Biology of Soil - Lesson 11 - History of Soil
Target Grade Level / Age Range:
Students will understand the various ways soil is formed, soil formation factors, and the amount of time necessary to build or rebuild soils.
- Science journals
- Computer with internet connection
- Pan of brownies or cake
- Sprinkles or mini chocolate chips
- Hard serve ice cream (well frozen)
- Ice cream cones
- Plates, spoons, forks
- Iowa Soils information sheet printed
Essential Files (maps, charts, pictures, or documents)
- Iowa Soils information sheet: http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1029&context=farmprogressreports
- History of Soil.pptx
Vocabulary (with definitions)
- Glacial till: soil parent material deposited from a moving glacier
- Glaciation: the process, condition, or result of being covered by glaciers or ice sheets
- Soil profile: the vertical section of soil from the ground surface downward where the soil meets the underlying rock
- Parent material: the underlying geological material in which soil horizons form
- Leach: to drain away from or through soil by the action of percolating liquid, especially rainwater
- Loess: a loosely compacted yellowish-gray deposit of windblown sediment
- Alluvium: a deposit of clay, silt, sand, and gravel left by flowing streams in a river valley or delta
- Moraine: a mass of rocks and sediment carried and deposited by a glacier, typically along its edges
- Kettle hole: a landform made by a glacier, in which ice broke away, melted, and formed a depression, sometimes resulting in a lake
- Kame: a small, round hill of sediment like sand and gravel deposited by a glacier
- Esker: a long, thin hill of sediment like sand and gravel deposited by a glacier
Background – Agricultural Connections (what would a teacher need to know to be able to teach this content)
Soil is formed in many different ways, and can include many different characteristics. There are five soil forming factors: parent material, organisms (vegetation), climate, time, and relief (topography) (ClORPT). Because of these varieties, there is a whole taxonomy of naming soils that includes 12 different soil orders. Beyond that, there are prefixes and suffixes to determine if the soils are frozen, if they have a high salt content, if there is a histic epipedon, and many other complicated things. If students are interested in learning more about soil taxonomy, you can direct them to the USDA NRCS publications here: https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/survey/class/taxonomy/?cid=nrcs142p2_053580 The main soil order in Iowa is the Mollisol order. This is a temperate grassland soil with a dark surface layer (A horizon). The native prairie or grassland vegetation encourages the dark soil structure to build because of the abundant organic matter in the grasses and their roots.
On a more practical level, there are also soil series. These have common names, like Tama, Colo-Ely, Nicollet, Storden, etc. These soil series are mapped across the state. Soil scientists will use soil surveys like the Web Soil Survey ( https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm) to map an area of interest and learn more about the soil types in that area. Each soil series has a typical texture, color, or formation. They can be referred to by name, or by their map unit symbol, which is a number. This number may be followed by a letter, that refers to the slope of that land. For example, a Storden loam with 6-10% slopes would have a map unit of 62C.
The web soil survey is an interesting way to see the differences of soil throughout the state and across the country. Another interesting way to compare soil series across the country is with by comparing state soils via this link: https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/edu/?cid=stelprdb1236841. Each state has a state soil, which represents their state in some type of meaningful capacity. In many states, it’s the most common or most productive. At this website, you can visually see the differences in soil color, depth, and parent material. For example, the state soils in New Hampshire, Maine, and Vermont are shallower, have an E horizon characteristic of native forests, and have a much rockier subsoil. This is because of the soil formation factors they’ve had in the northeast as compared to the soil formation factors we’ve had in the Midwest.
This lesson will start with a short video from a professor showing some characteristics of various soil profiles. He will point out different horizons of soil and some characteristics of each. The lesson instructs you to pause at certain times to have the class speculate about why the soil may have these characteristics in these horizons. The top horizon of soil is the O horizon. This just includes organic material, like leaves that have fallen. Beneath that is the A horizon. This horizon is darker than the rest due to organic matter content. Below the A horizon is generally the B horizon. The B horizon is where clay particles accumulate. It is lighter in color and holds more water (because of the extra clay content). In some soils – primarily in forested areas – there is a thin, chalky, white layer in between A and B called the E horizon. We usually do not see this in Iowa, but you would see it more in places like New England. Beneath the B horizon, we will see the C horizon, or the soil parent material (what the surface soil formed from).
