Soil and Environmental Analysis: Physical Methods - Chapter 15 (end) pdf

Whereany test is time-consuming or costly it may be possible to undertake it at only afew spots; examination of the soil is required to select a representative area.Field techniques have

con-A General Background

Expressions used to describe the field characteristics of soil go back to the nings of a settled agriculture When manual work was required to till the soil andremove weeds, differences in particle size were readily detected by contact withthe foot and hand ‘‘Light’’ and ‘‘heavy,’’ expressions still in use, did not refer tosoil bulk density but to the stickiness of wet soil, which is texture related.Despite the wide range of instruments available to measure physical prop-erties of soils, there are many circumstances where such tests cannot be done.The equipment may not be available, the cost may be high, and the time taken tocomplete a test may be so long that the results cannot be available in time to dealwith a practical problem Unless the soil is examined first, samples taken for

begin-subsequent analysis may be taken from material that crosses physical boundariesand includes layers with dissimilar properties There are also situations where thelateral distribution of a particular physical condition must be determined Whereany test is time-consuming or costly it may be possible to undertake it at only afew spots; examination of the soil is required to select a representative area.Field techniques have been widely used in pedology and soil surveys, inland evaluation for crop growth, and in the use and management of soils For thesepurposes, techniques have been developed with specific emphasis on particularproperties.

1 Pedology and Soil Surveys

The identification of soil horizons and their sequence feature prominently in ies of soil genesis, soil distribution, and soil classification For these purposes,there is an emphasis on criteria such as soil color and texture, which are relativelypermanent, and on the examination of soils under ‘‘natural’’ conditions A soilclassification name may be given to the profile as a whole, based on the sequence

stud-of horizons that are identified Although the names and nomenclature may differbetween classification systems, they share a common core of diagnostic criteria toidentify a particular horizon The methods used for describing soils in the field,including any for diagnostic horizons, are described in detail in soil survey manu-als or reports accompanying soil surveys Although local or national systems ofclassification may reflect more accurately the circumstance of a particular territory(e.g., Glentworth and Muir, 1963; Taylor and Pohlen, 1976; Avery, 1990; SoilSurvey Staff, 1993), there are two major soil classification systems that are usedworldwide, U.S Soil Taxonomy (Soil Survey Staff, 1975, 1998) and FAO (1998).Some systems of soil classification rely on features that can be identified inthe field (e.g., Avery, 1990); others require climatic data or laboratory analysis tosupplement the field-based descriptions (e.g., Soil Survey Staff, 1993)

2 Land Evaluation

Key features that are required for the evaluation of land quality are related to thegrowth of crop plants and are climate specific These include the amount of avail-able water within the potential rooting zone (based on soil texture, aeration, andconsistence), drainage class (based on color, texture, and porosity), and soil ero-dibility (based on soil texture) (Corbett and Tatler, 1970; FAO, 1976; Bibby et al.,1982; MAFF, 1988)

Where the physical properties of soil are altered, for example as the result oftillage or the application of mechanical pressure, it is often necessary to find out

what changes have taken place These could include soil compaction, surfacecrusting, erosion, structure degradation, or reduced permeability to air or water(Simpson, 1983; Davies and Payne, 1988; Batey, 1988, 1989; Daniells et al.,1996) Strictly speaking, permeability to air relates to the gaseous diffusivity(Chap 13), and permeability to water to the saturated hydraulic conductivity ofthe soil (Chap 4), although the earlier but less precise terminology has persisted

in the literature on applications With the advent of heavy machinery in agricultureand forestry, considerable emphasis is placed on the assessment of compactionand whether remedial deep tillage is required There is also an accompanying need

to evaluate the effects of a test run after soil has been loosened to confirm thatlandwork is effective Such investigations must be done on the spot and the resultsevaluated immediately so that appropriate action can be taken

