Making Soil Monoliths
How To Make Soil Models
Models of soils help us to observe soil properties and explain how soil they form. They are also valuable teaching tools when it is difficult to look at soils in the field. Soil monoliths are a kind of soil model that professional soil scientists use. A soil monolith is a vertical section that is removed from a soil profile in the field that is prepared for mounting. Other types of models of soil can also be made that would help capture its profile properties.Each soil profile is different because soils differ greatly from each other and from one place to another, due to interactions between the Five Soil-Forming Factors. Each section of soil on a landscape has its own unique characteristics. The face of a soil, or the way it looks if you cut a section of it out of the ground, is called a soil profile, like the profile of a person's face. Every soil profile is made up of layers called soil horizons. Soil horizons can be as thin as a few millimeters or thicker than a meter.Soil profiles and their horizons change as you move across a landscape, and also change as you move downward deeper into the soil at one location. In fact, soil samples taken at the surface may have entirely different characteristics and appearances from soil dug deeper in the soil profile. One common reason soil horizons are different as you dig deeper is because of mixing of organic material in the upper horizons and weathering and leaching in the lower horizons. Erosion, deposition, and other forms of disturbance might also affect the way a soil profile looks at a particular location. How many soil types are there? The United States Department of Agriculture (USDA) and other agencies map and collect soil data at many different scales. According to the USDA, there are over 50,000 different varieties of soil in the United States alone! Since parent material, topogrtaphy, climate, organisms, and the amount of time it takes for these to all interact varies worldwide, so soil profiles also vary worldwide.Having a soil model in you classroom might be a fun and informative way to learn about the soil in your area. There are many ways to create a model, ranging from the way professional soil scientists make them, to some simple methods to bring soil profiles into the class. Here are some options:
- Professional Method: Make a soil monolith.
- Easy Method: Put soil into a tube or a jar, and layer the horizons as they are extracted from the ground.
- Easy Method: Use the one meter long Hydrology turbidity tubes to fill with soil horizons, to get a full scale profile.
- Easiest Method: Draw a soil profile and color in the different horizons (perhaps using large pieces of paper glued to cardboard)
Let us know if you have discovered some other ways to make soil models that we can share!
Here is an example of a soil monolith in the making. This photo is from Thailand.
How To Make A Professional Soil Monolith Model
- Soil Monolith Procedures 1-5
- Soil Monolith Procedures 6-10
- Soil Monolith Procedures 11-15
- Soil Monolith Procedures 16-20
- Soil Monolith Procedures 21-23 and Final Procedures
Soil Monolith Procedures 1-5
1.) Excavate pit (4 feet deep) - a minimum of about 12 inches additional working space is required on each side of the profile extraction area. Moist soils are nearly essential. Smooth exposed profile face (this is the face you will eventually pick for your finished profile).
2.) Place a 2 foot angle-iron frame (with inside dimensions of 4" x 2" x 40") against exposed profile.
3.) Insert the cutting edge into the profile-face to mark the edges of the frame (hammer or other pressure).
4.) Remove frame and carve the profile to be captured from the pit face by removing soil from the area to the outside of the marked profile.
5.) Remove soil to a depth of at least 2" so that the frame fits perfectly over the sculptured profile.
Soil Monolith Procedures 6-10
6.) Attach a flat board (approximately 3/4" x 8" x 48") over the flat side of welded angle iron frame with C-clamps.
7.) Depress the frame over the sculptured profile being careful to allow no side- to-side or vertical slippage and maintain constant pressure until profile is free from the pit face.
8.) Excavate along both sides and the bottom of the frame with the objective of capturing 2-6 inches more profile than is eventually required to fill the frame.
9.) When both sides are essentially mined to the center, insert a tiling spade at the surface about 4-6 inches beyond the frame and complete the separation of the profile from the pit face. Care must be exercised to insure the sculptured profile does not slip downward or is otherwise pulled from the frame.
This is a very precarious step. The frame containing the separated profile is simply tilted away from the face in one smooth motion.
10.) Then it is lifted from the pit. Place the frame, board-side down, on a flat surface. Gently yet firmly depress the extruding profile downward into the frame and against the backing board.