This lesson talks specifically about the soil formation factors as they relate to Iowa. Iowa’s climate is humid continental, giving us hot summers, cold winters, and adequate precipitation. This means that our soils freeze in the winter (during which time our soil life also takes a break), and thaw in the summer (during which time soil life becomes more active). Precipitation impacts erosion and leaching of nutrients and minerals through the soil profile.
Iowa’s native vegetation (or organisms, in the CLORPT mnemonic) is prairie or grassland. Iowa’s native vegetation had thick, deep root systems that reached into the sediment, provided structure, and contributed a huge amount of organic material. This provided lots of food for microbes, insects, and larger vertebrate animals that ate the plants and left waste on the soil surface.
Iowa’s relief or topography impacts the way it weathers. For example, in parts of the state that were glaciated pre-Illinoian period, hills were cut into the landscape. Over time, those hills eroded and became steeper, which we now see in southern Iowa with the Southern Iowa Drift Plain region. However, in the Des Moines Lobe region that was glaciated during the Wisconsinan period, the glacier moved much faster and leveled the land. Without pre-existing hills, erosion does not work as quickly.
Iowa has a few sources of parent material. Parent material is what the soil originally formed from, and can be seen in modern soils in the C horizon of the soil profile. Iowa’s main parent materials are loess (wind-blown sediment), glacial till, and alluvium (deposited by water). Here’s a chart from the USDA that explains the various soil parent material types (link to article in Sources section):
Time itself doesn’t change things, but time allows for the other soil forming factors to take place. Over time, weathering will take place according to the location’s climate, relief, and native vegetation. These processes can create and/or destroy soils over time. If we think about a residual parent material soil starting as bedrock, time is needed for organisms like moss and lichen to slowly break down that rock into smaller particles and organic material. However, soils also erode and leach over time, which can degrade them. In general, young soils are said to be shallower and have fewer soil horizons (maybe just a shallow A horizon and a C horizon). Old soils are more weathered, and minerals have leached, creating a deeper soil with more horizons that are distinctly different from each other.
After students learn about the soil forming factors, they will get to see some glaciation in action using a pan of brownies and an ice cream cone. The ice cream will act as a model glacier that will deposit ice cream and sprinkles over the brownies. Students should be able to see glacial landforms, like moraines, kettle holes, kames, and/or eskers.
A moraine is a hill where a glacier has pushed a pile of sediment. Think about laying fabric on a table and pushing on it. You will create a wrinkle at your fingertips. This is like a moraine. Kettle holes are low areas in otherwise flat areas where chunks of ice had broken off and melted, leaving a depression. Kames and eskers are both hills of miscellaneous unsorted sediment, though kames are smaller and rounder, and eskers are longer and thinner.
All of Iowa has been glaciated at one point or another. Iowa’s oldest glaciation covered the whole state between 500,000 and 2.5 million years ago. The glacier moved slowly (a couple of miles per year) and cut hills into the landscape. Over time, those hills were weathered, eroded, and have become steeper, especially in the southern half of the state.
Iowa’s newest glaciation is very new in terms of soil formation. The Laurentide Ice Sheet of the Wisconsinan Glaciation moved through the north-central third of the state about 10,500 – 30,000 years ago. It moved very quickly (about a mile each year), and leveled the land, leaving a flat landform that we now call the Des Moines Lobe of the Laurentide Ice Sheet (or colloquially, “The Lobe”).
During the Wisconsinan Glaciation, there were lots of extreme weather events happening. This weathered the hills to the east and west of the glacier to make them less steep, and more rolling. These are the Northwest Iowa Plains to the west and the Iowan surface to the east. This extreme weathering also caused so much erosion in the far northeast corner of Iowa that bedrock is now exposed. This area is called the Paleozoic Plateau.
