Teaching soil morphologyto introductory soil science students

M. J. Vepraskas,* P. A. McDaniel, and J. J. Camberato

ABSTRACT ly, students must be given an opportunity to gain "hands- on" .experience in describing soil morphological Introductorysoil sciencestudents should receive practicalin- characteristics. Students need to practice identifying and structionin soil morphologybecause it can be a tool to assess describing horizons on "real soils" where they can use soil limitations for variousland uses. "Realsoils" shouldbe soil characteristics such as color and texture (by feel) examinedwhenever possible. Soil corescollected froma topose- delineate horizons. Monoliths consisting of soil profiles quenceare well-suitedfor classroominstruction because the soils permanently glued.to boards or trays (Malo and Nielsen, frequently exhibit a wide range of morphologicalfeatures associatedwith differencesin drainage.Cores of soil 1.0 mlong 1985) are not well-suited for this purpose because only can be collected quickly using a hydrauliccoring machine and certain visual properties (e.g., color, structure) can can be stackedand stored conveniently on special trays. In the assessed. Other important morphological properties such laboratory,soil profile descriptionsare completedas wouldbe as texture and consistence cannot be evaluated in donein the field by describinghorizon type, thickness,color, monoliths. Use of soil pits for soil morphologyinstruc- texture, structure, etc. Toposequencecross-sections can be tion is ideal but can impose logistical problems in preparedfrom prof’tle descriptionsto showvariations in horizon transporting students to the site, providing adequate space propertiesdown the hillslope. Soil series andsoil boundaries for students to workin the pit, and waiting for reasonable can be readily seen in the cross-sections. Surveysof students weather to work outdoors. Such constraints often make in introductorysoil science coursesindicated that these exer- the use of soil pits impractical for 2-h laboratory sessions. cises using soil cores werean enjoyableand meaningful way An alternative to soil pits for teaching morphologyis to study soil morphology. to bring undisturbed soil cores into the laboratory where they can be evaluated and described by students (Lemme, 1983). If enough cores are taken, students can work in- OIL MORPHOLOGYis the branch of soil science that dividually or in small groups to complete exercises. While S deals with the identification and description of soil it is not feasible to let students workintensively on a large horizons. Many land management decisions for both numberof different soils, cores can be collected fromsoils nonagricultural and agricultural uses are based on soil that possess important characteristics commonto many conditions that can be determined by examiningsoil mor- soils. Such characteristics include having the major soil phology. For example, gray soil colors usually indicate horizons (Ap, E, and B) present, as well as having tex- the soil is seasonally waterlogged, and this condition can tures common to the area. Another important cause septic systems to malfunction, kill plants, produce characteristic that should be apparent amongcores is the wetness problems for basgments, and indicate the soil is change in soil color caused by seasonal waterlogging. Well susceptible to flooding (Buol et al., 1980). By learning and poorly drained soils (soils with udic and aquic to recognize gray soil colors, a soils student maybe alerted moisture regimes, respectively) must be managed dif- to these potential problemson land he or she is consider- ferently for various agricultural and nonagricultural uses, ing buying or is trying to manage. Soil morphology can and it is important that students be able to see the funda- also be used to help make tillage decisions. In some mental morphological differences between these soils. coarse-textured soils where the E horizons are compact All the above soil characteristics can be found in soils and form a tillage pan, the root system is restricted and along a toposequence. A toposequence is a group of yields maybe low during dry years due to plant moisture related soils that differ from each other primarily because stress (Vepraskas et al., 1987). Crop yields can be in- of topography as a soil-forming factor (SSSA, 1975). creased in these soils by subsoiling to rip or fracture the Soils along a hillslope comprise a toposequence, and often pan, but such tillage should only extend to the top of the differ from one another in drainage class (Fig. 1). B horizon to avoid eventually deepening the compacted believe that soil cores collected along a toposequencepro- layer (Trouse, 1983). These two examples illustrate that soil morphologyshould be of practical interest to begin- TYPICPALEUDULT ning soil science students. (Well- drained) Webelieve that soil morphology should be taught to I AOUlCPALEUDULT introductory soil science students. To do this effective- ~ ~,,~rotely Well- drained) M.J. Vepraskasand P.A. McDaniel,Dep. of Soil Science,Box 7619, ~ TYPIC PALEAQUULT NorthCarolina State Univ., Raleigh, NC27695-7619; and J.J...... -. ~ (Poorly-drained) Camberato,Tennessee Valley Authority, MuscleShoals, AL35661. Seasonal High ~" "--. ~’~, I ...... ~’~ ~.,. ~ " . 1 Paperno. 11390of the JournalSeries of the NorthCarolina Agric. WaterTable Res. Service, Raleigh, NC27695-7601. Received 26 Oct. 1987. ~.._~__ V_. - - -~7"~ *Correspondingauthor. Fig.1. Representativetoposequence consisting of threesoils withdif- Publishedin J. Agron.Educ. 17:93-96 (1988). ferentsoil moistureregimes and drainage classes.