For whatever purpose, properties that can be determined in the field by sight

or by handling the soil have an important part to play in soil physical analysis.Some tests such as soil texture are of general applicability; others have been de-veloped for situations where the physical properties have been changed by man-kind’s use of the soil Such includes use as urban parks, playing fields, sportsgrounds, and paths and tracks as well as for crop production, grazing, or forestry.Profile examination is particularly appropriate for land that has been subject tohigh mechanical pressure under wet conditions, e.g., during harvesting of rootcrops, or to major disturbance such as extraction of minerals, renewal of land-scapes, or installation of pipelines (e.g., Lowe, 1993), or after prolonged periods

of industrial use

B Advantages of Direct Field Assessment

of Soil Physical Conditions

The advantages of making assessments of soil physical conditions directly in thefield are as follows:

1 The examination and evaluation can be done on the spot in a relativelyshort time, and the results are immediately available

2 The examination can be comprehensive and thorough

3 The methods are flexible and can deal with a wide range of situations.They can be done at any time of the year whether the land is bare, undercrop, grassland, or forest

4 Little equipment is required —simply a means to dig a hole in theground, by spade or mechanical digger, followed by dissection of theprofile with a knife or pointed trowel For some properties, further in-formation can be obtained from examination of the soil extracted by anauger

5 Slight changes in physical conditions can often be detected that may bedifficult to determine by other means.

6 Values for some key physical characteristics can be estimated by bining data on related properties determined in the field, for examplesaturated hydraulic conductivity, from field assessments of soil texture,structure, and porosity

A Techniques of Field Examination and Evaluation

To be effective, examination of soils in the field requires access to a soil profile,the vertical face of which has been carefully prepared to expose both natural ho-rizons and any features created as a result of the use and management of the land.The techniques described below are based on Batey (1975, 1988), Hodgson(1978), Simpson (1983), Pizer (1990), and McKenzie (1998)

In some circumstances it may be possible to use extremes of weather, such

as drought or heavy rain, to supplement the information obtained from profileexamination The reaction of soil to heavy rain can be used to assess its hydraulicconductivity, its erodibility and the stability of soil structure A wet and soft sur-face present after heavy rain may be caused by an impermeable compact layerbelow (Sec II.E) If the surface of the land is bare, the degree of breakdown ofstructure and the degree of slumping can be determined (Sec II.C)

2 Where to Look

This depends on the reason for the examination Unless the diagnosis of a specificproblem is the objective, care should be taken to avoid gateways, tracks, head-lands, wheelings, and other abnormally disturbed ground A representative area ofland should be selected that is uniform in appearance

When undertaking soil examination to determine the reason for a variation

in crop growth, the pattern of growth can be a useful diagnostic feature and ables holes to be made in areas of good and poor growth In times of drought,areas of shallow, rocky, or gravelly land (and archaeological foundations) may beshown up by pale or stunted vegetation A similar appearance can be caused bysoil compaction Deeper soils may be shown up by darker, more vigorous plantgrowth Photographs are recommended as a means of recording permanently thedistribution of variations in soil color or of crop appearance, whether caused byinherent differences in soils or by the effects of management of the land Thesemay be taken at field level, from high ground or buildings, or from the air, and can

en-be used subsequently to locate areas for detailed soil examination Satellite agery can be used to record variations in soil properties or plant growth It can bevery informative to dig a trench at right angles across the principal direction oftillage or harvest so that any compaction related to wheeltrack patterns can bemore readily identified

a Digging a Hole and Preparation of the Profile Face

A mechanical digger is recommended, provided that there is access to the locationrequired without causing excessive crop damage Alternatively, a hole can be dugwith a clean sharp spade, supplemented if necessary with a pickaxe or crowbar

