NSSH Part 618

Soil Properties and Qualities

Definition and Purpose (618.00)

Soil properties are measured or inferred from direct observations in the field or laboratory. Soil properties include, but are not limited to, particle-size distribution, cation exchange capacity, and salinity.
Soil qualities are behavior and performance attributes that are not directly measured. They are inferred from observations of dynamic conditions and from soil properties. Soil qualities include, but are not limited to, corrosivity, natural drainage, frost action, and wind erodibility.
Soil properties and soil qualities are the criteria used in soil interpretation rating guides, as predictors of soil behavior, and for classification and mapping of soils. The soil properties entered should be representative of the soil for the dominant land use for which interpretations will be based.

Policy and Responsibilities (618.01)

Soil property data are collected, tested, and correlated as part of soil survey operations. These data are reviewed, supplemented, and revised as necessary.
The soil survey project office is responsible for collecting, testing, and correlating soil property data and interpretive criteria.
The MLRA office is responsible for the development, maintenance, quality assurance, correlation, and coordination of the collection of soil property data that are used as interpretive criteria. This includes all data elements listed in part 618.
The National Soil Survey Center is responsible for the training, review, and periodic update of soil interpretation technologies.
The state soil scientist is responsible for ensuring that the soil interpretations are adequate for the field office technical guide and that they meet the needs of federal, state, and local programs.

Collecting and Testing Soil Property Data (618.02)

The collection and testing of soil property data is based on the needs described in the soil survey memorandum of understanding for individual soil survey areas. The collection and testing must conform to the procedures and guides established in this handbook.

Soil Properties and Soil Qualities (618.03)

The following sections list soil properties and qualities in alphabetical order and provide some grouping for climatic and engineering properties and classes. A definition, classes, significance, method, and guidance for NASIS database entry are given. The listing includes the soil properties and qualities in the National Soil Information System. For specifics of data structure, attributes, and choices in NASIS, refer to http://nasis.nrcs.usda.gov/documents/metadata/5_1/

Previous databases of soil survey information used metric or English units for soil properties and qualities. The National Soil Information System (NASIS) transferred English units to metric units on conversion, except for crop yields in the database. All future edits and entries in NASIS will use metric units, except yields and acreage.

Ranges of soil properties and qualities, posted in the NASIS database for map unit components, may extend beyond the established limits of the taxon from which the component gets its name, but only to the extent that interpretations do not change. However, the representative value (RV) is within the range of the taxon.

Albedo, Dry (618.04)

Definition. Albedo, dry, is the estimated ratio of the incident shortwave (solar) radiation that is reflected by the air-dry, less than 2 mm fraction of the soil surface to that received by it.
Significance. Soil albedo, as a function of soil color and angle of incidence of the solar radiation, depends on the inherent color of the parent material, organic matter content, and weathering conditions.
Estimates of the evapotranspiration rates and for predicting soil water balances require the albedo. Evapotranspiration and soil hydrology models that are part of Water Quality and Resource Assessment programs require this information.
Measurement. Instruments exist that measure albedo.
Estimation. Approximate the values by use of the following formula:
Soil Albedo=0.069 x (Color Value} - 0.ll4.

For albedo, dry, use dry color value. Surface roughness has a separate significant impact on the actual albedo. The equation above is the albedo of <2.0 mm smoothed soil condition, but if the surface is rough because of tillage, the albedo differs.
Entries. Enter the high, low, and representative values of the map unit component using the above formula. Allowable entries range from 0.00 to 1.00, with 2 decimal places.

Available Water Capacity (618.05)

Definition. Available water capacity is the volume of water that should be available to plants if the soil, inclusive of fragments, were at field capacity. It is commonly estimated as the amount of water held between field capacity and wilting point, with corrections for salinity, fragments, and rooting depth.
Classes. Classes of available water capacity are not normally used except as adjective ratings that reflect the sum of available water capacity in inches to some arbitrary depth. Class limits vary according to climate zones and the crops commonly grown in the areas. The depth of measurement also is variable.
Significance. Available water capacity is an important soil property in developing water budgets, predicting droughtiness, designing and operating irrigation systems, designing drainage systems, protecting water resources, and predicting yields.
Estimates. The most common estimates of available water capacity are made in the field or the laboratory as follows:
1. Field capacity is determined by sampling the soil moisture content just after the soil has drained following a period of rain and humid weather, after a spring thaw, or after heavy irrigation. The Soil Survey Investigation Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, provides more information.
2. The 15-bar moisture content of the samples is determined with pressure membrane apparatus.
3. An approximation of soil moisture content at field capacity is commonly made in the laboratory using 1/3-bar moisture percentage for clayey and loamy soil materials and 1/10-bar for sandy materials. Recently, some soil physicists have been using 1/10-bar instead of 1/3-bar for clayey and loamy soil materials and 1/20-bar for sandy soil materials.
4. Measure the bulk density of the moist soil. The Soil Survey Investigation Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, provides more information.
5. Calculate available water capacity (AWC) using the following formula:
  AWC = (W_1/3 - W₁₅) X (Db_1/3) X cm / 100
  
  Where
  
  AWC = volume of water retained in 1 cm³ of whole soil between 1/3-bar and 15-bar tension; reported as cm cm^-1, i.e., numerically equivalent to inches of water per inch of soil (in in^-1)
  
  W_1/3 = weight percentage of water retained at 1/3-bar tension
  
  W₁₅ = weight percentage of water retained at 15-bar tension
  
  Db_1/3 = bulk density of <2-mm fabric at 1/3-bar tension
  
  Cm = Vol moist <2-mm fabric (cm3) / Vol moist whole soil (cm3)
  
  Procedure 3B2 is used to determine Vol moist <2-mm fabric (cm3).
  
  AWC (cm cm^-1 or in in^-1 horizon) = AWC (cm cm^-1 or in in^-1) X horizon thickness
6. If data are available, estimates are based on available water capacity measurements. If data are not available, data from similar soils are used as a guide. The relationship between available water capacity and other soil properties has been studied by many researchers. Soil properties that influence available water capacity are particle size; size, shape, and distribution of pores; organic matter; type of clay mineral; and structure.
7. amount of water available to plants is nearly zero. Available water capacity values are zero for layers that exclude roots. If roots are restricted but not excluded, estimates of available water capacity are reduced according to the amount of dense material in the layers and the space available for root penetration. Depending on the ability of roots to enter the soil mass and utilize the water, values for the soils with these dense layers may be significantly less than for soils of similar texture that do not have pans. Entries are made for all soil layers below dense layers only if roots are present.
8. Depending on their abundance and porosity, rock and pararock fragments reduce available water capacity. Nonporous fragments reduce available water capacity in proportion to the volume they occupy, for example, 50 percent nonporous cobbles reduces available water capacity as much as 50 percent. Porous fragments, such as sandstone, may reduce available water capacity to a lesser proportion.
9. Several factors contribute to a lower amount of plant growth on saline soils. However, as a rough guide, available water capacity is reduced by about 25 percent for each 4 mmhos cm-1 electrolytic conductivity of the saturated extract.
10. Soils high in gibbsite or kaolinite, such as Oxisols and Ultisols, may have available water capacity values that are about 20 percent lower than those with equal amounts of 2:1 lattice clays.
11. Soils high in organic matter have higher available water capacity than soils low in organic matter if the other properties are the same.
Entries. Enter high, low, and representative values for available water capacity in cm per cm for each horizon. Enter “0” for layers that exclude roots. The range of valid data entries is 0.00 to 0.70 cm per cm.

Bulk Density, One-Tenth Bar or One-Third Bar (618.06)

Definition. Bulk density one-tenth bar or one-third bar is the oven-dried weight of the less than 2 mm soil material per unit volume of soil at a water tension of 1/10 bar or 1/3 bar.

Significance. Bulk density influences plant growth and engineering applications. It is used to convert measurements from a weight basis to a volume basis. Within a family particle size class, bulk density is an indicator of how well plant roots are able to extend into the soil. Bulk density is used to calculate porosity.

Plant growth. Bulk density is an indicator of how well plant roots are able to extend into the soil. Root restriction initiation and root limiting bulk densities are shown below for various family particle size classes.

