Soil Survey Manual - Chapter Six (Part 3 of 4)

Interpretations

Table of Contents

Page 1
Approaches to Generalizing Relative Soil Behavior
Interpretive Systematics
    Management Groups
    National Specific-use Placements
    Local Relative Placements

Page 2
Interpretive Soil Properties
    Assignment Guidelines
    Setting
    Field Water Characterization
    Particle Size Distribution
    Fabric-Related Analysis
    Engineering Classification
    Chemical Analysis
    Physical Features or Processes
    Erosion
    Corrosivity

Page 3
Interpretative Applications
    National Inventory Groupings
    Land-Use Planning
    Farmland
    Rangeland
    Forest land
    Windbreaks
    Recreation
    Wildlife Habitat
    Construction Materials
    Building Sites
    Waste Disposal
    Water Management

Page 4
Areal Application of Interpretations
    Map Units
    Areal Extension of Interpretations
    Areal Generalization
Illustrative Map Units

Interpretative Applications

In this section the process of developing soil interpretations for land uses is discussed and the kinds of soil interpretations or groupings of soils are presented.

Soil interpretations may be developed at many levels of generalization or abstraction. Traditionally, interpretations have been developed for national application. These criteria, however, may be too general for applications at the local or regional levels. The national criteria, however, may be the basis from which to narrow limits or add further criteria for the local situation.

The process of developing interpretations for a specific land use follows a scientific method as do other processes in a soil survey. The soil scientist or group preparing the criteria reviews the literature, interviews experts, makes observations of soil performance under the specific use, develops a set of criteria using the basic soil properties, tests the criteria, and finally adopts the system. The process rarely becomes static. As new technologies become available, the criteria must be reevaluated.

Soil interpretations are a paradigm of how soils respond for a specific use. They are models that use a set of rules or criteria that are based on, or fed by, the basic soil properties, modeled properties, or classes of properties. In some cases it may be necessary to model a subset or intermediate interpretation to evaluate potential frost action, corrosivity, or potential for mass movement.

The interpretations are most often developed in response to user needs, thus the development process must include input from the user and professionals from other disciplines. User feedback is crucial in the iterative process of refining a specific interpretation.

Example.—Table 6-1 contains the criteria for interpreting soils for septic tank filter fields.

Table 6-1. Interpretive Soil Properties and Limitation Classes for Septic Tank Soil Absorption Suitability

Table 6-2 illustrates how the criteria are applied to the map unit of Sharpsburg soils in the appendix. These tables are used to illustrate the process of developing an interpretation. (N.B.: The classes of hydraulic conductivity are those used currently in interpretations and are not coincident with the new class limits given in ch. 3.)

Table 6-2. Values of applicable interpretive properties for septic tank suitability for Sharpsburg silty clay loam, 5 to 9 percent slopes; Soil Survey of Lancaster County, Nebraska, (Brown et al., 1980).

The soil scientist or group of individuals developing an interpretation first determine a list of soil properties that are known, or are thought to be, important for septic tank filter fields. Depth to water table, permeability, depth to bedrock, depth to cemented pan, depth to permafrost, slope, flooding, ponding, susceptibility to downslope movement, and susceptibility to pitting are considered important properties in this case. After determining the list of soil properties, the soil scientist or group of individuals develop limits for each property and each class (slight, moderate, severe). This iterative phase is often the most difficult. The initial set of criteria is tested in different areas of the country using a wide array of soil conditions. Results of the tests may require adjustments to the criteria and retesting. Once the limits are set they may be arrayed in the table according to degree of severity or importance.

Testing and reevaluation.—The interpretative paradigm is under continuous scrutiny by user feedback, ranging from local homeowners' associations and units of government to national environmental agencies and organizations. Soil scientists continue the testing of interpretations through observations and discussions with local user groups during the soil survey process.

