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

15 Soil Profile Description and Evaluation Tom Batey University of Aberdeen, Aberdeen, Scotland I. INTRODUCTION The preceding chapters cover a wide range of soil physical measurements. In con- trast, this chapter deals with the often neglected topic of the visual and tactile methods of assessment that can be made directly in the field. Both have their place. Systematic examination of soil in the field should be a basic skill to evaluate its physical state. This was one of the conclusions of the international conference called Problems in Modern Soil Management (van Ouwerkerk et al., 1992). Infor- mation obtained in such a way can be used independently or can be used to com- plement and supplement measurements made by instruments in the field or the laboratory. Field examination should also precede the collection from the field of samples that are to be subject to other tests in the laboratory. A. General Background Expressions used to describe the field characteristics of soil go back to the begin- nings of a settled agriculture. When manual work was required to till the soil and remove weeds, differences in particle size were readily detected by contact with the foot and hand. ‘‘Light’’ and ‘‘heavy,’’ expressions still in use, did not refer to soil 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 to complete a test may be so long that the results cannot be available in time to deal with a practical problem. Unless the soil is examined first, samples taken for Copyright © 2000 Marcel Dekker, Inc. subsequent analysis may be taken from material that crosses physical boundaries and includes layers with dissimilar properties. There are also situations where the lateral distribution of a particular physical condition must be determined. Where any test is time-consuming or costly it may be possible to undertake it at only a few spots; examination of the soil is required to select a representative area. Field techniques have been widely used in pedology and soil surveys, in land evaluation for crop growth, and in the use and management of soils. For these purposes, techniques have been developed with specific emphasis on particular properties. 1. Pedology and Soil Surveys The identification of soil horizons and their sequence feature prominently in stud- 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 relatively permanent, and on the examination of soils under ‘‘natural’’ conditions. A soil classification name may be given to the profile as a whole, based on the sequence of horizons that are identified. Although the names and nomenclature may differ between classification systems, they share a common core of diagnostic criteria to identify 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 of classification may reflect more accurately the circumstance of a particular territory (e.g., Glentworth and Muir, 1963; Taylor and Pohlen, 1976; Avery, 1990; Soil Survey Staff, 1993), there are two major soil classification systems that are used worldwide, U.S. Soil Taxonomy (Soil Survey Staff, 1975, 1998) and FAO (1998). Some systems of soil classification rely on features that can be identified in the field (e.g., Avery, 1990); others require climatic data or laboratory analysis to supplement 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 the growth 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, and consistence), 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). 3. Soil Management Where the physical properties of soil are altered, for example as the result of tillage or the application of mechanical pressure, it is often necessary to find out 596 Batey Copyright © 2000 Marcel Dekker, Inc. what changes have taken place. These could include soil compaction, surface crusting, 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 of the soil (Chap. 4), although the earlier but less precise terminology has persisted in the literature on applications. With the advent of heavy machinery in agriculture and forestry, considerable emphasis is placed on the assessment of compaction and 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 that landwork is effective. Such investigations must be done on the spot and the results evaluated 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, sports grounds, and paths and tracks as well as for crop production, grazing, or forestry. Profile examination is particularly appropriate for land that has been subject to high mechanical pressure under wet conditions, e.g., during harvesting of root crops, 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 the field are as follows: 1. The examination and evaluation can be done on the spot in a relatively short 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, under crop, grassland, or forest. 4. Little equipment is required—simply a means to dig a hole in the ground, by spade or mechanical digger, followed by dissection of the profile with a knife or pointed trowel. For some properties, further in- formation can be obtained from examination of the soil extracted by an auger. Soil Profile Description and Evaluation 597 Copyright © 2000 Marcel Dekker, Inc. 5. Slight changes in physical conditions can often be detected that may be difficult to determine by other means. 6. Values for some key physical characteristics can be estimated by com- bining data on related properties determined in the field, for example saturated hydraulic conductivity, from field assessments of soil texture, structure, and porosity. II. METHODS AND APPLICATIONS 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). 