Family particle-size	Bulk Density (g cm^-3)
Classes	Restriction-initiation	Root-limiting
Sandy	1.69	>1.85
Loamy
coarse-loamy	1.63	>1.80
fine-loamy	1.60	>1.78
coarse-silty	1.60	>1.79
fine-silty	1.54	>1.65
Clayey*
35-45% clay	1.49	>1.58
>45% clay	1.39	>1.47

*Oxidic and andic materials can initiate restriction at lower bulk densities

Engineering applications. Soil horizons with bulk densities less than those indicated below have low strength and would be subject to collapse if wetted to field capacity or above without loading. They may require special designs for certain foundations.

Family particle-size	Bulk Density (g cm^-3)
Sandy	<1.60
Loamy
coarse-loamy	<1.40
fine-loamy	<1.40
coarse-silty	<1.30
fine-silty	<1.40
Clayey	<1.10

Estimates. The weight applies to the oven-dry soil, and the volume applies to the soil at or near field capacity. For non-expansive soils, the 1/10-bar and 1/3-bar bulk densities are the same. Bulk density is a use dependent property. The entry should represent the dominant use for the soil.
Entries. Enter bulk density at one tenth bar or one third bar with the low, high, and representative values for each horizon. The range of valid entries is 0.02 to 2.60 g cm-3. Values should be estimated to the nearest 0.05 g cm-3.

Bulk Density, 15 Bar (618.07)

Definition. Bulk density 15 bar (ρb1500) is the oven dry mass per unit volume of the <2 mm soil material at 15 bar water tension.
Significance. Bulk density 15 bar is used in resource assessment models such as water erosion prediction.
Estimation. The value is derived by equation 1.

Where:

ρb₃₃₍₁₀₎-Bulk density at one-third bar (33kPa) or one-tenth bar (10kPa) moisture content, acquired from lab data, by direct field measurement (e.g., core samples, compliant cavity), or estimated from lab data of similar soils.

ρb_od-Bulk density at oven-dry moisture content, acquired from lab data, by direct field measurement (e.g., core samples, compliant cavity), estimated from lab data of similar soils, or derived by the equation given in NSSH 618.08.

θ_m33(10)-Gravimetric water content at one-third bar or one-tenth bar, in weight percent, from lab data, estimated from lab data of similar soils, or derived from equation 2.

where:

MRD - Moisture retention difference, derived from equation 3.

EQUATION 3 - MRD = (θ _m33(10) - θ_m1500)*¹⁰⁰/_ρb33(10)

ρb33 (10) - Bulk density at one-third bar (33kPa) or one-tenth bar (10kPa) moisture content, acquired from lab data, by direct field measurement (e.g., core samples, compliant cavity), or estimated from lab data of similar soils.

V>2mm - fraction greater than 2mm, percent by volume.

θm1500 Gravimetric water content at 15 bar, in percent by weight. Acquired from lab data, estimated from lab data of similar soils, or derived from equation 4.

where:

OM - Organic matter, weight percent.

θ_mad - Air dry gravimetric water, in weight percent Acquired from lab data, estimated from lab data of similar soils, or derived from equation 5.

where:

AD/OD - Ratio of air-dry mass to oven-dry mass. Acquired from lab data, estimated from lab data of similar soils, or derived from equation 6.

Entries. Enter the high, low, and representative value for each horizon. Valid entries range from 0.02 to 2.60 and 2 decimal places are allowed.

Bulk Density, Oven Dry (618.08)

Definition. Bulk density oven dry (Pb_od) is the oven dry weight of the less than 2 mm soil material per unit volume of oven-dry soil.
Estimation. The value Pb_od is derived by the following formula:
Pb_od = [(linear extensibility percent/100) + 1]³

where linear extensibility percent is adjusted to a <2 mm basis.
Entries. Enter the high, low, and representative value for each horizon. Valid entries range from 0.02 to 2.60 and 2 decimal places are allowed.

Calcium Carbonate Equivalent (618.09)

Definition. Calcium carbonate equivalent is the quantity of carbonate (CO3) in the soil expressed as CaCO3 and as a weight percentage of the less than 2 mm size fraction.
Significance. The availability of plant nutrients is influenced by the amount of carbonates in the soil. This is a result of the effect that carbonates have on soil pH and of the direct effect that carbonates have on nutrient availability. Nitrogen fertilizers should be incorporated into calcareous soils to prevent nitrite accumulation or ammonium-N volatilization. The availability of phosphorus and molybdenum is reduced by the high levels of calcium and magnesium which are associated with carbonates. In addition, iron, boron, zinc, and manganese deficiencies are common in soils that have a high calcium carbonate equivalent. In some climates, soils that have a high calcium carbonate equivalent in the surface layer are subject to wind erosion. This effect may occur in soils that have a calcium carbonate equivalent of more than 5 percent. Strongly or violently effervescent reaction to cold dilute HCL defines calcareous in the wind erodibility groups because of the significance of finely divided carbonates.
Measurement. Calcium carbonate equivalent is measured by method 6E1 as outlined in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS. It also may be measured in the field using calcimeters.
Entries. Enter the high, low, and representative values for each horizon listed. Round values to the nearest 5 percent for horizons that have more than 5 percent CaCO3 and to the nearest 1 percent for those with less than 5 percent. Enter 0 if the horizon does not have free carbonates.

Cation Exchange Capacity NH₄OAc pH7 (618.10)

Definition. Cation-exchange capacity is the amount of exchangeable cations that a soil can adsorb at pH 7.0.
Significance. Cation-exchange capacity is a measure of the ability of a soil to retain cations, some of which are plant nutrients. Soils that have a low cation-exchange capacity hold fewer cations and may require more frequent applications of fertilizer than soils that have a high cation-exchange capacity. Soils that have high cation-exchange capacity have the potential to retain cations, which reduces the risk of the pollution of ground water.
Measurement. Cation-exchange capacity is measured by the methods outlined in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004. The ammonium acetate method 5A8 gives the cation-exchange capacity value for soils that have pH >5.5 or contain soluble salts. Cation-exchange capacity is expressed in milliequivalents per 100 grams (me 100g-1), of soil. If the pH is less than 5.5, use effective cation-exchange capacity (refer to part 618.18).
Entries. Enter the high, low, and representative values of the estimated range in cation exchange capacity, in meq 100g-1, for each horizon with pH >5.5. Values in tenths are allowed. Valid entries range from 0.0 to 400.0. A NASIS calculation is available and can be viewed in Appendix 2.

Climatic Setting (618.11)

Climatic setting includes frost free period, precipitation, temperature, and evaporation. These elements are useful in determining the types of natural vegetation or crops that grow or can grow in an area and in planning management systems for vegetation. Climatic data are observed nationally by the National Weather Service Cooperative Network, which consists of approximately 10,000 climate stations. The records are available from the Climatic Data Access Facility (CDAF) at Portland, Oregon. Climatic data are delivered to the field through a Climatic Data Access Network. The Climatic Data Access Network consists of climatic data liaisons established in each state and at National Headquarters. Climatic data that are input into NASIS are obtained from the respective climatic data liaison. Climatic data may also be obtained from project weather stations or from the state climatologist. NRCS has selected the standard “normal” period of 1971 to 2000 for climate database entries. Always check with your state’s climatic data liaison before using a climate station that has less than 30 years of records or that is located outside a county. Footnote the source of the data, the station, and the starting and ending year of record. Means are given as a range to represent the change of the climate over the geographic extent of the assigned soil.

Frost-Free Period
1. Definition. Frost-free period is the expected number of days between the last freezing temperature (0° C) in spring (January-July) and the first freezing temperature (0° C) in fall (August-December). The number of days is based on the probability that the values for the standard “normal” period of 1971 to 2000 will be exceeded in 5 years out of 10.
2. Entries. Enter the high, low, and representative values for the map unit component. Enter 365 for each value for taxa that are frost-free all year and 0 for those that have no frost-free period. Entries are rounded to the nearest 5 days.
Precipitation, Mean Annual.
1. Definition. Mean annual precipitation is the arithmetic average of the total annual precipitation taken over the standard “normal” period, 1971-2000. Precipitation refers to all forms of water, liquid or solid, that fall from the atmosphere and reach the ground.
2. Entries. Enter the high, low, and representative values in millimeters of water, as integers to represent the spatial range for the map unit component.
Air Temperature, Mean Annual.
1. Definition. Mean annual air temperature is the arithmetic average of the daily maximum and minimum temperatures for a calendar year taken over the standard “normal” period, 1971-2000.
2. Entries. Enter the high, low, and representative values as integers for the map unit component to represent the spatial range in degrees centigrade. Use a minus sign to indicate below zero temperatures.
Daily Average Precipitation.
1. Definition. Daily average precipitation is the total precipitation for the month divided by the number of days in the month for the standard “normal” period, 1971-2000.
2. Entries. Enter the high, low, and representative value in mm. The range of allowed entries is 0 to 750 mm.
Daily Average Potential Evapotranspiration
1. Definition. Daily average potential evapotranspiration is the total monthly potential evapotranspiration divided by the number of days in the month for the standard “normal” period, 1971-2000.
2. Entries. Enter the high, low, and representative value in mm. The range of allowed entries is 0 to 300 mm.

