PURPOSE Soil potentials are index numbers assigned to specific soil map units in a survey area to indicate performance (e.g., yield) and costs (e.g., drainage, irrigation, maintenance) for using each soil for a specific purpose. The gathering of output and input data used in calculation of soil potentials requires interdisciplinary cooperation of soil scientists and soil conservationists with agrono­mists, assessors, contractors, economists, engineers, planners, and many others. Soil potentials have economic units or economic implications, because feasibility of development of soil areas is usually directly related to costs and returns (Beatty et aI., 1979). Th is exercise is designed to acquaint students and others with soil potentials, and to provide practice in procedures to calculate ratings of soils in an area for their relative development potentials. Soil potentials are used (Soil Survey Staff, 1978):

1. To provide a common set of terms, applicable to all kinds of land uses, for rating the quality of a soil for a particular use relative to other soils in the area.

2. To identify the corrective measures needed to overcome soil limitations and the degree to which the measures are feasible and effective.

3. To enable local preparation of soil interpre­tations, using local criteria to meet local needs.

4. To provide information about soils that emphasizes feasibility of use rather than avoidance of problems.

5. To assemble in one place information on soils, corrective measures, and the relative costs of corrective measu res.

6. To make soil surveys and related information more applicable and easily used in resource planning.

7. To strengthen the resource planning effort through more effective communication of the information provided by soil surveys and properly relating that information to modern technologies.

DEFINITION The soil potential index (SPI) is a numerical rating of relative suitability or quality (Soil Survey Staff, 1978; Vinar, 1980). It is derived

Soil potentials

from evaluations of soil performance, cost of corrective measures, and costs for continuing limitations. The soil potential index is expressed by the equation

SPI = P - (CM + CL)

where P = index of performance or yield as a locally estab­

lished standard CM = index of costs of corrective measures to overcome

or minimize the effects of soil limitations CL = index of costs reSUlting from continuing limitations

Generally, the index number of the best soil map unit within a survey area is adjusted to 100 for the highest rating, and all other soils are expressed as lesser numbers or a percentage of the highest rating. In some cases index numbers can be greater than 100 if expressed in absolute units such as 120 bushels per acre (Kotlar, 1981).

EXAMPLES Soils with the highest potential index numbers are those with favorable attributes for the uses under consideration. The cost of corrective measures (eM) are the added costs above standard installation and management systems to overcome particular soil problems. The following list illustrates some soil limitations with general and specific measures to correct them:

Broad Categories of Limitations Corrective Measures

Wetness Drainage

Steep slope Construction grading Erodes easily Erosion control

High shrink- Strengthened foundation swell

Floods Flood control

Low Supported foundation strength

D rough ty Irrigation

More Specific Corrective Measures

Surface drainage Tile drainage Drainage land grading Cuts and fills Permanent vegetation Grassed waterways Terraces Conservation tillage Reinforced slab Extended footings Moisture control Raised foundation Dikes Improved channels Widened footings Extended footings Slab foundation Sprinkler irrigation Furrow irrigation Border irrigation

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G. W. Olson, Field Guide to Soils and the Environment Applications of Soil Surveys© Dowden & Culver, Inc. 1984

Continuing limitations (CL) are those that persist after corrective measures have been applied and have adverse effects on social, economic, or environmental values. Examples illustrating the derivation of continuing limitations (CL) include (Soil Survey Staff, 1978):

1. If the local performance standard is 2,000 pounds per acre, a potential production of only 1,500 pounds per acre from rangeland in a normal year, obtained through use of all feasible corrective measures for yield increase, is substandard by 500 pounds. Where P is 100, an appropriate index value for CL is

25 (2000 - 1500 X 100\ 2000 ')

2. If flooding of a dwelling remains a probability after feasible measures are installed, an esti­mate of damage and inconvenience from a flood event divided by the frequency of flooding might provide an annual cost for conversion to index values. For example, damages of $6,000 might be estimated to result from floods occurring 1 year in 10. This represents an annual cost of $600 and a serious continuing limitation. An approximate value for CL might be 60 if the index for P is 100.

