Soil Survey Manual - Chapter Three (Part 8 of 9)

Examination and Description of Soils

Table of Contents

Chapter 3 Full Table of Contents

Concentrations
    Describing Concentrations Within the Soil
Consistence
    Rupture Resistance Block-like Specimens
    Rupture Resistance Plate-Shaped Specimens
    Plasticity
    Toughness
    Stickiness
    Manner of Failure
    Penetration Resistance
    Excavation Difficulty

Concentrations

The features discussed here are identifiable bodies within the soil that were formed by pedogenesis. Some of these bodies are thin and sheet-like; some are nearly equidimensional; others have irregular shapes. They may contrast sharply with the surrounding material in strength, composition, or internal organization. Alternatively, the differences from the surrounding material may be slight. Soft rock fragments which have rock structure but are weakly cemented or noncemented are not considered concentrations. They are excluded on the basis of inference as to a geological as opposed to a pedological origin.

Masses are noncemented concentrations of substances that commonly cannot be removed from the soil as a discrete unit. Most accumulations consist of calcium carbonate, fine crystals of gypsum or more soluble salts, or iron and manganese oxides. Except for very unusual conditions, masses have formed in place.

Plinthite consists of reddish, iron-enriched bodies that are low in organic matter and are coherent enough to be separated readily from the surrounding soil. Plinthite commonly occurs within and above reticulately mottled horizons. Plinthite has higher penetration resistance than adjacent brown or gray bodies or than red bodies that do not harden. Soil layers that contain plinthite rarely become dry in the natural setting. The bodies are commonly about 5 to 20 mm across their smallest dimension. Plinthite bodies are firm or very firm when moist, hard or very hard when air dry, and become moderately cemented on repetitive wetting and drying. They occur as discrete nodules or plates. The plates are oriented horizontally. The nodules occur above and the plates within the upper part of the reticulately mottled horizon. The plates generally have a uniformly reddish color and have sharp boundaries with the surrounding brown or gray material. The part of the iron-rich body that is not plinthite normally stains the fingers when rubbed while wet, but the plinthite center does not. It has a harsh, dry feel when rubbed, even if wet. Horizons containing plinthite are more difficult to penetrate with an auger than adjacent horizons at the same water state and clay content but which do not contain plinthite. Plinthite generally becomes less cemented after prolonged submergence in water. An air dry sample can be dispersed by normal procedures for particle-size distribution.

Nodules and concretions are cemented bodies that can be removed from the soil intact. Composition ranges from material dominantly like that of the surrounding soil to nearly pure chemical substances entirely different from the surrounding material. Their form is apparently not governed by crystal forms based on examination at a magnification of 10X as is the case for crystals and clusters of crystals. It is impossible to be sure if some certain nodules and concretions formed where they are observed or were transported.

Concretions are distinguished from nodules on the basis of internal organization. Concretions have crude internal symmetry organized around a point, a line, or a plane. Nodules lack evident, orderly, internal organization. A typical example of a concretion organized around a point is illustrated in figure 3-33. The internal structure typically takes the form of concentric layers that are clearly visible to the naked eye. A coat or a very thin outer layer of an otherwise undifferentiated body does not indicate a concretion.

Figure3-33 (Click here or on image for high resolution 267 KB image.)

Nodules and concretions: an unbroken nodule at the left, a broken concretion at the right, a hollow nodule in the middle

Crystals are considered to have been formed in place. They may occur singly or in clusters. Crystals of gypsum, calcite, halite, and other pure compounds are common in some soils. These are described as crystals or clusters of crystals, and their composition is given if known.

Ironstone is an in-place concentration of iron oxides that is at least weakly cemented. Ironstone nodules are commonly found in layers above plinthite. These ironstone nodules are apparently plinthite that has cemented irreversibly as a result of repeated wetting and drying. Commonly, the center of iron-rich bodies cements upon repeated wetting and drying but the periphery does not.

Describing Concentrations Within the Soil

Any of a large number of attributes of concentrations within the soil may be important; the most common are number or amount, size, shape, consistence, color composition, kind, and location. Not all of these attributes are necessarily described. The order as listed above is convenient for describing them, as for example: "many, fine, irregular, hard, light gray, carbonate nodules distributed uniformly through the horizon." The conventions for describing kind have been indicated in this section. Descriptions of consistence and color are discussed in other parts of this chapter.

Amount or quantity of concentrations refers to the relative volume of a horizon or other specified unit occupied by the bodies. The classes used for quantity of mottles are also used for these features.