1. When to Look The techniques can be applied at any time of the year. If it is possible to choose the timing, examination should be made preferably when the maximum amount of information can be obtained. Under annual crops, this would be when the crops are close to their peak of vegetative growth and while the soil is moist. In many climates this would be in late spring. However, other factors may dictate the timing, such as access to the land. Postharvest examination is frequently made both because of easy access and because of the need to assess soil compac- tion, so that remedial deep tillage may be done timeously prior to the establish- ment of the next crop. Some of the information obtained may be limited by the conditions under which the examination is made. For example, if the soil is very dry, it is difficult to distinguish between layers that are hard because they are com- pact and those that are hard because they are dry. If the land is very wet, it may not be possible to prepare a hole without excessive damage to the soil in its vi- cinity, nor to make an adequate examination under soft and wet ground conditions. In some circumstances it may be possible to use extremes of weather, such as drought or heavy rain, to supplement the information obtained from profile examination. The reaction of soil to heavy rain can be used to assess its hydraulic conductivity, 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 layer below (Sec. II.E). If the surface of the land is bare, the degree of breakdown of structure and the degree of slumping can be determined (Sec. II.C). 598 Batey Copyright © 2000 Marcel Dekker, Inc. 2. Where to Look This depends on the reason for the examination. Unless the diagnosis of a specific problem is the objective, care should be taken to avoid gateways, tracks, head- lands, wheelings, and other abnormally disturbed ground. A representative area of land 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 en- 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 be shown up by pale or stunted vegetation. A similar appearance can be caused by soil compaction. Deeper soils may be shown up by darker, more vigorous plant growth. Photographs are recommended as a means of recording permanently the distribution of variations in soil color or of crop appearance, whether caused by inherent differences in soils or by the effects of management of the land. These may be taken at field level, from high ground or buildings, or from the air, and can be used subsequently to locate areas for detailed soil examination. Satellite im- agery can be used to record variations in soil properties or plant growth. It can be very informative to dig a trench at right angles across the principal direction of tillage or harvest so that any compaction related to wheeltrack patterns can be more readily identified. 3. How to Look a. Digging a Hole and Preparation of the Profile Face A mechanical digger is recommended, provided that there is access to the location required without causing excessive crop damage. Alternatively, a hole can be dug with 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 equip- ment suited to soil examination are given in Sec. b below. The dimensions of a hole depend on the question being asked and on how far the deepest zone of interest lies below the surface. Rarely would the depth be less than 50 cm, and it could be 1.2 meters or more. There should be enough space at the bottom of the hole to accept waste soil taken off the face during examination. While digging, two edges of the hole should be left untrampled, and the soil dug out should be kept well away from these sides. One or more vertical faces should then be cleared of any soil that was smeared or compressed while the hole was dug. The next objective is to highlight the physical characteristics of the soil. 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 features and 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. Soil Profile Description and Evaluation 599 Copyright © 2000 Marcel Dekker, Inc. 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 top soil almost at eye level. Safety aspects must be considered and local regulations followed when working in trenches more than 1 m deep. Care must be taken when digging 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 last remnants lying on the upper surface of the suspect compact layer brushed or flicked off. b. Equipment for Examining Soils in the Field The suggestions made in this paragraph are based on the author’s experience (see also ADAS, 1971). Catalogs of equipment for field use can be obtained from sev- eral of the suppliers listed in Table 1 of Chap. 10. These contain a much wider range of equipment than that described here, with some dedicated for specific purposes. Local suppliers may also be able to provide suitable equipment. Some examples of equipment are shown in Fig. 1. Spades: A conventional rectangular spade, typically 20 ϫ 25 cm, is often used. 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. A smaller spade 15 ϫ 20 cm in size with a concave face is also often used. Screw augers: These are usually modified wood-boring augers of 2 or 2.5 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, the auger can more easily penetrate soils which are slightly stony. Because a large pull is often required to extract the auger from the soil, care must be taken to avoid back strain or injury. Screw augers are suitable for taking samples for tests where structure is of no significance. The soil core retained on the screw can be examined for 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 cm diameter. 