National Inventory Groupings

Technical soil groupings have been developed as criteria for the application of national legislation concerned with the environment and with agricultural commodity production. Groupings may pertain to agricultural productivity and diversity, erosion potential, surface and ground water quality, maintenance of wetlands, or other groups to meet national needs. Four national groupings are described as examples: prime farmlands, unique farmlands, hydric soils, and highly erodible lands. Specific criteria in the National Soils Handbook may be studied to demonstrate how various taxonomic and nontaxonomic map unit criteria, coupled with interpretative soil properties, have been employed to construct definitions for national inventory purposes.

Prime farmland. This is land that has the best combination of physical and chemical characteristics for producing food, feed, forage, fiber, and oilseed crops. It must also be available for these uses. It has the soil quality, growing season, and moisture supply needed to produce economically sustained high yields of crops when treated and managed according to acceptable farming methods, including water management. In general, prime farmlands have an adequate and dependable water supply from precipitation or irrigation, a favorable temperature and growing season, acceptable acidity or alkalinity, acceptable salt and sodium content, and few or no rocks. They are permeable to water and air. Prime farmlands are not excessively erodible or saturated with water for a long period of time, and they either do not flood frequently or are protected from flooding.

Unique farmland. This is land other than prime farmland that is used for the production of specific high value food and fiber crops. It has the special combination of soil quality, location, growing season, and moisture supply needed to produce economically sustained high quality and/or high yields of a specific crop when treated and managed according to acceptable farming methods. Examples of crops are tree nuts, olives, cranberries, citruses and other fruits, and vegetables.

Hydric soils. These are soils that are saturated, flooded, or ponded long enough during the growing season to develop anaerobic conditions in the upper part. They make up part of the criteria for the identification of wetlands.

Highly erodible land. These lands have been defined in order to identify areas on which erosion control efforts should be concentrated. The definition is based on Erosion Indexes derived from certain variables of the Universal Soil Loss Equation (Wischmeier and Smith, 1978) and the Wind Erosion Equation (Woodruff and Siddoway, 1965). The indexes are the quotient of tons of soil loss by erosion predicted for bare ground divided by the sustainable soil loss (T factor).

Land-Use Planning

Land-use planning is the formulation of policies and programs for guiding public and private land use in areas of any size where different uses compete for land. The word "land" in this context implies attributes of place and other factors besides soil. Planners must consider place, size of area, relation to markets, social and economic development, skill of the land users, and other factors. Soil surveys can help in land-use planning by serving as an introduction to the soil resources of the area and by providing a source of information for the evaluation of the environmental and economic effects of proposed land uses. Soil surveys can be interpreted for land-use planning through groupings or ratings of soils according to their limitations, suitabilities, and potentials for specified uses.

Local planning.—Local government units such as those of cities, towns, and counties do local planning. The planning applies to complexes of farms and ranches, to housing developments, shopping centers, industrial parks, and to entire communities or political units.

Local planners use interpretations of soils and other information to develop recommendations on alternatives for land use, patterns of services, and public facilities. Planners may need interpretative maps at different scales depending on the objective. Interpretations of small areas for local planning rate limitations, identify management or treatment needs, and predict performance and potential of individual kinds of soils identified on detailed soil survey maps. Interpretations of areas that include entire governmental units evaluate the soils for all competing uses within the planning area. These maps are smaller in scale, and the units are associations of soil series or of higher taxa. Local planners commonly need ratings of the whole association for alternative uses. Special maps showing the location of areas having similar potentials or limitations for certain uses may be helpful for planners. Information about the amounts and patterns of soils having different potentials within each association can be given in tables or in the text of a soil survey report.

Regional Planning.—Certain problems pertain to areas that cover several political units. For these situations, regional planning is appropriate. The principal functions of regional planning are the collection, analysis, and dissemination of planning and engineering information, preparation of long-range plans, and coordination among the agencies involved.

Most soil maps for regional planning are small scale maps generalized from detailed soil survey maps. Soil interpretations show the differences between the map units in terms of suitabilities and limitations for the principal competing uses. The distribution of map units having similar behavior for a given use are commonly shown on special maps. An accompanying text describes the units, explains the basis for the ratings, and may also describe the effect of the pattern of associated soils on the use of specific parcels. Regional planners commonly need more specific information about the suitability of small parcels than can be obtained from generalized soil maps. For example, they may find an area that is generally good for recreation, but they also need to know that a potential site for a reservoir has soils suitable for storing water before the regional plan can be completed.

