SOIL SURVEY

The purpose a soil survey is to:

determine characteristics of the soils in a given area,

classify the soils according to a standard system of classification,

plot boundaries of the soils on a map, and

make predictions about the behavior and best management of the soils.

Information contained in a soil survey makes it a valuable tool for planning and managing various enterprises including crop production, forestry, urban planning, and other soil uses.

Soils, as defined in soil survey, are considered as natural bodies with unique properties. Thus, the soil map, as part of the survey, shows the distribution of different kinds of soil as an integration of all properties rather than the distribution of a single soil property such as texture, slope, pH, or depth. From such a map of soils rather than soil properties, maps showing the distribution of properties, expected response to management, and suitability for various uses can easily be constructed. Such interpretive maps can also be easily revised without additional field work as new information becomes available or as new uses for the soil are considered.

Single-purpose soil inventories can be made at a lower cost than a survey of soils. These single-purpose maps, however, can rarely be used to evaluate an area for a use other than the one for which the map was constructed. Additionally, single-purpose maps may not contain the information needed to evaluate new management strategies and new technology applied to the land use for which the inventory was made. Thus, while less costly in the short-term, additional field work needed to update single-purpose inventories every few years is often more expensive in the long term.

Taxonomic units versus cartographic units

A taxonomic unit is a category in an arbitrary system for classifying individuals (soils, plants, animals, bricks, etc.). An individual must meet a strict set of criteria to be within the taxonomic unit, and if it fails any one of these criteria, it is not part of the taxonomic unit. Taxonomic units are strictly arbitrary and do not really exist in nature. They only exist in people's minds.

A cartographic unit (map unit) is a real naturally occurring body that can be delineated on a map of some form. Cartographic units, in most cases, contain individuals from several taxonomic units. Thus, they are not pure.

An example: One can go into a pasture and select an individual grass plant and place the plant into a taxonomic unit based on its characteristics; fescue for example. The pasture as a whole is the cartographic unit. Fescue may be the dominant grass species, and thus, the pasture is called a "fescue pasture". The pasture, however, contains many other plant species that can be considered as inclusions.

Soil map units follow a similar concept. One or more soils may be dominant in the landscape unit delineated on the map, but there are always other soils present as inclusions. In most cases, the inclusions present are not the fault of the soil surveyor making the map. Natural systems are seldom pure, and understanding soil distribution, especially at a small scale, is a challenging task.

Inclusions

Soil map units are named for one or, in some cases, two or more kinds of soil that are most abundant in the area delineated on the map. However, there are almost always inclusions of soils with properties that differ from the rigorous definition of the named soil(s). In general, there are two types of inclusions.The first type are soils that are differ from the named soil by only one or two properties (similar soils) and that are expected to behave and respond to management in a similar manner to the named soil. The second type of inclusion are soils that have properties that are greatly different from those of the named soil(s) and that may behave and respond to management much differently than the named soil. The goal of a soil survey is to delineate the soils in an area in such a manner as to limit the amount of dissimilar soils in the map units.

Inclusions occur in map units for a number of reasons. Similar inclusions may be the result of incomplete understanding of subtle differences in landscape position, vegetation or other factors influencing soil distribution. Additionally, because soils may be a continuum over short distances, the transition from one soil to the next is sometimes gradual and the line separating two soils cannot be placed exactly on the base map.

Similar inclusions may occur as a choice rather than by chance. Because behavior and management are expected to be similar, separation of similar soils from the named soil would result in a map that was cluttered and difficult to use. Too many delineations can be as bad as too few.

Dissimilar soils occur in map units because of poor understanding of soil landscape relationships or, more commonly, because of map scale. If the minimum size area that can be shown on the map is 4 ha, an area of a contrasting soil that is 2 ha in size cannot be shown even if the soil surveyor knew of its presence.

Map Scale / Minimum Size Delineation*
ha
1:1,200 / 0.005
1:5,000 / 0.10
1:12,000 / 0.41
1:24,000 / 2.3
1:62,500 / 15.8
1:250,000 / 252
1:1,000,000 / 4,000
1:5,000,000 / 101,000

* Minimum size is based on 6 X 6 mm delineation on a map (size of pencil eraser).

Soils are best known by their characteristics derived from small samples. This includes both descriptions of properties such as depth, thickness, texture, structure, and arrangement of horizons that are observed in the field and physical, chemical, and mineralogical properties measured in the laboratory. Because of time and expense involved, few samples can be completely characterized in the laboratory. Field observable morphological properties are valid predictors of laboratory-measured properties, however. Thus, analysis of one or two pedons from a soil can provide the basis for interpreting the behavior and response to management of that soil wherever it occurs. Because of this, laboratory characterization is a necessary part of any soil survey and one or two pedons of each soil in the survey area (depending on extent and complexity) should be analyzed. Soil type then provides the link between laboratory analyses and application of the data to field-sized areas.

Soil Survey as a Paradigm-Based Science

Hudson, B.D. 1991. Soil Survey as a Paradigm-based science. Soil Sci. Soc. Am. J. 56:836-841.

A "paradigm" is a broad explanatory structure that provides a foundation for an entire field of scientific inquiry.For a scientific field to make progress, a dominant guiding concept or paradigm must eventually arise. This enables a group of scientists to focus its efforts on a narrow range of problems. When the paradigm is accepted, there is no need to explain the meaning of each concept.