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 be used to examine the texture, color, root numbers, and, to a certain extent, structure. Crescent-shaped open corers (sometimes called cheese corers): These are semicircular in cross-section and some 2 to 2.5 cm in diameter. The length of the core may be limited to a specific distance of 15 or 30 cm for taking samples to that 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 undisturbed soil profile can be retained on the corer for examination. Such corers 600 Batey Copyright © 2000 Marcel Dekker, Inc. 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 to insert and extract long cores may be too great for manual use. Mechanical corers and split samplers: Where cores are required of a size or depth that exceed human endeavor, mechanical equipment as used in geology or Soil Profile Description and Evaluation 601 Fig. 1 Augers used for obtaining soil samples. From left to right: gouge, screw, Dutch auger. Copyright © 2000 Marcel Dekker, Inc. engineering can be used. Those used for extracting cores for root measurements are shown in Chap. 12. 4. What to Look For—Examination and Interpretation Physical properties that can be determined by tactile and visual examination di- rectly in the field are described in the following sections of this chapter. To assess their characteristics, it is convenient to divide the soil into four layers: the soil surface, 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)]. a. The Soil Surface If a bare soil has been exposed to rain, any disintegration of aggregates can be used as an indication of the stability of the structure. Individual aggregates may have partially collapsed, and if severe, a smooth surface can be the result (see Sec. 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 as coarse sand or small stones can sometimes be seen firmly embedded in the crust and 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 is low, 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 tracked vehicles. 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 the topsoil and even deeper (see Sec. II.C.5). These are known as hardsetting soils and may be found occasionally in temperate regions where intensive management has 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 cm below 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 602 Batey Copyright © 2000 Marcel Dekker, Inc. plow, or by a rotary, tined or other implement. Multiple cultivations are common and may take place at a range of depths. Most types of cultivation implement can form a thin zone of compressed soil, 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 into the 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 cultivated layer 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 discontinu- 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 of visibly dense soil, with reduced or no visible pores. When dry, this would be detected as a hard layer. Thick pans usually have a greater adverse effect than thin ones 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. On very sandy soils an unusual method to detect thin compact layers is to remove 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 as thin 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 and can be seen seeping out and running down the side of an inspection pit. On sloping land, if water cannot drain through a pan, the risk of erosion is considerably en- hanced. Other changes may also accompany soil compaction; for example, dark gray anaerobic pockets with a malodorous smell may be seen where recent crop residues 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 root and 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 plowing that may be responsible for compaction. Wheels of tractors, harvesters and loaded trailers running on the surface can transmit pressure to this depth (or even below) and can cause severe compaction (Soane and van Ouwerkerk, 1994; Hakansson and Petelkau, 1994; McKenzie, 1998). The signs of compaction are high density as determined by probing, reduced hydraulic conductivity leading to an accumulation of water above the compact layer, a marked discontinuity in structural form often with horizontal laminated or platy 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- Soil Profile Description and Evaluation 603 Copyright © 2000 Marcel Dekker, Inc. tuous root paths with common horizontal segments provide a good indication of compaction (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 the crop of any suspect compact layer. In a crop growing under unrestricted physical conditions, 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 nutrient status. The concentration of roots is usually greatest in the topsoil, with a relatively steady reduction in numbers with depth (Gregory, 1988). A compact or smeared layer can restrict the number of roots penetrating below it. A mat or an increased density 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, a sharply differentiated moisture profile may develop, with dry soil within and above the compact layer and moist below. This is caused by the lack of roots below the compaction to extract moisture. On