Farmland

Soil surveys in agricultural areas identify the soil characteristics that determine the suitability and potential of soils for farming. Interpretations for farming involve placement of the soils in management groups (land capability system) and identification of the important soil properties that pertain to crop production, application of conservation practices, and other aspects of agriculture. The other aspects of agriculture include yield potential, susceptibility to erosion, depth to layers that restrict roots, available water capacity, saturated hydraulic conductivity, the annual pattern of soil-water states (including soil drainage class, inundation, and free water occurrence), qualities that describe tilth, limitations to use of equipment (including slope gradient and complexity, rock fragments, outcrops of bedrock, and extremes in consistence), salinity and sodium adsorption ratio, presence of toxic substances, deficiency of plant nutrients, capacity to retain and release plant nutrients, capacity to retain soluble substances that may cause pollution of ground water, capacity to absorb or deactivate pesticides, and pH as related to plant growth and the need for liming.

The fate of added nutrients and pesticides, as related to farm management and cropping systems, is an important consideration in nonpoint water pollution. The identification of critical soil properties as related to resource management systems is crucial in the wise use of the land. The Land Capability System shows the suitability of soils for agricultural uses (Klingebiel and Montgomery, 1961). The system classifies soils for mechanized production of the more commonly cultivated field crops—corn, small grains, cotton, hay, potatoes, and field-grown vegetables. It does not apply directly to farming systems that produce crops, such as some fruits and nuts that require little cultivation, or to crops that are flooded, such as rice and cranberries. It also cannot be used for farming systems that depend on primitive implements and extensive hand labor.

Soil productivity is the output of a specified plant or group of plants under a defined set of management practices. It is the single most important evaluation for farming. In general, if irrigation is an optional practice, yields are given with and without irrigation. Productivity can be expressed in quantity of a product per unit land area, such as kilograms or metric tons per hectare. For pasture, productivity can be expressed as the carrying capacity of standard animal units per unit area per season or year—or as live-weight gain. Productivity may be expressed as a rating or index related to either optimum or minimum yields, or it may be indexed to a set of soil qualities (properties) that relate to potential productivity. Productivity indices have the advantage of being less vulnerable to changes in technology than are expressions of productivity based on yields.

Productivity ratings express the predicted yields of specified crops under defined management as percentages of standard yields. They are calculated as follows:

Productivity rating = predicted yield per unit area x 100 / (standard yield per unit area)

Such a rating provides a scale for comparing the productivity of different kinds of soils over large areas. Ratings lend themselves to numerical treatment. Productivity ratings permit comparison of the productivity of crops having yields that differ markedly in numerical values. For example, a certain soil has a yield of corn for silage of 60,000 kg/ha and of 9,000 kg/ha for grain corn. These entities represent similar levels of production so the productivity ratings would be similar. Selection of the standard yield of a crop depends on the purpose of the rating. For national comparison, the standard yields should be for a high level of management on the best soils of the region for the crop. If potential production is of interest, yields under the best combination of practices are used.

Productivity ratings for individual crops can be combined to obtain a general rating for the soil over its area of occurrence. The individual ratings are weighted by the fraction of the area occupied by each crop, and a weighted average is calculated that characterizes the general productivity of the soil.

Productivity indices tied to soil properties are used as a relative ranking of soils. Soil properties important to favorable rooting depth and available water capacity normally are chosen. Some productivity models rely on a few critical soil properties such as pH and bulk density to rate soils (Kiniry et al., 1983). The EPIC model is a comprehensive productivity calculator that integrates many soil and climatic processes (Williams et al., 1989). Giving a relative ranking to soils, as well as calculating the impact of cropping systems on soil erosion and productivity, is worthwhile.

The soil fertility capability classification (FCC) system is a technical Soil Classification system that focuses quantitatively on the physical and chemical properties of the soil that are important to fertility management (Sanchez et al., 1982). Information required by the system is obtained from pedon descriptions and associated field data, laboratory characterization data, and Soil Taxonomy. The system is applicable to upland and wetland rice crops, pasture, forestry, and agroforestry needs under high- or low-input systems. The system does not rank soil, but rather it states the soil properties important to management decisions which will differ by crop type and management system. The system provides management statements for the classified soil and lists the general adaptability of various crops.

