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Soil Survey Manual - Chapter Four (Part 4 of 4)Mapping TechniquesTable of Contents
EquipmentThe efficient operation of a soil survey requires the use of certain kinds of equipment, some easy, some difficult to obtain. There are three major kinds of needs: Tools to examine the soil profile and soil testing, measuring and recording devices for mapping, and transportation vehicles. Some of these are described in the following section. Tools for Examining the SoilA soil scientist examines the soil often in the course of mapping. Examination of both horizontal and vertical variations is essential. The most commonly used tools are spades and soil augers. Backhoes, spades, and shovels are used to expose larger soil sections for examinations, sampling, and photography. Augers are used in most areas for routine mapping. In some areas, however, a spade is used to examine the soil. In soils free of rock fragments, probes provide samples that are quick and relatively easy to obtain. Where a probe or auger is regularly used for examining the soil, some profiles need to be exposed in a pit and examined as a check. Power equipment is often used to save time and effort. Various small instruments can also be used to examine the soil. Spades, shovels, picks, and bars.—Especially after a preliminary excavation has been made, a flat-bladed, square- pointed spade is most convenient for collecting samples. The best spade for ordinary use in mapping is a tile spade or a post-hole spade that has been modified by cutting off the sharp corners. A tile spade has a rounded point and tapers at the end. It is superior to a post-hole spade for stony and gravelly soils. The blades of post-hole and tile spades are commonly 30 to 45 cm in length. Where deep holes are required, a long-handled spoon-type shovel is useful. A heavy crowbar and/or pick may be needed to penetrate dry, cemented, or compact layers. A mattock is especially useful for making holes in soils that are hard, dry, stony, or gravelly. A small army-issue trenching pick will serve satisfactorily in some soils, but commonly the heavier conventional mattock with a long handle is better. One end of the mattock is pointed and the other is a chisel. For moist soils and those containing many woody roots, the chisel point is useful; for dry soils, the sharp point is more effective. A geologist's hammer, one end of which can be used as a pick, is also useful in examining rocks and the soil in cuts. A post-hole digger is useful in removing deep soil material for examination. A digger is heavy and is used mainly for sampling at the bottom of pits where digging is difficult. It removes the soil with less disturbance of structure than most kinds of augers. Augers.—The screw, or worm, soil auger is essentially like a wood auger and ranges from about 2 1/2 to 4 cm in diameter (fig. 4-10a). The worm part is about 15 cm in length, and the distance between flanges is about the same as the diameter. If the distance between flanges is less, removing the soil with the thumb is difficult. In clayey soils, a bit of 2 1/2 cm may work better than the larger ones. The shaft is commonly 100 to 150 cm in length. Extra lengths can be added for deep boring. The bit will become tapered as it wears, therefore, it should be replaceable. A scale can be marked on the shaft of the auger to measure depth to the tip. Screw augers made especially for examining soils are available, but they can also be made from a wood auger bit and lengths of pipe. The auger bit is welded to a steel rod or iron pipe with a crosspiece at the top for a handle. A screw auger is easily carried. It can be used to examine gravelly or stony soils and to bore holes rapidly. It cannot be used in dry or sandy soil because the soil material will not adhere to the bit. It is difficult to pull from the bored hole. The extracted soil material is disturbed more by a screw auger than by other augers and probes. Several kinds of barrel augers are used. Barrel augers are known as post-hole augers, bucket augers, orchard augers, core augers, and various other names. They have a cylinder, or barrel, to hold the soil, which is forced into the barrel by cutting lips at the lower end (fig. 4-10b). The upper end of the cylinder is attached to a length of pipe with a crosspiece for turning at the top. Although both ends of the cylinder are open, the soil generally packs so that it stays in place while the auger is removed from the hole. A few taps of the cylinder on the ground or on a board will loosen the soil for removal. Barrel augers with special closed cutting blades are available for use in sandy soils, very wet loose soils, and very dry soils. Tips should be made of hardened steel to resist wear. Barrel augers disturb the soil less than screw augers. Soil structure, porosity, consistence, and color can be observed better. Barrel augers work well in loose or sandy soils and in compact soils. They are not well suited to use in wet or clayey soils, though an open-sided barrel is available that works well. They also work poorly in stony and gravelly soils. Barrel augers bore more slowly than screw augers or probes. Generally, they are more bulky to carry. They are easy to pull from the hole. Tips wear excessively if not made of hardened steel. Where animals are grazing, the holes must be filled.
