This unit is part of the NCGIA Core Curriculum in Geographic Information Science. These materials may be used for study, research, and education, but please credit the author, Peter H. Schut, and the project, NCGIA Core Curriculum in GIScience. All commercial rights reserved. Copyright 1998 by Peter H. Schut.

Your comments on these materials are welcome. A link to an evaluation form is provided at the end of this document.

---

Advanced Organizer

Topics covered in this unit

• Overview of soil classification
• The mapping of soils
• Types and contents of soil surveys
• Structure of soil data in GIS databases

Learning Outcomes

• After learning the material covered in this unit, students should be able to:
• describe the spatial nature of soil
• explain how soil data is collected
• list the characteristics of different scales of soil surveys
• describe how soil data is stored in GIS databases
• list some uses of soil data in GIS applications

Instructors' Notes

Full Table of Contents

Metadata and Revision History

1. Overview of soil and soil classification

1.1. Definition of Soil

• Soil results from the interaction of the surficial deposits (bedrock or sediments) with soil-forming processes, including between climate, living organisms, and relief
• Soil consists of breakdown products of surficial deposits, and accumulation of organic matter
• Material less than 15 cm deep is not considered to be soil (arbitrary definition)
• Materials at depths greater than one meter are not considered to be part of the soil although tree roots and soil dwelling organisms may extend below 1 meter (arbitrary definition)

1.2. Soil Classification systems

• Several classification systems exist; all based on observable / measurable parameters
• Canadian System of Soil Classification
• US 7^th approximation
• Russian
• FAO
• Soil classification systems are still undergoing development
• Classification systems are hierarchical in nature
• Orders, which include
• Great groups, which include
• Subgroups, which include
• Series.
• Since soil has a continuum of properties
• classification systems have to have somewhat arbitrary boundaries to separate different soil types (e.g. >19%clay = one series, 20% = another)
• extends right up to the level of Order, so two soils from different orders can actually be quite similar, separated by just a few relatively minor characteristics.
• The classification of a soil is often, but not always, a good predictor of its agricultural productivity

2. Mapping soils and landscapes

2.1. Spatial Nature of Soil

• Soil is a continuum, spread thinly over bedrock in some places, thickly in others
• Soil properties can vary dramatically over the space of a few centimeters, often due to a discontinuity in the parent material (e.g. edge of sand deposited by an old delta)
• in other locations soil properties can undergo minor transitions over a distance of kilometers, where similar parent material, landscape, and climate combine to create a relatively homogeneous soil (e.g. level clay plain)
• Soil has an important 3 dimensional aspect, and characteristics vary with depth. Identifiable differences are used to divide the soil into layers or soil horizons
• Soils are typically intermingled, due to local drainage effects and the distribution of the parent material
• mapping at a farm/field farm / field scale is only scale at which one soil can be mapped per polygon. Scales greater than 1:5,000 normally require assigning more than one soil per polygon

2.2.  Soil Surveys

• Soil is normally surveyed by county
• Normally undertaken by a federal government agency
• Take a long time to perform (typically many months of fieldwork and lab analysis)
• Survey is an interpretation, so adjacent polygon coverages may not line up if they were produced by different soil surveyors.
• Concepts and analytical methods have changed over time, so adjacent surveys may not contain compatible information, and quite often older surveys will be missing key attributes
• Few countries have complete coverage at anything regional scale

2.3.  Scale and soil survey

2.3.1.  Generalized surveys

• scale: 1:1,000,000 and smaller
• Contents
• Regional landforms
• Local surface forms
• Typical slopes
• Parent material mode of deposition
• example soils
• Used for national/provincial scale analysis
• Estimates of productivity
• climate change modeling
• Examples
• Land potential database
• Soil landscapes of Canada
• STATSGO

2.3.2.  Reconnaissance surveys

• scales 1:250,000 – 1:750,000
• Contents
• Soil capability for agriculture
• Used for regional scale analysis
• general land use planning
• land inventory
• Examples
• Canada Land Inventory (CLI)
• US SURGO

2.3.3.  Detailed soil surveys

• scales 1:20,000 – 1:125,000
• Contents
• Soil type
• texture
• Drainage
• stoniness
• Used for
• Municipal Zoning
• Environmental impact mitigation
• Tax assessments
• Examples
• US STATSGO

2.3.4.  Farm/field surveys

• scales 1:1,000 – 1:10,000
• Contents vary widely
• Soil type
• Nutrients
• texture
• Used for
• precision agriculture
• farm environmental plans

2.4.  Soil associations (catenas)

• Soil characteristics vary in a somewhat predictable way as a function of landscape position.

• Well drained soils are found at the top of slopes
• Poorly drained soils tend to be found at the bottom of slopes or in depressions
• Drainage affects soil development (physical and chemical characteristics) and thus potential productivity

• Knowledge of topography and landscape effects can be combined with soil survey information to estimate locations of individual soils

2.5.  Properties recorded in surveys

• The following soil properties are typically recorded in soil surveys

• Parent material mode of deposition
• Stoniness
• Rockiness
• Coarse fragment content
• Percent sand, silt, clay
• Organic carbon
• Bulk density
• pH
• base saturation
• calcareousness
• cation exchange capacity
• water retention
• drainage
• depth to water table
• rooting restriction
• electrical conductivity

• The following landscape properties are typically recorded in soil surveys

• Slope
• Local surface form
• Regional land form

2.6.  Coding of soil properties in GIS databases

• Normally stored as vector coverages
• Polygon boundaries are often indistinct. This may be represented by an uncertainty (or fuzziness) factor
• Since soils are so complex, soil GIS databases normally rely on more than one data table (these may go by different names or be normalized somewhat differently)

• Polygon Table
• Contains information applicable to each specific polygon
• Rock outcroppings

• Map Unit Table
• contains attributes which apply to a group of similar polygons

• Soil Component Table
• Identifies one or more soil types found in a map unit or polygon
• Identifies percentage occurrence of each component with polygon, or uses terms like dominant, co-dominant, subdominant
• Contains one or more descriptors like slope, stoniness

• Soil Name Table
• contains information applicable to that named soil
• Parent material
• Drainage characteristics
• classification

• Soil Pedon Table
• Contains information about the smallest mapped unit of soil
• Soil Layer Table
• Contains information specific to soil horizons or layers

2.7.  Mapping of soil and landscape attributes

• Commonly the dominant soil (or soil characteristic) is mapped
• Alternatively the percent distribution of a particular soil or characteristic is sometimes mapped
• For numeric data, spatially weighted averages can be employed

3.  Applications of soils data

• Land use planning
• Agriculture
• Forestry
• Agricultural production
• Suitable cropping practices
• Pesticide registration
• Similar soils need only be tested once
• Engineering
• Foundation stability
• Site selection for radio towers
• Electrical conductivity can affect radio wave transmission
• Weather modeling
• Texture and organic content affects heat retention
• soil degradation risk assessment
• wind erosion
• water erosion
• salinization

4. Summary

• Overview of soil classification
• The mapping of soils
• Types and contents of soil surveys
• Structure of soil data in GIS databases

5. References

• suggestions accepted!

6. Exam and Discussion Questions

• suggestions accepted!

Evaluation

Citation

Peter H. Schut (1998) Soil Data for GIS,  NCGIA Core Curriculum in GIScience, http://www.ncgia.ucsb.edu/giscc/units/u091/u091.html, DRAFT, posted October 21, 1998.