Resiliency of soils is an interpretation that relates to the ability of a soil to rebound from depletion of plant nutrients or organic matter or to rebound from the degradation of physical or chemical properties. The resiliency ratings are based on estimates of the natural fertility of the soil, available water capacity, favorable rooting depth, particle size distribution, and distribution of salts in the profile, if present. Resiliency ratings are important in evaluating alternative management systems that are based on lower chemical inputs. Traditional practices that use high inputs of chemical fertilizers and pesticides often mask properties of the soils that are important to crop production. Resiliency of soils is also important in evaluating long-term affects of management systems on soils.

Rangeland

Rangeland has a native vegetation of grasses, grasslike plants, forbs, and shrubs. In many areas, introduced forage species are also managed as rangeland. The vegetation is suitable for grazing and browsing by animals. Rangeland includes natural grasslands, savannahs, many wetlands and deserts, tundra, and certain shrub and forb communities. Soil-range site correlation within a soil survey gives the suitability of the soil to produce various kinds, proportions, and amounts of plants (fig. 6-4). This knowledge is important in developing management alternatives needed to maintain site productivity. Rangeland interpretations are given as range sites.

Figure 6-4 (Click here or on picture for high resolution 127 KB image)

Two range sites on different soils.

Range sites are ecological subdivisions into which rangeland is divided for study, evaluation, and management. A range site is, therefore, a distinctive kind of rangeland that differs from other kinds of rangeland in its ability to produce a characteristic natural plant community. It is typified by an association of plant species that differs from that of other range sites in the kind or proportion of species or in total annual production. The natural plant community, in the presence of natural disturbances—fire, insects, drought—and the absence of abnormal disturbances and physical site deterioration, is the climax plant community for the range site.

A range-site description commonly contains the following information:

1. Landscape factors that describe the geographic location, physiography, and associated stream and nonstream water features of the site;
2. Climate factors that consider the soil moisture and temperature regimes, ambient climate, and moisture and temperature distribution patterns;
3. Soil factors that are most important in developing soil-vegetation relationships and that affect plant growth—the major soil families, geologic formation, features of the soil surface, surface horizon and texture, soil depth, major root zone thickness and its available water capacity, kind and amount of accumulations, profile rock fragments, reaction, salinity, sodicity, soil-water states, water table, and flooding;
4. Vegetation factors that describe the various kinds of tree, shrub, forb, grass, and grasslike, components of the plant community:

   The vegetative factors are the percent of cover and the composition and production of the plant community. Percent composition is expressed as a range of percent for each plant species identified by air-dry weight. Production is the total annual yield of air-dry forage, expressed as a range of values that reflect long-term weather variations. Yields are usually based on measured values;

5. Species list of wildlife that are associated with the site or are expected to use the site in the climax situation and will directly influence plant community dynamics;
6. Community dynamics that describe known or expected time relationships attributed to natural disturbances—periods between wild fires, cyclic insect infestations, or other disturbances to the composition of the plant community;
7. Site interpretations that give the potential importance of the site for each of its major uses and the feasibility of restoring depleted areas by seeding suitable species.
8. Listing of soils grouped into the site by soil survey area, map unit symbol, and soil name and phase.

Forest land

Forest land is dominated by native or introduced trees with an understory that consists of many kinds of woody plants, forbs, grasses, mosses, and lichens. Some forest communities produce, at least occasionally, enough understory vegetation suitable for forage to permit grazing.

Soil-forest site correlation within a soil survey gives the suitability of the soil to produce wood products. If forest land is important in a soil survey, the estimated productivity of the common trees is given for each individual soil. The understory vegetation is described at the expected canopy density most representative of forest stands having a normal production of wood. Determination of the soil productivity for forest products requires close collaboration between foresters and soil scientists.

