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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 componentsof soil. Thus, it is desirable to have an understanding of the origin of parent material when considering theorigin of soils.

Soil parent material may be referred to as a geological formation (a layer of material varying from a fewfeet to many feet in thickness). Wherever a geological formation is exposed at the earth’s surface, eithersoil 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 plantlife. Parent material from which these productive soils are formed may be classified as residual, trans-ported, or cumulose (see Figure 1).

* Underlined words are defined in the Glossary of Terms.

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Residual Materials

Residual materials develop from underlying bedrock materials. Most rock formations at the earth’s surfaceare sedimentary. Examples of sedimentary formations are limestone, sandstone, shale, and slate. Otherrock formations are classified as either igneous or metamorphic. Metamorphic rocks include marble andslate, 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, ormarine materials. Alluvial materials are sediments that were transported by flowing water of streams andrivers. Lacustrine materials are sediments that were deposited into freshwater lakes due to erosion of sur-rounding uplands during the glacial period. The greatest lacustrine deposits in the United States border theGreat Lakes. Marine materials are sediments that were washed out to sea by rivers and then later liftedabove sea level. The principal marine sediments in the United States occur along the Gulf Coast and theAtlantic Coast.

Parent material that was transported from its place of origin by wind is classified as either eolian depositsor loess deposits. Eolian deposits are sand dunes or deposits that were formed by dust storms. Loess depos-its are soil materials that were deposited during interglacial periods. Loessial soil materials are silt-loam intexture 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 threetopography types: moraines, till plains, and outwash plains. Geologists claim that ice deposits wereformed four times during our country’s geologic history. A glacier, one mile thick in places, movedsouth from Canada and covered the northern United States. As the ice moved, it ground rocks, gougedvalleys, 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 containunderlying 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 ava-lanches 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 andmuck. Peat is the term used for cumulose materials when the plants are recognizable. Muck describes thecumulose materials when plants are decayed beyond recognition.

Weathering

Parent material is constantly being transformed into soil as rocks and minerals are weathered by bothphysical and chemical processes. Physical weathering processes include constant temperature changes,climatic conditions, rainfall, erosion, and action of animals, plants, and humans. Chemical processes in-clude such factors as oxidation, reduction, solution, hydrolysis, hydration, and carbonation.

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SOIL COMPOSITION

It should be emphasized at this point that soils develop from parent material by processes of soil formationthat differ from processes of rock weathering, which produce the parent material. As a soil develops fromparent material, certain changes occur that produce the various layers of soil that are characteristic of thesoil’s profile.

Soil consists of living and nonliving materialsin three forms: solid, liquid, and gas. Organicmatter and mineral matter in soil are the solids.Air and water, which move through soil pores,are the gas and liquid. Approximately 45% of asoil’s volume is mineral matter, approximately5% is organic matter, and approximately 50% iswater and air (see Figure 2).

Soil minerals are inorganic and contain approxi-mately sixty-four elements. These soil minerals,with the exception of those introduced by hu-mans through fertilizers and lime, come fromweathered parent material and decayed plants andanimals. Sixteen elements are essential to plantgrowth. These elements are carbon, hydrogen,oxygen, phosphorus, potassium, nitrogen, sul-fur, calcium, iron, magnesium, chlorine, copper,boron, manganese, molybdenum, and boron. Airand water provide carbon, hydrogen, and oxy-gen; 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 arehigh in soil microorganisms, which affect the decomposition rate of plants. Microorganisms obtain theirenergy, as do animals, by oxidizing organic matter. The amount of decayed organic matter, often calledhumus, occurring naturally in the soil depends on climatic conditions and water drainage. Dry soils inwarm 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 highorganic 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

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Water and air (liquid and gas) in the soil are necessary for plant growth. Soil adsorbs a great amountof water during rainfall, and water that is not adsorbed by the soil drains away unless measures aretaken to conserve the soil. The amount of moisture and air in the soil varies depending on climaticconditions and water drainage. As soil adsorbs water, less air becomes available in the soil; as soildries, air replaces the water. Plants receiving excessive water for a long period of time usually sufferor die because of oxygen deficiency. Soil in good physical condition drains better and holds more airthan 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 changesoil properties or characteristics. However, agricultural producers can make considerable improvementsin 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 soilprofile. Topsoil colors are classified as dark, moderately dark, light, or very light. Subsoil colors are clas-sified 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 organicmatter. Light-colored soils generally are low in organic matter content, high in sand particles, or containreduced iron or manganese oxide.