An auger maybe used to extract soil from depth Details of augers and other ment suited to soil examination are given in Sec b below

equip-The dimensions of a hole depend on the question being asked and on howfar the deepest zone of interest lies below the surface Rarely would the depth beless than 50 cm, and it could be 1.2 meters or more There should be enoughspace at the bottom of the hole to accept waste soil taken off the face duringexamination While digging, two edges of the hole should be left untrampled, andthe soil dug out should be kept well away from these sides One or more verticalfaces should then be cleared of any soil that was smeared or compressed while thehole was dug The next objective is to highlight the physical characteristics of thesoil Using a small pointed trowel or penknife, the face should be gently probed,beginning at the surface then working down the face to restore natural featuresand to search for any human-induced changes Where coarse blocky structure(Sec II.C.1) is found, this can be levered out with a spade, beginning near the base

of the hole

Where a mechanical digger is used, a trench can be dug readily to a depth

of 1.5 m or more, to provide a hole wide enough to walk along and to have topsoil almost at eye level Safety aspects must be considered and local regulationsfollowed when working in trenches more than 1 m deep Care must be taken whendigging a hole in loose soil or where marked vertical fissures are present; the risk

of injury as a result of wall collapse must be evaluated

If compaction of the soil just below plow or cultivation depth is suspected,most of the loose soil above can be lifted off by spade or trowel and the lastremnants lying on the upper surface of the suspect compact layer brushed orflicked off

b Equipment for Examining Soils in the Field

The suggestions made in this paragraph are based on the author’s experience (seealso ADAS, 1971) Catalogs of equipment for field use can be obtained from sev-eral of the suppliers listed in Table 1ofChap 10 These contain a much widerrange of equipment than that described here, with some dedicated for specificpurposes Local suppliers may also be able to provide suitable equipment Someexamples of equipment are shown inFig 1

Spades: A conventional rectangular spade, typically 20⫻ 25 cm, is oftenused However, this may be difficult to push into firm or dry soil It can be modi-fied to penetrate more easily by cutting off the corners to make it U-shaped Asmaller spade 15⫻ 20 cm in size with a concave face is also often used

Screw augers: These are usually modified wood-boring augers of 2 or2.5 cm diameter with a screw length of 20 cm, to which a stem has been welded

to increase its length to 1 m or possibly longer If the original point is cut off, theauger can more easily penetrate soils which are slightly stony Because a largepull is often required to extract the auger from the soil, care must be taken to avoidback strain or injury Screw augers are suitable for taking samples for tests wherestructure is of no significance The soil core retained on the screw can be examinedfor texture and color but not for structure

Dutch augers: These are specially designed for soil examination and have

an open twist tapered head about 20 cm long, typically of 3 or preferably of 5 cmdiameter The head is at the end of a stem 1.2 m long Despite their larger diame-ter, they usually take less force to insert and pull out of the soil than screw augers

A core can be extracted that is partly intact, and about 15 cm long; this can beused to examine the texture, color, root numbers, and, to a certain extent, structure.Crescent-shaped open corers (sometimes called cheese corers): These aresemicircular in cross-section and some 2 to 2.5 cm in diameter The length of thecore may be limited to a specific distance of 15 or 30 cm for taking samples tothat depth Alternatively it may extend to 1.0 m, the whole length of the corer.After insertion into the soil and giving it a half rotation, an entire core can be held

on the corer when it is pulled out By cutting the exposed part off with a blade, an

cannot be used where stones are present They can be used to extract deep cores

in wet or soft soil such as peat, but in mineral or dry soils the force required toinsert and extract long cores may be too great for manual use

Mechanical corers and split samplers: Where cores are required of a size or

Fig 1 Augers used for obtaining soil samples From left to right: gouge, screw, Dutchauger

engineering can be used Those used for extracting cores for root measurementsare shown inChap 12.

4 What to Look For—Examination and Interpretation

Physical properties that can be determined by tactile and visual examination rectly in the field are described in the following sections of this chapter To assesstheir characteristics, it is convenient to divide the soil into four layers: the soilsurface, the layer disturbed by normal cultivations, the soil just below the culti-vated soil, and subsoil undisturbed by normal cultivations Visual and tactile ex-amination can also help to locate the optimum position for instrumental measure-ments to be made, or for samples to be taken for testing later in the laboratory[e.g., bulk density (Chap 8) or gas movement (Chap 13)]