Wood production or yield is commonly expressed as the site index or as some other measure of the volume of wood produced annually. Site index is the average height of dominant and codominant trees of a given species at a designated age. Measurements of site index are usually extended to a number of like soils where data are unavailable. The site index is correlated to each soil and may be further interpreted in terms of cubic meters per hectare.

Soils may be grouped using the ordination system. The symbols that make up the system indicate productivity potential and the major limitations for the use and management of individual soils or groups of like soils. The first part of the ordination symbol is the class designator. This is a number that denotes potential productivity in terms of the nearest whole cubic meter of the wood growth per hectare per year for the soil based on the site index of an indicator tree species. For a number of species, data are available for converting site index to average annual wood growth. The second part of the ordination symbol, or subclass, indicates soil or physiographic characteristics that limit management—stoniness or rockiness, wetness, or restricted rooting depth. A third component of the ordination symbol, or group, is sometimes employed to distinguish groups of soils that respond similarly to management. When the group symbol is used, soils that have about the same potential productivity are capable of producing similar kinds of trees and understory vegetation and have similar management needs.

Soils may be rated for such factors as susceptibility to mechanical compaction or displacement during forestry operations, limitations that result from burning, hazards from soil-borne pests and diseases and limitations imposed by specific soil properties such as wetness. The management of trees begins with an understanding of the soil on which the trees grow or are to be grown. Soil surveys include information that can be used effectively in the management of forest land; for example,

Erosion hazard. This is the probability that erosion damage may occur as a result of site preparation and the aftermath of cutting operations, fires, and overgrazing.

Equipment limitations. These are limits on the use of equipment either seasonally or year-round due to soil characteristics such as slope, surface rock fragments, wetness, and surface soil texture.

Seedling mortality. This rating considers soil properties that contribute to the mortality of naturally occurring or planted tree seedlings such as droughtiness, drainage class, and slope-aspect. It does not consider plant competition.

Windthrow hazard. This is based on soil properties that affect the likelihood of trees being uprooted by wind as a result of insufficient depth of the soil to give adequate root anchorage. Depth of the soil may be affected by a fragipan, bedrock, gravel, or a high water table. Differences in root systems related to tree species are not considered. The rating is usually independent of the probability of high winds unless the soil is typically on landscape positions susceptible to high winds.

Plant competition. This is the likelihood of invasion or growth of undesirable plants in openings in the tree canopy. Soil properties such as depth to the seasonal water table and available water capacity have the most affect on natural regeneration or suppression of the more desirable plant species.

Trees to plant. This is a list of one or more adapted species for producing tree crops.

Windbreaks

Windbreaks are made up of one or more rows of trees or shrubs. Well-placed windbreaks of suitable species will protect soil resources, control snow deposition, conserve moisture and energy, beautify an area, provide wildlife habitat, and protect homes, crops, and livestock. The plant species used in windbreaks are not necessarily indigenous to the areas that are planted. Each tree or shrub species has certain climatic and physiographic limits and, within these limitations, a particular species may be well or poorly suited because of soil characteristics. Correlation of soil properties and adaptable windbreak species, therefore, is essential.

A listing of adaptable species is given for each kind of soil where windbreaks will serve a useful purpose—such as open field-planting, interplanting in existing woodland, and for environmental modifications like wind or water barriers and wildlife habitat. The plant species identified for these purposes are stratified by height classes at twenty years of age.

Recreation

Interpretations in heavily populated areas are made for golf fairways, picnic sites, and playgrounds; in sparsely populated areas for paths, trails, and campsites. Interpretations for ski slopes and snowmobile trails are made in some places. Ratings are usually made on the basis of restrictive soil interpretative properties such as slope, occurrence of internal free water, and texture of surface horizons.

Interpretations for recreation must be applied cautiously. Many recreational areas in the United States have only Order 3 or more general soil surveys. Map units for such soil surveys are commonly associations or complexes of soils that may differ markedly in their limitations and suitabilities. Furthermore, general suitability of the map unit must take into consideration not only the qualities of the individual kinds of soil but also the soil pattern and potential interactions. Suitability may depend on a combination of several kinds of soil in a pattern appropriate to the intended use. Finally, factors other than soils are important in recreational planning. Aesthetic considerations, location, accessibility, land values, access to water and to public sewer lines, presence of potential impoundment sites, and location relative to existing facilities may be important even though none of these factors is evaluated for map units.

