An Introduction to Soils for Environmental Professionals

Duane L. Winegardner

• Publisher: Routledge
• Year: 2019
• Print ISBN: 9780367448851
• eBook ISBN: 9781351466936
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Agriculture
• Index: Technology & Engineering

An Introduction to Soils for Environmental Professionals assembles and presents the basic principles of each of the major soil science fields. It introduces fundamental concepts and shows the interrelationships between the various branches of soil science - from mineralogy to soil physics. Each chapter was reviewed by a professional in the particul

CHAPTER 1

The Fundamental Concept of Soil

From a purely pragmatic viewpoint, soil can be considered everything that is included in the superficial covering of the earth’s land area. Based on environmental perspectives, soil is an aggregate of unconsolidated mineral and organic particles produced by the combined physical, chemical, and biological processes of water, wind, and life activity.

In common usage, the definition of soil varies according to the user. For agriculturists (and related scientists), the important factors in soil focus on the upper few feet of the soil column, which are important to plant growth. Civil engineers consider soil a structural material with definable physical and chemical properties that can be manipulated (or tolerated) for construction purposes.

Microbiologists are interested in the interaction of microbes in the life cycle of the soil. Soil chemists study the detailed chemical reactions that result from the continuously changing underground laboratory. Questions asked by geoscientists include: Where did it originate?, How did it get here?, What is the next stage of its development?, and Of what is it composed?

The science of soil physics, which evolved from agricultural studies, considers the mechanical functions of soil such as fluid flow, interparticle relationships, and the complex interaction between the soil atmosphere, water, mineral surfaces, and organic matter.

Scientists in the individual disciplines of soil science identified in the preceding paragraphs tend to focus their efforts in specific directions toward the solution of well-defined problems. The environmental specialist is expected to be a “jack-of-all-trades” or (in medical terms) a “general practitioner.” Environmental science recognizes that all terrestrial life depends on soil for its existence. It is important that environmental specialists have at least a deep appreciation of the complex interactions of all of the various happenings occurring within the “soil sphere” (no pun intended).

The chemical composition and physical structure of the soil at any given location is determined by: (a) the type of geological material from which it originated, (b) the vegetative cover, (c) length of time that the soil has been weathered, (d) topography, and (e) the artificial changes caused by human activities. Land surfaces almost everywhere are covered by this unconsolidated debris (soil) sometimes called the regolith. This blanket above the bedrock may be very thin, or it may extend to depths of hundreds of feet. Its physical and chemical composition may vary, not only horizontally, but vertically, and its geological origin is not always the same, even within the local area. Some soils are formed from the bedrock (or other geological material) which immediately underlies it. Other soils develop as the result of transported materials being deposited in their current location by the action of water, ice, or wind.

Regardless of the origin, most soils consist of four basic components: mineral matter, water, air, and organic matter. These materials are present in a fine state of subdivision (individual particles) and intimately mixed. In fact, the mixture is so completely blended that separation is difficult. The more solid part of the soil, and naturally the most noticeable, is composed of mineral fragments in various stages of decomposition and disintegration. A variable amount of organic matter, depending on the horizon observed, is blended with inorganic substances. Normally, the largest amount of organic matter is in the surface layer.

The mineral material has its genesis in the parent material. Some soil minerals persist almost intact, while others are quickly transformed into new minerals in response to the current soil environment. Various sizes of mineral particles occur, ranging from coarse sand (2 mm in diameter) to finely divided clay particles (< 0.002 mm in diameter).

Organic matter represents the accumulation of plant and animal residues (generally in an active state of decay), especially in shallow horizons. Most organic matter is distributed in finely divided clay-sized particles. The organic content of soil ranges from less than 0.05% to greater than 80% (in highly organic soil such as peat), but is most often found between 2 and 5%.

All soil, even the most dense, has significant pore volume between soil grains. Soil pores are variable as to continuity, local dimensions, and total volume. These void spaces may be filled with air or water. The proportion of water-filled or air-filled pores is dependent upon the character of the soil, the conditions of formation, and when the last water was added to the system. Soft clay may have void spaces of over 60% by volume, while a well-compacted, uniformly sized sand can have pore volumes in the 25% range.

Water in the soil structure can be either transitory or fixed (at least temporarily). The part of the water that is retained is held with varying degrees of tenacity. Surface contact between the water and soil particles is the key to many chemical and physical reactions. Soil water is never without solutes (dissolved components) as its solvent properties, along with varying acidic and oxidation potential, causing it to be a major player in the dynamic nature of soil.

The general components of the soil atmosphere are nitrogen, oxygen, and carbon dioxide. Nitrogen is relatively inert unless fixed by soil bacteria. Free oxygen is present unless it has reacted with mineral matter or organic matter (especially living tissue).

Carbon dioxide can result from respiration of microbes, or abiotic chemical reactions. The concentration of carbon dioxide in the soil air is usually greater than the outside atmosphere. The proportion of oxygen, nitrogen, and carbon dioxide not only varies inversely with the amount of water present in the soil, but is extremely variable as to the ratio of the gases present.

