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Soil Survey Manual - Chapter Six (Part 2 of 4)InterpretationsTable of Contents
Interpretive Soil PropertiesSoil survey interpretations are provided for specific soil uses. Interpretations for each soil use are based on a set of interpretative soil properties. These properties include site generalities such as slope gradient, measurements on individual horizons (e.g., particle size distribution), temporal repetitive characteristics that pertain to the soil as a whole (e.g., depth to free water), and potential for diastrophic events (e.g., down-slope movement). Most of the interpretative soil properties are included in the description text and are on the tables associated with a particular map unit (fig. 6-1).
Abbreviated descriptions follow for the more commonly used interpretative soil properties. Formal classes have been assigned to several of the interpretative soil properties. These classes generally are not given unless they are used in field morphological descriptions. All of the classes are in the National Soils Handbook of the Soil Conservation Service. Local conditions may dictate other interpretative soil properties or a greater emphasis on a subdivision of some of the interpretative properties here listed. Assignment GuidelinesIn principle, the system permits the assignment of interpretative soil properties and related interpretations by map unit based on the named components, generally phases of soil series. Properties of the minor components of a map unit may be included if adequate sampling techniques are used to characterize the map units of a survey area. Commonly, representative values are specified by layers or horizons. These layers normally reflect the mode for depths and sequences of horizons for the soil series. Only major horizons are delimited and are generally related to surface layer, subsoil, and underlying material. Ranges are commonly attached to numerical quantities. The ranges are designated to encompass both the actual variation to be expected within the modal concept of the named soil and the expected analytical variation. Setting
Field Water CharacterizationAvailable water capacity.—This is the volume of water that should be available to plants if the soil, inclusive of rock fragments, were at field capacity. Volumes are expressed both as a volume fraction and as a thickness of water. The standard of reference is the water retention difference (under 4C in Soil Survey Laboratory Staff, 1992). Reductions are made in the water retention difference for incomplete root ramification that is associated with certain taxonomic horizons and features such as fragipans, and for chemical properties that are indicative of root restriction such as low available calcium and high extractable aluminum. Corrections for the osmotic effect of high salt concentrations also may be made. The amount of available water to the expected maximum depth of root exploration, commonly either 1 or 1 1/2 m, or a physical or chemical root limitation, whichever is shallower, has been formulated into a set of classes. For the class sets, the depth of rooting that is assumed and the class limits that are stipulated differ among the taxonomic moisture regimes. Drainage class.—This class (ch. 3) places major emphasis on the relative wetness of the soil under natural conditions as it pertains to wetness due to a water table. Flooding.—This refers to inundation by flowing water. Frequency and duration classes are employed. These are described in chapter 3 (fig. 6-2).
Free water occurrence.—This includes the depth to, kind, and months of the year that a zone of free water is present within the soil (ch. 3). Hydrologic soil groups.—This is a set of classes that pertain to the relative infiltration rate of soil under conditions of maximum yearly wetness. It is assumed that the ground surface is bare and ice does not impede infiltration and transmission of water downward (ch. 3). Ponding.—This refers to inundation by stagnant water. The duration and month(s) of the year that ponded water occur are recorded (ch.3). Particle Size DistributionUSDA particle size classes (based on < 2mm fraction).—This is the relative proportion by weight of the particle separate classes <2 mm in diameter (textural classes) as modified by adjectival classes based on the proportion, size, and shape of rock fragments and by the proportion of organic matter if high. The classes are defined in chapter 3. Measurement is described under 1A2, 3A, and 3B (Soil Survey Laboratory Staff, 1992). Fraction >250 mm (based on whole soil).—This quantity is expressed as a weight percent and is inclusive of unattached pieces of rock up to an unspecified upper limit, but it does not exceed the size of the pedon. The rocks more than 250 mm do not affect the Unified or AASHTO classifications, but they may have a large influence on suitability for certain soil uses (ch. 3). Fraction 75 - 250 mm (based on whole soil).—This quantity is expressed as a weight percent of the whole soil, inclusive to an undefined upper limit which is less than the size of the pedon. Consult chapter 3 and methods 1A2 and 3B (Soil Survey Laboratory Staff, 1992). The quantity does not affect the Unified and AASHTO placements. It may, however, have a large influence on suitability for certain uses. Percent passing sieve numbers 4, 10, 40, and 200 (based on < 75mm fraction).—These quantities are the weight percent passing sieves with openings of 4.8 mm, 2.0 mm, 0.43 mm, and 0.075 mm in diameter, respectively. The quantities are expressed as a percentage of the less than 75 mm material. The percent passing the number 4 and 10 sieves may be estimated in the field (ch. 3), or measured in the office or laboratory under methods 1A2 and 3B (Soil Survey Laboratory Staff, 1992). The material passing the 40 and 200 sieves may be measured directly in the laboratory (designation D 422-063, ASTM, 1984) or estimated from the USDA particle separate measurements made as described under 3A (Soil Survey Laboratory Staff, 1992). Clay (based on < 2mm fraction).—This is the <0.002 mm material as the weight percent of the total <2 mm. The pipette method under 3A (Soil Survey Laboratory Staff, 1992) is the standard. For soils that disperse with difficulty, the clay percentage commonly is evaluated from the 1500 kPa retention under 4B (Soil Survey Laboratory Staff, 1992). Carbonate of clay size is included. Fabric-Related AnalysisMoist bulk density.—This is the oven dry weight in megagrams divided by the volume of soil in cubic meters at or near field capacity, exclusive of the weight and the volume of fragments >2 mm. Method 4A1 in (Soil Survey Laboratory Staff, 1992), the so-called clod density method, is the common laboratory reference determination. Shrink-swell potential.—These are a set of classes of reversible volume change between field capacity and oven-dryness for a composition inclusive of rock fragments. Actual shrink-swell, in contrast, is dependent on the minimum water content that occurs under field conditions. The standard laboratory method 4D (Soil Survey Staff Laboratory, 1992), involves computation of the strain from the volume decrease of bulk density clods that are oven-dried from the water content at the suction selected to estimate field capacity, (fig. 6-3).