The Loess Hills are another Iowa landform to mention, but are an indirect product of glaciation as opposed to a direct descendant from it. The Loess Hills started forming during the Wisconsinan Glaciation because when the glacier started melting, the Missouri River got quite full. As the water levels went back down, there were excess deposits of glacial till that were now susceptible to wind. As we know, there are three soil particle sizes, and the wind is not very effective at moving sand (the largest), so that stayed put. The wind was very effective at moving clay (the smallest), and that was distributed across the whole state. The silt, however, (the “baby bear” size of this Goldilocks story) was moved from the river bed to just a few miles away from the river, creating a band of interestingly shaped hills of rich, yellow-colored silt.
Iowa is also home to two alluvial plains along the Missouri River and the Mississippi River, which are essentially ancient flood plains.
Interest Approach – Engagement (what will you do to engage students at the beginning of the lesson)
Begin class with a demonstration on how soil particles settle. Have two to three Mason jars about half full of soil from the schoolyard or garden. Potting soil should not be used as it is mostly humus. Fill the jar the rest of the way full with water. Shake the jars prior to class. One jar should be well shaken and left to settle many hours before class. One should be shaken at the beginning of class to show the difference. If more than two are used, stagger the start times so that students can observe the differences over time.
In this demonstration, large particles like pebbles and sand will settle first. This is because they are the largest and heaviest. Next, silt will settle out, as it is the second-largest soil particle size. After that, clay will settle out. Clay will take much longer to settle as it is so much smaller than the other particle sizes. Organic matter or humus will float on top, and the rest of the cloudy look to the water is attributed to dissolved minerals.
When soils form, different particles will react differently. Let students observe the jars, measure where they think sand, silt, and clay begin and end, and brainstorm ways that this idea could contribute to soil formation and weathering.
- Start class by bringing up this video from Purdue Extension about soil profiles. Let it play until about 1:25. Dr. Graveel said the top portion of the soil is darker than the second portion of the soil. Why is that? https://www.youtube.com/watch?v=xoTd7ctj-e0
- Let students think for a minute. They should recall from the organic material lesson that organic content in the soil is what makes soils dark. The top portion of the soil has the most organic material from plant roots, organisms dying and decaying, and so forth. Thus, the top portion of the soil (the A horizon) has the most organic material and the darkest color.
- Let the presentation play again. He begins talking about how soils are different colors. Pause at 1:51. Ask the class why they think soils can be different colors?
- Take suggestions from the class. They might say things like lack of organic matter, overabundance of a specific element like iron in red soils, or other ideas.
- Go back to the video. He begins talking about the horizons of soil. He mentions that the A horizon has a high organic matter content and makes soils dark, confirming what students should have mentioned in step A. He then mentions that the B horizon is the horizon of accumulation and says that the B horizon has a higher clay content. Pause the video around 3:00 and ask students why they think clay accumulates in the B horizon.
- Students should recall that clay is the smallest soil particle size, and therefore is more easily moved. With rains and gravity, clay particles tend to leach through the soil profile and collect in the subsoil.
- Lead into class discussion on soil formation factors. In lesson one, it was mentioned that soils can form in three ways. Can students remember what those are?
- Chemical processes, physical processes, and biological processes
- Soil scientists look at soil formation using the five soil forming factors. These are climate, organisms, relief, parent material, and time. You can remember these with the mnemonic device ClORPT.
- Walk through each of the factors as a class, writing down some examples for each of factors and answering questions as they arise. Encourage students to take notes in their science notebooks, as well.
- Climate relates to temperature, precipitation, and other factors. Depending on these factors, microbial activity and leaching of nutrients and soil particles through the profile can be faster or slower. For example, if a climate is cold, microbes will likely not be very active, and biological breakdown of materials will take longer.
- Organisms means the type of vegetation that naturally grows in the area. For example, prairie grass soils are much different than naturally forested soils. Why might that be? (Different root structures, different life cycles, etc.)
- Relief means the topography of the land. For example, land that is on a tectonic plate boundary where mountains form weathers much differently than land that is naturally flat and is not near these boundaries. Discuss some of these geologic differences and why mountains weathering and flat lands weathering might look different.
