highly decomposed organic material. The 'a' is used only with the O master horizon. The rubbed fiber content < 17 % of the volume.

b: Buried genetic horizon. It is not used in organic soils or to identify a buried O master horizon.

c: Concretions of hard nonconcretionary nodules. This symbol is used only for iron, aluminium, manganese, or titanium cemented nodules or concretions.

d: Physical root restriction. It is used to indicate naturally occuring or humanly induced layers such as basal till, plow pans, and other mechanically compacted zones. Roots do not enter except along fracture planes.

e: Organic material of intermediate decomposition. This symbol is only used in combination with an O master horizon with rubbed fiber content between 17 - 40 % of the volume.

d: Frozen soil. The horizon must contain permanent ice.

g: Gleying: This symbol is used in B and C horizons to indicate low chroma color (<= 2), caused by reduction of iron in stagnant saturated conditions. The iron may or may not be present in the ferrous form (Fe²⁺). The g is used to indicate either total gleying or the presence of gleying in a mottled pattern. It is not used in E horizons, which are commonly of low chroma, or in C horizons where the low chroma colors are inherited form the parent material and no evidence of saturation is apparent.

h: Illuvial accumulation of organic matter: Used only in B horizons. The h indicates an accumulation of illuvial, amorphous, dispersible organic matter with or without sequioxide component. If the sequioxide component contains enough iron so that the color value and chroma exceed 3 additionally a s is used (hs). The organosequioxide complexes may coat sand and silt particles, or occur as discrete pellets, or fill voids and cement the horizon (use of m).

i: Slightly decomposed organic material. Used only in combination with an O master horizon to designate that the rubbed fiber content is > 40 % of the volume.

k: Accumulation of carbonates, usually calcium carbonate. Used with B and C horizons.

m: Cementation or induration: Used with any master horizon, except R, where > 90 % of the horizon is cemented and roots penetrate only through cracks. The cementing material is identified by the appropriate letter:

km: carbonate

qm: silica

sm: iron

ym: gypsum

kqm: both lime and silica

zm: salts more soluble than gypsum

n: Accumulation of sodium: This symbol is used on any master horizon showing morphological properties indicative of high levels of exchangeable sodium.

o: Residual accumulation of sesquioxides.

p: Tillage or other cultivation disturbance (e.g. plowing, hoeing, discing). This symbol is only used in combination with the master horizon A or O.

q: Accumulation of silica: This symbol is used with any master horizon, except R, where secondary silica has accumulated.

r: Weathered soft bedrock: This symbol is only used in combination with the master C horizon. It designates saprolite or dense till that is hard enough that roots only penetrate along cracks, but which is soft enough that it can be dug with a spade or shovel.

s: Illuvial accumulation of sesquioxides and organic matter. This symbol is only used in combination with B horizons. It indicates the presence of illuvial iron oxides. It is often used in conjunction with h when the color is =< 3 (chroma and value).

ss: Presence of slickensides. They are formed by shear failure as clay material swell upon wetting. Their presence is an indicator of vertic characteristics.

t: Accumulation of silicate clay: The presence of silicate clay forming coats on ped faces, in pores, or on bridges between sand-sized material grains. The clay coats may be either formed by illuviation or concentrated by migration within the horizon. Usually used in combination with B horizons, but it may be used in C or R horizons also.

v: Plinthite: This symbol is used in B and C horizons that are humus poor and iron rich. The material usually has reticulate mottling of reds, yellows, and gray colors.

w: Development of color and structure. This symbol is used for B horizons that have developed structure or color different, usually redder than that of the A or C horizons, but do not have apparent illuvial accumulations.

x: Fragipan character: This symbol is used to designate genetically developed firmness, brittleness, or high bulk density in B or C horizons. No cementing agent is evident.

y: Accumulation of gypsum. This symbol is used in B and C horizons to indicated genetically accumulated gypsum.

z: Accumulation of salts more soluble than gypsum. This symbol is used in combination with B and C horizons.

Note: Arabic numerals can be added as suffixes to the horizon designations to identify subdivisions within horizons. For example, Bt1 - Bt2 - Bt3 indicated three subsamples of the Bt horizon.