Size may be measured directly or given by the classes listed below. The dimension to which size-class limits apply depends on the shape of the body described. If the body is nearly uniform, size is measured in the shortest dimension, such as the effective diameter of a cylinder or the thickness of a plate. For irregular bodies, size refers to the longest dimension unless that creates an erroneous impression; measurements can be given if needed.

The following size classes are used:

• fine: < 2 mm
• medium: 2 - 5 mm
• coarse: 5 - 20 mm
• very coarse: 20 - 76 mm
• extremely coarse: > 76 mm

Shape of concentrations is variable both among kinds of concentrations and commonly within a concentration. The following terms are suggested:

rounded: Approximately equidimensional, a few sharp corners, and at least approximately regular.

cylindrical: At least crudely cylindrical or tubular; one dimension is much greater than the other two.

platelike: Shaped crudely like a plate; one dimension is very much smaller than the other two. The term "platelike" is used to avoid confusion with platy structure.

irregular: Characterized by branching, convoluted, or mycelial form.

The terms listed apply to all concentrations. Individual crystals of a particular mineral usually implies a shape.

Composition of bodies is described if known and if important for understanding their nature or the nature of the soil in which they are observed. Some of the physical attributes of the interior of a feature are implied by the name. Other features, such as enclosed mineral grains, patterns of voids, or similarity to the surrounding soil, may be important.

A distinction is made between bodies that are composed dominantly of a single substance and those that are composed of earthy material impregnated by various substances. For many bodies, the chemical composition cannot be determined with certainty in the field. The following set of terms, however, is useful for describing composition.

If the substance dominates the body, then the body is described as a substance body. If the substance impregnates other material, the body is described as a body of substance accumulation.

Carbonates and iron are common substances that dominate or impregnate nodular or concretionary bodies. Discrete nodules of clay are found in some soils; argillaceous impregnations are less common. Materials dominated by manganese are rare, but manganese is conspicuous in some nodules that are high in iron and mistakenly called "manganese nodules."

Consistence

Soil consistence in the general sense refers to "attributes of soil material as expressed in degree of cohesion and adhesion or in resistance to deformation on rupture." As employed here consistence includes: (1) resistance of soil material to rupture, (2) resistance to penetration, (3) plasticity, toughness, and stickiness of puddled soil material, and (4) the manner in which the soil material behaves when subject to compression. Although several tests are described, only those should be applied which may be useful.

A word may be in order about the similar term, consistency. Consistency was used originally in soil engineering for a set of classes of resistance to penetration by thumb or thumbnail (test designation D 2488, ASTM, 1984). The term has been generalized to cover about the same concept as "consistence." The set of tests specified, however, is different from those given here.

Consistence is highly dependent on the soil-water state and the description has little meaning unless the water state class is specified or is implied by the test. Previously class sets were given for "dry" and "moist" consistence of the soil material as observed in the field. "Wet" consistence was evaluated for puddled soil material. Here the terms used for "moist" consistence previously are applied to the wet state as well. The previous term "wet consistence" is dropped. Stickiness, plasticity, and toughness of the puddled soil material are independent tests.

For determinations on the natural fabric, variability among specimens is likely to be large. Multiple measurements may be necessary. Recording of median values is suggested in order to reduce the influence of the extremes measured.

Rupture Resistance Block-like Specimens

Table 3-14 contains the classes of resistance to rupture and the means of determination for specimens that are block-like. Different class sets are provided for moderately dry and very dry soil material, and for slightly dry and wetter soil material. Unless specified otherwise, the soil-water state is assumed to be that indicated for the horizon or layer when described. Cementation is an exception. To test for cementation, the specimen is air-dried and then submerged in water for at least 1 hour. The placements do not pertain to the soil material at the field water state.

The blocklike specimen should be 25-30 mm on edge. Direction of stress relative to the in-place axis of the specimen is not defined unless otherwise indicated. The specimen is compressed between extended thumb and forefinger, between both hands, or between the foot and a nonresilient flat surface. If the specimen resists rupture by compression, a weight is dropped onto it from increasingly greater heights until rupture. Failure is at the initial detection of deformation or rupture. Stress applied in the hand should be over a 1-second period. The tactile sense of the class limits may be learned by applying force to top loading scales and sensing the pressure through the tips of the fingers or through the ball of the foot. Postal scales may be used for the resistance range that is testable with the fingers. A bathroom scale may be used for the higher rupture resistance.

Specimens of standard size and shape are not always available. Blocks of specimens that are smaller than 25-30 mm on edge may be tested. The force withstood may be assumed to decrease as the reciprocal of the dimension along which the stress is applied. If a block specimen with a length of 10 mm along the direction the force is applied were to be ruptured, the force should be one-third that for an identical specimen 30 mm on edge. If the specimen is smaller than the standard size, the evaluated rupture resistance should be recorded and the dimensions of the specimen along the axis the stress is applied should be indicated.

