Soil Classification

A Global Desk Reference

Hari Eswaran, Robert Ahrens, Thomas J. Rice, B.A. Stewart

• Publisher: CRC Press
• Year: 2002
• Print ISBN: 9781138627536
• eBook ISBN: 9781000611137
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

Developments in soil classification have accompanied parallel progress in our understanding of the soil system. However the theories behind the classifications and the purposes for which they were created have changed over time. The editors hope that this comprehensive synthesis will help to rally soil scientists around the world to develop an acce

PART 1

Concepts and Innovations in Soil Classification

CHAPTER 1

Philosophies of Soil Classifications: From Is to Does

S.W. Buol

CONTENTS

Abstract

Introduction

Indigenous Classifications

Anthology of Soil Classifications

Philosophy of Selecting a Unit of Soil to Classify

Philosophy of Classification to Accommodate Soil Mapping

Utilitarian Expectations of Soil Classification

Benchmark Philosophies of Soil Classification

Multiple Systems

Conclusions

References

ABSTRACT

Philosophies of soil classification are guided by existing knowledge and pragmatic circumstances. Soil is a natural entity that connects the inorganic minerals of earth to the organic organisms of life, and is therefore germane to several academic disciplines. Each of these disciplines seeks to identify and classify soil in relation to its entity of study. Pedologists have established, through definition and classification, that soil is an entity worthy of independent academic recognition. People who seek nourishment and utilitarian support from soil identify and classify it by criteria that relate to what it does as it interacts with their attempts to utilize land resources. Political entities seek guidance in evaluating the impact of policies and regulations on soil-related natural resources. Soil classification provides a link between soil samples and the natural entities on the land surfaces of earth, and is a communicator of soil properties as spatially represented on maps of diverse scales and in soil property aggregation. No single classification can equally serve all who seek to study and obtain sustenance from soil. Several classifications, each guided by a philosophy of service to an identified audience, are required.

INTRODUCTION

Show me your [classification] system and I will tell you how far you have come in the perceptions of your research (Kubiëna, 1948).

Soil is so universal that all people know of its existence, and therefore have individual concepts of what soil is and what soil does. The universality of soil, the individuality of people, and the limited geographic exposure to soil each person has experienced ensure an infinite array of philosophies about how to classify soils. To some people, soil is a singular entity, but many have some appreciation for more than one kind of soil, abstractly referring to simplistic classifications of fertile, wet, black, red, sandy, clayey, etc. Others classify soil by association with geology, geography, climate, and vegetation. Philosophies of soil classification are fascinating.

INDIGENOUS CLASSIFICATIONS

Everyone who uses soil classifies it. Most indigenous classifications are in reference to a specific use. Indigenous people are directly interested in what a soil does with respect to their intended use of the land. Indigenous classifications are often quite colorful and informative. At an early age, I understood that “push-dirt” in southern Wisconsin referred to soil that adhered to the plow, and, if slightly too wet, would not be inverted by a moldboard plow. I later learned “push-dirt” was an apt classification for some Newglarus (fine-silty over clayey, mixed, superactive, mesic Typic Hapludalfs) soils, wherein a moldboard plow sometimes reached into the upper part of the clay-textured 2Bt2 horizon that, early in the spring, was often wetter than was predicted by looking at the surface soil. The wet, sticky 2Bt2 material adhered to the moldboard and pushed forward, rather than inverting as intended. “Black Jack,” “pipe clay,” and “Beeswax” land (Hearn and Brinkley, 1912) referred to distinct soils on the piedmont of North Carolina. Local farmers were said to also call them “dinner-bell” soils because they were too wet to plow in the morning and so dry and hard in the afternoon that a mule could not pull the plow. Plowing was best done at noon, i.e., dinner-time on the farm. We now know “dinner-bell” or “Black Jack” soils as an Iredell (fine, mixed, active, thermic Oxyaquic Vertic Hapludalfs).

Indigenous people who till the soil are concerned with what a soil does when it is interacting with their activities. Soil scientists attempt to identify what a soil is and to classify it using measurable chemical and physical criteria.

ANTHOLOGY OF SOIL CLASSIFICATIONS

Scientists in the latter part of the 19^th century were actively seeking to define soil in a context that separated soil science from geology. Dokuchaev (1883) related soil properties to elements of vegetation and climate, in addition to the influence of geologic parent material. Vegetation and climate were not considered to influence geology, and soil could be defined as a scientific entity apart from geology. In 1927, Glinka (quoted in Jenny, 1941) reported that Russian soil scientists regarded soil type as a summary of the external and internal properties of a soil, so as to include climate, vegetation, and animal life as a unit for soil classification, rather than as volumes of soil, i.e., modern day pedons.

Although he clearly recognized soil as an independent entity, Hilgard (1914) defined soil as “the more or less loose and friable material in which plants, by means of their roots, may or do find a foothold and nourishment, as well as other conditions of growth.” Thus he considered soil primarily to be a means of plant production. Perhaps the inscription on Hilgard Hall on the University of California-Berkeley campus best relates his concern for the concepts of soil held by indigenous people: “TO RESCUE FOR HUMAN SOCIETY THE NATIVE VALUES OF RURAL LIFE.”