Corrosion (618.12)

Various metals and other materials corrode when they are on or in the soil, and some metals and materials corrode more rapidly when in contact with specific soils than when in contact with others. Corrosivity ratings are given for two of the common structural materials, uncoated steel and concrete.

Uncoated steel.
1. Definition. Risk of corrosion for uncoated steel is the susceptibility of uncoated steel to corrosion when in contact with the soil.
2. Classes. The risk of corrosion classes are low, moderate, and high.
3. Significance. Risk of corrosion on uncoated steel pertains to the potential soil-induced electrochemical or chemical action that converts iron into its ions, thereby dissolving or weakening uncoated steel.
4. Guides. Exhibit 618-1 gives the relationship of soil moisture, soil texture, acidity, and content of soluble salts (as indicated by either electrical resistivity at field capacity or electrolytic conductivity of the saturated extract of the soil) to corrosion classes.
  1. Soil reaction (pH) correlates poorly with corrosion potential; however, a pH of 4.0 or less almost always indicates a high corrosion potential.
  2. Ratings, which are based on a single soil property or quality, that place soils in relative classes for corrosion potential must be tempered by knowledge of other properties and qualities that affect corrosion. A study of soil properties in relation to local experiences with corrosion helps soil scientists and engineers to make soil interpretations. Special attention should be given to those soil properties that affect the access of oxygen and moisture to the metal, the electrolyte, the chemical reaction in the electrolyte, and the flow of current through the electrolyte. A constant watch should be maintained for the presence of sulfides or of minerals, such as pyrite, that can be weathered readily and thus cause a high degree of corrosion in metals.
  3. The possibility of corrosion is greater for extensive installations that intersect soil boundaries or soil horizons than for installations that are in one kind of soil or in one soil horizon.
  4. Using interpretations for corrosion without considering the size of the metallic structure or the differential effects of using different metals may lead to wrong conclusions. Activities, such as construction, paving, fill and compaction, and surface additions, that alter the soil can increase possibility of corrosion by creating an oxidation cell that accelerates corrosion. Mechanical agitation or excavation that results in aeration and in a discontinuous mixing of soil horizons may also increase the possibility of corrosion.
5. Entries. Enter the appropriate class of risk of corrosion for uncoated steel for the whole soil. The classes are LOW, MODERATE, or HIGH.
Concrete.
1. Definition. Risk of corrosion for concrete is the susceptibility of concrete to corrosion when in contact with the soil.
2. Classes. The risk of corrosion classes are low, moderate, and high
3. Significance. Risk of corrosion on concrete pertains to the potential soil-induced chemical reaction between a base (the concrete) and a weak acid (the soil solution). Special cements and methods of manufacturing may be used to reduce the rate of deterioration in soils that have a high risk of corrosion. The rate of deterioration depends on (i) soil texture and acidity, (ii) the amount of sodium or magnesium sulfate present in the soil, singly or in combination, and (iii) the amount of sodium chloride (NaCl) in the soil. The presence of NaCl is one of the factors evaluated not because of its corrosivity of cement but because it is used to identify the presence of seawater. Seawater contains sulfates, which are one of the principal corrosive agents. A soil that has gypsum requires special cement. The calcium ions in gypsum react with the cement and weaken the concrete.
4. Guides. Exhibit 618-2 gives the relationship of soil texture, soil acidity, sulfates, and NaCl to corrosion classes.
5. Entries. Enter the appropriate class of risk of corrosion for concrete for the whole soil. The classes are LOW, MODERATE, or HIGH.

Crop Name and Yield (618.13)

Definition. Crop name is the common name for the crop. Crop yield units is crop yield units per unit area for the specified crop.
Classes. The crop names and the units of measure for yields that are allowable as data entries are listed in the data dictionary of the National Soil Information System. (http://nasis.nrcs.usda.gov/documents/metadata/4_1/home.shtml).
Significance. Crop names and units of measure are important as records of crop yield. The crops and yield often are specific to the time when the soil survey was completed, but the ranking and comparison between soils within a soil survey is helpful. These crops and yield data are used to evaluate the soil productive capabilities, cash rent, and land values. Generally, only the most important crops are listed and only the best management is reflected.
Estimates. Crop names and yields are specific to the soil survey area. The listing of crop names is not limited to any number but only the most important crops in the survey area should be used. The yields are derived in a number of ways but should represent a high level of management by leading commercial farmers, which tends to produce the highest economic return per acre. This level of management includes using the best varieties; balancing plant populations and added plant nutrients to the potential of the soil; controlling erosion, weeds, insects, and diseases; maintaining optimum soil tilth; providing adequate soil drainage; and ensuring timely operations.
Generally only a representative value is used for each map unit component for non MLRA soil survey areas. MLRA soil survey areas use the high and low representative value from map unit components of non MLRA soil survey areas. High and low values represent the range of representative values for a high level of management across the survey area or across several surveys.
Entries. Enter the common crop name and units of measure. Enter the corresponding irrigated and/or nonirrigated yields as appropriate for the component. Yields can be posted as high, low, and representative values for the map unit component.

Diagnostic Horizon Feature Depth to Bottom (618.14)

Definition. Diagnostic horizon feature depth to bottom is the distance from the top of the soil to the base of the identified diagnostic horizon or to the lower limit of the occurrence of the diagnostic feature.
Measurement. Distance is measured from the top of the soil which is defined as the top of the mineral soil, or, for soils with “O” horizons, the top of any “O” layer that is at least partially decomposed. For soils that are covered by 80 percent or more rock or pararock fragments, the top of the soil is the surface of the fragments. See pages 63-64 in the Soil Survey Manual for a complete discussion.
Entries. Enter the high, low, and representative values in whole centimeters. The high value represents either the greatest depth to which the base of the diagnostic horizon or feature extends or, for horizons for features extending beyond the limit of field observation, it is the depth to which observation was made (usually no more than 200 cm). In the case of the lithic contact, paralithic contact, and petroferric contact, the entries for depth to the bottom of the diagnostic feature will be the same as the entries for depth to the top of the feature, since the contact has no thickness.

Diagnostic Horizon Feature Depth to Top (618.15)

Definition. Diagnostic horizon feature depth to top is the distance from the top of the soil to the upper boundary of the identified diagnostic horizon or to the upper limit of the occurrence of the diagnostic feature.
Measurement. Distance is measured from the top of the soil, which is defined as the top of the mineral soil, or, for soils with “O” horizons, the top of any “O” layer that is at least partially decomposed. For soils that are covered by 80 percent or more rock or pararock fragments, the top of the soil is the surface of the fragments. See pages 63-64 in the Soil Survey Manual for a complete discussion.
Entries. Enter the high, low, and representative values in whole centimeters.

Diagnostic Horizon Feature Kind (618.16)

Definition. Diagnostic horizon feature kind is the kind of diagnostic horizon or diagnostic feature present in the soil.
Significance. Diagnostic horizons and features are a particular set of observable or measurable soil properties that are used in Soil Taxonomy to classify a soil. They have been chosen because they are thought to be the marks left on the soil as a result of the dominant soil forming processes. In many cases they are thought to occur in conjunction with other important accessory properties. The utilization of diagnostic horizons and features in the classification process allows the grouping of soils that have formed as a result of similar genetic processes. The grouping, however, is done on the basis of observable or measurable properties, rather than speculation about the genetic history of a particular soil.
Entries. The diagnostic horizons and features are listed in the latest Keys to Soil Taxonomy. Allowable codes are given in the NASIS data dictionary.