The Dutch auger is a modified barrel auger having two connected straps with lips (fig. 4-10d). The cylinder is about 5 to 10 cm in diameter. The cutting blades are so constructed that the soil is loosened and forced into the cylinder of the auger as it cuts into the soil. The Dutch mud auger works well in moist or wet soils of moderately fine or fine texture. This auger works poorly in other moist or wet soils and in all dry soil. Soil augers are simple in design and somewhat crude in appearance, but considerable skill is required to use them effectively and safely. They must be pulled from the soil by using a technique that puts stress on the leg muscles, rather than the back muscles, to avoid serious back injury. Twisting the auger firmly while pulling takes advantage of the inclined plane of the screw to break the soil loose. A pair of pipe wrenches is needed to add and remove lengths of shafts and bits. Examinations of deep deposits of peat are made with special tubelike samplers. A peat sampler designed by the Macaulay Institute for Soil Research, Aberdeen, Scotland, takes a relatively undisturbed volume that can be used for measurement of bulk density. The Davis peat sampler, consists of 10 or more sections of steel rods, each 60 to 120 cm in length, and a cylinder of brass or Duralumin, approximately 35-cm long with an inside diameter of about 1.9 cm (fig. 4-10e). The cylinder has a plunger, cone-shaped, at the lower end and a spring catch near the upper end. The sampler is pressed into the peat until the desired depth for taking the sample is needed. Then the spring catch is released, allowing the plunger to be withdrawn from the cylinder. With the plunger withdrawn, the cylinder is filled by forcing it further downward. The cylinder protects the sample from contamination and preserves its structure when the sample is removed. With this instrument, one can avoid the error of thinking that firm bottom has been hit when actually a buried log is encountered. Probes.—Probes consist of a small-bore tube that has a tempered sharp cutting edge slightly smaller in bore but larger in outside diameter than the barrel (fig. 4-10). Approximately one-third of the tube is cut away above the cutting edges so that the soil can be observed and removed. Probes are about 2.5 cm in diameter and about 20 to 40 cm in length. The tube is attached to a shaft with a "T" handle at the opposite end. Shaft length can be varied by adding or removing sections. Probes can be used to examine the soil to a depth of 2 meters. A pedal that is attached to the shaft is available to allow the operator to apply body weight. Some workers carry rubber or plastic mallets to drive the tube into the soil. A pair of pipe wrenches is needed to add and remove lengths of shaft. Probes work well in moist, medium textured soils that are free of gravel, stones, and dense layers. Under these conditions, the soil can be examined faster than with an auger. Probes are very difficult to use in dry, dense, or poorly graded soil, and in soil containing gravel or stones. Probes disturb the soil less than augers, but they retrieve less soil for examination. Probes are light and easily carried, and they pull from the hole more easily than screw augers. Often a special punch or dowel must be used to clear the tube. Use of a soil probe is the fastest method to collect samples of surface layers for analysis. Probes used with power equipment have wide applications in soil surveys (fig. 4-11 (no longer available)). Power equipment.—Power equipment is used for rapid excavation or for extracting cores and samples rapidly and from depths that are difficult to reach with hand tools. The use of power equipment results in large savings in time and permits deeper and larger excavations with better exposure of the various horizons than can be attained with hand tools. Not all sites, however, are accessible to power equipment. Most of this equipment is powered by the motor of the tractor or truck on which the equipment is mounted, although some of the heavier types have separate power units. A backhoe (fig. 4-12 (no longer available)) is used to expose vertical sections of soil. The width of the bucket, or shovel, ranges from 30 centimeters on the smaller models to more than 75 cm on the larger ones. Small backhoes are available that mount on the back of small trucks or on small self-propelled vehicles. The larger backhoes are mounted on tractors. Excavations can be made rapidly to depths of 2 or 3 m performing in a matter of minutes a task that would take two people most of a day. Backhoes can be used effectively in gravelly and stony soils as well as in soils that are stone-free. They ensure good horizontal and vertical exposure of the soil profile. Backhoes have limitations. Maintenance costs are high, and time must be taken for maintenance. Operators must be trained, and safety standards must be met. Some property owners do not want large equipment on their property. There is a tendency to dig pits so deep that site walls are weakened. This practice is dangerous for anyone in the pit. Rental costs for backhoes are high in the areas where machines are available for rent. Power augers are commonly mounted on a small truck and are powered by the engine of the truck. Some have independent power plants and can be mounted on a trailer. The auger can be raised to permit soil to be taken from the bit for examination and can be reinserted in the hole for continued sampling. The bits are 5 cm to more than 15 cm in diameter and generally are threaded over lengths of 50 cm or more. Some augers, such as that in figure 4-13 (no longer available), are threaded their entire length and have extensions that permit sampling to depths of a few meters. Power augers can be equipped with barrel-type bits. The barrels are usually larger and heavier than those on hand augers. Most power barrel augers have a cylinder that can be opened for removing the sample. Power-operated probes (figs. 4-11 (no longer available) and 4-14) are used in moist soils that have few stones. They are usually mounted on a truck and are forced into the soil by hydraulic drivers that are powered by the engine of the truck and act against the weight of the truck and its load. The tubes are usually 2.5 to 10 cm in diameter. They can effectively remove undisturbed cores of soil to a depth of 2 m or more. The top of the tube can be taken off. The tube is open on one side, which permits removal of the core. Power probes are especially useful in moist stone-free soil material, such as loess. They function poorly in dry soils and in soils having cemented layers.