The Bureau of Soils in the United States recognized the need to communicate soil science to indigenous people with regard to how it interacted with various crops and management practices, and produced practical reports that “deal with everyday problems (of the indigenous farmer) and are written with as little use of technical terms as possible” (Whitney, 1905, p. 11).

While concerns for what a soil does for human endeavors to obtain sustenance dominated soil mapping efforts in the United States, soil classification efforts sought to establish what is a natural entity worthy of independent recognition. Perhaps the infant science of soil reached a status of scientific adulthood, independent of its geologic parent, with the widely read treatise of Jenny (1941). The expression S = ƒ (cl, pm, r, o, t) served to identify soil (S) as a distinct natural entity, but identified it in terms of climate (cl), parent material (pm), relief (r), organisms (o), and time (t). Jenny (1941) concluded that the requirements necessary for establishing correlation of soil formation factors to soil properties were possible only under controlled experimental conditions, and in the field we must be satisfied with approximations and general trends. When the soil-forming factors are considered over time, each becomes a criterion that cannot be measured. Climate consists of weather averages, but one extreme weather event may leave an indelible imprint in a soil. Parent material may be the material below the soil, but incremental aerosol additions made to the soil are often not represented. Tectonic movements and erosion alter relief. Organisms in and on the soil change with natural succession, fire, wind, and human intervention. Jenny (1941) reasoned that the use of soil-forming factors in classification was handicapped by an inability to determine the exact composition of the initial (parent material) and the final (mature soil) stages of soil development. However, in view of the many correlations between soil properties and soil-forming factors, many soil classifications identified soils by association with soil-forming factors, i.e., forest soils, prairie soils, tropical soils, tundra soils, etc.

PHILOSOPHY OF SELECTING A UNIT OF SOIL TO CLASSIFY

Cursory observation of a soil reveals a vertical arrangement of soil components that change, often gradually, as one traverses the landscape. Our understanding of soil is limited without the use of chemical, mineralogical, biological, and physical quantification of soil samples. Soil can be dismembered, sampled, and autopsied, but only if we know where each soil sample is located within a body of soil do the analyses help us understand both what a soil is and what a soil does. Identification of the depth and vertical arrangement of a soil’s component parts through several degrees of sophistication, i.e., topsoil, subsoil, generic horizon nomenclature, and diagnostic horizons, is necessary in all attempts to classify soil.

Perhaps no single problem has plagued soil classification more than the identification of the spatial boundaries of a soil individual on the landscape that is to be sampled for study and used as a unit of classification. As soil science struggled to claim adulthood as an independent science, a basis of identifying a soil individual suitable for classification was at the root of many philosophies. Soil classification is closely allied with soil mapping, and most practitioners intertwined pragmatic realities of mapping into their philosophies of identifying an appropriate volume of soil for classification.

From the inception of soil surveys in the United States, three categories were employed in classifying soils in the field: series, type, and phase. Series names were place names of cities, towns, etc. in the area where the soil was first defined. Type identified the texture of the surface horizon, and phase referred to slope, rockiness, and other features of the area being identified. The grouping of soils into categories above series was not discussed in the 1937 Soil Survey Manual (Kellogg, 1937). At that time, a series was defined as “a group of soils having genetic horizons similar as to differentiating characteristics and arrangement in the soil profile, and developed from a particular type of parent material” (Kellogg, 1937, p. 88). It is clear from the discussion that differentiae from higher categories were not imposed on series. Instead, “All mappable differences in the profile significant to the growth of plants should be recognized in the classification” (Kellogg, 1937, p. 89). Clearly, a spatial area that could be identified on a soil map was considered germane to the concept of series. Most soil mapping in the United States at that time was done on 1:63,360 scale maps.

The concept of “mappable differences” as classification criteria caused severe confusion. Mappable differences in one area (in which soils belonged to a named series because of “genetic horizon” similarities) often were not “mappable” in other areas, or at different mapping scales. By 1951, problems associated with aligning “genetic horizon” criteria with “mappability” were recognized, but no clear protocol was established to overcome the problems (Soil Survey Staff, 1951). It was simply stated that the time and expense involved in separating two soil series could not always be justified. Moreover, the criteria for using only similarity of “genetic horizons” to define series were extended to other, often geologic, features below the soil profile. This was pragmatically justified in response to increasing use of soil survey information for engineering purposes that relied on soil properties not always present in the genetic horizons but violating a basic tenet of classification, i.e., that criteria used to classify an object should be observable within the object classified.

Identification of an individual volume of soil that could be used for classification had to be addressed before progress could be made in the development of quantitative soil classification. The conceptual philosophy for identifying an individual soil paralleled the concept of the unit cell used to identify minerals. The unit cell of a mineral identifies all the elements present in that mineral, and is the basic unit for identification and classification. That exact unit cell composition, however, may not be present in each mineral particle belonging to that mineral class. In the process of developing Soil Taxonomy, the concept of pedon, “the smallest volume that can be c...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Part I Concepts and Innovations in Soil Classification
• Part II Developments in Classification Systems
• Index