Drainage Class (618.17)

Definition. Drainage class identifies the natural drainage condition of the soil. It refers to the frequency and duration of wet periods.
Classes. The seven natural drainage classes are: (1) excessively drained, (2) somewhat excessively drained, (3) well drained, (4) moderately well drained, (5) somewhat poorly drained, (6) poorly drained, and (7) very poorly drained. Chapter 3 of the Soil Survey Manual provides a description of each natural drainage class.
Significance. Drainage classes provide a guide to the limitations and potentials of the soil for field crops, forestry, range, wildlife, and recreational uses. The class roughly indicates the degree, frequency, and duration of wetness, which are factors in rating soils for various uses.
Estimates. Infer drainage classes from observations of landscape position and soil morphology. In many soils the depth and duration of wetness relate to the quantity, nature, and pattern of redoximorphic features. Correlate drainage classes and redoximorphic features through field observations of water tables, soil wetness, and landscape position. Record the drainage classes assigned to the series.

Entries. Enter the drainage class name for each map unit component. Utilize separate map unit components for different drainage class phases or for drained versus undrained phases where needed.

Drainage Class
Excessively
Somewhat Excessively
Well
Moderately well
Somewhat poor
Poorly
Very poorly

Effective Cation-Exchange Capacity (618.18)

Definition. Effective cation-exchange capacity is the sum of NH4OAc extractable bases plus KCl extractable aluminum (method 5A3b, SSIR #42).
Significance. Cation exchange capacity is a measure of the ability of a soil to retain cations, some of which are plant nutrients. Soils that have a low cation exchange capacity hold fewer cations and may require more frequent applications of fertilizer and amendments than soils that have a high cation exchange capacity. Soils that have high cation exchange capacity have the potential to retain cations. Effective CEC is a measure of CEC that is particularly useful in soils whose ion exchange capacity is largely a result of variable charge components such as allophane, kaolinite, hydrous iron and aluminum oxides, and organic matter, which results in the soil’s CEC being not a fixed number but a function of pH. Examples of such soils might include some andic soils, Oxisols, and more weathered Ultisols with kaolinitic mineralogy.
Measurement. Effective cation exchange capacity is measured by the methods outlined in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November, 2004. Method 5A3b gives the effective cation exchange capacity value for soils that have pH <5.5 and that are low in soluble salts. For soils that have a pH of 5.5 or greater, the ECEC equals the sum of NH4OAc extractable bases.
Entries. Enter the high, low, and representative values of the estimated range in effective cation exchange capacity at the field pH of the soil, in meq 100g¬1, for the horizon. Values in tenths are allowed. Valid entries range from 0.0 to 400.0. A NASIS calculation is available and can be viewed in Appendix 2.

Electrical Conductivity (618.19)

Definition. Electrical conductivity is the electrolytic conductivity of an extract from saturated soil paste.

Classes. The classes of salinity are:

Classes	Electrical Condutivity (mmhos cm^-1)
Nonsaline	0-2
Very slightly saline	≥2-4
Slightly saline	≥4-8
Moderately saline	≥8-16
Strongly saline	≥16

Significance. Electrical conductivity is a measure of the concentration of water-soluble salts in soils. It is used to indicate saline soils. High concentrations of neutral salts, such as sodium chloride and sodium sulfate, may interfere with the absorption of water by plants because the osmotic pressure in the soil solution is nearly as high or higher than that in the plant cells. Salts may also interfere with the exchange capacity of nutrient ions, thereby resulting in nutritional deficiencies in plants.
Measurement. The electrolytic conductivity of a saturated extract is the standard measure used to express salinity as millimhos per centimeter (mmhos cm^-1) at 25 degrees C. The laboratory procedure used to measure is described in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS.
Estimates. Field estimates of salts are made from observations of free salt on structural faces or on the soil surface, from plant growth indicators, or from field salinity meters. The occurrences of bare spots, salt-tolerant plants, and uneven crop growth are used as indicators of salinity and high electrical conductivity. When keyed to measurements, these observations help to estimate the amount of salts.
Entries. Enter the high, low, and representative values for the range of electrolytic conductivity of the saturation extract during the growing season for each horizon. Use the following classes: 0-2, 2-4, 4-8, 8-16, and 16-32; or use a combination of classes, for example, 2-8 for the high and low values. The allowable range is 0 to 99.

Elevation (618.20)

Definition. Elevation is the vertical distance from mean sea level to a point on the earth’s surface.
Significance. Elevation, or local relief, exerts a modifying influence of the genesis of natural soil bodies. Elevation also may affect soil drainage within a landscape, salinity or sodicity within a climatic area, or soil temperature.
Estimates. Elevation is normally obtained from U.S. Geological Survey topographic maps or measured using altimeters or global positioning systems.
Entries. Enter the high, low, and representative values for each map unit component. The minimum entry is -300 meters and the maximum entry is 8550 meters. Record elevation to the nearest integer.

Engineering Classification (618.21)

AASHTO group classification.
1. Definition. AASHTO group classification is a system that classifies soils specifically for geotechnical engineering purposes that are related to highway and airfield construction. It is based on particle-size distribution and Atterberg limits, such as liquid limit and plasticity index. This classification system is covered in AASHTO Standard No. M 145-82 and consists of a symbol and a group index. The classification is based on that portion of the soil that is smaller than 3 inches in diameter.
2. Classes. The AASHTO classification system identifies two general classifications: (i) granular materials having 35 percent or less, by weight, particles smaller than 0.074 mm in diameter and (ii) silt-clay materials having more than 35 percent, by weight, particles smaller than 0.074 mm in diameter. These two divisions are further subdivided into seven main group classifications. Exhibit 618-4 shows the criteria for classifying soil in the AASHTO classification system.
  The group and subgroup classifications are based on estimated or measured grain-size distribution and on liquid limit and plasticity index values.
3. Significance. The group and subgroup classifications of this system are aids in the evaluation of soils for highway and airfield construction. The classifications can help to make general interpretations relating to performance of the soil for engineering uses, such as highways and local roads and streets.
4. Measurements. Measurements involve sieve analyses for the determination of grain-size distribution of that portion of the soil between a 3 inch and 0.074 mm particle size. ASTM methods D 422, C 136, and C 117 have applicable procedures for the determination of grain-size distribution. The liquid limit and plasticity index values (ASTM method D 4318) are determined for that portion of the soil having particles smaller than 0.425 mm in diameter (No. 40 sieve). Measurements, such as laboratory tests, are made on most benchmark soils and on other representative soils in survey areas.
5. Estimates. During soil survey investigations and field mapping activities, the soil is classified by field methods. This classification involves making estimates of particle-size fractions by a percentage of the total soil, minus the greater than 3-inch fraction. Estimates of liquid limit and plasticity index are based on clay content and mineralogy relationships. Estimates are expressed in ranges that include the estimating accuracy as well as the range of values for the taxon.
6. Entries. Enter classes and separate them by commas for each horizon, for example, A-7, A-6. Acceptable entries are A-1, A-l-A, A-l-B, A-2, A-2-4, A-2-5, A-2-6, A-2-7, A-3, A-4, A-5, A-6, A-7, A-7-5, A-7-6, and A-8.
AASHTO group index.
1. Definition. The AASHTO group and subgroup classifications may be further modified by the addition of a group index value. The empirical group index formula was devised for approximate within-group evaluation of the “clayey granular materials” and the “silty-clay” materials.
2. Significance. The group index is an aid in the evaluation of the soils for highway and airfield construction. The index can help to make general interpretations relating to performance of the soil for engineering uses, such as highways and local roads and streets.
3. Measurement. The group index is calculated from an empirical formula:
  GI = (F-35) [0.2 + 0.005 (LL-40)] + 0.01 (F-15) (PI-10)
  
  where:
  F = Percentage passing sieve No. 200 (75 micrometer), expressed as a whole number
  LL = Liquid limit
  PI = Plasticity index
  