Available water capacity.—This is the volume of water that should be available to plants if the soil, inclusive of rock fragments, were at field capacity. Values are expressed both as a volume fraction and as a thickness of water per thickness of soil. The standard of reference is the water retention difference under 4C (Soil Survey Laboratory Staff, 1992). Reductions are made in the water retention difference for incomplete root ramification associated with certain taxonomic horizons and features such as fragipans, and for chemical properties indicative of root restriction such as low available calcium, and high extractable aluminum. Corrections for the osmotic effect of high salt concentrations also may be made. Saturated hydraulic conductivity.—This class placement pertains to the amount of water that would move downward through a unit area of saturated in-place soil in unit time under unit hydraulic gradient (ch. 3). Estimates are based on models that relate laboratory measurements on soil cores to the interpretative soil properties and morphology (O'Neil, 1952; Baumer, 1986). The quantity has been referred to as "permeability." Engineering ClassificationLiquid limit.—This is the water content at the change between the liquid and the plastic states. It is measured on thoroughly puddled soil material that has passed a number 40 sieve (0.43 mm) and is expressed on a dry weight basis (ASTM method D 4318-83 in ASTM, 1984). Plasticity index.—This is the range in water content over which soil material is plastic. The value is the difference between the liquid limit and the plastic limit of thoroughly puddled soil material that has passed a number 40 sieve (0.43 mm). The plastic limit is the water content at the boundary between the plastic and semisolid states. The measurement of the plastic limit is described in ASTM method D 4318-83 (ASTM, 1984). Unified classification.—This is a classification of soil material designed for general construction purposes. It is dependent on the particle size distribution of the <75 mm, the liquid limit, and the plasticity index and on whether the soil material is high in organic matter (ASTM test D 2487, in ASTM, 1984). There are three major divisions: mineral soil material having below 50 percent particle size <0.074 mm (pass 200 mesh), mineral soil material having 50 percent or more particle size <0.074 mm, and certain highly organic soil materials. The major divisions are subdivided into groups based on the liquid limit, plastic index, and the coarseness of the material that exceeds 0.074 mm (retained on 200 mesh). AASHTO classification.—This is a classification of soil material for highway and airfield construction (Procedure M 145-73. In Am. Assoc. of State Highway and Transportation Officials, 1984). It is based on the particle size distribution of the <75 mm and on the liquid limit and the plastic index. The system separates soil materials having 35 percent or less, which is <0.074 mm, from those soil materials having over 35 percent. Each of these two divisions are subdivided into classification groups based on guidelines that employ particle size, liquid limit, and volume change. A group index may be computed based on the liquid limit and plasticity index in addition to the percent <0.074 mm. The group index is a numerical quantity based on a set of formulas. Chemical AnalysisCalcium carbonate equivalent.—The methods under 6E (Soil Survey Laboratory Staff, 1992) are the standard of reference. Cation exchange capacity.—The methods of reference are 5A3b for soil with a pH below 5.5 and method 5A8 if the pH is 5.5 or above (Soil Survey Laboratory Staff, 1992). Gypsum.—The quantity pertains to the <20 mm. The methods of reference are under 6F (Soil Survey Laboratory Staff, 1992). Organic matter.—The methods of reference are under 6A (Soil Survey Laboratory Staff, 1992). Measured organic carbon is multiplied by a factor of 1.72 to obtain organic matter. Reaction (pH).—The standard is the 1:1 water pH (method 8C1f, Soil Survey Laboratory Staff, 1992). For organic soil materials the pH in 0.01M CaCl2 is employed. Classes are in chapter 3. Salinity.—A set of classes is employed for the concentration of dissolved salts in a water extract. The classes are expressed as electrical conductivity. The measurement of reference is made on water extracted from a saturated paste (method under 8A, Soil Survey Laboratory Staff, 1992). Units are decisiemens per meter (dS/m). Sodium adsorption ratio.—This is evaluated for the water extracted from a saturated soil paste. The numerator is the concentration of water soluble sodium and the denominator is the square root of half of the sum of the concentrations of water soluble calcium and magnesium (5E, Soil Survey Laboratory Staff, 1992). Sulfidic materials.