- Parent material refers to the material that the soils formed from. If we look at a soil profile, like in the video we watched earlier, this is what we find in the C horizon. There are a few sources of parent material, including glacial till, alluvium (brought up from a river), colluvium (brought down from a higher landform – think mudslide), and loess (wind-blown sediment).
- Time is an important soil forming factor, because old soils can behave much differently than young soils. Ask students why this might be. Older soils have more time to leach, erode, and degrade. However, time can also help minerals break down and give microbes time to decompose materials. Time can both build and degrade soils in natural environments.
- Hand out the Iowa Soil sheet ( http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1029&context=farmprogressreports) to students. Tell them they will need to read this sheet and write in their science journals about Iowa’s soil forming factors. Have them sketch out climate, organisms, relief, parent material, and time in a way that makes sense to them in their notebooks.
- Optional: Post-it notes, extra pieces of paper, and tape can be provided for this step to allow students to make flip-up sections to add an extra dimension.
- Students should find the following about Iowa’s soil forming factors based on the reading:
- Climate: 26-38” precipitation (via Encyclopedia Brittanica), humid continental
- Organisms: prairie grass
- Relief: most acres are 0-5% slope
- Parent material: loess, glacial till, and alluvium
- Time: cultivation during last 160 years has reduced natural organic matter content and accelerated erosion
- Talk through those factors as a class. What questions do they have (about terminology, etc.)? What surprised them? What didn’t?
- Point out that a major parent material in Iowa is glacial till. This means that glaciers pushed new sediment over the state. Glaciers can move in different ways, but can create moraines, kettle holes, and kames.
- Glacial till with brownies activity:
- For this demonstration, you will need a pan of soft brownies or sheet cake with sprinkles or mini chocolate chips on top, hard serve ice cream (well-frozen), and ice cream cones.
- Bring out your pan of brownies. Explain to the class that this pan of brownies is to represent Iowa prior to glaciation.
- Next, bring out the ice cream. Scoop some ice cream into an ice cream cone, packing it in as tightly as you can. Explain to the class that this ice cream is representing the glaciers that moved through Iowa.
- The first glaciers that moved through Iowa came before the Illinois period, in between 500,000 to 2,500,00 years ago. This was a pretty typical glacier that moved very slowly (about .5 miles per year). Since it moved slowly, it created rolling hills that you can still see in the southern half of Iowa.
- Holding on to the ice cream cone, flip the cone over and slowly push the ice cream across the pan of brownies. Tell students to observe what’s happening with the pan of brownies. As the sprinkles are moved and deposited and as the ice cream melts, try to point out what those landforms would be. If there are puddles of ice cream, they would be kettle holes. Where you stop pushing the ice cream would create a moraine in the brownie. If there are any piles of sprinkles or brownie crumbs deposited by the ice cream, they would be kames or eskers depending on shape.
- If the ice cream in the cone has begun to melt, put it in a bowl and set aside. Then scoop a fresh cone. Explain that this ice cream cone represents the most recent glacier that came through Iowa during the Wisconsinan period.
- Move this ice cream cone similar to the previous one, but be sure to press firmly and move faster. Explain that this glaciation happened only 10,000 years ago (very recent in geology) and moved very fast – two whole miles per year. This glaciation pushed the land flat, creating what we now call the Des Moines Lobe of the Laurentide Ice Sheet, or just the Des Moines Lobe.
- This part of Iowa has very young soils that have not had a lot of time to erode. There are also more glacial landforms here, like moraines and kettle holes.
- Fun fact: the terminal moraine of this glacier is the hill that our capitol was built on!
- Optional: After the demonstration and observations, serve the brownies and ice cream to students who would like some.
- While students eat their brownies, have a class discussion. What did the activity show them? What is glacial till?
- The activity should show some ways that glaciers impact the land. Glacial till is the sediment deposited by a glacier moving across the land.
- Have students take out their science notebooks and journal about their observations and takeaways while they finish their snack.
- After students are finished journaling, review some of Iowa’s soil formation factors as a class. How do these factors impact our soils?