Soils have significant variations in appearance, fertility, and chemical characteristics, depending on the mineral and plant materials from which they were formed and continue to be transformed. The soil realm has been compared to an elaborate chemical laboratory where a large number of reactions occur simultaneously. A few of the reactions are relatively simple and well understood, while the vast majority are not yet completely explained. The reactions range from simple solution and substitution to complex biologically mitigated multistep processes. Many reactions depend on the participation of water, mineral, and biological factors in a dynamic setting.

Soil, then, is a dynamic environment; almost a living structure. Continuous processes (although relatively slow) are active in even the most remote settings. Soil has been called “the bridge between life and the mineral world.” All life owes its existence to a few elements that must be ultimately derived from the earth’s crust. After weathering and other processes create soil, plants (including microbes) perform the intermediary role of assimilating these necessary elements, making them available to animals and humans.

All mineral energy sources on earth, as we know it, come from plants that have grown in the soil while obtaining their energy from the sun. Most “natural” materials used by man are derived in some way from the soil.

As concern for the environment increases, it is comforting to recognize that soil, when properly used, can offer an unlimited potential for disposal and recycling of waste materials. Knowledge of physical, chemical, and biological reactions is more important to us today than ever before.

CHAPTER 2

Classification Systems

2.1 INTRODUCTION

The ability to describe a soil with sufficient clarity so that a fellow observer can recognize the soil is an age-old problem. The level of detail required is usually based primarily on the purpose of the description. If, for example, a gardener is writing to a newspaper horticulture columnist, a sufficient description for a soil might be “sandy loam.” That level of detail, along with the geographic location, may enable the columnist to conclude that the garden would benefit from the addition of lime, water, and fertilizer to improve green bean production.

On the other hand, an environmental geologist making a detailed investigation of a potential hazardous waste disposal site should (ideally) invest a considerable amount of time and effort describing the minute details of the soil column. The appropriate level of detail in this case will most assuredly include visual observations of structure, texture, color, and apparent soil type, plus laboratory analysis of particle size distribution, and a host of chemical components.

A common practice of inexperienced field professionals is the assumption of values for these parameters based on average characteristics reported in the literature. The probability of failure of a remediation design based on assumed values is great.

Classification of soils should be based on the needs of the user. If insufficient detail or accuracy is presented, the user must either reinvestigate the site, or design the final use with enough safety factors to compensate for weak data. At the other extreme is the scholarly approach that generates more data than the project demands, or the client is willing to afford.

Several of the most common systems of soil classification are presented in this chapter. While each system was developed for a specific purpose, some data included in each can be transferred to other uses. The focus of the discussion in this chapter is oriented to application needs of environmental specialists.

2.2 PARTICLE SIZE ANALYSIS

For environmental purposes, a soil is defined as virtually every type of non-cemented or partially cemented inorganic material found in the subsurface. This wide definition often includes the entire column that can be drilled by a drilling rig equipped with hollow stem augers. Often, weathered shale, sandstone, and other semiconsolidated materials are considered soil for environmental and construction purposes. While not a sophisticated definition, it has a practical value because it defines material that behaves like soil and can be excavated with common earthmoving equipment.

Most subsurface materials that cannot be augured are sufficiently brittle to contain natural fractures. Fluids (gas or water) migrating preferentially through fractures have limited contact with most of the soil grains, and thus have very few opportunities to interact with each grain.

Soil is composed of two basic components: solid matter and pore fluids (water and soil atmosphere). Since pore fluids are usually relatively minor components, they are considered secondary as far as classification is concerned. Solid particles consist of mineral grains or organic matter of various sizes and shapes, occurring in a variety of organizational forms.

Solid particles can be divided into various components based on their size and shape (i.e., sand, silt, or clay), each of which interacts with water to give the soil its individual physical characteristics. The following sections discuss the procedures used to describe the physical characteristics in a scientific manner.

2.3 PARTICLE SIZE DISTRIBUTION

Probably the most basic tool of soil description is that of the size range of mineral components. A casual observer may indicate that a soil is “sand,” “c...

Table of contents

• Cover
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Chapter 1 The Fundamental Concept of Soil
• Chapter 2 Classification Systems
• Chapter 3 Soil Mineralogy
• Chapter 4 Soil Mechanics
• Chapter 5 Soil Physics
• Chapter 6 Soil Chemistry
• Chapter 7 Microbiology
• Chapter 8 Sampling Techniques
• Chapter 9 Selection of Analytical Procedures
• Chapter 10 Agricultural Considerations
• Chapter 11 Management, Presentation and Interpretation of Soil Data
• Chapter 12 Case Histories and Applications
• Appendix A Field Methods for Soil Classification According to the USCS System
• Appendix B Glossary of Soil Terms
• Bibliography
• Index