  In calculating the group index of A-2-6 and A-2-7 subgroups, only the PI portion of the formula is used.
4. Entries. The group index is reported to the nearest integer. If the calculated group index is negative, the group index is zero (0). The minimum index value is 0 and the maximum is 120. A NASIS calculation is available and can be viewed in Appendix 5.
Unified soil classification.
1. Definition. The unified soil classification system is a system for classifying mineral and organic mineral soils for engineering purposes based on particle-size characteristics, liquid limit, and plasticity index.
2. Classes. The Unified Soil Classification System identifies three major soil divisions: (i) coarse-grained soils having less than 50 percent, by weight, particles smaller than 0.074 mm in diameter; (ii) fine-grained soils having 50 percent or more, by weight, particles smaller than 0.074 mm in diameter, and (iii) highly organic soils that demonstrate certain organic characteristics. These divisions are further subdivided into a total of 15 basic soil groups. The major soil divisions and basic soil groups are determined on the basis of estimated or measured values for grain-size distribution and Atterberg limits. ASTM D 2487 shows the criteria chart used for classifying soil in the Unified system and the 15 basic soil groups of the system and the plasticity chart for the Unified Soil Classification System.
3. Significance. The various groupings of this classification have been devised to correlate in a general way with the engineering behavior of soils. This correlation provides a useful first step in any field or laboratory investigation for engineering purposes. It can serve to make some general interpretations relating to probable performance of the soil for engineering uses.
4. Measurement. The methods for measurement are provided in ASTM Designation D 2487. Measurements involve sieve analysis for the determination of grain-size distribution of that portion of the soil between 3 inches and 0.074 mm in diameter (No. 200 sieve). ASTM methods D 422, C 136, and C 117 have applicable procedures that are used where appropriate for the determination of grain-size distribution. Values for the Atterberg limits (liquid limit and plasticity index) are also used. Specific tests are made for that portion of the soil having particles smaller than 0.425 mm in diameter (No. 40 sieve) according to ASTM methods D 423 and D 424. Measurements, such as laboratory tests, are made on most benchmark soils and on other representative soils in survey areas.
5. Entries for measured data. For measured Unified data, enter up to four classes for each horizon. ASTM D 2487 provides flow charts for classifying the soils. Separate the classes by commas, for example, CL-ML, ML. Acceptable entries are GW, GP, GM, GC, SW, SP, SM, SC, CL, ML, OL, CH, MH, OH, PT, CL-ML, GW-GM, GW-GC, GP-GM, GP-GC, GC-GM, SW-SM, SW-SC, SP-SM, SP-SC, and SC-SM.
6. Estimates. The methods for estimating are provided in ASTM Designation D 2488. During all soil survey investigations and field mapping activities, the soil is classified by field methods. The methods include making estimates of particle-size fractions by a percentage of the total soil. The Atterberg limits are also estimated based on the wet consistency, ribbon or thread toughness, and other simple field tests. These tests and procedures are explained in ASTM D 2488. If samples are later tested in the laboratory, adjustments are made to field procedures as needed. Estimates are expressed in ranges that include the estimating accuracy as well as the range of values from one location to another within the map unit. If an identification is based on visual-manual procedures it must be clearly stated so in reporting.
7. Entries for estimated soils. For estimated visual-manual Unified data, enter up to four classes for each horizon. ASTM D 2488 provides flow charts for classifying the soils. Separate the classes by commas, for example, CL, ML, SC. Acceptable entries are GW, GP, GM, GC, SW, SP, SM, SC, CL, ML, CH, MH, OL/OH, PT, GW-GM, GW-GC, GP-GM, GP-GC, SW-SM, SW-SC, SP-SM, and SP-SC.

Erosion Accelerated, Kind (618.22)

Definition. Erosion accelerated, kind, is the type of detachment and removal of surface soil particles as largely affected by human activity.
Significance. The type of accelerated erosion is important in assessing the current health of the soil, and in assessing its potential for different uses. Erosion, whether natural or induced by humans, is an important process that affects soil formation and may remove all or parts of the soils formed in the natural landscape.

Classes.

Accelerated erosion Class
Water erosion, sheet
Water erosion, rill
Water erosion, gully
Water erosion, tunnel
Wind erosion

Entries. Enter the appropriate class for each map unit component. Multiple entries are allowable, but a representative value should be indicated.

Erosion Class (618.23)

Definition. Erosion class is the class of accelerated erosion.
Significance. The degree of erosion that has taken place is important in assessing the health of the soil and in assessing the soil’s potential for different uses. Erosion is an important process that affects soil formation and may remove all or parts of the soils formed in natural landscapes.
Removal of increasing amounts of soil increasingly alters various properties and capabilities of the soil. Properties and qualities affected include bulk density, organic matter content, tilth, water infiltration. Altering these properties affects the productivity of the soil.
Estimation. During soil examinations, estimate the degree to which soils have been altered by accelerated erosion. The Soil Survey Manual describes the procedures involved.

Classes.

Erosion Class
none - deposition
Class 1
Class 2
Class 3
Class 4

Entries. Enter the appropriate class for each map unit component.

Excavation Difficulty Classes (618.24)

Definition. Excavation difficulty is an estimation of soil layers, horizons, pedons, or geologic layers according to the difficulty in making an excavation into them. Excavation difficulty, in most instances, is strongly controlled by water state, which should be specified.

Classes. The excavation difficulty classes are:

Classes	Definition
Low	Excavations can be made with a spade using arm-applied pressure only. Neither application of impact energy nor application of pressure with the foot to a spade is necessary
Moderate	Arm-applied pressure to a spade is insufficient. Excavation can be accomplished quite easily by application of impact energy with a spade or by foot pressure on a spade.
High	Excavation with a spade can be accomplished with difficulty. Excavation is easily possible with a full length pick, using an over-the-head swing.
Very High	Excavation with a full length pick, using an over-the-head swing, is moderately to markedly difficult. Excavation is possible in a reasonable period of time with a backhoe mounted on a 40 to 60 kW (50-80 hp) tractor
Extremely High	Excavation is nearly impossible with a full length pick using an over-the-head arm swing. Excavation cannot be accomplished in a reasonable time period with a backhoe mounted on a 40 to 60 kW (50-80 hp) tractor

Significance. Excavation difficulty classes are important for evaluating the cost and time needed to prepare shallow excavations.
Estimates. Estimates of excavation difficulty classes are made from field observations.
Entries. Enter the appropriate class for each horizon. The allowable entries are Low, Moderate, High, Very high, and Extremely high.

Extractable Acidity (618.25)

Definition. Extractable acidity is a measure of soil exchangeable hydrogen ions that may become active by cation exchange.
Significance. Extractable acidity is important for soil classification and for certain evaluations of soil nutrient availability or of the effect of waste additions to the soil.
Measurement. Extractable acidity is determined by method 6H5a, as outlined in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS.
Entries. Enter the range of extractable acidity as milliequivalents per 100 grams (meq 100g-1) of soil for the horizon. Valid entries range from 0.0 to 250.0. Tenths are allowed. A NASIS calculation is available and can be viewed in Appendix 4.

Extractable Aluminum (618.26)

Definition. Extractable aluminum is the amount of aluminum extracted in one normal potassium chloride.
Significance. Extractable aluminum is important for soil classification and for certain evaluations of soil nutrient availability and of toxicities. An aluminum saturation of about 60 percent is usually regarded as toxic to most plants. It may be a useful measurement for assessing potential lime needs for acid soils.
Measurement. Extractable aluminum is determined by method 6G9d, as in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS. Units of measure are milliequivalents per 100 grams (meq 100g-1).
Entries. Enter the range of extractable aluminum as milliequivalents per 100 grams (meq 100g-1) of soil for the horizon. Valid entries range from 0.0 to 150.0. Tenths are allowed.

Flooding Frequency, Duration, and Month (618.27)

Definition. Flooding is the temporary covering of the soil surface by flowing water from any source, such as streams overflowing their banks, runoff from adjacent or surrounding slopes, inflow from high tides, or any combination of sources. Shallow water standing or flowing that is not concentrated as local runoff during or shortly after rain or snow melt is excluded from the definition of flooding. Chapter 3 of the Soil Survey Manual provides additional information. Standing water (ponding) or water that forms a permanent covering is also excluded from the definition.

Classes. Estimates of flooding class are based on the interpretation of soil properties and other evidence gathered during soil survey field work. Flooding hazard is expressed by (1) flooding frequency class, (2) flooding duration class, and (3) time of year that flooding occurs. Not considered here, but nevertheless important, are velocity and depth of floodwater. Frequencies used to define classes are generally estimated from evidence related to the soil and vegetation. They are expressed in wide ranges that do not indicate a high degree of accuracy. Flooding frequencies that are more precise can be calculated by performing complex analyses used by engineers. The class very frequent is intended for use on areas subject to daily and monthly high tides.