9.2.4) Diagnostic Subsurface Horizons

The accumulation of substances such as silica, iron, aluminium, carbonate, and other salts can result in cemented layers, which change the physical, chemical, and biological behavior of the soil. For example, a cemented layer retards percolation and restrict root activity. Furthermore, the availablility of nutrients for plant growth is reduced, i.e., the cation exchange capacity is reduced. There are accumulations in the soil which show the enrichment of one substance and / or the depletion of another substance. This can be expressed by diagnostic subsurface horizons, which are listed in alphabetically order below. It should be stressed that some characteristics can be measured only in the laboratory and not in the field.

Agric horizon: It is formed directly under the plow layer and has silt, clay, and humus accumulated as thick, dark lamallae.

Albic horizon: Typically this is a light-colored E horizon with the color value >= 5 (dry) or >=4 (moist).

Argillic horizon: It is formed by illuviation of clay (generally a B horizon, where the accumulation of clay is denoted by a lower case 't') and illuviation argillans are usually observable unless there is evidence of stress cutans. Requirements to meet an argillic horizon are:

1/10 as thick as all overlying horizons

>= 1.2 times more clay than horizon above, or:

If eluvial layer < 15 % clay, then >= 3 % more clay, or:

If eluvial layer > 40 % clay, then >= 8 % more clay.

Calcic horizon: This layer has a secondary accumulation of carbonates, usually of calcium or magnesium. Requirements:

>= 15 cm thick

>= 5 % carbonate than an underlying layer

Cambic horizon: This subsurface often shows weak indication of either an argillic or spodic horizon, but not enough to qualify as either. It may be conceptually regarded as a signature of early stages of soil development, i.e soil structure or color development. Requirements:

Texture: loamy very fine sand or finer texture 

Formation of soil structure

Development of soil color

Duripan: It is a subsurface horizon cemented by illuvial silica. Air-dry fragments from more than 50 % of the horizon do not slake in water or HCl but do slake in hot concentrated KOH.

Fragipan: These subsoil layers are of high bulk density, brittle when moist, and very hard when dry. They do not soften on wetting, but can be broken in the hands. Air-dry fragements slake when immersed in water. Fragipan genesis as outlined in Soil Taxonomy is largely dependent on physical processes and requires a forest vegetation and minimal physical disturbance. Desiccation and shrinking cause develoment of a network of polygonal cracks in the zone of fragipan formation. Subsequent rewetting washes very fine sand, silt, and clay-sized particles from the overlying horizons into the cracks. Upon wetting, the added materials and plant roots growing into the cracks result in compression or the interprism materials. Close packing and binding of the matrix material with clay is responsible for the hard consistence of the dry prisms. Iron is usually concentrated along the bleached boundaries of the prisms. It has also been postulated that clay and sequioxides cements to be binding agents in fragipans. 

  Glossic horizon: It occurs usually between an overlying albic horizon and an underlying argillic, kandic, or natric horizon or fragipan. Requirements:

>= 5 cm thick

Albic material between 15% to 85 %, rest: material like the underlying horizon

Kandic horizon: It is composed of low activity clays, which are accumulated at its upper boundary. Clay skins may or may not be present. It is considered that clay translocation is involved in the process of kandic formation, however, clay skins may be subsequentlz disrupted or destroyed by physical and chemical weathering, or they may have formed in situ. Requirements:

Within a distance of < 15 cm at its upper boundary the clay content increases by > 1.2 times

Abrupt or clear textural boundary to the upper horizon

At pH 7: low-activity clays with CEC of <= 16 cmol/kg and ECEC (effective CEC) of <= 12 cmol/kg

Natric horizon: It is a subsurface horizons with accumulation of clay minerals and sodium. Requirements:

Same as argillic horizon

Prismatic or columnar structure

> 15 % of the CEC is saturated with Na⁺, or:

More exchangeable Na⁺ plus Mg²⁺ than Ca²⁺

  Oxic horizon: Requirements:

>= 30 cm thick

Texture: sandy loam or finer

At pH 7: CEC of <= 16 cmol/kg and ECEC of <= 12 cmol/kg (i.e., a high content of 1:1 type clay minerals)

Clay content is more gradual than required by the kandic horizon

< 10 % weatherable minerals in the sand

< 5 % weatherable minerals by volume rock structure (i.e., indicative of a very strongly weathered material)

Petrocalcic horizon: It is an indurated calcic horizon. Requirements:

At least 1/2 of a dry fragment breaks down when immersed in acid but does not break down when immersed in water

Petrogypsic horizon: This is a strongly cemented gypsic horizon. Dry fragments will not slake in H₂O.

Placic horizon: This is a dark reddish brown to black pan of iron and / or manganese. Requirements:

2 - 10 mm thick

It has to lie within 50 cm of the soil surface

Boundary: wavy

Slowly permeable

Salic horizon: This is an subsurface horizon accumulated by secondary soluble salts. Requirements:

>= 15 cm thick

Enrichment of secondary soluble salts such that electrical conductivity exceeds 30 dS/m more than 90 days each year

Sombric horizon: Formed by illuviation of humus (dark bron to black color) but not of aluminium or sodium. Requirements:

At pH 7: base saturation < 50 %

Not under an albic horizon

Free-draining horizon

Spodic horizon: This horizon has an illuvial accumulation of sequioxides and / or organic matter. There are many specific limitations dealing with aluminium, iron, and organic matter content, and clay ratios, depending on wheather the overlying horizon is virgin or cultivated.

Sulfuric horizon: This is a very acid mineral or organic soil horizon. Requirements:

pH < 3.5

Mottles are present (yellow color: jarosite)

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9.3) Boundary

The boundary between the horizons can be described considering the distinctness and topography. Distinctness refers to the degree of contrast between two adjoining horizons and the thickness of the transition between them. Topography refers to the shape or degree of irregularity of the boundary. In Figure 9.3.1 examples for several boundaries are shown.

Figure 9.3.1. Boundaries between soil horizons.

Table 9.3.1. Classification of horizon boundaries.

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9.4) Soil Color

Color reflects an integration of chemical, biological and physical transformations and translocations that have occurred within a soil . In general, color of surface horizons reflects a strong imprint of biological processes, notably those influenced by the ecological origin of soil organic matter (SOM). Soil organic matter imparts a dark brown to black color to the soil. Generally, the higher the organic matter content of the soil, the darker the soil. A brigh-light color can be related to an eluvial horizon, where sequioxides, carbonates and/or clay minerals have been leached out.

Subsoil color reflects more strongly in most soils the imprint of physico-chemical processes. In particular, the redox status of Fe and to a lesser extent Mn, strongly influence the wide variation found in subsoil color. Soil color can provide information about subsoil drainage and the soil moisture conditions of a soils. The colors of the Fe oxides and hydroxides were described in chapter 7.2 . In well aerated soils, Fe³⁺ is present which give soil a yellow or redish color. In more poorly drained soils (anaerobic conditions) iron compounds are reduced and the neutral gray colors of Fe²⁺ or bluish-green colors of iron sulfides, iron carbonates, or iron phosphates are visible. A black color in the subsoil can be related to an accumulation of manganese.

In arid and semi-arid environments, the influence of soluble salts (carbonates, sulfates, chlorides etc.) may impart a strong influence on soil color. For example, in arid or subhumid regions, surface soils may be white due to evaporation of water and soluble salts.

Colors associated with minerals inherited from parent materials may also influence color in horizons that have not been extensively weathered. For examle, light gray or nearly white colors is sometimes inherited from parent material, such as marl or quartz. Parent material, such as basalt, can imprint a black color to the subsoil horizons.

Table 9.4.1. Soil colors accociated with soil attributes.

Soil color is usually registered by comparison of a standard color chart (Munsell Book of Colors). The Munsell notation distinguishs three characteristics of the color: hue, value, and chroma.

Hue: It is the dominant spectral color, i.e., wheather the hue is pure color such as yellow, red, green, or a mixture of pure colors.