di-a The Soil Surface

If a bare soil has been exposed to rain, any disintegration of aggregates can beused as an indication of the stability of the structure Individual aggregates mayhave partially collapsed, and if severe, a smooth surface can be the result (seeSec II.B) The presence of such a layer can be confirmed by probing and levering

up the surface with a pointed blade Such a crust may act as a seal on the surface,which excludes air when it is wet; when dry, it may become hard and impenetrable

to emerging seedlings More stable aggregates and large mineral particles such ascoarse sand or small stones can sometimes be seen firmly embedded in the crustand projecting above the otherwise smooth surface Below a crust, aggregates can

be firmly attached to the underside of the crusted portion Soils with a high content

of fine or very fine sand and silt, particularly where the organic matter content islow, are prone to show this feature (Davies, 1974) If rain is heavy and prolonged

or the land is flooded for a while, a crust may develop into a layer 3 –5 cm thick.Compaction of the surface is widespread, caused by the treading of animals(including wildlife and human activity) and by the passage of wheeled or trackedvehicles The surface is depressed by the pressure applied and the pattern is related

to the movement of the animals or machines The effects are worst when the land

is soft The primary effect is a decrease in porosity and infiltration that may lead

to water flowing downslope and inducing erosion In hot, dry regions of the world,hard and compact soil may be found extending from the surface throughout thetopsoil and even deeper (see Sec II.C.5) These are known as hardsetting soilsand may be found occasionally in temperate regions where intensive managementhas reduced soil organic matter content (Mullins et al., 1987)

b Within the Cultivated Layer

This refers to the layer disturbed by cultivation, usually to between 20 and 30 cmbelow the surface (i.e., the depth to which the deepest working implement oper-ates) The term ‘‘cultivation’’ includes any operation done by a moldboard or disc

plow, or by a rotary, tined or other implement Multiple cultivations are commonand may take place at a range of depths.

Most types of cultivation implement can form a thin zone of compressedsoil, just below their operating depth (often called a cultivation pan) In this zone,

a pan may be detected by the relative resistance to a probe pushed manually intothe soil (a spade, auger, or stick can be used) Such pans occur not only just be-low plow depth (see the next section) but can also be found within the cultivatedlayer due to shallower secondary cultivations or the use of shallower implements

in the later stages of seedbed preparation A pan can often be seen from above as

a smooth, slightly shiny smeared surface, which may be continuous or ous, and may bear the imprint of the blade or implement responsible for its for-mation In a thick panned layer, aggregates pack tightly together to form a slab ofvisibly dense soil, with reduced or no visible pores When dry, this would bedetected as a hard layer Thick pans usually have a greater adverse effect than thinones on water or root penetration, but the depth at which they occur is important.Shallow pans can have more severe effects (see Sec II.D) Soils of all types, in-cluding sands and peats, may exhibit smeared or compacted layers

discontinu-On very sandy soils an unusual method to detect thin compact layers is toremove carefully an entire spadeful of dry soil and lay it on its side, tap the spade,and blow away any loose sand If compact layers are present they may be seen asthin or thick layers separated by cleavage planes often lying parallel to the surface(Harrod, 1975)

In wet weather, water may build up above a compacted or smeared layer andcan be seen seeping out and running down the side of an inspection pit On slopingland, if water cannot drain through a pan, the risk of erosion is considerably en-hanced Other changes may also accompany soil compaction; for example, darkgray anaerobic pockets with a malodorous smell may be seen where recent cropresidues have been incorporated into the compact soil (Sec II.F)

c Just Below the Base of the Cultivated Layer

This is the position of the classic plow-pan; it is one of the most critical for rootand water penetration Above it, the soil is loosened regularly by normal cultiva-tions; within and below it, soil is rarely disturbed However, it is not only plowingthat may be responsible for compaction Wheels of tractors, harvesters and loadedtrailers running on the surface can transmit pressure to this depth (or even below)and can cause severe compaction (Soane and van Ouwerkerk, 1994; Hakanssonand Petelkau, 1994; McKenzie, 1998)