Flooding frequency class. Flooding frequency class is the number of times flooding occurs over a period of time and expressed as a class. The classes of flooding are defined as follows:

Class	Definition
None	No reasonable possibility of flooding; near 0 percent chance of flooding in any year or less than 1 time in 500 years.
Very Rare	Flooding is very unlikely but possible under extremely unusual weather conditions; less than 1 percent chance of flooding in any year or less than 1 time in 100 years but more than 1 time in 500 years.
Rare	Flooding unlikely but possible under unusual weather conditions; 1 to 5 percent chance of flooding in any year or nearly 1 to 5 times in 100 years
Occasional	Flooding is expected infrequently under usual weather conditions; 5 to 50 percent chance of flooding in any year or 5 to 50 times in 100 years
Frequent	Flooding is likely to occur often under usual weather conditions; more than a 50 percent chance of flooding in any year or more than 50 times in 100 years, but less than a 50 percent chance of flooding in all months in any year.
Very Frequent	Flooding is likely to occur very often under usual weather conditions; more than a 50 percent chance of flooding in all months of any year.

Flooding duration classes. The average duration of inundation per flood occurrence is given only for occasional, frequent, and very frequent classes.

Class	Duration
Extremely brief	0.1 to 4.0 hours
Very brief	4 to 48 hours
Brief	2 to 7 days
Long	7 to 30 days
Very long	≥30 days

Assignment. Yearly flooding frequency classes are assigned to months to indicate the months of occurrence and not the frequency of the flooding during the month, except for the very frequent class. The time period expressed includes two-thirds to three-fourths of the occurrences. Time period and duration of the flood are the most critical factors that determine the growth and survival of a given plant species. Flooding during the dormant season has few if any harmful effects on plant growth or mortality and may improve the growth of some species. If inundation from flood water occurs for long periods during the growing season, the soil becomes oxygen deficient and plants may be damaged or killed.

Significance. The susceptibility of soils to flooding is an important consideration for building sites, sanitary facilities, and other uses. Floods may be less costly per unit area of farmland as compared to that of urban land, but the loss of crops and livestock can be disastrous.
Estimates. The most precise evaluation of flood-prone areas for stream systems is based on hydrologic studies. The area subject to inundation during a flood of a given frequency, such as one with a 1 percent or 2 percent chance of occurrence, generally is determined by one of two basic methods.
1. The first method is used if stream flow data are available. In this method, the data are analyzed to determine the magnitude of floods of different frequencies. Engineering studies are made to determine existing channel capacities and flow on the flood plain by the use of valley cross sections and water surface profiles.
2. The second method is used if stream flow data are not available. In this method, hydrologists make an estimate of flood potential from recorded data on rainfall. They consider such factors as (i) size, slope, and shape of the contributing watershed, (ii) hydrologic characteristics of the soil, (iii) land use and treatment, and (iv) hydraulic characteristics of the valley and channel system.
3. With the use of either method, soil surveys can aid in the delineation of flood-prone areas. Possible sources of flooding information are (i) NRCS project-type studies, such as PL 556, FP, RB, or RC&D; (ii) flood hazard analyses; (iii) Corps of Engineers flood plain information reports; (iv) special flood reports; (v) local flood protection and flood control project reports; (vi) HUD flood insurance study reports; (vii) maps by USGS, NRCS, TVA, COE, NOAA; (viii) studies by private firms and other units of government; and (ix) USGS quadrangle sheets and hydrologic atlases of flood-prone areas and stream gauge data.
4. General estimates of flooding frequency and duration are made for each soil. However, in intensively used areas where construction has materially altered the natural water flow, flood studies are needed to adequately reflect present flooding characteristics.
5. Soil scientists collect and record evidence of flood events during the course of the soil survey. The extent of flooded areas, flood debris in trees, damage to fences and bridges, and other signs of maximum water height are recorded. Information that is helpful in delineating soils that have a flood hazard is also obtained. Hydrologists may have flood stage predictions that can be related to kinds of soil or landscape features. Conservationists and engineers may have recorded elevations of high flood marks. Local residents may have recollections of floods that can help to relate the events to kinds of soil, topography, and geomorphology.
6. Certain landscape features have developed as the result of past and present flooding and include former river channels, oxbows, point bars, alluvial fans, meander scrolls, sloughs, natural levees, backswamps, sand splays, and terraces. Most of these features are easily recognizable on aerial photographs by comparing the photo image with on-the-ground observations. Different kinds of vegetation and soils are normally associated with these geomorphic features.
7. The vegetation that grows in flood areas may furnish clues to past flooding. In central and southeastern United States, the survival of trees in flood-prone areas depends on the frequency, duration, depth, and time of flooding and on the age of the tree.
8. Past flooding may sometimes leave clues in the soil, such as (1) thin strata of material of contrasting color or texture, or both; (2) an irregular decrease in organic matter content, which is an indication of a buried surface horizon; and (3) soil layers that have abrupt boundaries to contrasting kinds of material, which indicate that the materials were laid down suddenly at different times and were from different sources or were deposited from stream flows of different velocities.
9. Laboratory analyses of properly sampled layers are often helpful in verifying these observations. Organic carbon and particle-size analyses are particularly useful in verifying flood deposits. Microscopic observations may detect preferential horizontal orientation of plate-like particles; micro-layering, which indicates water-laid deposits; or mineralogical differences between layers.
Entries. Flooding and frequency are posted for each month of the year for each map unit component. Flooding entries reflect the current existing and mapped condition with consideration for dams, levees, and other man-induced changes affecting flooding frequency and duration.
1. Enter the flooding frequency class name: none, very rare, rare, occasional, frequent, or very frequent.
2. Enter the flooding duration class name that most nearly represents the soil: extremely brief, very brief, brief, long, or very long.

Fragments in the Soil (618.28)

Definition. Fragments are unattached cemented pieces of bedrock, bedrock-like material, durinodes, concretions, and nodules 2 mm or larger in diameter; and woody material 20 mm or larger in organic soils. Fragments are separated into three types: rock fragments; pararock fragments, which are separated based on cementation; and wood fragments.
1. Rock fragments are unattached pieces of rock 2 mm in diameter or larger that are strongly cemented or more resistant to rupture. Rock fragments from 2 mm to 75 mm (3 inches) are considered when estimating the percent passing sieves as discussed in part 618.44.
2. Pararock fragments are unattached, cemented bodies or pieces of material 2 mm in diameter or larger that are extremely weakly cemented to moderately cemented. These fragments are not retained on sieves because of the sample preparation by grinding.
3. Wood fragments are woody materials that cannot be crushed between the fingers when moist or wet and are larger than 20 mm in size. Wood fragments are only used in organic soils. They are comparable to rock and pararock fragments in mineral soils.
Significance. The fraction of the soil 2 mm or larger has an impact on the behavior of the whole soil. Soil properties, such as available water capacity, cation exchange capacity, saturated hydraulic conductivity, structure, and porosity, are affected by the volume, composition, and size distribution of fragments in the soil. Fragments also affect the management of the soil and are used as interpretation criteria. Terms related to volume, size, and hardness of fragments are used as texture modifier terms.
Generally, the fraction of soil greater than 75 mm (3 inches) in diameter is not included in the engineering classification systems. However, it can be added as a descriptive term to the group name, for example, poorly graded gravel with silt, sand, cobbles, and boulders. Estimates of the percent of cobbles and boulders are presented in the soil descriptions for a group name. A small amount of these larger particles generally has little effect on soil properties. It may, however, have an effect on the use of a soil in certain types of construction. Often, the larger portions of a soil must be removed before the material can be spread in thin layers, graded, or compacted and graded to a smooth surface. As the quantity of this “oversized” fraction increases, the properties of the soil can be affected. If the larger particles are in contact with each other, the strength of the soil is very high and the compressibility very low. If voids exist between the larger particles, the soil will likely have high saturated hydraulic conductivity and may undergo some internal erosion as a result of the movement of water through the voids. Most of the smaller and more rapid construction equipment normally used in excavating and earthmoving cannot be used if the oversize fraction of a soil is significant.
Measurement. The fraction from 2 to 75 mm may be measured in the field. However, 50-60 kg of sample may be necessary if an appreciable amount of fragments near 75 mm are present. An alternative is to visually estimate the volume of the 20-to 75-mm fraction, then sieve and weigh the 2-to 20-mm fraction. The fraction 75 mm (3 inches) or greater is usually not included in soil samples taken in the field for laboratory testing. Measurements can be made in the field by weighing the dry sample and the portion retained on a 3-inch screen. The quantity is expressed as a weight percentage of the total soil. A sample as large as 200 pounds to more than a ton may be needed to assure that the results are representative. Measurements of the fraction from 75 to 250 mm (3 to 10 inches) and the fraction greater than 250 mm (10 inches) are usually obtained from volume estimates.
Estimates. Estimates are usually made by visual means and are on the basis of percent by volume. The percent by volume is converted to percent by weight, as shown in Exhibit 618-11, by using the average bulk unit weights for soil and rock. These estimates are made during investigation and mapping activities in the field. They are expressed as ranges that include the estimating accuracy as well as the range of values for a component.
Measurements or estimates of fragments less than strongly cemented are made prior to any rolling or crushing of the sample.
Rock Fragments greater than 10 inches (250 mm).
1. Definition. Rock fragment greater than 10 inches is the percent by weight of the horizon occupied by rock fragments greater than 10 inches (250 mm) in size. The upper limit is undefined, but for practical purposes it generally is no larger than a pedon, up to 10 meters square. For nonspherical material, the intermediate dimension is used for the 250 mm (10 inch) measurement. For example, a flat-shaped rock fragment that is 100 mm x 250 mm x 380 mm has an intermediate dimension of 250 mm, and is not counted as greater than 250 mm. A flat-shaped rock fragment that is 100 mm x 275 mm x 380 mm has an intermediate dimension of 275 mm, and is counted as greater than 250 mm.
2. Entries. Enter the high, low, and representative values as whole number percentages for each horizon as appropriate.
Rock fragments 3 to 10 inches (75 to 250 mm).
1. Definition. Rock fragments 3 to 10 inches is the percent by weight of the horizon occupied by rock fragments 3 to 10 inches (75 to 250 mm) in size.
2. Entries. Enter the high, low, and representative values as whole number percentages for each horizon as appropriate.
Fragment kind.
1. Definition. Fragment kind is the lithology/composition of the 2 mm or larger fraction of the soil.
2. Entries. Enter the appropriate class name for the kind of fragment present. More than one choice may be entered. The class names can be found in the NASIS data dictionary.