—On exposure to air the pH of soil materials that contain significant sulfides becomes very low. The requirements are defined in the latest edition of the Keys To Soil Taxonomy. Methods for total sulfur are under 6R (Soil Survey Laboratory Staff, 1992). Direct measurement of the pH after exposure to air is also employed. Physical Features or ProcessesDepth to bedrock.—This refers to the depth to fixed rock. Hard and soft bedrock are distinguished. Hard bedrock is usually indurated but may be strongly cemented, and excavation difficulty would be very high or higher. Soft bedrock meets the consistence requirements for paralithic contact (Soil Survey Staff, 1975). Depth to cemented pan.—This is the depth to a pedogenic zone that is weakly cemented to indurated. Thin and thick classes are distinguished. The thin class is less than 8 cm thick if continuous and less than 45 cm if discontinuous or fractured. Otherwise, the thick class applies. Mass movement.—Three kinds of rather large scale irreversible soil movement are recognized: downslope movement may occur if the soil is loaded, excavated below, or is unusually wet; ice-melt pitting may result from melting of ground ice after vegetative cover has been removed; and differential settling may occur related to wet-dry cycles. Total subsidence.—This is the potential decrease in surface elevation as a result of drainage of wet soils having organic layers or semifluid mineral layers. The subsidence may result from several causes: loss of water and resultant consolidation; mechanical compaction; wind erosion; burning; and of particular importance for organic soils, oxidation. Depth to permafrost.—The critical depth is determined by the active layer. utilities, footings, and so on are placed below the active layer. The minimum depth is affected by the depth of annual freezing. Permafrost depth may be strongly influenced by the soil cover. Potential frost action.—This pertains to the likelihood of upward or lateral movement of soil by formation of ice lenses and the subsequent loss of soil strength upon thawing. Large scale collapse to form pits is excluded and considered under mass movement. Soil temperature, particle size, and the pattern of water states are used to make predictions. ErosionThe K Factor.—The factor appears in the Universal Soil Loss Equation (Wischmeier and Smith, 1978) as a relative index of susceptibility of bare, cultivated soil to particle detachment and transport by rainfall. Measurements are made on plots of standard dimensions. Erosion is adjusted to a standard of 9 percent slope. K factors are currently measured by applying simulated rainfall on freshly tilled plots. Earlier measurements integrated the erosion for the year for cultivated plots under natural rainfall. K may be computed from the composition of the soil, saturated hydraulic conductivity, and structure. The T Factor.—This is the soil loss tolerance which can be used with the Universal Soil Loss Equation (Wischmeier and Smith, 1978). It is defined as the maximum rate of annual soil erosion that will permit crop productivity to be sustained economically and indefinitely. The T factors are integer values of from 1 through 5 tons per acre per year. The factor of 1 ton per acre per year is for shallow or otherwise fragile soils and 5 tons per acre per year is for deep soils that are least subject to damage by erosion. Wind erodibility groups.—This is a set of classes given integer designations from 1 through 8, based on compositional properties of the surface horizon that are considered to affect susceptibility to wind erosion. Texture, presence of carbonate, and the degree of decomposition of organic soils are the major criteria. Associated with each wind erodibility group is a wind erodibility index in tons per acre per year. The wind erodibility index is the theoretical, long-term amount of soil lost per year through wind erosion. It is based on the assumption that the soil is bare, lacks a surface crust, occurs in an unsheltered position, and is subject to the weather at Garden City, Kansas (Woodruff and Siddoway, 1965). Tillage frequency and practices are not specified. CorrosivityUncoated steel.—The rating depends on texture, drainage class, extractable acidity (methods under 6H, Soil Survey Laboratory Staff, 1992), and either resistivity of a saturated soil paste or electrical conductivity of the saturation extract (methods under 8E and 8A, respectively, Soil Survey Laboratory Staff, 1992). Concrete.—The ratings depend on texture, occurrence of organic horizons, pH, and the amounts of magnesium and sodium sulfate or sodium chloride in the saturation extract (methods under 8A, 8C, and 6, Soil Survey Laboratory Staff, 1992).
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