SOIL SCIENCE 8382 COMPONENTS AND PROPERTIES OF SOIL ORIGIN OF SOIL Agronomists customarily refer to the origin of parent material* of soils when refe...
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COMPONENTS AND PROPERTIES OF SOIL ORIGIN OF SOIL Agronomists customarily refer to the origin of parent material* of soils when referring to the components of soil. Thus, it is desirable to have an understanding of the origin of parent material when considering the origin of soils. Soil parent material may be referred to as a geological formation (a layer of material varying from a few feet to many feet in thickness). Wherever a geological formation is exposed at the earth’s surface, either soil formation is taking place or soil has already developed from the exposed material. Productive soils develop from parent materials that supply the essential elements needed to support plant life. Parent material from which these productive soils are formed may be classified as residual, transported, or cumulose (see Figure 1).

* Underlined words are defined in the Glossary of Terms. -1-

Residual Materials Residual materials develop from underlying bedrock materials. Most rock formations at the earth’s surface are sedimentary. Examples of sedimentary formations are limestone, sandstone, shale, and slate. Other rock formations are classified as either igneous or metamorphic. Metamorphic rocks include marble and slate, while granite and basalt are considered igneous rocks. Transported Materials Parent material that was transported from its place of origin by water is classified as alluvial, lacustrine, or marine materials. Alluvial materials are sediments that were transported by flowing water of streams and rivers. Lacustrine materials are sediments that were deposited into freshwater lakes due to erosion of surrounding uplands during the glacial period. The greatest lacustrine deposits in the United States border the Great Lakes. Marine materials are sediments that were washed out to sea by rivers and then later lifted above sea level. The principal marine sediments in the United States occur along the Gulf Coast and the Atlantic Coast. Parent material that was transported from its place of origin by wind is classified as either eolian deposits or loess deposits. Eolian deposits are sand dunes or deposits that were formed by dust storms. Loess deposits are soil materials that were deposited during interglacial periods. Loessial soil materials are silt-loam in texture and are found mainly in the North Central United States. Parent material that was transported from its place of origin by ice is commonly classified into three topography types: moraines, till plains, and outwash plains. Geologists claim that ice deposits were formed four times during our country’s geologic history. A glacier, one mile thick in places, moved south from Canada and covered the northern United States. As the ice moved, it ground rocks, gouged valleys, and scoured loose materials off hills mixing them with rocks. This mixture called glacial till, contained fine particles, stones, and huge boulders. Unlike residual material, glacial till did not contain underlying bedrock. Soil materials transported by gravity exist to some extent at the base of all slopes. These soil materials, called colluvial materials, are noticeable in mountainous topography where rock slides, slips, and avalanches are common. Cumulose Materials (Organic) - Peat and Muck Cumulose materials are plant residues that were developed in shallow lakes and are referred to as peat and muck. Peat is the term used for cumulose materials when the plants are recognizable. Muck describes the cumulose materials when plants are decayed beyond recognition. Weathering Parent material is constantly being transformed into soil as rocks and minerals are weathered by both physical and chemical processes. Physical weathering processes include constant temperature changes, climatic conditions, rainfall, erosion, and action of animals, plants, and humans. Chemical processes include such factors as oxidation, reduction, solution, hydrolysis, hydration, and carbonation.


SOIL COMPOSITION It should be emphasized at this point that soils develop from parent material by processes of soil formation that differ from processes of rock weathering, which produce the parent material. As a soil develops from parent material, certain changes occur that produce the various layers of soil that are characteristic of the soil’s profile. Soil consists of living and nonliving materials in three forms: solid, liquid, and gas. Organic matter and mineral matter in soil are the solids. Air and water, which move through soil pores, are the gas and liquid. Approximately 45% of a soil’s volume is mineral matter, approximately 5% is organic matter, and approximately 50% is water and air (see Figure 2). Soil minerals are inorganic and contain approximately sixty-four elements. These soil minerals, with the exception of those introduced by humans through fertilizers and lime, come from weathered parent material and decayed plants and animals. Sixteen elements are essential to plant growth. These elements are carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulfur, calcium, iron, magnesium, chlorine, copper, boron, manganese, molybdenum, and boron. Air and water provide carbon, hydrogen, and oxygen; the others come from the soil. Organic matter, another soil solid, is decomposed plant and animal matter originating from plant roots, above-ground crop residue, green mature crops, and livestock manure. Soils high in organic matter also are high in soil microorganisms, which affect the decomposition rate of plants. Microorganisms obtain their energy, as do animals, by oxidizing organic matter. The amount of decayed organic matter, often called humus, occurring naturally in the soil depends on climatic conditions and water drainage. Dry soils in warm climates have less organic matter than do wet soils in cool climates. The physical features, called tilth, of the soil are affected by the amount of organic matter in the soil. A high organic matter content provides the following benefits: • • • • • • • • •