Equipment is available that anchors the truck to the ground by means of a screw. Anchoring allows undisturbed cores to be taken at a greater depth and over a larger range of soil conditions. Power equipment for extracting samples for examination or analysis is necessary in soil surveys that require systematic sampling of deep layers, as in many areas where landscapes have low predictive value. In dry areas, deep layers that have no influence on present vegetation can be very important to the success of irrigation farming. Power equipment has made surveys of such areas much more accurate and much less physically demanding on fieldworkers. Power augers and probes have limitations. Generally, holes can be bored or probed only when the truck is level. If the truck is not equipped with four-wheel drive, off-road operation in wet areas is curtailed. Fences further restrict off-road movement. Equipment and maintenance costs of power augers and probes are high. Operators must be trained and safety standards met. Power augers mix the soil so that depths to different layers cannot be measured accurately. Dense soils and soils that contain large amounts of rock fragments are difficult to examine. Electrically powered jackhammers that quickly loosen compact or skeletal material are available. The loosened material can be thrown from the pit with a shovel. The jackhammers are similar to those used in street repair. A chisel bit is used. The power source is the truck generator or an independent gasoline-powered generator. Use of jackhammers is limited to areas that can be reached by truck. The initial cost is high. The scheduling of power equipment is important to ensure maximum use of it while the equipment is available and weather and soil-moisture conditions are advantageous. Small implements.—Many kinds of small implements are used for examining soil. Although personal preferences may influence choices, certain general types of implements are essential almost everywhere. A large knife is the most commonly used small tool for probing and digging in an exposed profile. A sheath knife having a blade about 10 to 15 cm in length and 2 1/2 to 5 cm in width—the kind available for hunting or camping—can be used for probing the soil, for cutting through peds to observe the interior, for removing small amounts of material, for cutting roots, for scraping the vertical or horizontal sections of the pedon, and for a variety of other purposes. Trowels, spatulas, putty knives, and other small instruments are used similarly. A geologist's or mason's hammer is useful, especially for breaking cemented layers and examining rock fragments and very strongly cemented nodules. The chisel-shaped end of the head can also be used for digging unconsolidated material. Various small instruments for measurement and observation are essential. A scale for measuring is indispensable. Graduated steel tapes that retract into a small case are most useful. A hand lens is very important. A 10X lens is most common, but lenses having magnifications that range from 4X to more than 50X are used. Some, mounted in a pen-sized tube, have high magnification but a small field. Some of these have battery-powered lights for illuminating the sample. A pocket magnet for separating magnetic material is useful in some areas. A soil thermometer is needed by most field scientists. Those having a metal probe and a dial on which the temperature is read are especially suitable for use in soil. A grid for area measurements or for point counts of features like stones is an important part of the kit of a soil scientist. The grid may be simply a piece of wire mesh with the spacing of wires chosen to fit the scale at which measurements are to be made. A hand tally or counter is useful for point counts; it is also useful for recording paces in measuring distance. Standard color charts and standard charts for the estimation of proportionate area are also necessary components of the kit. Mapping EquipmentVarious small pieces of equipment and instruments are used in mapping. The choices of fieldworkers vary, but certain kinds of equipment are essential. Various kinds of metal or wood clipboards or folders are used for holding the map (fig. 4-15). Some surveyors use an aluminum folder with a spring clip and a covering flap that is hinged at one edge. If the field sheets are large, a rotary map cylinder is useful.