Value: It describes the degree of lightness or brightness of the hue reflected in the property of the gray color that is being added to the hue.

Chroma: It is the amount of a particular hue added to a gray or the relative purity of the hue.

Figure 9.4.1. Munsell soil color chart.

The soil colors are given in the order: hue, value, and chroma. For example, 2.5YR 4/2 describes the hue 2.5YR, dark-grayish brown with a value 4 and a chroma of 2. It should be stressed that soil color is dependent on soil moisture, hence if soil color is recorded also the soil moisture conditions have to be described (e.g. soil color dry, soil color wet). In the upper midwest and other humid areas, colors are conventionally recorded moist. This convention may differ in other climatic regimes.

Many soils have a dominant soil color. Other soils, where soil forming factors vary seasonally (e.g. wet in winter, dry in summer) tend to exhibit a mixture of two or more colors. When several colors are present the term mottling or redoximorphic features (RMF) is used. In such a case, several soil colors have to be recorded, where the dominant color is first, following by a description of the abundance, size, and contrast of the other colors in the mottled pattern. Mottling/RMFs are described by three characteristics: contrast, abundance, and size of area of each color.

Redoximorphic features are a color pattern in a soil due to loss (depletion) or gain (concentration) of pigment compared to the matrix color. It is formed by oxidation / reduction of Fe and/or Mn coupled with their removal and translocation or a soil matrix color controlled by the presence of Fe²⁺. RMFs are described separately from other mottles or concentrations! Based on the Field Book for Describing and Sampling Soils (Schoeneberger et al., 1998) RMFs are described in terms of kind, color & contrast, qantity, size, shape, location, composition & hardness, and boundary. RMFs occur in the soil matrix, on or beneath the surface of peds, and as filled pores, linings of pores, or beneath the surface of pores.

Mottles are areas of color that differ from the matrix color. These colors are commonly lithochromic or lithomorphic attributes retained from the geologic source rather than from pedogenesis. Mottles exclude RMFs and ped & void surface features (e.g. clay films). Based on the Field Book for Describing and Sampling Soils (Schoeneberger et al., 1998) mottles are described in terms of quantity, size, color & contrast, moisture state, and shape. Example: Few, medium, distinct, reddish yellow moist (7.5YR 7/8), irregular mottles.

However, a variety of other features in a horizon may have colors different from the matrix, such as infillings of animal burrows (krotovinas), clay coatings (argillans) and precipitates of calcium carbonate. In all instances where specific soil features are described, the shape and spatial relationships of the feature (i.e., where is it located, on a ped face, in the matrix...) to adjacent features should be described in addition to its color, abundance, size and contrast.

Table 9.4.2. RMFs/mottles in soils are described in term of abundance, size, and contrast.

Contrast	Abbreviation	Visibility
faint	f	difficult to see, heu and chroma of matrix and mottles closely related
distinst	d	readily seen, matrix and mottles vary 1 - 2 hues and several units in chroma and value
prominent	p	conspicious, matrix and mottles vary several units in hue, value, and chroma

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9.5) Soil Texture

9.5.1. Classification

Texture refers to the amount of sand, silt,and clay in a soil sample. The distribution of particle sizes determines the soil texture, which can be assessed in the field or by a particle-size analysis in the laboratory. A field analysis is carried out in the following way: a small soil sample is taken, water is added to the sample, it is kneaded between the fingers and thumb until the aggregates are broken down. The guidelines to determine the particle class are as following:

Sand : Sand particles are large enough to grate against each other and they can be detected by sight. Sand shows no stickiness or plasticity when wet.

Silt: Grains cannot be detected by feel, but their presence makes the soil feel smooth and soapy and only very slightly sticky.

Clay: A characteristic of clay is the stickiness. If the soil sample can be rolled easily and the sample is sticky and plastic when wet (or hard and cloddy when dry) it indicates a high clay content. Note that a high organic matter content tend to smoothen the soil and can influence the feeling for clay.

Table 9.5.1.1. Soil texture classes.