Soil drainage refers to the movement of water through the soil layers or off the soil surface. Drainageaffects soil color and determines kinds of crops that can be planted. A well-drained soil is red or brown incolor; a fair-drained soil is yellow in color; and a poorly drained soil is gray in color. Mottled subsoilindicates 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 coarsenessof soil particles determined by the percentages of sand, silt, and clay in the surface layer of soil. Soilparticle size is important because it affects the water-holding capacity and workability of soil. Sand, thelarger particles in a soil, can be seen with the naked eye. Soils with a high percentage of sand are infertilebecause 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, theyprovide few nutrients for growing plants. After rainfall, topsoil with a high percentage of silt tends to runtogether 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 moisturethan sand or silt. Clay particles are microscopic in size; their expansion and contraction depend on theamount of moisture present. In presence of water, a film of moisture separates clay particles; as the mois-ture evaporates and the soil becomes dry, the soil contracts and leaves cracks in the ground. Clay soils dryout slowly and become cloddy unless properly managed.

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Most soils contain mixtures of sand, silt, and clay; it isunusual to find a soil containing only one of these. Soiltextural classes denote the average or combined effect ofthe soil particle sizes in a given sample. In the laboratory,a mechanical process of separating the soil into groups ofgrain sizes determines soil texture. Further analysis is usedto determine the percentages of different size groups ofsoil particles. Diameter ranges (see Figure 3) determinethese groups of soil particles.

Each of the soil particles has a distinctive “feel.” Withknowledge of twelve soil textural classes and some ex-perience, one can determine, by “feel,” the texture of asoil. By working some moist soil in the hands, differentparticle sizes can be felt. Sand particles feel gritty andcoarse and will form a mold; however, molded sandmay fall apart when the hand is opened. Silt particlesare sticky and gummy in moist soils but floury in drysoils. As moist silt particles are worked between thethumb and index finger, they stick together and fall offthe finger in large pieces. Clay particles are very stickywhen wet. When worked between the thumb and indexfinger, moist clay forms a ribbon of soil up to one inchin length before it falls off the finger. (Refer to page 12for the ribbon test.)

For convenience in determining the texturalname of a soil from the mechanical analy-sis, an equilateral triangle has been adopted(see Figure 4). The textural triangle givespercentages of sand, silt, and clay in each ofthe textural classes.

Because of their large size, sand and largesilt particles have a low specific surface (sur-face area per unit mass). Because most ofthe important chemical and physical reac-tions in soil take place at the particle sur-face, the amount of surface area affects theability of soils to react to chemicals. Soil witha high percentage of large particles lacksplasticity and cannot retain large amounts ofwater or nutrients. However, the organicmatter content of the soil can be increased,thus increasing the amount of nutrients andmoisture available to plants. Soil containinga high percentage of clay or fine silt particleshas greater chemical activity necessary tomaintain good soil structure.

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SOIL STRUCTURE

Soil structure, the arrangement of soil particles into various sizes and shapes, is important because it affectssoil tilth. Agricultural producers can control soil structure through methods of cultivation, times of cultiva-tion, and applications of organic matter and lime.

Soils with good structure contain a large number of crumbs or aggregates, which indicates good tilth. Goodsoil 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, heavygrowth of grass and securing a sample from an area in continuous cultivation. The sample from the grassyarea will be grainy and contain a large amount of aggregates. The sample from the cultivated field will befirm, hard, and lack aggregates, or crumbs. Soils scientists classify structural aggregates of soils into threemain 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 canbe observed by separating a handful of soil. A loose soil is granular in structure. The other structure typesare reserved for clumps of soil that have to be broken apart. If the cracks or lines of cleavage are mostlyvertical, the soil has a prismatic or columnar structure. When cleavage is mostly horizontal, a platy struc-ture exists. If cleavage is about equal (both horizontal and vertical cracks), the soil has a blocky or subangularblocky structure.