The part of the map being used is exposed on the board, and the unused part is rolled into a cylinder attached to the edge of the board. The cylinder protects the unused part of the map and provides a work surface. Some field scientists make these map holders to suit their own needs (fig. 4-16 (no longer available)). The use of aerial photographs as mapping bases has almost eliminated the need for compasses for finding bearings. In areas where keeping located is difficult, a compass must be used to orient the map and to take bearings from which the soil scientist can plot location. A traverse board consists of a map board that has a compass attached at one edge and that rotates on a tripod. In mountainous areas, an altimeter can be used to determine elevation and establish location relative to contours on topographic maps. Altimeters measure altitude by measuring changes in barometric pressure as related to elevation and must be adjusted at a point of known elevation to the barometric pressure at the time. An instrument is needed for measuring slope gradient. The Abney hand level (fig. 4-17) is commonly used. For convenience, the scale is graduated in percent of slope or in both percentage and degrees. This instrument consists of a small spirit bubble level pivoted above a graduated arc and is operated by rotating the level until the bubble, visible through the eyepiece by means of mirrors, indicates that the level is horizontal. The barrel of the level is sighted parallel to the soil surface. The gradient is read directly from the graduated arc. Clinometers are used in some places to measure slope gradient. In a clinometer, a weighted string swings across a graduated arc. Clinometers are lighter in weight, more convenient to carry, and slightly faster to use than an Abney level.
A scale for measuring distances on the map is needed. A supply of pencils that have different degrees of hardness should be carried. The hardness chosen is determined by temperature and humidity and by the material of the mapping base. If aerial photographs are used, the pencils should leave a fine dark line that does not smudge easily on handling, but it should not be hard enough to cut the emulsion. Because soil boundaries must be adjusted and the symbols changed frequently, the pencils should make marks that can be erased without smudging and without damaging the mapping base. TransportationField operations of the soil survey require transporting workers, equipment, supplies, and soil samples. Vehicles are provided to the soil survey party for their daily operations. The time spent by soil scientists traveling to and from the field is lengthy and mainly unproductive. Enough vehicles are provided to keep travel time as short as possible. Additional equipment used for special purposes or for short periods is usually rented or supplied as needed. A passenger van, for example, may be furnished by one of the agencies during a field review. Aircraft may be rented to visit areas not readily reached by ground transport. The uses of vehicles vary widely from one area to another. In some areas, travel is mainly on roads; in other areas, vehicles must be used to travel across country during mapping or to reach remote sites for soil studies. Some vehicles must carry power equipment or pull trailers. All vehicles that are provided for use should carry workers efficiently and in comfort and safety, hold the equipment that is used regularly, have some reserve capacity to accommodate an extra load, and protect workers and equipment from adverse weather. In many areas, pickup trucks are desirable. Trucks are available with optional equipment that may be useful in some areas. Optional equipment includes four-speed transmissions for mountainous and off-road travel; four-wheel drive for off-road travel under adverse conditions; high clearance for travel over rough or stony areas; oversize radiators for use in hot climates, for use where the truck engine will be idled for long periods, or for use with power probes, augers, or winches powered by the truck engine; special tires and wheels for unusual wet, rocky, sandy conditions; and special bodies or truck beds for mounting and storing special equipment (fig. 4-11 (no longer available)). In some remote areas, vehicles are equipped with two-way radios. The various kinds of optional equipment have various disadvantages and limitations such as increased initial cost, increased operating and maintenance cost, increased downtime for the truck, difficulty in obtaining replacement parts, a decrease in the truck's handling qualities, and a decrease in road speed. In areas with good roads and little off-road travel is required, passenger vehicles are adequate. Passenger vehicles also are used to transport groups for field reviews. Specialized vehicles are necessary in some areas. Tracked vehicles and all-terrain vehicles (ATVs) may be needed in very rugged areas (fig. 4-18) Marsh buggies with large buoyant tires and airboats are used in swamps and marshes. Snowmobiles provide access in winter to some northern swamps where travel is impossible or impractical in other seasons. Trail bikes or ATVs can be used in areas that could otherwise be reached only by walking. Specialized vehicles must be reliable in relatively inaccessible areas. The equipment must be transported to the use area. Costs of buying or renting the equipment, maintaining it, and training operators can be high. Time is needed for transport, maintenance, and training. Some kinds of equipment are hazardous to operate. Sensitive ecosystems may be damaged by the equipment.
Aircraft, particularly helicopters, are used in some soil surveys to transport workers and equipment and to provide broad views of landscape and vegetation. Aircraft are useful for photographing landscapes, soil patterns, and land use. Availability, cost, and lack of conventional landing sites are the main limitations.
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