A variety of systems are used to define the size ranges of particles, where the ranges of sand, silt, and clay that define a particle class differs among countries. In the U.S. the soil texture is classified based on the U.S.D.A. system, which is used in this course. The classification of particle sizes are the following (units: mm):

xxclay: < 0.002xx

xxxx xxsilt: 0.002 - 0.05xx

xxxxxxxxxxfine sand: 0.05 - 0.1xx

x xxxxxxxxxxxxxxmedium sand: 0.1 - 0.5xx

xxxxxxxxxxxxxxxxxxxxxcoarse sand: 0.5 - 1.0xxx x

xxxxxxxxxxxxxxxxxxxxxxxxxvery coarse sand: 1.0 - 2.0xxx

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxgravel: 2.0 - 762.0xx

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxcobbles: > 762.0xx

Soil texture in the field is determined using a texture triangle (Figure 9.5.1.1.). For example, a particle size distribution of 33 % clay, 33 % silt, and 33 % sand would result in the soil texture class 'clay loam'.

 Figure 9.5.1.1. Trinagular diagram of soil textural classes (USDA triangle).

Particles greater than 2 mm are removed from a textural soil classification. The presence of larger particles is recognized by the use of modifiers added to the textural class (e.g. gravelly, cobbly, stony) (Table 9.5.1.2. and 9.5.1.3).

Table 9.5.1.2. Terms for rock fragments. 

Shape and size [mm]	Adjective
Spherical and cubelike: 2 - 75 2 - 5 5 - 20 20 - 75 75 - 250 250 - 600 > 600	xxx gravelly fine gravelly medium gravelly coarse gravelly cobbly stony bouldery
Flat: 2 - 150 150 - 380 380 - 600 > 600	channery flaggy stony bouldery

Table 9.5.1.3. Modifier for rock fragments.

Rock fragments by volume [%]	Adjectival modifier
< 15	no modifier
15 - 30	gravelly loam
30 - 60	very flaggy loam
> 60	extremely bouldery loam

The distinction between a mineral and an organic horizon is made by the organic carbon content. Layers which contain > 20 % organic carbon and are not water saturated for periods more than a few days are classed as organic soil material. If a layer is saturated for a longer period it is considered to be organic soil material if it has:

> = 12 % organic carbon and no clay, or

>= 18 % organic carbon and >= 60 % clay, or

12 - 18 % organic carbon and 0 - 60 % clay.

Figure 9.5.1.2. Relationship between soil texture and pore size.

9.5.2) Significance of Soil Texture

The fine and medium-textured soils (e.g. clay loams, silty clay loams, sandy silt loams) are favorable from an agricultural viewpoint because of their high available retention of water and exchangeable nutrients. In fine pores the water is strongly adsorbed in pores but not available for plants, i.e. cohesion and adhesion water occupy the micropore space and they are retained in soil by forces that exceed gravity. In medium-sized pores the available water content is high, whereas in macropores water is more weakly held and percolation is high (gravitational water). In silty soils the distribution of macropores, medium-sized, and fine pores is optimal relating to available water content.

Table 9.5.2.1. Pore size distribution in soils different in texture (Scheffer et al., 1989).

In general, coarse-textured soils permit rapid infiltration because of the predominance of large pores, while the infiltration rates of finer-textured soils is smaller because of the predominance of micropores. Other factors, like the compaction of the soil, managment practices, vegetation, saturation of the soil have also a significant impact on infiltration and have to be considered.

Soil texture has an impact on soil temperature. Fine-textured soils hold more water than coarse-textured soils, which considering the differences in the specific heat capacity results in a slow response of warming up of fine-textured soils compared to coarse-textured soils.

Another issue to address is the effect that with decreasing particle size the surface area increases. Many important chemical and biological properties of soil particles are functions of particle size and hence surface area. For example, the adsorption of cations (nutrients) or the microbial activity are dependent on surface area.

Reference

Scheffer F., and Schachtschabel P., 1989. Lehrbuch der Bodenkunde. Enke-Verlag Stuttgart.