To calculate soil potential index numbers for dwellings without basements, for example (Soil Survey Staff, 1978), assume single-family resi­dences each with 1,500 to 2,000 square feet of living space with slab construction, lawns, gardens, landscape design, play areas, on lot sizes of ~ acre or less. Evaluating criteria are:

--------------------Degree of Limitation

F actors Affecting Use Slight Moderate Severe

Depth to water table (inches) > 30 18-30 < 18 Flooding None None All years Slope (percent) 0-8 8-15 > 15 Shrink-swell Low Moderate High

Approximate index values and costs for correct­ing soil problems are:

Index Value

2 4 8

12 16 20

Cost Classes for Corrective Measures and Continuing Limitations

< 250 250-500 500-1,000

1,000-2,000 2,000-3,000 3,000-4,000 4,000-5,000

152 I Field Guide to Soils and the Environment

Corrective measures and their approximate costs for houses without basements are:

Corrective Measures Costs Index

Drainage of footing $ 300-500 4 Drainage of footing and slab 600-800 7 Excavation and grading

8-15% slopes 100-300 2 15-30% slopes 300-500 4

Rock excavation and disposal (fractured) 0-8% slopes 1 ,000-1 ,400 12 8-15% slopes 700-900 8

Reinforced slab Moderate shrink-swell 1,500-2,000 17 High shrink-swell 3,600-4,200 39

Surface drainage (per lot) 100-200 2 Importing topsoil for lawn

and garden 1,000-1,400 11

Each soil potential index number and each cost value, of course, must be formulated in specific or general terms most applicable to each survey area. Consultations among all people concerned are vital to the derivation of accurate and reliable data. These values presented here are examples only, and must be modified to fit each local situation. The numbers are most valuable when relatable to logical applications: these index values, for example, are one percent of estimated costs. The costs of continuing limitations are established for the 50-year life span of the dwelling to be com­patible with costs of corrective measures.

When soil potential index numbers have been derived for all soil map units in a survey area for each specific use being considered, then general rating classes can be used to group soil potentials within the area. General definitions of such classes include:

1. Very high potential. Production or perfor­mance is at or above local standards, because soil conditions are exceptionally favorable. Installation and management costs are low and there are no soil limitations.

2. High potential. Production or performance is at or above the level of local standards. Costs of measures for overcoming soil limitations are judged locally to be favorable in relation to the expected performance or yields. Soil limitations continuing after corrective mea­sures are installed do not detract appreciably

from environmental quality or economic returns.

3. Medium potential. Production or performance is somewhat below local standards, or costs of measures for overcoming soil limitations are high. Soil limitations continuing after correc­tive measures are installed detract from environmental quality or economic returns.

4. Low potential. Production or performance is significantly below local standards. Measures required to overcome soil limitations are very costly. Soil limitations continuing after corrective measures are installed detract appre­ciably from environmental quality or econom­ic returns.

5. Very low potential. Production or perfor­mance is much below local standards. There are severe soil limitations for which econom­ically feasible measures are unavailable. Soil limitations continuing after corrective mea­sures are installed seriously detract from environmental quality or economic returns.

ASSIGNMENT Each student should derive soil potential index numbers for each soil map unit within the student's study area, for several land uses. Local contractors and other experts should be consulted to obtain cost figures and index numbers that are realistic and useful. Modifications should be freely made in the examples given here and in the references, to fit the local situations. Students

should use their imagination and initiative freely, and make assumptions for feasible scenarios. A brief written report should be prepared explaining how the index numbers and cost figures were derived, and how the information can be used to evaluate the development potentials of different soil map units. A brief oral report (10 minutes) should also be presented by each student to the class. The instructor should arrange the assignment so that each student does not repeat the work of other students, and so that each student has ample opportunity to improvise in illustrating the application of soil potentials.

REFERENCES

Beatty, M. T., G. W. Petersen, and L. D. Swindale. 1979. Planning the uses and management of land. Monograph 21, American Society of Agronomy, Madison, WI. 1028 pages.

Kotlar, K. 1981. Application of the soil potential rating system to land use planning: A New York example. M.S. thesis, Dept. of Agronomy, Cornell University, Ithaca, NY. 77 pages.

Soil Survey Staff. 1978. Application of soil survey informa­tion: Policy guide. Parts I and II of Section 404 and Soil Potential Ratings of National Soils Handbook. U.S. Government Printing Office, Washington, DC. 45 pages.

Vinar, K. R. 1980. Soil potential ratings for the soils of Westchester County, New York. Soil Conservation Service, U.S. Dept. of Agriculture, White Plains, NY. 107 pages.

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