Soil structure complicates the evaluation of rupture resistance. If a specimen of standard size can be obtained, report the rupture resistance of the standard specimen and other individual constituent structural units as desired. Usually the constituent structural units must exceed about 5 mm in the direction the stress is applied; expression must exceed weak for the rupture resistance to be evaluated.

If structure size and expression are such that a specimen of standard size cannot be obtained, then the soil material overall is loose. Structural unit resistance to rupture may be determined if the size is large enough (exceed about 5 mm in the direction stress is applied) for a test to be performed.

Table 3-14. Rupture resistance classes for blocklike specimens

Rupture Resistance Plate-Shaped Specimens

Tests are described that are applicable to plate-shaped specimens where the length and width are several times more than the thickness. Test procedures were developed for surface crusts but are applicable to plate-shaped bodies at greater depth in the soil. An alternative method of directly measuring plate-shaped specimens is to break them into a crudely blocked form. If the dimensions of the resulting block specimens are smaller than 25-30 mm on edge, it would be assumed that the measured rupture resistance is lower by 25.

Rupture Resistance by Crushing.—This test was designed primarily for air dry surface crust, but it may be used for other soil features. The morphological description of surface crust is discussed earlier in this chapter. The specimen should be 10 to 15 mm on edge and 5 mm thick or the thickness of occurrence if less than 5 mm. If surface crust, the thickness is inclusive of the crust proper and the adhering soil material beneath. The specimens are small to make the test applicable to crusts with closely spaced cracks. The specimen is grasped on edge between extended thumb and first finger. Force is applied along the longer of the two principal dimensions. Table 3-15 contains a set of classes. Compression to failure should be over about one second. A scale may be used to both rupture the specimens directly and develop the finger tactile sense. Force is applied with the first finger through a bar 5 mm across on the scale to create a similar bearing area to that of the plate-like specimen. The specimen is compressed between thumb and first finger while simultaneously exerting the same felt pressure on the scale with the first finger of the other hand. The scale is read at the failure of the specimen. For specimens that cannot be broken between thumb and forefinger, the resistance to rupture may be evaluated using a small penetrometer. The specimen is formed with the two larger surfaces parallel and flat. The specimen is placed with a larger face downward on a nonresilient surface and force is applied through the 6 mm diameter penetrometer tip until rupture occurs.

Table 3-15. Rupture resistance classes applied to crushing plate-shaped specimens

For plate-shaped bodies that are durable enough to withstand handling, such as fragments of fissile sedimentary rock, a modulus of rupture estimation is an appropriate test (Reeve, 1965). In practice, modulus of rupture tests commonly would be used to acquire a tactile sense which then would be used directly in the field. Insufficient experience has been obtained to provide classes.

The tests to follow are hand-held tests. The configuration of the tests do not conform rigorously to the requirements for measurement of modulus of rupture. Furthermore, the amount of force applied may be only roughly approximated. For these reasons, the test results are only a crude measure of the modulus of rupture.

In one test, a specimen is held in contact with a small diameter cylindrical shaft (pencil, nail, and so on) placed near the center of the specimen. Stress is applied by pressing in opposite directions with the two first fingers and the thumbs until rupture occurs. The equation for the modulus of rupture in MPa is:

S = (0.15FL)/(bd²)

where F is the force in newtons, L is the distance between the shaft and the inside edge of the area over which the force is applied on either side of the shaft with the fingers, b is the width of the specimen (in centimeters), and d is the depth or thickness in the direction of the load (in centimeters). The force application is based on the tactile sense and hence is approximate.

In the other approach, the specimen is grasped firmly at one end with pliers and force is applied downward at an established distance (to the nearest 1 cm) from the edge of the pliers. The area over which the force is applied should be small. The flat-end rod penetrometer described in the section on micropenetration resistance works well. A chisel point may be mounted over the tip. Modulus of rupture, S, expressed in megapascals (MPa), is calculated by:

S = (0.6FL)/(bd²)

where F is the force in newtons, L is the distance between the end of the jaws of the pliers and the inside edge of the area where the force is applied, b is width, and d is the thickness. The dimensions are all in centimeters. Length and width are estimated to 1 cm and thickness to 1 mm.

Plasticity

Plasticity (table 3-16) is the degree to which puddled soil material is permanently deformed without rupturing by force applied continuously in any direction. Plasticity is determined on material smaller than 2 mm.