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 9.6) Soil Structure

9.6.1) Classification

Structure refers to the arrangement of soil particles. Soil structure is the product of processes that aggregate, cement, compact or unconsolidate soil material. In essence, soil structure is a physical condition that is distinct from that of the initial material from which it formed, and can be related to processes of soil formation. The peds are separated from the adjoining peds by surfaces of weakness. To describe structure in a soil profile it is best to examine the profile standing some meters apart to recognize larger structural units (e.g. prisms). The next step is to study the structure by removing soil material for more detailed inspection. It should be stressed that soil moisture affects the expression of soil structure. The classification of soil structure considers the grade, form, and size of particles.

The grade describes the distinctiveness of the peds (differential between cohesion within peds and adhesion between peds). It relates to the degree of aggregation or the develoment of soil structure. In the field a classification of grade is based on a finger test (durability of peds) or a crushing of a soil sample.

The form is classified on the basis of the shape of peds, such as spheroidal, platy, blocky, or prismatic. A granular or crumb structure is often found in A horizons, a platy structure in E horizons, and a blocky, prismatic or columnar structure in Bt horizons. Massive or single-grain structure occurs in very young soils, which are in an initial stage of soil development. Another example where massive or single-grain structure can be identified is on reconstruction sites. There may two or more structural arrangements occur in a given profile. This may be in the form of progressive change in size/type of structural units with depth (e.g. A horizons that exhibit a progressive increase in size of granular peds that grade into subangular blocks with increasing depth) or occurrence of larger structural entities (e.g. prisms) that are internally composed of smaller structural units (e.g. blocky peds). I such a case all discernible structures should be recorded (i.e. more rather than less detail).

The size of the particles have to be recorded as well, which is dependent on the form of the peds.

Table 9.6.1.1. Classification of soil structure considering grade, size, and form of particles.

Size	Angular and subangular blocky structure [mm] diameter	Granular and crumb structure [mm] diameter	Platy structure [mm] width	Prismatic and columnar structure [mm] diameter
Very fine	< 5	< 1	< 1 (very thin)	< 10
Fine	5 - 10	1 - 2	1 - 2 (thin)	10 - 20
Medium	10 - 20	2 - 5	2 - 5	20 - 50
Coarse	20 - 50	5 - 10	5 - 10 (thick)	50 - 100
Very coarse	> 50	> 10	> 10 (very thick)	> 100

Figure 9.6.1.1. Soil structures (Foth, 1984)

The three characteristics of soil structure are conventionally written in the order grade, size, and shape. For example, weak fine subangular blocky structure.  

 The distribution of different particle sizes in a soil influence the distribution of pores, which can be characterized by their abundance, size, and shape.

Table 9.6.1.2. Abundance, size, and shape of pores.

9.6.2) Significance of Soil Structure

Soil formation starts with a structureless condition, i.e., the structure is single-grained or massive. Soil development also means development of soil structure, which describes the formation of peds and aggregates. Soil structure forms due to the action of forces that push soil particles together. Subsurface structure tends to be composed of larger structural units than the surface structure. Subsoil structure also tend to have the binding agents on ped surfaces rather than mixed throughout the ped.

Climatically-driven physical processes that result in changes in the amount, distribution and phase (solid, liquid, vapor) of water exert a major influence on formation of soil structure. Phase changes (shrinking-swelling, freezing-thawing) result in volume changes in the soil, which over time produces distinct aggregations of soil materials.

Physico-chemical processes (e.g., freeze-thaw, wet-dry, clay translocation, formation/removal of pedogenic weathering products) influence soil structure formation through out the profile. However, the nature and intensity of these processes varies with depth below the ground surface. The structure and hydrological function of plant communities, texture, mineralogy, surface manipulation and topography all serve to modify local climatic effects through their influence on infiltration, storage and evapotranspiration of water.

Biological processes exert a particularly strong influence on formation of structure in surface horizons. The incorporation of soil organic matter is usually largest in surface horizons. Soil organic matter serves as an agent for building soil aggregates, particularly the polysaccharides appear to be responsible for the formation of peds. Plant roots exert compactive stresses on surrounding soil material, which promotes structure formation. Soil-dwelling animals (e.g., earth worms, gophers) also exert compactive forces, and in some cases (e.g., earth worms) further contribute to structure formation via ingestion/excretion of soil material that includes incorporated organic secretions.