Wildlife Habitat

Soils influence wildlife primarily through control over the vegetation. Description of the soil as wildlife habitat has two parts. In one part, the suitability class for different vegetation groups is recorded. These vegetation groups are called habitat elements. Each habitat element is a potential component of the environment of wildlife. Hardwood trees and shallow water areas are examples of habitat elements. In the other part of the description, soils are rated separately for several kinds of wildlife, including animals adapted to open land, woodland, wetland, and rangeland (fig. 6-5). Current land use and existing vegetation are not considered, because these factors are subject to change and cannot be determined from a soil map. Wildlife population is also disregarded because of the mobility of wildlife and the possibility of a changing population over the year. The ratings show where management for wildlife can be applied most effectively and which practices are appropriate. The ratings may also show why certain objectives may not be feasible; for example, the production of pheasants. Some soil surveys include explicit management recommendations. These may be particularly useful in planning land acquisition for recreational uses.

Figure 6-5 (Click here or on picture for high resolution 124 KB image)

Ducks in a rice field

Construction Materials

Soil survey interpretations estimate suitability of the soil as construction material and show where to locate material that can be mined. Material that compacts readily and has high strength and low shrink-swell potential is preferred as base material under roads and foundations. Gravel and sand are used for concrete, road surfacing, filters in drainage fields, and other uses. Organic soil material is used widely as horticultural mulch, potting soil, and soil conditioner. Mineral soil material of good physical condition, is generally rich in organic matter and is applied to lawns, gardens, roadbanks, and the like. Soils can be rated as probable sources of these materials. The quality of a particular site, however, usually cannot be specified.

Building Sites

Interpretations are made for the construction of small buildings; for the installation of roads, streets, and utilities; and for the establishment of lawns and the landscaping of the grounds around the building. Such soil uses involve high capital expenditures in relatively small areas. Usually, onsite evaluation is necessary.

Soil survey interpretations are useful for comparing alternative sites, in planning onsite investigations and testing, and in land-use planning. Soil maps can assist in selecting building sites that are near areas suitable for utilities, parks, and other needs.

The preparation of building sites may alter soil properties markedly. To this extent, some interpretative soil properties for the undisturbed sites must be applied cautiously. Upper horizons may have been removed and locally translocated, which might either increase or decrease the depth to horizons important to behavior. The pattern of soil-water states may be changed. Areas may have been drained and, therefore, are not as wet as indicated. On the other hand, irrigation may be employed to establish and maintain vegetation leading to a more moist soil and possible deep movement of water. Pavements, roofs, and certain other aspects of construction increase runoff and may cause inundation at lower elevations where the soil survey does not indicate such a hazard.

Building construction: Construction and maintenance of buildings belongs primarily to architecture and engineering. Additionally, large multistory structures are generally supported by footings placed below the depth of soil survey examination. Soil survey interpretations are not, therefore, a definitive source of information for building construction. Important interpretative soil properties for small buildings and accessory installations such as roads and utilities include slope, inundation, mass movement, potential frost action, depth to bedrock and to cemented pans, shrink-swell, rock fragments >75 mm, erodibility, subsidence, and soil strength (fig. 6-6).

Figure 6-6 (Click here or on picture for high resolution 137 KB image)

Damaged house.

Roads, streets, and utilities: The performance of local roads and streets, parking lots, and similar structures is often directly related to the performance of the underlying soil, (fig. 6-7). Pipelines and conduits are commonly buried in soil at shallow depth. The properties of the soil may affect cost of installation and rate of corrosion. Soil material is used directly as topsoil, roadfill, and aggregate for concrete. Soil interpretations can predict some suitabilities and limitations of different kinds of soil for these uses, although soil interpretations cannot predict performance of highways, major streets, and similar structures. For such construction, onsite testing is necessary. Use of soil surveys information, however, may reduce the number of borings and engineering tests.