J. Agron. Educ., Vol. 17, no. 2, 1988 93 vide the best examples for beginning students to use to of soil profile and major horizons were provided prior study soil morphology. In addition, the use of topose- to the laboratory. Criteria used to describe a soil core are quence soils allows some of the concepts of soil forma- shown in Table 1. Students usually worked in groups of tion to be integrated into a laboratory exercise. five or less in completing descriptions. This paper presents how basic soil morphological con- Students were instructed to begin profile descriptions cepts can be taught to introductory soil science students by dividing each core into horizons. This was done largely using soil cores collected along a toposequence. The pro- on the basis of color and texture. The Ap horizons were cedures discussed have been used successfully for more easily identified on the basis of color, and the upper than 7 yr. boundary of the B horizon was determined on the basis of texture by feeling for the depth at which the clay con- MATERIALS AND METHODS tent increased. The E horizons normally were present in the soils examined and were determined last. Transitional Soil cores were collected along a toposequence consisting of zones between horizons could be difficult to describe, three soils as shown in Fig. 1. Cores were collected from the because students had to decide whether these were three hillslope positions indicated by arrows. At each of the three separate horizons (e.g., EB) or were simply gradual or hillslope positions, cylindrical soil cores approximately 1 .O m diffuse boundaries between the major horizons. Horizon long and 0.08 m in diam. were collected using a hydraulic cor- ing machine (Giddings Machine Co., Fort Collins, CO).' Ex- boundary topography (e.g., smooth, wavy) cannot be tracted cores were stored in special trays (Fig. 2), which were realistically described from a core. Low chroma (Le., lined with aluminum foil. Each intact core was placed over the gray) drainage mottles were given special attention foil in a tray, and the foil was then tightly wrapped around the because of the obvious changes down the hillslope. Soil core. The foil served as a moisture barrier and kept the soil moist structure was usually weakly developed in the soils of the for several weeks. Without the foil, the cores would quickly toposequence used, but an estimate of the type of struc- dry out and become hard, making them too difficult to pick ture visible was made by the students. apart. The trays were designed so that they could be stacked Students used the profile descriptions of each core to when filled with a core (Fig. 3). This facilitated both transport plot a cross-section of the toposequence. The profile from the field as well as laboratory storage. In the laboratory, descriptions were first diagrammed on a sketch of the the trays can be stacked 10 high until needed. Ten soil cores were normally collected from each of the three hillslope (Fig. 4A), and then the horizons were connected soils over a 4-h period by three people. This number of cores was required for a class of approximately 100 students contain- ing seven laboratory sections of approximately 15 students each. Each laboratory section was provided with one set of three cores. Three extra sets of cores were collected for a variety of pur- poses including display, replacement of damaged cores, and sharing with other courses.

LABORATORY USE OF CORES

Each student was expected to prepare profile descrip- tions of the three soils of the toposequence. Definitions 'The use of trade names in this publication does not imply endorse- ment by the North Carolina Agricultural Research Service of the pro- ducts named, nor criticism of similar ones not mentioned.

hole for dowel used in stacking trays

MATERIALS A - PVC Pipe, 98mm I. D., cut in half B - Tray support, 51 x102mm (2x4in) stock C - Tray end, cut from tray support Pieces ossernbled with wood screws. l$ewc

MB

4.254 ml- Fig. 3. A stack of soil trays stabilized by wooden dowels. Trays filled Fig. 2. Materials and dimensions of trays used to store soil cores. with soil cores can be stacked at least 10 high using dowels.