Classes of Soil Structure

Soil structure class is determined by sizes of soil aggregates. The three common classes of soil structure arecoarse, 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 bymeasuring 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). Otherdescriptive grades of soil structure are weak, moderate, and strong.

In order to describe soil structure, scientists named each structural shape that soil particles could arrangethemselves. Six common types of soil structure are single grain, blocky, platy, massive, prismatic, andgranular (see Figure 5).

Soil structure influences many important functions of soil. One such function is infiltration rate ofwater. Soil with a single grain or granular structure possesses a rapid infiltration rate. Soil with ablocky or prismatic structure has a moderate infiltration rate. Platy and massive soil structures haveslow infiltration rates.

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Because soil structure is important to soil management, agricultural producers and students should becomefamiliar with the procedure by which soil particles are grouped into aggregates by organic polymers. Whileone polymer arranges soil particles loosely into clusters, another polymer binds particles firmly into aggre-gates. Before soil particles can be bound into aggregates, they must be grouped into loose clusters. Floccu-lation, the first step in forming a soil aggregate, is caused by nature. Flocculation pulls clay particles closetogether and allows them to be cemented with organic polymers. The second step in aggregate formation isthe binding of clay particles in the flocs into aggregates.

Removal of plant cover from the soil surface destroys aggregation and organic sources of cementation. Thelonger the soil is without a cover, the more break down there is in the aggregates. As plants return cover tothe soil surface, aggregates start to rebuild and the break down eventually stops.

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SOIL CONSISTENCE

Consistence refers to cohesiveness of soil (or its resistance to rupture). Soil consistence and structure areinterrelated. Structure deals with shape, size, and distinctness of natural aggregates while consistence re-fers to the force required to rupture soil material.

Consistence is described under wet, moist, or dry soil moisture conditions. When the soil is wet, consis-tence is described by degrees of stickiness and plasticity. A soil’s friability and firmness, its tendency tobreak into smaller masses, characterized consistence under moist conditions, and its ability to cohere againwhen 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 fertilitylevels, plant food must be returned to soil if it is to remain fertile. As soil nutrient levels decline, plantgrowth decreases.

Soil productivity is the present capability of soil to produce a specified plant or sequence of plants under adefined set of management practices. Productivity is measured in terms of outputs or harvests in relation toproduction 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 climaticconditions 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.

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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. EnglewoodCliffs, NJ: Prentice Hall.

Donahue, R., R. Follet, and R. Tulloch. Our Soils and Their Management. Danville, IL: The InterstatePrinters and Publishers, Inc.

GLOSSARY OF TERMS

Cleavage - The tendency of crystalline rocks or mineral particles to split along thin parallel lines intothin 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 largermasses 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 soilprofile are formed.

Polymer - A chemical compound or mixture of compounds formed by a combination of moleculesforming larger molecules.

Soil profile - A vertical section of soil from the surface through all its horizons.

Soil tilth - The condition of the soil structure.

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SELECTED STUDENT ACTIVITIES

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

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

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

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

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

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6. 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.

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

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

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

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

7. 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?____________________________________________________________________________

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

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

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

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ADVANCED ACTIVITIES

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

1. Soil falls apart; will not form a ball: sand

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

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

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

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

2. After determining the textural class, refer to the soil triangle (see Figure 4). Use the triangle todetermine possible percentage of clay, silt, and sand. Discuss possible soil characteristics. Be sureto 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?

3. Soil texture and composition has played a major role in the success or failure of many man-madestructures. The Teton Dam in Idaho is an example of structural failure because of soil compositionand texture. Research and report on the failure of this dam. What caused the dam to fail? What werethe results of this failure? How could it have been prevented?

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Reproduction prohibited without written permission.Instructional Materials Service

Texas A&M University2588 TAMUS

College Station, Texas 77843-2588http://www-ims.tamu.edu

2001