Figure 6-7 (Click here or on picture for high resolution 154 KB image)

Damaged sidewalk.

Soil information in conjunction with engineering testing can identify those soils that can be stabilized in place for a road base and establish where gravel or crushed stone will be needed. Soil surveys can be helpful in deciding methods of stabilizing cuts and fills. Soil properties may affect the cost of installation and length of service of buried pipelines and conduits. Shallow bedrock, for instance, greatly increases the cost of installation. Rate of corrosion is related to wetness, electrical conductivity, acidity, and aeration (fig. 6-8). Differences in properties between adjacent horizons, including aeration, enhances corrosion in some soils. Soil properties affect the cathodic protection provided by sacrificial metal buried with pipes. Rock fragments can break protective coatings on pipes. Shrinking and swelling of some soils may preclude the use of certain kinds of utility pipe.

Figure 6-8 (Click here or on picture for high resolution 141 KB image)

Corrosion of buried pipe

Soil survey interpretations may be particularly useful in the prediction of problems likely to be encountered along proposed routes. Hydrologic information and other data combined with interpretative soil properties, such as the hydrologic group, can be helpful for the estimation of potential runoff for design of culverts and bridges. The probability of bedrock and unstable soils that require removal or special treatment can be determined from soil surveys.

Lawns and landscaping: Soil survey interpretations give general information about planning, planting and maintaining grounds, parks, and similar areas. Particularly important is the suitability of the soil for turf, ornamental trees and shrubs, the ability to withstand trampling and traffic, the suitability for driveways and other surfaced areas, and the ability to resist erosion. A number of soil chemical properties may be critical, especially for new plantings. Interpretations for particular plants and the treatments for a specific site require other disciplines.

Many lawn and ornamental plantings are made in leveled areas on exposed subsoil or substratum or on excavated material that has been spread over the ground. Interpretations can be made as to the suitability of such soil materials for lawns and other plantings, the amount of topsoil that is necessary, and other treatments required for satisfactory establishment of vegetation. Highway departments use soil interpretations to establish and maintain plantings on subsoil material in rights-of-way.

Waste Disposal

Waste disposal is divided on the basis of whether the practice places the waste in a relatively small area or distributes the waste at low rates over larger areas of soil.

Localized placement. Waste in this context includes a wide range of material from household effluent, through solid waste, to industrial wastes of various kinds. Effluent from septic tanks is distributed in filter fields. Liquid wastes are stored and treated in lagoons constructed in soil material. Solid wastes are deposited in sanitary landfills and covered with soil material.

The criteria for septic tank absorption fields is given in table 6-1. Extremes in saturated hydraulic conductivity and free water at a shallow depth limit the use of soil for septic tank absorption fields. Sewage lagoons require a minimum saturated hydraulic conductivity to prevent rapid seepage of the water, a slope within certain limits, and slight or no possibility of inundation or the occurrence of free water at shallow depths.

Soils are used to dispose of solid wastes in landfills, either in trenches or in successive layers on the ground surface. For trench disposal, properties that relate to the feasibility of digging the trench—depth to bedrock, slope—and factors that pertain to the likelihood of pollution of ground water—shallow zone of free water, inundation occurrence, and moderate and high saturated hydraulic conductivity—have particular importance. For disposal on the soil surface, saturated hydraulic conductivity, slope, and inundation occurrence are important.

Low-intensity distribution. Soil is used to render safe, either solid or liquid, waste that is spread on the ground surface or injected into the soil. Manures, sewage sludge, and various solids and waste waters are included, the latter particularly from factories that process farm products. In general, the physical process of distribution of the waste is limited by steep slopes, rock fragments > 75 mm and rock outcrops, and wetness. The rate at which wastes can be applied without contamination to ground water or surface water is called loading capacity. Low infiltration values limit the rate at which liquid wastes can be absorbed by the soil. Similarly, low saturated hydraulic conductivity through most of the upper meter limits the rate at which liquid wastes can be injected. Shallow depth of a hardpan or bedrock or coarse particle size reduces the amount of liquid waste that a soil can absorb in a given period. The time that wastes can be applied is reduced by the soil being frozen or having free water at shallow depths. Low soil temperatures reduce the rate at which the soil can degrade the material microbiologically.