In soil survey, the paradigm is the soil-landscape model. This model enables a trained soil surveyor to delineate bodies of soil by observing less than 1/1000 of the soil below the surface. This paradigm has its origin in the soil factor equation outlined by Dokuchaeiv

S = C, PM, R, O, T

The equation implies that, by looking for changes in one or more of these factors as the landscape is traversed, one can accurately locate boundaries between different bodies of soil.

This equation and the soil-landscape paradigm require that one discard one common tenet of soil science; that soils are a continuum. The soil may behave as a continuum over short distances, but there are frequent, often abrupt, discontinuities that can be discerned by trained observers.

To understand the soil-landscape paradigm, one must understand the concept of soil-landscape units. These are natural landscapes that result from the interaction of the five soil forming factors. It is similar to a landform but more narrowly defined.

Summary of soil-landscape paradigm

1. Within a soil-landscape unit, the five factors interact in a distinctive manner which leads to development of the same kind of soil within the unit.

2. The more different two adjoining soil-landscape units, the more abrupt and striking the discontinuity between them.

3. The more similar two soil-landscape units are, the more similar their associated soils tend to be.

4. Adjacent areas of different soil-landscape units have a predictable spatial relationship.

5. Once the relationships among soils and landscape units have been determined for an area, the soil can be inferred by identifying the characteristic soil-landscape unit.

This paradigm makes soil mapping possible because of observable discontinuities between adjoining soil-landscape units. In general, the more different adjoining areas of soil are, the easier it is to locate the boundary between them accurately.

The largest weakness in the soil-landscape model is an extreme reliance on learning the paradigm and associated concepts through experience rather than the classroom.To the untrained eye, the landscape is nothing but a backdrop for objects of more interest; trees, houses, animals, etc.

To use the soil-landscape model to recognize and delineate soil bodies, the soil scientist must learn to see the landscape as an entity unto itself. This requires learning to see the physical world in a new way. Once one learns to "see" the landscape as a group of facets with unique characteristics and soils, it is impossible to look at a landscape in any other way.However, one cannot choose to see landscapes in this "new" way. Instead, it is something that "seizes" the person. This shift in perception of landscapes normally occurs within a few months of daily observation, but for a few people, it never happens.

Once the soil scientist learns to recognize landscapes and soil-landscape units, he/she must then learn to group these units into a smaller number of similarity groups called map units.This grouping is not made based on all properties of the soil-landscape units but is based a few characteristics. This is analogous to recognizing people’s faces. You do not recognize people by all their individual properties but by a few distinctive properties. Learning to group soils in this way is a process of trial and error.

It is impossible to observe and make measurements on the entire population of soils in a region. By observing soils at selected points in the landscape and relating the properties of the soil at that point to landscape features and other indicators, the soil surveyor can develop conceptual models that allow him or her to predict the properties of the soils in new areas with similar vegetation, geology, climate, and landscape characteristics. Observations of the soil are still needed, however, to confirm the model predictions and to refine the model if the observation reveals a soil different from that expected. Thus, making of a soil map is an iterative process. Models are developed by relating soil properties to other landscape components from a limited number of observations, and these models are used to predict soil occurrence in unknown areas. Additional observations are used confirm the model's prediction and to provide additional data to continually refine the model. The end result is a usable map of the distribution of soils in the region.

Numbers and locations of observations of soils depend on the complexity of the area, experience of the soil surveyor, and validity of the soil surveyor's model. Observations are not made randomly because soils are not distributed randomly. Observations are made at locations expected to best represent the soil on a particular segment of the landscape. From these observations, the properties of the soil can be described, and new soils identified as the landscape and map scale warrants. The sum of all observations within a particular landscape segment over the course of the soil survey is used to rigorously define the range in properties of each soil in the survey.

Even though the soil-landscape model is the basis for soil survey, it is rarely written in textbooks or technical journals. These concepts have mostly resided in the minds of perceptive, experienced soil surveyors. Few, however, can express the concepts verbally. This is because they were learned by experience by observing soils and landscapes daily over a long period in different areas. They can show you the soil-landscape relationships and where to separate two units, but they cannot tell you, in all cases, the basis for making the separation.

This concept is related to the statement made by a wise old soil scientist, "We all see the world through colored glasses. The challenge is to recognize the color of the glasses we are wearing." In other words, we see what we have been trained to see and only what we are looking for.

The lack of written expression of the soil-landscape relationships is one reason that soil mapping is often considered an art rather than a science. What is called scientific knowledge is first and foremost expressed in language rather than pictures. The result of studying and mapping soils is a map rather than written words. Unless you understand the soil-landscape relationships in the area, the map is nothing but an abstract representation of a spatial distribution. Maps are ineffective for conveying the scientific concepts and complex relationships shown on the imagery.

Until soil surveyors become more effective in relating the soil-landscape relationships they see and use daily to a wide audience through the written word, soil mapping will be considered by many as an art rather than a science (similar to the art of medicine). If the relationships can be conveyed effectively, soil mapping may be teachable in the classroom rather than through a long process of experience and one-on-one training by showing.

We as soil scientists (or people in any walk of life) must recognize the power of ideas and concepts. "A sensible philosophy controlled by relevant set of concepts can nearly act as a substitute for genius."

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