By Roseanne Williby
Time Allocated: 2 days
Soil is produced by soil forming processes that act on the materials deposited or accumulated by geologic activities. Soil characteristics are determined by physical and mineral composition of the parent material, climate during accumulation, plant and animal life on and in the soil, the relief or lay of the land, and length of time for soil formation.
The United States Department of Agriculture Soil Conservation Service in cooperation with the University of Nebraska Conservation and Survey Division has completed major fieldwork in 1963-1969 to compile soil survey information by county designation. The scientists looked for the kinds of soils, where they are located, and how they are can be used.
Soil suitability information obtained from the Soil Survey can be applied toward the selection of sites for farms, industry, construction of buildings, roads, and recreation. It is also used for the management of lands used in agriculture, ranching, and forestry.
For people who want a general idea of the soils in a county for comparisons, or for locating a large tract that is suitable for a certain kind of use, a general soil map with soil associations is provided. A soil association is a landscape that has a distinctive proportional pattern of soils composed of major and minor soil types. Refinement of a soil association is given as a soil series and soil profile. The soil profile is an important part of the description of each soil series. The soil profile is the sequence of layers from the surface downward to rock or other underlying material.
Understanding soil profiles is easier when they can be visually examined and evaluated. A soil micro-monolith is a miniature representation constructed for this purpose. A soil micro-monolith contains small-scale samples of the soil profile which are mounted for display. The soil micro-monolith was designed by Francis Belohavy, a research soil scientist at the University of Nebraska, Lincoln. This activity incorporates his design as part of this soil profile examination.
The soil profile is composed of soil horizons. The soil horizon is a layer of soil approximately parallel to the surface that has distinct characteristics produced by soil-forming processes. Such as:
- The O Horizon is a layer of organic matter on the surface of a mineral soil.
- The A Horizon is on the surface or just below an O Horizon. Living organisms are most active and are marked by the accumulation of humus. It may contain soluble salts and clay. This is also known as the topsoil.
- The B Horizon is below the A. This is in part a layer of change from the overlying A to the underlying C. There is often the accumulation of clay and humus, a blocky structure, redder or stronger colors than the A horizon. This is known as the subsoil.
- The C Horizon is below the B and is composed of the weathered rock immediately below and the soil is presumed to be like that from which the overlying horizons were formed.
- The R Layer is the consolidated rock beneath the soil. This rock usually underlies the C horizon, but may be immediately beneath the A or B horizon.
Students will create a detailed description of a soil profile through observations made in the field and the classroom, and create and display a scale model of the soil profile.
- several flat boards slightly larger than the 5 x 7 inch index cards for mounting the micro-monoliths (scrap wood can be easily retrieved from new home construction sites)
- yard sticks or meter sticks
- white glue (Elmer’s works well)
- 4 labeled containers for collecting soil samples
- Hand or garden spade
- Foam-core or poster board for mounting finished profile
- push pins
- newspaper to cover the work area and make clean-up easier
- magnifying glasses or stereomicroscopes
Procedure - In the Field
- Select a site for profile examination. Moist soil is the easiest to work with like that near a creek or river that has been cutting downward. If not available, new construction sites in initial stages of development will have exposed soil profiles.
- Examine the soil horizons. Labeling each, record the general location, depth in inches, color, and texture description of each horizon. (see data collection)
- Note the relief, recent land use, climate, and water drainage.(see data collection)
- Collect and label the samples of each horizon.
In the Classroom
- After returning to the classroom with the horizon samples and observations, proceed to create the micro-monolith.
- Allow the samples to dry for mounting and to identify texture and structure.
- Using the scale 1 inch = 1 foot, determine the length of each horizon on your micro-monolith. (divide by 12 to one decimal place)
- Tack the card onto the boards with the push-pins to avoid curling.
- Drawing a rectangle 2 inches wide x 6 inches long onto the card will delineate the outside edge of the monolith.
- With a pencil draw lines on the card to divide the rectangle into the correct scale lengths for each soil horizon within the profile. They should appear to be stacked one upon another in the same way as they were in the soil profile in the field.
- To prepare the card for the soil sample from each horizon, apply a layer of glue to the area of the card you will be working on, one horizon at a time.
- Sprinkle the soil over the glue. Invert the board and gently tap to remove any material not adhering to the glue. Continue with each horizon in the same manner.
- Blend between the horizons to give the profile a more natural appearance.
- Allow the card to dry over night.
- Mount the profile on foam-core or poster board.
- Label the monolith with the name of soil (check the soil survey book for the county of origin), the location, and features or horizons.
- Examine the top soil under a magnifying glass or microscope.
Teacher Helping Hints- How and When?
The placement for this activity should be early in a soil or ecology unit for it develops an understanding of how soils form.
Also, it was helpful to utilize the Nebraska Soil Map to show the soil classification scheme for local soils. Since it is from the soil associations, like those provided in the legion on the map, that soil profiles are created; the map is a helpful tool for seeing the larger scheme of the soil picture.
The most difficult portion of this activity is the preparation. Identifying a location with well-defined soil horizons can prove challenging since so often developers begin their work by removing the top soil. In addition, banks cut by rivers and creeks are often over-grown with vegetation, so that observation is difficult. So, it is very helpful to do quite a bit of scouting ahead of time, but close to the activity date, since the appearance of the landscape is so dynamic.
Since this activity begins outside, it is more suitable to perform the activity when the soil is fairly dry and the temperature moderate. Also, it will be most likely that transportation to a selected site is necessary. So it will be necessary to correlate weather, season, location, and transportation.
In lieu of transporting an entire class to a location, photographs or videotapes can be made of an appropriate site with field-view and close-up views of the profile. This could be done by the teacher ahead-of-time with a few interested volunteers after school. Samples of each horizon could be obtained at this time and transported to class.
Modifications or Clarification
When students begin working on creating the profile in the classroom, be sure to have extra glue available so that adequate amounts of glue may be applied to insure the quality of profile. It is important to stress that the glue should be applied generously so that the soil adheres to the card and appears dense enough for discerning the different soil horizons
Benefits of the Activity
In the pre-lab activity, it became evident that many students have had little opportunity to work with soils, so as a result only some of the students were aware of how soils form and what constitutes a soil horizon. By the time the activity was near completion, students had benefited from creating their own soil mico-monolith for it served as a manipulative by which the students could visualize the soil forming process. The micro-monolith also became a personal remembrance of this learning experience which some of them were proud to take home.
In the past, the teaching of this concept would have been simply presented through lecture and discussion without the benefit of a hands-on activity. This activity provided an initial lesson on soils so that further learning about soils could continue. This content was presented in conjunction with an Ecology unit when students study biomes. Students have demonstrated the benefit of this activity by their above average performance on this activity and section tests within this unit.