Soils differ in their capacity to retain pollutants until deactivated or used by plants. Highly pervious soils may permit movement of nitrates to ground water. Similarly, saturated or frozen soils allow runoff to carry phosphates absorbed on soil particles or in waste deposited on the soil directly to streams without entering the soil. Soils that combine a limited capacity to retain water above slowly permeable layers and a seasonal water excess may allow water that is carrying pollutants to move laterally at shallow depths. Such water may enter streams directly.

Large quantities of waste may change the soil. Heavy loading with liquid waste may reduce the oxygen supply so that yields of certain crops are depressed. On the other hand, heavy loadings can provide beneficial irrigation and fertilization for other kinds of soil and crop combinations. Animal wastes improve most soils, but the effects differ according to the kind of soil.

The first step in making interpretations of soils for disposal of wastes is usually to determine how disposal systems for each kind of waste have performed on specific kinds of soil in the area. Experience may have been acquired in practical operations or by research. Soil scientists and specialists in other disciplines determine what properties are critical and how to appraise the effects of the properties. Limiting values of critical properties can be determined through experience and may be used in making interpretations where data on soil performance are scarce or lacking.

Water Management

Water management in this context is concerned with the construction of relatively small or medium impoundments, control of waterways of moderate size, installation of drainage and irrigation systems, and control of surface runoff for erosion reduction. These activities may involve large capital expenditures. Onsite evaluation commonly should be conducted, particularly of soil properties at depth. The usual Order 2 or Order 3 soil survey can be helpful in the evaluation of alternative sites, but onsite investigations are required to design engineered projects.

Ponds and reservoirs. Soil information is used in predicting the suitability of soils for ponds and reservoir areas. Impoundments contained by earthen dikes and fed by surface water have somewhat different soil requirements than those that are excavated and fed by ground water. Separate interpretations are commonly made.

Seepage potential of the soil, as determined by the minimum saturated hydraulic conductivity and the depth to pervious soil material, is an important factor for design of ponds and reservoirs. Slope also is of importance because it affects capacity of the reservoir. The hydrologic group of the soil (ch. 3) pertains to the prediction of runoff into a pond or reservoir.

Embankments, dikes, and levees. These are raised structures made of disturbed soil material constructed to impound water or to protect land from inundation. The soils are evaluated as sources of material for the construction. Particle size distribution and placement in the Unified system are important considerations. The interpretations do not consider whether the soil in place can support the structure. Performance and safety may require onsite investigation to depths greater than are usual in a soil survey.

Irrigation. Important considerations for the design of irrigation systems are feasible water application rates, ease of land leveling and the resultant effect on the soils, possibility of erosion by irrigation water, physical obstructions to use of equipment, and susceptibility to flooding. To meet these considerations, an Order 1 soil survey may be needed to include both deeper than customary observations and measurements of infiltration rates. Soil properties that may be the basis for the interpretations are saturated hydraulic conductivity, available water capacity, erodibility, slope, stoniness, effective rooting depth, salinity, sodium adsorption ratio (SAR), gypsum, and properties that may affect the level of response of crops (fig. 6-9).

Figure 6-9 (Click here or on picture for high resolution 178 KB image)

Irrigation of grain sorghum

Interpretations for irrigation in arid and semiarid regions may be more complex than for humid regions, because irrigation changes the soil-water regime more in arid and semiarid areas. Salinity and the SAR of the soils can be particularly significant, as can the quality of the irrigation water. In arid and semiarid areas, small differences in slope and elevation can lead to an accumulation of salt-laden drainage water in low places or the creation of a high water table if a proper drainage system is not provided.

Drainage. The term refers to the removal of excess water from soils for reclamation or alteration. Drainage construction criteria are established by engineers. The criteria include spacing and depth of subsurface drains, depth and width of open ditches and their side slopes, and allowable gradient. Properties of soils important to drainage include water transmission, soil depth, soil chemistry, potential frost action, slope, and presence of rock fragments greater than 75 mm.

< Back to Technical References...