Increases the soil’s porosity Supplies nitrogen and other nutrients to growing plants Holds water in the soil to protect growing plants against droughts Aids in soil moisture content management Furnishes food for soil organisms Serves as a store house for mineral nutrients Minimizes leaching of nutrients Serves as a source of nitrogen and growth promoting substances Improves the soil’s structure


Water and air (liquid and gas) in the soil are necessary for plant growth. Soil adsorbs a great amount of water during rainfall, and water that is not adsorbed by the soil drains away unless measures are taken to conserve the soil. The amount of moisture and air in the soil varies depending on climatic conditions and water drainage. As soil adsorbs water, less air becomes available in the soil; as soil dries, air replaces the water. Plants receiving excessive water for a long period of time usually suffer or die because of oxygen deficiency. Soil in good physical condition drains better and holds more air than does soil in poor physical condition. PROPERTIES OF SOIL Although soils in general contain the same components, they differ in their properties or characteristics. These differences affect or determine the management of soils. Soils differ in color, texture, structure, consistence, and fertility and productivity. Many years of proper soil management are required to change soil properties or characteristics. However, agricultural producers can make considerable improvements in the structure, consistence, and fertility and productivity of their soils. Soil Color Color is one of the most easily recognized soil characteristics and is very apparent in the horizons of a soil profile. Topsoil colors are classified as dark, moderately dark, light, or very light. Subsoil colors are classified as red, yellow, brown, gray, or mottled. Color usually is a good indicator of organic matter content, moisture availability, and texture of a soil. The dark color of topsoil usually indicates presence of organic matter. Light-colored soils generally are low in organic matter content, high in sand particles, or contain reduced iron or manganese oxide. Soil drainage refers to the movement of water through the soil layers or off the soil surface. Drainage affects soil color and determines kinds of crops that can be planted. A well-drained soil is red or brown in color; a fair-drained soil is yellow in color; and a poorly drained soil is gray in color. Mottled subsoil indicates it is saturated with water at certain times of the year and dry at other times of the year. Soil Texture Soil texture refers to the sizes of individual soil particles. It is often thought of as the fineness or coarseness of soil particles determined by the percentages of sand, silt, and clay in the surface layer of soil. Soil particle size is important because it affects the water-holding capacity and workability of soil. Sand, the larger particles in a soil, can be seen with the naked eye. Soils with a high percentage of sand are infertile because they do not provide nutrients to growing plants. Sandy soils dry out quickly and are droughty. Silt particles are between sand and clay particles in size. Until they weather to fine-sized particles, they provide few nutrients for growing plants. After rainfall, topsoil with a high percentage of silt tends to run together and forms a surface crust. Silt type soils erode very easily. Clay, the smallest of the soil particles, holds soil nutrients, influences soil acidity, and holds more moisture than sand or silt. Clay particles are microscopic in size; their expansion and contraction depend on the amount of moisture present. In presence of water, a film of moisture separates clay particles; as the moisture evaporates and the soil becomes dry, the soil contracts and leaves cracks in the ground. Clay soils dry out slowly and become cloddy unless properly managed.


Most soils contain mixtures of sand, silt, and clay; it is unusual to find a soil containing only one of these. Soil textural classes denote the average or combined effect of the soil particle sizes in a given sample. In the laboratory, a mechanical process of separating the soil into groups of grain sizes determines soil texture. Further analysis is used to determine the percentages of different size groups of soil particles. Diameter ranges (see Figure 3) determine these groups of soil particles. Each of the soil particles has a distinctive “feel.” With knowledge of twelve soil textural classes and some experience, one can determine, by “feel,” the texture of a soil. By working some moist soil in the hands, different particle sizes can be felt. Sand particles feel gritty and coarse and will form a mold; however, molded sand may fall apart when the hand is opened. Silt particles are sticky and gummy in moist soils but floury in dry soils. As moist silt particles are worked between the thumb and index finger, they stick together and fall off the finger in large pieces. Clay particles are very sticky when wet. When worked between the thumb and index finger, moist clay forms a ribbon of soil up to one inch in length before it falls off the finger. (Refer to page 12 for the ribbon test.) For convenience in determining the textural name of a soil from the mechanical analysis, an equilateral triangle has been adopted (see Figure 4). The textural triangle gives percentages of sand, silt, and clay in each of the textural classes. Because of their large size, sand and large silt particles have a low specific surface (surface area per unit mass). Because most of the important chemical and physical reactions in soil take place at the particle surface, the amount of surface area affects the ability of soils to react to chemicals. Soil with a high percentage of large particles lacks plasticity and cannot retain large amounts of water or nutrients. However, the organic matter content of the soil can be increased, thus increasing the amount of nutrients and moisture available to plants. Soil containing a high percentage of clay or fine silt particles has greater chemical activity necessary to maintain good soil structure. -5-