Reference

Foth H.D., 1984. Fundamentals in Soil Science. John Wiley & Sons, Inc.

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9.7) Consistence 

Consistence refers to the cohesion among soil particles and adhesion of soil to other substances or the resistence of the soil to deformation. Whereas soil structure deals with the arrangement and form of peds, consistence deals with the strength and nature of the forces between particles. Consistence is described for three moisture levels: wet, moist, and dry. The stickiness describes the quality of adhesion to other objects and the plasticity the capability of being molded by hands. Wet consistence is when the moisture content is at or slightly more than field capacity. Moist consistence is a soil moisture content between field capacity and the permanent wilting point. When recording consistence it is important to record the moisture status as well. Cementation is also considered when consistence is described in the field. Cementing agents are calcium carbonate, silica, oxides of iron and aluminium.

Table 9.7.1. Classification of consistence (Buol et al., 1997).

Reference

Buol S.W., Hole F.D., McCracken R.J., and Southard R.J., 1997. Soil Genesis and Classification. Iowa State University Press.

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9.8) Roots

Plant roots give evidence of the plant root activity and the penetration. For example, it is important to record if roots only penetrate through cracks, are retarded by waterlogged layers or cemented layers. Other reasons for limited root penetration can be soil compaction or the absence of nutrients. If there is no obstacle to root growth in the soil the roots may be distributed evenly in a soil. It is important to record the quantity and diameter of roots.

Table 9.8.1. Classification of roots.

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9.9) PH and Effervescence

The acidity of a soil can be tested using a simple field test set for fast pH determination. The pH is important for the pH dependent charge of silicates and organic material, therefore for the cation exchange capacity. Furthermore, the pH determines which buffering system is active, i.e. how soils can cope with additional H⁺ ions. For example, buffering systems are carbonates, organic matter, silicates, or iron and aluminium oxihydroxides.

Using HCl on a small soil sample the reaction (effervescence) can give clues of the calcium carbon content in the sample.

2 HCl + CaCO₃ <--> CaCl₂ + H₂CO₃ (effervescence)

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9.10) Special Features

Special features occur is soils which should be recorded additionally. Ped exteriors include clay coats, organic matter coats, silt coats, sand coats, carbonate coats, manganese coats, slickensides, stress surfaces, and clay bridges between sand grains. Ped interiors include concentrations of oxides, nodules, soft accumulations, pseudo-rock fragements, plinthite, and streaks. In particular, concretions are resulting from alternate periods of reducing and oxidizing regimes. Another special feature might be the evidence of animal activity by burrowing animals or high eaethworm acitivty. 

Concentrations

Def: Soil features that form by accumulation of material during pedogenesis. Processes involved: Chemical dissolution/precipitation, oxidation and reduction, physical and/or biological removal, transport, and accumulation

 Types:

Finely disseminated materials:

Small precipitates (e.g. salts, carbonates) dispersed throughout the matrix of a horizon

Concentrations

Masses:

Noncemented bodies of accumulation of various shapes that cannot be removed as discrete units (e.g. crystalline salts)

Nodules:

Cemented bodies of various shapes that can be removed as discrete units from soil

Concretions:

Cemented bodies similar to nodules, except for the presence of visible, concentric layers of material around a point, line, or plane

Crystals:

Macro-crystalls forms of relatively soluble salts (e.g. gypsum, carbonates) that form in situ by precipitation from soil solution

Biological concentrations:

Discrete bodies accumulated by a biological process (e.g., fecal pellets, insect casts)

Ped & Void Surface Features

These features are coats/films or stress features formed by translocation and deposition, or shrink-swell processes on or along surfaces. They are described in terms of kind, amount, continuity, distinctness, location, and color.

 Examples:

Ferriargillans (Fe ³⁺ stained clay films)

Mangans (black, thin films of Mn)

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