The signs of compaction are high density as determined by probing, reducedhydraulic conductivity leading to an accumulation of water above the compactlayer, a marked discontinuity in structural form often with horizontal laminated orplaty units within the compact layer, which may have a smooth shiny upper sur-face, and the absence of pores, fissures, roots, or earthworm holes within it Tor-

tuous root paths with common horizontal segments provide a good indication ofcompaction (McCormack, 1986) The upper surface may also bear the imprint

of cultivator tines or the lugs of tractor tires Such imprints may even be found

in prehistoric fields that have not been subsequently cultivated (Ashmore, 1996),which is an indication of the potential longevity of unrelieved compaction.The pattern of roots can be used directly to assess the significance to thecrop of any suspect compact layer In a crop growing under unrestricted physicalconditions, the root pattern would be related to the species and variety of the crop,

to the soil water regime, to acidity, and sometimes to differences in soil nutrientstatus The concentration of roots is usually greatest in the topsoil, with a relativelysteady reduction in numbers with depth (Gregory, 1988) A compact or smearedlayer can restrict the number of roots penetrating below it A mat or an increaseddensity of roots may be found on the upper surface of severely compacted soil.Roots that are able to grow a short way into compacted soil are often much thick-ened and distorted

If roots have been unable to grow much into or below a compact soil, asharply differentiated moisture profile may develop, with dry soil within andabove the compact layer and moist below This is caused by the lack of roots belowthe compaction to extract moisture On the other hand, if the soil has dried to somedepth in the subsoil below the compaction, this may be a useful indication thatroots have been able to penetrate and extract moisture However, the change inconsistence at the base of the cultivated layer may be mistaken for the upper sur-face of a compacted layer, particularly in late summer when subsoils may be dryand hard The unloosened subsoil is harder than the topsoil without necessarilyhaving been compacted (discussed further in Sec II D.3)

Although the upper surface of a compacted layer may be readily located, it

is more difficult to determine how far down the compaction persists The mostcompacted soil is found on the upper surface The severity of compaction thendeclines with depth until the layer merges with unaffected soil at some depth be-low If possible, a comparison should be made between the physical properties ofsoil nearby that has not been compacted The effects that compact layers may have

on crops and on soil properties is discussed in Sec II.D

d The Subsoil

This section deals mainly with the identification of natural soil features, as thephysical properties of subsoil are not normally affected by grazing or cropping.However, there is increasing evidence that the continued use of tractors and har-vesters of large mass transmit pressure deep into the subsoil (McKenzie et al.,1990; Hakansson and Petelkau, 1994; Sullivan and Montgomery, 1998) Theseeffects have yet to be fully evaluated The signs to look for are increased density,lack of porosity, and reduced penetration of water or roots

The agronomic role of the subsoil is to provide entry and egress for waterand to permit the entry and extension of crop roots to extract water and nutrients.Roots can grow into quite stiff soil by deforming it but tend to grow mainly downpores and cracks in structured soils if the peds are fairly dense Pores and fissuresmay be up to several mm across and can be observed directly by eye, and by thepresence within them of roots either living (white when young) or dead (brown).

In some soils, fissures and pores may have been created many years ago when theland was in forest or marsh and when trees or other species were growing withroots much thicker than those of the present vegetation The imprint of roots canpersist on the faces of fissures for long periods, possibly centuries Earthwormholes often contain roots and darker colored topsoil, and sometimes follow formerroot channels Particularly where they are numerous, they may significantly im-prove root penetration and drainage

In some subsoils dominated by sand, root penetration may be very poor,without obvious signs of compaction or hardness In such soils roots may extendonly 8 –10 cm into the subsoil sand and also show a characteristic swollen appear-ance (Batey, 1988) This phenomenon is thought to be due to the close packing ofthe grains and their resistance to moving apart to create the space needed for roots

to expand and grow normally (Hettiaratchi, 1987) Loamy subsoils in their naturalstate usually provide excellent conditions for root growth, unless affected byacidity or waterlogging