SOIL STRUCTURE Soil structure, the arrangement of soil particles into various sizes and shapes, is important because it affects soil tilth. Agricultural producers can control soil structure through methods of cultivation, times of cultivation, and applications of organic matter and lime. Soils with good structure contain a large number of crumbs or aggregates, which indicates good tilth. Good soil structure permits deep root penetration and a large particle area from which plants can secure nutrients. Two different soil structures can be compared by securing a soil sample from an area with a dense, heavy growth of grass and securing a sample from an area in continuous cultivation. The sample from the grassy area will be grainy and contain a large amount of aggregates. The sample from the cultivated field will be firm, hard, and lack aggregates, or crumbs. Soils scientists classify structural aggregates of soils into three main categories based on their shapes, sizes, and strengths. Types of Soil Structure The main soil structure types determined by shape and arrangement of aggregates are granular, prismatic, columnar, blocky, subangular blocky, and platy. Evidence of any of these soil structure types usually can be observed by separating a handful of soil. A loose soil is granular in structure. The other structure types are reserved for clumps of soil that have to be broken apart. If the cracks or lines of cleavage are mostly vertical, the soil has a prismatic or columnar structure. When cleavage is mostly horizontal, a platy structure exists. If cleavage is about equal (both horizontal and vertical cracks), the soil has a blocky or subangular blocky structure. Classes of Soil Structure Soil structure class is determined by sizes of soil aggregates. The three common classes of soil structure are coarse, medium, and fine. Classes of soil structure also vary according to types of soil structure. Grades of Soil Structure Soil structure grade is determined by strength of aggregates. Soil aggregate strength is determined by measuring a soil’s resistance to crushing. Soil has no structure if coarse soil particles fail to cling together (single grain structure) or if fine soil particles break into large cement-like clods (massive structure). Other descriptive grades of soil structure are weak, moderate, and strong. In order to describe soil structure, scientists named each structural shape that soil particles could arrange themselves. Six common types of soil structure are single grain, blocky, platy, massive, prismatic, and granular (see Figure 5). Soil structure influences many important functions of soil. One such function is infiltration rate of water. Soil with a single grain or granular structure possesses a rapid infiltration rate. Soil with a blocky or prismatic structure has a moderate infiltration rate. Platy and massive soil structures have slow infiltration rates.


Because soil structure is important to soil management, agricultural producers and students should become familiar with the procedure by which soil particles are grouped into aggregates by organic polymers. While one polymer arranges soil particles loosely into clusters, another polymer binds particles firmly into aggregates. Before soil particles can be bound into aggregates, they must be grouped into loose clusters. Flocculation, the first step in forming a soil aggregate, is caused by nature. Flocculation pulls clay particles close together and allows them to be cemented with organic polymers. The second step in aggregate formation is the binding of clay particles in the flocs into aggregates. Removal of plant cover from the soil surface destroys aggregation and organic sources of cementation. The longer the soil is without a cover, the more break down there is in the aggregates. As plants return cover to the soil surface, aggregates start to rebuild and the break down eventually stops.


SOIL CONSISTENCE Consistence refers to cohesiveness of soil (or its resistance to rupture). Soil consistence and structure are interrelated. Structure deals with shape, size, and distinctness of natural aggregates while consistence refers to the force required to rupture soil material. Consistence is described under wet, moist, or dry soil moisture conditions. When the soil is wet, consistence is described by degrees of stickiness and plasticity. A soil’s friability and firmness, its tendency to break into smaller masses, characterized consistence under moist conditions, and its ability to cohere again when pressed together. Consistence of soil under dry conditions is characterized by degrees of hardness, maximum resistance to pressure, and inability of crushed material to cohere again when pressed together. SOIL FERTILITY AND PRODUCTIVITY Soil fertility is the ability of soil to provide nutrients for plant growth. Regardless of soil fertility levels, plant food must be returned to soil if it is to remain fertile. As soil nutrient levels decline, plant growth decreases. Soil productivity is the present capability of soil to produce a specified plant or sequence of plants under a defined set of management practices. Productivity is measured in terms of outputs or harvests in relation to production inputs (fertilizer, lime, etc.) for a specific kind of soil. The capability of a soil under a particular management system is dependent upon factors such as climatic conditions and the soil’s natural fertility, organic matter content, texture, and structure. Acknowledgements Kristi Falco, Graduate Assistant, Department of Agricultural Education, Texas A&M University, researched and developed this topic. Dr. Joe Dettling, Curriculum Specialist, Instructional Materials Service, Texas A&M University, edited and reviewed this topic. Vickie Marriott, Office Software Associate, Instructional Materials Service, Texas A&M University, edited and prepared the layout and design for this topic. Christine Stetter, Artist, Instructional Materials Service, Texas A&M University, prepared the illustrations for this topic.