- Precipitation can erode soils and cause minerals and soil particles to leach through the soil profile, but also feed plants that contribute to soil health. Native prairie soils have lots of organic matter and a thriving microbe population. Flatter lands don’t erode as quickly, but also don’t drain water as well. Glacial till and loess deposits have left fertile sediment for the soil to build from. Very old glaciations have since been weathered. Newer glaciations have not been eroded as badly. In the last couple hundred years, erosion has increased more due to human intervention.
- Our current soil biology is dependent on the native vegetation of our soils. The first plants (that may have been moss or lichen) started the cycle of contributing organic material that feeds microbes and builds the A horizon of a new soil. Eventually grasses began to grow, reaching their roots deep into the glacial till, further providing structure, organic material, and feeding microbial life.
- In the last few minutes of class, have students brainstorm how soils could be different if each of Iowa’s soil forming factors were to change.
Did You Know? (Ag facts)
- Iowa’s capitol building is built on the terminal moraine of the Laurentide Ice Sheet of the Wisconsinan Glaciation.
- Iowa’s principal soil order is Mollisol.
- Iowa’s state soil is the Tama soil series.
Extension Activities (how students can carry this beyond the classroom)
- Explore the soil profile: reach out to an area Natural Resource Conservation Service (NRCS) or Extension and Outreach office to see if they could dig your class a soil pit or procure a soil core for your class to observe. If there is construction going on near your school building, any soil pit where the profile is visible and soil has been largely undisturbed will work well.
- Take the class to the pit or bring the soil core to the classroom. Gather students around it and ask them questions about what they see. Where does the A horizon start and end? Where does the B Horizon start and end? Where is the C horizon or parent material? What does the parent material look like? Does this soil have lots of organic material or not (is the A horizon dark)? Can we see bedrock or the R horizon? Do we see an E horizon?
- Talk with students about what this means about the soil around them. In Iowa, most of our soils’ parent material is glacial till, which is a bit rocky. Iowa’s native plants are mostly prairie grasses, which made our soils deep, dark, and rich. In some areas of the state, trees have been more present, which could form an E horizon. In some areas of the state, the soil may be shallower or more eroded.
- Have students write a one-page summary on their observations of their local soil, and have them draw conclusions about the soil forming factors that created this soil.
Suggested Companion Resources (books and websites)
- Iowa’s Climate: https://www.britannica.com/place/Iowa-state/Climate
- Why Iowa Isn’t All Flat: https://iowaagliteracy.wordpress.com/2016/08/11/why-iowa-isnt-all-flat/
- Iowa’s State Soil, Tama: https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1236956.pdf
- State Soils, NRCS: https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/edu/?cid=stelprdb1236841
- The Climate of Iowa: https://www.weather-us.com/en/iowa-usa-climate
- This material is based upon work supported by the Natural Resources Conservation Service, U.S. Department of Agriculture, under number NR196114XXXXC003. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.
- The grant by which this project is funded is administered by the Conservation Districts of Iowa.
- The Origin and Development of Soil: https://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1130447038&topicorder=2&maxto=10&minto=1
- Soil Genesis and Development, How Parent Material Affects Soil Profile Development: https://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1130447021&topicorder=7&maxto=7&minto=1
- Iowa’s Loess Hills: http://www.iowapbs.org/iowapathways/mypath/iowa%e2%80%99s-loess-hills
- Introduction to Geological Process in Illinois: https://www.dnr.illinois.gov/education/Documents/OnlineGeology(7-8).pdf
- Kame: http://www.landforms.eu/cairngorms/kame.htm
- When Does Rock Become Soil: https://soilsmatter.wordpress.com/2017/08/15/when-does-rock-become-soil/
Iowa Agriculture Literacy Foundation
Agriculture Literacy Outcomes
- T5.9-12.g: Evaluate and discuss the impact of major agricultural events and agricultural inventions that influenced world and U.S. history.
Iowa Core Standards
Iowa Core Science Standards:
- HS-ESS2-5: Plan and conduct an investigation of the properties of water and its effects on Earth materials and surface processes.
This work is licensed under a Creative Commons Attribution 4.0 International License.