In some clay soils distinctive and characteristic vertical cracks develop due

to shrinkage of the clay when it dries (see also Sec II.C.4) These cracks quently re-form in the same position each year and roots therefore grow also inthe same position The degree and depth of fracturing is related to the magnitude

fre-of the soil water deficit, to the clay content, and to the type fre-of clay present kinson, 1975) Topsoil often falls down cracks in summer, and whether this is abeneficial effect is equivocal Chemical fertility may be enhanced in the subsoil,but the extra material may give rise to a tighter seal when the clay expands as itrewets (Smart, 1998; Batey and McKenzie, 1999) Because roots can be so readilyseen on crack faces in the subsoil, their presence is a good guide to the absence ofany major limiting feature higher up the soil profile

(Wil-e Cemented and Indurated Layers

Hard layers may develop by natural processes In northern latitudes, induratedlayers occur in many sandy and loamy soils within 30 –50 cm of the surface; theseare relics of the Ice Age (Fitzpatrick, 1956; Glentworth and Muir, 1963) Theirpresence is rarely in doubt, as they are extremely hard even when wet; a strongblow with a spade may penetrate less than 1 cm If shallow, they may adverselyaffect drainage, the growth of crops, and land capability However, their directsignificance for crop growth is often less than expected because of the cooler,

wetter climate in which they are usually found (Batey, 1988) Dense layers canalso occur due to pedogenic processes These include the downward movement ofclay, and cementing by iron and other oxides and oxyhydroxides (plinthite) (SoilSurvey Staff, 1993).

B Soil Texture

The expression ‘‘soil texture’’ is used to describe the feel and molding teristics of moist soil Words such as clay, sand, and loam have been used to dis-tinguish soils with different properties since the beginnings of a settled agricul-ture Hand texturing is one of the most important single tests that can be done inthe field

charac-Four terms are used in varying combinations: sand, loam, silt, and clay,

together with adjectives qualifying the size of the sand grains, to describe just over

20 different classes of texture Texture must not be confused with soil structure,which describes the way the individual particles are assembled and bound intogroups, usually called aggregates

Soil texture gives a guide to many soil characteristics The textural classprovides an indication of soil water retention and the available water capacity(Chap 3); particle size distribution (Chap 7); the likely development and stabil-ity of soil structure; cation exchange capacity (and hence nutrient retention andavailability, and the activity and retention of residual soil-acting herbicides); ero-dibility by wind or water; stickiness and ease of cultivation; drainage characteris-tics, saturated hydraulic conductivity and suitability for mole draining; croppingsuitability; and thermal properties of soils (Chap 14)

1 Soil Texture Classes and Particle Size

The size ranges of soil particles are classified into three groups, sand, silt and clay,with the upper limit of ‘‘soil’’ set at 2 mm However, there is no general consensusregarding the size range of each group, as discussed in Chap 7 and by Hodgson(1978) One system that is widely accepted classifies particles as follows (Hodg-son, 1974):

Sand: between 2 mm and 60mm

Silt: between 60 and 2mm

Clay: less than 2mm

Particles larger than 2 mm, i.e., stones (2 to 600 mm in size) and boulders(⬎600 mm), are important where present in a significant proportion Stone sizescan be further subdivided into very small (2 – 6 mm), small (6 –20 mm), me-dium (20 – 60 mm), large (60 –200 mm) and very large (200 – 600 mm) (Hodg-son, 1974)

There are two types of method available to determine soil particle size tribution, laboratory analysis and field assessment To avoid confusion betweenthe two, it is recommended that the term particle-size class be used to express thedescriptive names applied to different mixtures of sand, silt, and clay size particlesbased on laboratory analysis The term soil texture is then reserved for the esti-mation based on a field test as described in Sec II.B.2 below.