REFERENCES Brady, N. and R. Weil. The Nature and Properties of Soils. Upper Saddle River, NJ: Prentice Hall, 1999. Donahue, R., R. Miller, and J. Shickluna. Soils: An Introduction to Soils and Plant Growth. Englewood Cliffs, NJ: Prentice Hall. Donahue, R., R. Follet, and R. Tulloch. Our Soils and Their Management. Danville, IL: The Interstate Printers and Publishers, Inc. GLOSSARY OF TERMS Cleavage - The tendency of crystalline rocks or mineral particles to split along thin parallel lines into thin sheets. Cumulose - Something that has accumulated or has been accumulated. Floc - A cluster of soil particles. Flocculation - The process by which suspended colloidal or very fine particles are assembled into larger masses or floccules which eventually settle out of suspension. Geological - Relating to the earth’s surface features. Infiltration - To pass into or through (a substance) by filtering or permeating. Moraine - An accumulation of soil and rock deposited by a glacier. Organic matter - The plant or animal residue at varying stages of decomposition in the soil. Parent material - The unconsolidated mass of mineral or rock from which the upper layers of the soil profile are formed. Polymer - A chemical compound or mixture of compounds formed by a combination of molecules forming larger molecules. Soil profile - A vertical section of soil from the surface through all its horizons. Soil tilth - The condition of the soil structure.



Identify the layers of a soil profile as shown in the illustration.


What are the three groups in which soil parent material is classified? ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________


List the four general components of an average soil and indicate the percentage of each part. ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________


What is the importance of soil particle size? ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________


What are the characteristics or properties by which soils differ? ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________

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Complete the following statements that relate to soil differences by circling the appropriate term(s). a.

The darker color of topsoil usually indicates the (absence, presence) of organic matter.


The color of a well-drained soil is more apt to be (brown, yellow, gray).


Soil texture is determined by the percentages of (sand, silt, clay) in a soil.


The arrangement of soil particles in various sizes and shapes determines soil (depth, structure, color).


Soil (consistence, productivity) refers to the cohesiveness of soil or its resistance to rupture or deformation.


Assume that you have three one-gallon containers filled with sand, silt, and clay, respectively. Which of the filled containers will hold the most water? ____________________________________________________________________________ ____________________________________________________________________________


Review the figure of the soil textural triangle in this topic. Which soil texture has about equal amounts of sand, silt, and clay? ____________________________________________________________________________ ____________________________________________________________________________


What important soil function does soil structure influence? ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________

10. Briefly explain how soil fertility and productivity are related. ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________

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Obtain a soil sample and determine the soil texture using the “ribbon test.” Begin by kneading a walnut size sample of moist clay into a putty-like consistency; adding water if necessary. Once you have reached the desired consistency, form the soil sample into a ball. Squeeze the ball between your forefinger and thumb, making a ribbon of soil. Make the ribbon as long as possible until it breaks under its own weight. Record your observations. Using the following criterion, interpret your observations. 1.

Soil falls apart; will not form a ball: sand


Soil forms a ball; will not form a ribbon: loamy sand.


Soil ribbon is dull and breaks off at less than 2.5cm long and a. Grinding noise can be heard; grittiness is prominent feel: sandy loam b. Smooth, floury feel prominent; no grinding can be heard: silt loam c. Only slight grittiness and smoothness; grinding not clearly heard: loam


Soil is exhibits moderate stickiness and firmness; forms ribbon 2.5 to 5 cm long and a. Grinding noise can be heard; grittiness is prominent feel: sandy clay loam b. Smooth, floury feel prominent; no grinding can be heard: silty clay loam c. Only slight grittiness and smoothness; grinding not clearly heard: clay loam


Soil exhibits dominant stickiness and firmness, forms shiny ribbon longer than 5 cm and a. Grinding noise can be heard; grittiness is prominent feel: sandy clay b. Smooth, floury feel prominent; no grinding can be heard: silty clay c. Only slight grittiness and smoothness; grinding not clearly heard: clay


After determining the textural class, refer to the soil triangle (see Figure 4). Use the triangle to determine possible percentage of clay, silt, and sand. Discuss possible soil characteristics. Be sure to answer the following questions in your discussion. Will this soil have a high nutrient content? How is this soil affected by rain? Is this soil likely to have increased chemical activity?


Soil texture and composition has played a major role in the success or failure of many man-made structures. The Teton Dam in Idaho is an example of structural failure because of soil composition and texture. Research and report on the failure of this dam. What caused the dam to fail? What were the results of this failure? How could it have been prevented? ALL RIGHTS RESERVED Reproduction prohibited without written permission. Instructional Materials Service Texas A&M University 2588 TAMUS College Station, Texas 77843-2588 2001 - 12 -

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