Fragment roundness.

Definition. Fragment roundness is an expression of the sharpness of edges and corners of fragments.
Significance. The roundness of fragments impacts water infiltration, root penetration, and macropore space.

Classes. The fragment roundness classes are:

Roundness class
Angular
Subangular
Subrounded
Rounded
Well-rounded

Entries. Enter the appropriate class name for the roundness class(es) present. A representative value may be designated.

Fragment rupture resistance cemented.

Definition. Fragment rupture resistance cemented is the rupture resistance of a fragment of specified size that has been air dried and then submerged in water.
Measurements. Measurements are made using the procedures and classes of cementation that are listed with the rupture resistance classes in the Soil Survey Manual. Classes are described for block-like specimens about 25-30 mm on edge, which are air-dried and then submerged in water for at least 1 hour. The specimen is compressed between extended thumb and forefinger, between both hands, or between the foot and a nonresilient flat surface. If the specimen resists compression, a weight is dropped onto it from progressively greater heights until it ruptures. Failure is considered at the initial detection of deformation or rupture. Stress applied in the hand should be over a 1-second period. The tactile sense of the class limits may be learned by applying force to top loading scales and sensing the pressure through the tips of the fingers or through the ball of the foot. Postal scales may be used for the resistance range that is testable with the fingers. A bathroom scale may be used for the higher rupture resistance range.
Significance. The rupture resistance of a fragment is significant where the class is strongly cemented or higher. These classes can impede or restrict the movement of soil water vertically through the soil profile and have a direct impact on the quality and quantity of ground water and surface water.

Classes. The classes are:

Rupture resistance class
Extremely weakly
Very weakly
Weakly
Moderately
Strongly
Very strongly
Indurated

Entries. Enter the appropriate class name(s) for the fragments present. A representative value may be designated.

Fragment shape.
1. Definition. Fragment shape is a description of the overall shape of the fragment.
2. Significance. Fragment shape is important for fragments that are too large to be called channers or flagstones.
3. Classes. The classes are:
  Flat
  Nonflat
4. Entries. Enter the appropriate class name for the class(es) present. Multiple entries may be made. A representative value may be designated.

Fragment size.

Definition. Fragment size is the size based on the multiaxial dimensions of the fragment.
Significance. The size of fragments is significant to the use and management of the soil. Fragment size is used as criteria for naming map units. It affects equipment use, excavation, construction, and recreational uses.

Classes. Classes of fragment size are subdivided according to flat and non-flat fragments.

Flat fragment classes	Length (mm)
Channers	2-150
Flagstones	150-380
Stones	380-600
Boulders	≥600

Non-flat fragment classes Diameter (mm)

Pebbles 2-75

   fine pebbles 2-5

   medium pebbles 5-20

   coarse pebbles 20-75

Cobbles 75-250

Stones 250-600

Boulders ≥600

For fragments that are less than strongly cemented, “para” is added as a prefix to the above terms; i.e., paracobbles or fine parapebbles.

Entries. Enter the minimum, maximum, and representative values in whole numbers of each size class being described. Entries are in millimeters and range from 2 to 3,000 mm.

Fragment volume.
1. Definition. Fragment volume is the volume percentage of the horizon occupied by the 2 mm or larger fraction.
2. Significance. The volume occupied by the 2 mm or larger fraction is important for naming textural modifiers; i.e., gravelly, very gravelly, extremely paragravelly.
3. Entries. Enter the high, low, and representative values, in whole numbers, for the percent volume present for each class of fragments being described.

Free Iron Oxides (618.29)

Definition. Free iron oxides are secondary iron oxides, such as goethite, hematite, ferrihydrite, lepidocrocite, and maghemite. This form of iron may occur as discrete particles, as coatings on other soil particles, or as cementing agents between soil mineral grains. It is the iron extracted by dithionite-citrate from the fine earth fraction.
Significance. The amount of iron that is extractable by dithionite-citrate is used in Soil Taxonomy in the Ferritic, Feruginous, Parasesquic, and Sesquic mineralogy classes. The ratio of dithionite-citrate (free) iron to total iron in a soil is a measure of the degree of soil weathering. Free iron oxides are important in the soil processes of podzolization and laterization and play a significant role in the phosphorous fixation ability of soils.
Measurement. Free iron oxides are measured as the amount extracted by dithonite citrate using method 6C2b as outlined in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS.
Entries. Enter high, low and representative values as percentages for each horizon for which data is available. Valid entries range from 0.00 to 99.99, and hundredths are allowed.

Frost Action, Potential (618.30)

Definition. Potential frost action is the rating for the susceptibility of the soil to upward or lateral movement by the formation of segregated ice lenses. It rates the potential for frost heave and the subsequent loss of soil strength when the ground thaws.

Classes. Classes are used in regions where frost action is a potential problem. Exhibit 618-5 provides more information. The classes are low, moderate, and high and are defined as follows:

Potential frost action classes	Definition
Low	Soils are rarely susceptible to the formation of ice lenses.
Moderate	Soils are susceptible to the formation of ice lenses, which results in frost heave and subsequent loss of soil strength.
High	Soils are highly susceptible to the formation of ice lenses, which results in frost heave and subsequent loss of soil strength.

Significance. Damage from frost action results from the formation of segregated ice crystals and ice lenses in the soil and the subsequent loss of soil strength when the ground thaws. Frost heave damages highway and airfield pavements. It is less of a problem for dwellings and buildings that have footings which extend below the depth of frost penetration. In cold climates, unheated structures that have concrete or asphalt floors can be damaged by frost heave. Driveways, patios, and sidewalks can heave and crack. The thawing of the ice causes a collapse of surface elevation and produces free water perches on the still frozen soil below. Soil strength is reduced. Back slopes and side slopes of cuts and fills can slough during thawing. Seedlings and young plants of clover, alfalfa, wheat, and oats can be raised out of the soil or have their root systems damaged by frost heave.
Estimates. Freezing temperatures, soil moisture, and susceptible soils are needed for the formation of segregated ice lenses. Ice crystals begin to form in the large pores first. Water in small pores or water that was adsorbed on soil particles freezes at lower temperatures. This super cooled water is strongly attracted to the ice crystals, moves toward it, and freezes on contact with them. The resulting ice lens continues to grow in width and thickness until all available water that can be transported by capillary has been added to the ice lens and a further supply cannot be made available because of the energy requirements.
Soil temperatures must drop below 0° C for frost action to occur. Generally, the more slowly and deeply the frost penetrates, the thicker the ice lenses are and the greater the resulting frost heave is. Exhibit 618-6 provides a map that shows the design freezing index values in the continental United States. The values are the number of degree days below 0° C for the coldest year in a period of 10 years. The values indicate duration and intensity of freezing temperatures. The 250 isoline is the approximate boundary below which frost action ceases to be a problem. Except on the West Coast, the frost action boundary corresponds closely to the mesic-thermic temperature regime boundary used in Soil Taxonomy. More information is provided in the U.S. Army Engineer School, Student Reference, 1967, Soil Engineering, Section I, Volume II, Chapters VI-IX, Fort Belvoir, Virginia.