dis-Particle size analysis provides precise values for the proportions of particles

in a number of size classes The terms already used to describe soil texture classesare then used to describe soils with different proportions of sand, silt, and clay.The conversion is done using a triangular or orthogonal diagram (Chap 7) Details

of size classes and of the naming of various mixtures of these are discussed byHodgson (1978) However, particle size analysis is conventionally done after re-moving cementing materials such as organic matter, carbonate, and iron and alu-minum oxides and hydroxides Laboratory results therefore cannot always be ex-pected to relate accurately to the field behavior of soils Furthermore, the texturaldiagrams commonly used take no account of the range of particle sizes within aclass, so that important qualifying adjectives such as ‘‘coarse’’ or ‘‘fine’’ cannot

be applied Care must be taken when interpreting soil surveys where particle sizeanalysis figures are used to verify field estimates of texture (Avery, 1990)

In some red tropical soils, estimating the texture by hand gives a result ofsilty loam, whereas a laboratory determination shows a high content of clay Thisdifference is due to the intense microaggregation of the clay particles, whichmasks some of the cohesive properties of the clay when manipulated by hand

In such soils, the results of hand texturing give a much better indication of thefield behavior and capability of the soil (Trapnell and Webster, 1986)

When hand-based assessment of soil texture has been tested against ratory analysis, the results showed that, for soils within a limited geographic re-gion, those with experience of hand assessment could confidently estimate theparticle size distribution of a wide range of samples (Hodgson et al., 1976; Pizer,1990)

labo-2 Field Method

Methods for assessment of soil texture are given in national and international soilsurvey manuals (e.g., Hodgson, 1974, 1978) A brief description of one methodthat has been used successfully over many years for field evaluation in Englandand Wales is given here (further details can be found in ADAS, 1971; Batey, 1988;and Pizer, 1990) The field properties associated with each class are described inthe next section

The procedure is as follows: Take about half a handful of soil, and if it isdry, add water gradually until the particles hold together to form a moist ball Noexcess water should be present The assessment is made by kneading the moist

soil between fingers and thumb It is important to work the soil thoroughly toeliminate any small lumps (aggregates) present The assessment is then done byestimating the contribution that the different particles, sand, silt, and clay, make

to the feel of the soil as a whole

The physical properties of the individual fractions that determine the texture

of a soil are as follows

Sand consists of grains that feel gritty and are large enough to grate againsteach other; they may be detected individually by both touch and sight Four sub-grades can be distinguished, coarse, medium, fine, and very fine

With silt, individual grains cannot be detected; silt feels smooth, soapy, orsilky It adheres readily to the fingers

Clays are sticky; some dry clays require a great deal of moistening andworking between the fingers before they develop their maximum stickiness Claycoheres and can come away fairly cleanly from the fingers A moist surface willtake a slight polish when a finger or thumb is rubbed firmly across it

Each class of soil texture has a characteristic feel (Table 1) and is bestthought of as a single entity For those unfamiliar with the technique of handtexturing, expert advice should be sought initially until experience is gained tomake accurate assessments Practicing on samples that have already been classi-fied is a good way to gain experience

3 Soil Texture Descriptions and Associated Physical Properties

In Table 1, one column describes the tactile characteristics of each class and theother the physical characteristics associated with each class Although this isbased on U.K experience, the method can be applied with only minor modifica-tions to the soils of most countries In Table 1, particles between 20 and 60mm insize are referred to as very fine sand In some systems (Hodgson, 1974) this sizerange is called coarse silt

4 Hand Assessment of Soil Texture

The characteristics of the different textural classes are set out below:

a Sandy Soils

Those with a significant amount of grittiness Test the binding and cohesion

Readily molded into a cohesive ball, does not form threads Sandy loamModerately cohesive, sticky and plastic, forms threads, will take a

Very cohesive, very sticky, forms long threads, will take a high polish

For each of the sand groups it is also important to identify the grade of sand, andthe main classes should be prefixed accordingly, though for sandy clay loam andsandy clay it is less easy to identify the sand grades.