Soil Monolith Procedures 11-15
11.) Carefully cut away, shave, or otherwise remove all soil above the cutting edge of the frame; a machete is an appropriate tool for this job. This is the surface that will be attached to the mounting board. (At this time the monolith may be transported in the frame if desired).
Average time to this point is about 3 hours, but it can range from 1-6 hours depending on the ease of digging, soil texture, etc.
12.) Prepare mounting board (cabinet grade plywood 3/4"x 8" x 48" is used) by outlining, in pencil, the 4" x 40" area to contain the monolith on both sides of the board.
13.) From the back side, drive approximately 24+ finishing nails (1" length) through the board being careful to stay more than 1" from the sides and about 2" from the top and bottom of the mount area.
14.) Apply a generous coat of Elmer's Contact Cement to the mounting board and protruding nails.
.jpg)
.jpg)
15.) Also apply a generous coat of Elmer's Contact Cement to the smoothed soil profile face exposed in the frame. (Usually, this must be accomplished in less than 15 minutes due to rapid drying time- some folks also choose to place 1-2 layers of cheesecloth between the board and the profile as an interface to hold additional glue.)
Soil Monolith Procedures 16-20
16.) Align mounting board with soil profile and depress board with protruding nails into the profile with firm even pressure. Take steps to insure board is squarely aligned with profile before applying firm pressure.
17.) Immediately invert frame and mounting board combination so that mounting board is now on the bottom. (Usually this step requires a coordinated effort between two people).
18.) Remove C-Clamps and temporary backing board from the frame.
19.) Place a piece of board (cut to pass through the metal frame) on the soil profile exposed inside the frame.
20.) Press down on the board and pull up on the frame, simultaneously, to remove the frame from the soil profile.
Soil Monolith Procedures 21-23
21.) As the frame is raised about 3/4", quarter-round molding can be placed (tacked lightly) along both sides and ends to hold profile intact until glue has dried. This step may or may not be needed depending on texture, soil moisture content, and disposition of the monolith. Ideally, if molding is used, it should be loosened before glue completely hardens although it can be removed later or allowed to remain.
(Monoliths can be transported at this time if care is exercised in keeping them flat and relatively free from drastic bumps).
22.) Ideally, the exposed profile is picked or dressed to show desired structure or finish before the soil dries; however, it can be remoistened for this process. (Dentists' picks, walnut picks, or other fine tools are used for this process . Avoid tool smear marks on finished surface - they will be very obvious on the finished monolith).
Pick the profile, allowing about 1-1 1/2 inches of soil to remain. Remove all loose soil particles from the dressing process with low air pressure.
23.) Allow soil monolith to air dry for 2-3 weeks - it is absolutely essential that all moisture be removed from the monolith before applying "hardening compound", otherwise very undesirable and almost irreversible chemical reactions occur.
HARDENING COMPOUND PREPARATION AND FINAL SOIL MONOLITH PROCEDURES
- Prepare hardening compound at least 24 hours in advance of pouring the monolith (consists of mixing 1 pint acetone- high strength- with 37 grams vinyl chloride/vinyl acetate resin {Union Carbide, Corp.}. This VC powder should dissolve completely - it may take 2-3 days to dissolve.
- In a well-ventilated area, pour the hardening compound over the soil, completely saturating the surface, and taking care not to "pond" more liquid in structural cavaties than can readily be absorbed. Due to textural differences and absorption rates, the amount to pour and elapsed time between pours varies considerably.
- The objective is to completely saturate the soil profile. Usually on fine-loamy soils, pour 2-3 times and use about 1-1 1/2 quarts of hardening compound per monolith. On some occasions, a second pour is done within one hour of the initial, followed by a third pour 24 hours later.
- Allow profile to dry for about one week. Remove excess glue, which usually requires sanding.
- Paint or finish mounting board as desired and apply soil name plaque and/or lettering, horizonation, depth, etc. (A flat, black background is best for emphasizing the soil profile).
Note: While there may be simpler, easier methods of extracting soil profiles and procedures requiring less time for preparing soil monoliths than this process, this process results in a better end product. Total time is estimated at 20-40 hours per each monolith.
Permission to digitize and display the images and text of "How to Make a Soil Monolith" was granted by Dr. Maxine Levin, USDA's Agricultural Research Service.
Web development by: Christy Spector