The determination is made on thoroughly puddled soil material at a water content where maximum plasticity is expressed. This water content is above the plastic limit, but it is less than the water content at which maximum stickiness is expressed. The water content is adjusted by adding water or removing it during hand manipulation. The closely related plastic limit that is used in engineering classifications is the water content for < 0.4 mm material at which a roll of 3 mm in diameter which had been formed at a higher water content breaks apart (method D 4318 in ASTM, 1984).

Table 3-16. Plasticity classes

Toughness

Toughness is related to plasticity. Table 3-17 contains a set of classes. The classes are based on the relative force necessary to form with the fingers a roll 3 mm in diameter of < 2 mm soil material at a water content near the plastic limit (test D 2488 in ASTM, 1984).

Table 3-17. Toughness classes

Stickiness

Stickiness refers to the capacity of a soil to adhere to other objects. Table 3-18 contains a set of classes. The determination is made on puddled <2 mm soil material at the water content at which the material is most sticky. The sample is crushed in the hand; water is applied while manipulation is continued between thumb and forefinger until maximum stickiness is reached.

Table 3-18. Stickiness classes

Manner of Failure

The manner in which specimens fail under increasing force ranges widely and usually is highly dependent on water state. To evaluate the manner of failure, a roughly cubical specimen 25-30 mm on edge is pressed between extended forefinger and thumb and/or a handful of soil material is squeezed in the hand. Table 3-19 contains sets of classes and related operations. Some soil materials although wet are brittle; some may be compressed markedly without cracks appearing; others, if wet, behave like liquids; and still others smear if stressed under shear to failure.

Soil in the slightly moist or dry states, if coherent, is nearly always brittle and probably would not exhibit smeariness; consequently, manner of failure is probably only useful for moderately moist or wetter soil material.

Table 3-19. Manner of failure classes

Penetration Resistance

Penetration resistance is the capacity of the soil in its confined state to resist penetration by a rigid object. Shape and size of the penetrating object must be defined. Penetration resistance depends strongly on the water state, which should be specified.

The classes in table 3-20 pertain to the pressure required to push the flat end of a cylindrical rod with a diameter of 6.4 mm a distance of 6.4 mm into the soil in about 1 second (Bradford, 1986). Orientation of the axis of insertion should be specified. A correction should be made for the weight of the penetrometer if the axis of insertion is vertical and the resistance is small. If rock fragments are present, the lower values measured are probably more descriptive of the fine earth fabric.

Table 3-20. Penetration resistance classes

A standard instrument is the pocket penetrometer shown in Bradford (1986). Penetrometers with the same 6.4-mm diameter flat end tip and a dial reading device are available. The resistance can be read with less variability using the dial device. The scale on the barrel of the pocket penetrometers should be converted to units of force. The supplied scale on such instruments commonly is based on a regression between penetration resistance and unconfined, compressive strength measurements and has no application in the context here. Penetration resistance is expressed in units of pressure. The preferred unit is the megapascal; the symbol is MPa. For the 6.4-mm diameter tip, the measured force in kilograms is multiplied by 0.31 to obtain the pressure in megapascals. To extend the range of the instrument, weaker and stronger springs may be substituted. Values in megapascals obtained with any diameter of flat-end rod are used to enter the set of classes in table 3-20. Cone-shaped tips may be mounted on the penetrometers with flat ends as well as other penetrometers. Two 30-degree cone penetrometer tips are specified by the American Society of Agricultural Engineers (1982). One has a base area of 1.3 cm²; the other, 3.2 cm². Insertion should be to where the base of the cone is flush with the soil surface. Insertion times of 2 seconds and 4 seconds, respectively, should be used for the smaller and the larger cones. A relationship between the cone tips and the specified rod with a flat end must be established before table 3-20 can be used to enter cone measurements.

Determination of penetration resistance while the soil layer is at or near its maximum water content is a useful strategy for evaluation of root limitations. The relationship between penetration resistance and root growth has been the subject of numerous studies—Blanchar et al., 1978; Campbell et al., 1974; Taylor et al., 1966; and Taylor and Ratliff, 1969. These studies suggest the following generalities, which may need modification for particular plants and soils. First, if the soil material is wet or very moist and there are no closely spaced vertical structural planes, the limit of 2 MPa (6.4 mm flat-end rod) indicates strong root restriction for several important annual crops. This is the basis for the penetration resistance criterion in the criteria for physical root restriction. Secondly, between 2 and 1 MPa, root restriction may be assumed to decrease roughly linearly. Finally, below 1 MPa, root restriction may be assumed to be small.

Excavation Difficulty

Excavation of soil is a very common activity. Table 3-21 lists classes for recording the difficulty of making an excavation. The classes may be employed to describe horizons, layers, or pedons on a one-time observation or over time. In most instances, excavation difficulty is related to and controlled by a water state.

Table 3-21. Excavation difficulty classes