Medium sand Moderately harsh feel (e.g., sea shore sand) 0.6–0.2 mm

Very fine sand Smooth and powdery, only just visible to the naked eye 0.06–0.02 mm

b Clayey Soils

Those which are not gritty, but are strongly cohesive, form threads and rings easilyand have a surface that readily takes a polish when rubbed with thumb or finger

Extremely sticky, moderately smooth, difficult to deform Silty clay loamExtremely cohesive, forms long threads and rings, high degree of

Extremely cohesive, high polish, also smooth and silky Silty clay

c Silty Soils

Those dominated by a smooth, soapy slipperiness or silkiness, moderately sive Silt adheres readily and fingers become very dirty; clay coheres, i.e., sticks

cohe-to itself and fingers remain relatively clean

Smoothness and silkiness dominant Silty loam

Where none of the above fractions, sand, silt, or clay, imparts a dominant feel

Moderately smooth and can be rolled into short threads; no polish can be

C Soil Structure

In its broadest sense, soil structure refers to the physical organization of soil terials as expressed by the arrangement of solid particles and voids (Avery, 1990).Field descriptions place emphasis on the degree of development, and on the size,shape, and arrangement of naturally formed aggregates that are separated fromeach other by voids or planes of weakness (Hodgson, 1974; Avery,1990).Soil structure has also been described as the architecture of the soil (Russell,1961) Certainly it has to do with space, construction, stability, pathways, and

ma-Table 1 Soil Textural Descriptions and Associated Physical Properties

SANDY SOILS: soils dominated by

sands; divided into three groups (sands,

loamy sands, and sandy loams),

de-pending on the proportion of sand

present Each group is then subdivided

into four (coarse, medium, fine, very

fine), according to the dominant size

of the sand grains

SANDS feel gritty, lack any cohesion,

loose when dry, not sticky at all when

wet, do not stain the fingers

Low retention of water and nutrients

COARSE SAND (2 – 0.6 mm): harsh

to the touch

Very droughty, fast draining, readilyeroded by water

MEDIUM SAND (600 –200mm):

sands of the seashore

Very droughty, erodible by wind and ter, root entry difficult

wa-FINE SAND (200 – 60mm): dune sand Very erodible by wind and water, root

en-try difficultVERY FIND SAND (60 –20mm): lo-

ess, barely visible to the naked eye,

powdery

Very erodible, root entry difficult

LOAMY SANDS feel gritty, slight

cohe-sion— can be molded into a ball when

sufficiently moist, do not stick to the

ero-ture, liable to collapse in heavy rain,crusts and caps on surface, very erod-ible by wind and water

LOAMY VERY FINE SAND: very

fine powder

Very weak structure, collapses readily,easily compacted, forms hard surfacecap

Texture Description Associated Physical Properties

SANDY LOAMS feel gritty, show a fair

degree of cohesion, can be molded

quite readily into a ball when just

FINE SANDY LOAM: slight

gritti-ness, firmly molded

Fast draining, free working, good tion of water—high proportion avail-able, erodible by water, structureslightly weak, liable to capVERY FINE SANDY LOAM: gritti-

reten-ness barely detectable, firmly

molded, fine and powdery when dry

Moderately porous, weak structure andliable to cap, surface ponding com-mon, excellent retention of water, highavailable water capacity, erodible bywater, very high value in dry areas

LOAMS

LOAM: no fraction dominates the feel of

the soil, readily molded into a ball

al-though sand present, does not feel

ob-viously gritty; insufficient silt to

im-part silky feel, insufficient clay to

make it sticky or to take a polish

Good water retention, porous, easy ing, stable structure

work-SILTY LOAM: smooth silky feel, sticky

when wet, firmly cohesive

Good water retention, adhesive and cult to work when wet, structure usu-ally stable but may break down if over-worked, high value in dry areas, lessgood in wet

diffi-SILT: as silty loam but smoothness,

silki-ness, and adhesion more distinct,

sur-face takes a weak polish when rubbed

with finger

As silty loam but more sticky, ately slow draining

moder-CLAY LOAM: sticky, binds together

strongly when moist and resists

defor-mation, takes a polish on surface

Good water retention, slow draining,high retention of nutrients, stronglydeveloped stable structure, weathersinto fine aggregates on surface, highdraught requirement, readily smeared,may shrink on drying to form deepcracks