94 J. Agron. Educ., Vol. 17, no. 2, 1988 amongsoils. The horizons and special features (e.g., Approximately 61070 of the students (both semesters mottles) were colored appropriately to show how soil combined)felt that use of soil cores for teaching soil mor- properties changed across the toposequence (Fig. 4B). phology was more instructive than use of soil monoliths. estimate of the seasonal high water table was made by Sixty-seven percent of the students felt they could iden- drawing a line across the upper zone of gray mottles or tify Ap, E, and B horizons in the field after having com- horizons of gray color. Soil series could be readily seen pleted the laboratory exercise. Seventy-six percent of the and identified on the cross-section. In addition, the students enjoyed working with soil cores, although some gradual changes of one soil into another were readily parts of the exercise were frustrating. Approximately20o70 apparent. of the students complained that some soil horizons The value of having students plot a toposequencecross- (primarily transitional horizons) did not clearly conform section (as in Fig. 4B) is that such diagrams showthem to any horizon definition, and they disliked having to howsoil properties systematically changealong a hillsope. decide which definition applied best. Although making Ideally, students notice that morphological changes such such decisions can be frustrating, it has to be done fre- as soil color are predictable across a landscape. Some quently whendescribing soils in the field. It is beneficial students have even examined soils across hillslopes out- that students realize early in their career that such deci- side of class to see whether the expected changes actual- sions must be made when describing soils. ly occurred. CONCLUSION STUDENT EVALUATIONS We have found use of soil cores from a toposequence During the spring semester of 1985, 74 students en- to be a relatively simple and highly effective wayto teach rolled in the introductory soil science course were asked soil morphologyto introductory soil science students. The to evaluate the laboratory exercise that used soil cores approach described could also be easily modified for use to teach soil morphology. During the spring semester of by advanced students in upper-level courses. The soil 1986, 76 students evaluated the sameexercise. The results cores have an advantage in that they can be used in a of the student surveys were similar both semesters. laboratory setting while allowing students to develop the The exercise followed a similar exercise where students descriptive skills that are used in a field situation. reviewed soil horizon definitions and tried to apply them to soil monoliths. Students were polled after the monolith ACKNOWLEDGMENT and core exercises were completed. The soil trays were madeby Fred Averette, whowas also responsible for key design modifications, whichmade the trays ruggedand easy to use. Fred’s help is gratefully acknowledged.

Table 1. Criteria used to describe soils for profile descriptions. 1. Horizon type--A, Ap, E, B, C 2. Horizon depths 3. Horizon boundary-The thickness of the zone between two horizons Abrupt: < 25 mm Clear: 25-64 mm Gradual: 63-125 mm Diffuse: > 125 mm 4. Horizon color--Dominant color of horizon 5. Presence of gray mottles {spots of gray color). Estimate amount of gray mottles Few: < 2%of horizon Common: 2-20% B Many: > 20% 6. Textural class--Estimate of amount of sand and clay. Use one of the W,T.\~°cBoooooo ".’"::... terms shown belowt High Low o o o o o o o o-o o ¯ o o_o_o~l~_ sand sand ,aooooooooooooooo ooooooooonooooooq Low clay Sand ,oooooooooooooooo oooooooooooonooo( Loamy sand :~oooooooooooO~,OOqoooooooooooooooo Sandy loam Loam Sandy clay loam Clay loam High clay Sandy clay Clay ~BROWN 7. Soil structure--Type and strength of aggregates T_.~)e Strength ~LIGHT BLACK I~ GRAY Single grain ~LIGHT BROWN E~JBROWN with GRAY MOTTLES Massive Weak Granular Moderate W.T. -- Seosonolhigh wofer toble Platey Strong Fig. 4. Toposequencecross-section. (A) sketch of toposequenc~ sup- Blocky plied to students; (B) completeddiagram after profile description data Silty textures are not found in the soils consideredand werenot considered were plotted and horizon variations diagrammedacross hillslope. in the exercise.

J. Agron. Educ., Vol. 17, no. 2, 1988 95