the other hand, if the soil has dried to some depth in the subsoil below the compaction, this may be a useful indication that roots have been able to penetrate and extract moisture. However, the change in consistence 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 dry and hard. The unloosened subsoil is harder than the topsoil without necessarily having 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 most compacted soil is found on the upper surface. The severity of compaction then declines 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 of soil 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 the physical 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). These effects have yet to be fully evaluated. The signs to look for are increased density, lack of porosity, and reduced penetration of water or roots. 604 Batey Copyright © 2000 Marcel Dekker, Inc. [...]... soil mechanics model for agricultural soils Soil Use Manage 3 : 94 –105 Hodge, C A H., R G O Burton, W M Corbett, R Evans, and R S Seale 1984 Soils and Their Use in Eastern England Bull No 13 Harpenden, England: Soil Survey of England and Wales Hodgson, J M., ed 1974 Soil Survey Field Handbook Tech Monograph No 5 Harpenden, England: Soil Survey of England and Wales Hodgson, J M 1978 Soil Sampling and. .. 2000 Marcel Dekker, Inc Sand Loamy sand Sandy loam Sandy clay loam Sandy clay Soil Profile Description and Evaluation 609 For each of the sand groups it is also important to identify the grade of sand, and the main classes should be prefixed accordingly, though for sandy clay loam and sandy clay it is less easy to identify the sand grades Coarse sand Medium sand Fine sand Very fine sand Very harsh feel Moderately... D B 1974 Soil structure and crop production In: Mackney, D., ed Soil Type and Land Capability Tech Monograph No 4 Harpenden, England: Soil Survey of England and Wales, pp 117–124 Davies, D B 1975 Field behaviour of medium textured and ‘silty’ soils In: Soil Physical Conditions and Crop Production MAFF Tech Bull No 29 London: HMSO, pp 52 –75 Davies, D B., and D Payne 1988 Management of soil physical. .. into soil compaction (and other soil problems) on land reinstated after pipeline laying M.Sc thesis, Univ of Aberdeen, Scotland Mackney, D., ed 1974 Soil Type and Land Capability Tech Monograph No 4 Harpenden, England: Soil Survey of England and Wales MAFF (Ministry of Agriculture, Fisheries and Food) 1969 Field Drainage Experimental Unit Annual Report 1969 Cambridge, England: MAFF MAFF 1975 Soil Physical. .. 1999 Deep subsoil compaction—Letter to the editor Soil Use Manage 15 : 136 Bibby, J S., H A Douglas, A J Thomasson, and J S Robertson 1982 Land Capability Classification for Agriculture Soil Survey of Scotland Monograph Aberdeen, Scotland: Macaulay Land Use Res Inst Brewer, R., and J R Sleeman 1988 Soil Structure and Fabric Melbourne, Australia: CSIRO Div Soils Corbett, W M., and W Tatler 1970 Soils in... Agriculture, Fisheries and Food Avery, B W 1990 Soils of the British Isles Wallingford, England: CAB International Batey, T 1975 Soil examination in the field In: Soil Physical Conditions and Crop Production MAFF Tech Bull No 29 London: HMSO, pp 207–217 Batey, T 1988 Soil Husbandry Aberdeen, Scotland: Soil and Land Use Consultants Ltd Batey, T 1989 Control of compaction on the farm In: Soil Compaction as... between 0.0 (poor) and 2.0 (good) This system is used extensively by cotton agronomists in Australia to assess soil compaction and to provide options for soil loosening and soil management It has been shown to correlate well with soil aeration and soil strength mea- Copyright © 2000 Marcel Dekker, Inc 618 Batey Table 4 Numerical System for Classifying Soil Structure Firm, most soil: soil below the tilled... Bengough, and J G Ley 1987 Hard-setting soils Soil Use Manage 3 : 79 – 83 Mullins, C E., D A MacLeod, K H Northcote, J H Tisdall, and I M Young 1990 Hardsetting soils; Behaviour, occurrence and management Adv Soil Sci 2 : 37–108 Northcote, K H., G D Hubble, R F Ishbell, C F Thompson, and E Bettany 1975 A Description of Australian Soils Australia: CSIRO Nugis, E., and R Lehtveer, eds 1992 Soil Compaction and. .. Deep subsoil compaction of two cracking clays used for irrigated cotton production in Australia Soil Use Manage 14 : 56 –57 Taylor, N H., and I J Pohlen 1976 Soil Survey Method: NZ Handbook for the Field Study of Soils Bull No 25 New Zealand Soil Bureau, Wellington Trapnell, C G., and R Webster 1986 Microaggregates in red earths and related soils in East and Central Africa, their classification and occurrence.. .Soil Profile Description and Evaluation 605 The agronomic role of the subsoil is to provide entry and egress for water and 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 down pores and cracks in structured soils if the peds are fairly dense Pores and fissures may be up to several mm across and . development and stabil- ity of soil structure; cation exchange capacity (and hence nutrient retention and availability, and the activity and retention of residual soil- acting herbicides); ero- dibility. particular properties. 1. Pedology and Soil Surveys The identification of soil horizons and their sequence feature prominently in stud- ies of soil genesis, soil distribution, and soil classification. For. hand. In such soils, the results of hand texturing give a much better indication of the field behavior and capability of the soil (Trapnell and Webster, 1986). When hand-based assessment of soil