Water necessary for the formation of ice lenses may come from a high water table or from infiltration at the surface. Capillary water in voids and adsorbed water on particles also contribute to ice lens formation; but unless this water is connected to a source of free water, the amount generally is insufficient to produce significant ice segregation and frost heave.

The potential intensity of ice segregation is dependent to a large degree on the effective soil pore size and soil saturated hydraulic conductivity, which are related to soil texture. Ice lenses form in soils in which the pores are fine enough to hold quantities of water under tension but coarse enough to transmit water to the freezing front. Soils that have a high content of silt and very fine sand have this capacity to the greatest degree and hence have the highest potential for ice segregation. Clayey soils hold large quantities of water but have such slow saturated hydraulic conductivity that segregated ice lenses are not formed unless the freezing front is slow moving. Sandy soils, however, have large pores and hold less water under lower tension. As a result, freezing is more rapid and the large pores permit ice masses to grow from pore to pore, entombing the soil particles. Thus, in coarse-grained soils, segregated ice lenses are not formed and less displacement can be expected.

Estimates of potential frost action generally are made for soils in mesic or colder temperature regimes. Exceptions are on the West Coast, where the mesic-thermic temperature line crosses below the 250 isoline, as displayed in Exhibit 618-6, and along the East Coast, where the soil climate is moderated by the ocean. Mesic soils that have a design freezing index of less that 250 degree days should not be rated because frost action is not likely to occur. The estimates are based on bare soil that is not covered by insulating vegetation or snow. They are also based on the moisture regime of the natural soil. The ratings can be related to manmade modifications of drainage or to irrigation systems on an on site basis. Frost action estimates are made for the whole soil to the depth of frost penetration, to bedrock, or to a depth of 2 meters (6.6 feet), whichever is shallowest. Exhibit 618-5 is a guide for making potential frost action estimates. It uses the moisture regimes and family textures as defined in Soil Taxonomy.
Entries. Enter one of the following: LOW, MOD, or HIGH for the whole soil. If frost action is not a problem, enter NONE.

Gypsum (618.31)

Definition. Gypsum is the percent, by weight, of hydrated calcium sulfates in the <20 mm fraction of soil.
Significance. Gypsum is partially soluble in water and can be dissolved and removed by water. Soils with more than 10 percent gypsum, may collapse if the gypsum is removed by percolating water. Gypsum is corrosive to concrete. Corrosion of concrete is most likely to occur in soils that are more than about 1 percent gypsum when wetting and drying occurs.
Measurement. Gypsum is measured by method 6F4, as outlined in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS.
Entries. Enter the high, low, and representative values to represent the range in gypsum content as a weight percent of the soil fraction less than 20 mm in size. Round values to the nearest 5 percent for layers that are more than 5 percent gypsum and to the nearest 1 percent for layers that are less than 5 percent gypsum, for example, 0-1, 1-5, 5-10. If the horizon does not have gypsum, enter “0”. Entries range from 0 to 120.

Horizon Depth to Bottom (618.32)

Definition. Horizon depth to bottom is the distance from the top of the soil to the base of the soil horizon.
Measurement. Distance is measured from the top of the soil, which is defined as the top of the mineral soil, or, for soil with “O” horizons, the top of any “O” layer that is at least partially decomposed. For soils that are covered by 80 percent or more rock or pararock fragments, the top of the soil is the surface of the fragments. See pages 63-64 in the Soil Survey Manual for a complete discussion. Measurement should be estimated to a depth of 200 cm for most soils and to a depth at least 25 cm below a lithic contact if the contact is above 175 cm. For soils, including those that have a root restricting contact such as a paralithic contact, the lowest horizon bottom should extend to a depth of at least 25 cm below the contact or to a depth of 200 cm, whichever is shallower.
Entries. Enter the high, low, and representative values in whole centimeters. The high value represents either the greatest depth to which the base of the horizon extends or, for horizons extending beyond the limit of field observation, it is the depth to which observation was made (usually no more than 200 cm but at least 150 cm).

Horizon Depth to Top (618.33)

Definition. Horizon depth to top is the distance from the top of the soil to the upper boundary of the soil horizon.
Measurement. Distance is measured from the top of the soil, which is defined as the top of the mineral soil, or, for soils with “O” horizons, the top of any “O” layer that is at least partially decomposed. For soils that are covered by 80 percent or more rock or pararock fragments, the top of the soil is the surface of the fragments. See pages 63-64 in the Soil Survey Manual for a complete discussion.
Entries. Enter the high, low, and representative values in whole centimeters. Refer to the discussion under “horizon designations” as to how to list E/B and E and Bt type horizons.

Horizon Designation (618.34)

Definition. Horizon designation is a concatenation of three kinds of symbols used in various combinations to identify layers of soil that reflect the investigator’s interpretations of genetic relationships among layers within a soil.
Significance. Soils vary widely in the degree to which horizons are expressed. The range is from little or no expression to strong expression. Layers of different kinds are identified by symbols. Designations are provided for layers that have been changed by soil formation and for those that have not. Designations are assigned after comparison of the observed properties of the layer with properties inferred for the material before it was affected by soil formation. Designations of genetic horizons express a qualitative judgment about the kind of changes that are believed to have taken place. A more detailed discussion can be reviewed in the Soil Survey Manual, Chapter 3.
Entries. Enter combinations of symbols. Each horizon identified in a soil description can be entered or, if there are no significant differences in other data elements between two horizons, they may be combined. Enter only what the documentation can support. For example, if the only horizons that the data identify are an A, B, and C, then only enter those horizons. If, on the other hand, an Ap, A1, A2, Bt1, Bt2, Btk, C1, and C2 are documented, then enter those horizons. If the Bt1 and Bt2 horizons in the above example have no significant differences in the data element values, then they can be combined into a Bt horizon. For E/Bt and E&Bt horizon types, it is necessary to enter the horizons designations twice since each part will have a different set of data elements values associated with that portion of the horizon. This procedure is addressed in Chapter 7 of the Pedon Description Program User’s Guide. Allowable codes are listed in the NASIS data dictionary. Further discussion of rules for use can be found in the Soil Survey Manual, Chapter 3, and the Keys to Soil Taxonomy, Ninth Edition, 2003.

Horizon Thickness (618.35)

Definition. Horizon thickness is a measurement from the top to bottom of a soil horizon throughout its areal extent.
Measurement. Soil horizon thickness varies on a cyclical basis. Measurements should be made to record the range in thickness as it normally occurs in the soil.
Entries. Enter the high, low, and representative values in whole centimeters. The minimum allowable entry is 1 cm. For horizons extending beyond the limit of field observation, thickness is calculated only to the depth to which observation was made.

Hydrologic Group (618.36)

Definition. The complete definition and official criteria for hydrologic soil group is available online at http://directives.sc.egov.usda.gov/media/pdf/H_210_630_7.pdf (U.S. Department of Agriculture, Natural Resources Conservation Service. 2007. National Engineering Handbook, Title 210-VI, Part 630, Chapter 7, Hydrologic Soil Groups. Washington, DC.).
Hydrologic group is a group of soils having similar runoff potential under similar storm and cover conditions. Soil properties that influence runoff potential are those that influence the minimum rate of infiltration for a bare soil after prolonged wetting and when not frozen. These properties are depth to a seasonally high water table, and saturated hydraulic conductivity after prolonged wetting, and depth to a layer with a very slow water transmission rate. Changes in soil properties caused by land management or climate changes also cause the hydrologic soil group to change. The influence of ground cover is treated independently.
Classes. The soils in the United States are placed into four groups, A, B, C, and D, and three dual classes, A/D, B/D, and C/D.
Significance. Hydrologic groups are used in equations that estimate runoff from rainfall. These estimates are needed for solving hydrologic problems that arise in planning watershed-protection and flood-prevention projects, for planning or designing structures for the use,

Technical References

National Soil Survey Handbook