This month, we are getting our hands dirty as we explore Pedology, the study of soil. Soil, not to be confused with plain Jane "dirt", is a complex and important component of the natural world. Let's dig up the dirt on soil to find out what it's all about.
Feel the Difference
Soil is composed of five main parts: humus, gravel, sand, silt, and clay. The three primary particles which determine soil texture are sand, silt, and clay. These three particles range in size, with sand the largest and clay the smallest. Most soils have a combination of the three different particles in varying amounts and that combination is what creates different soil textures. As amounts of each particle changes, the soil texture also changes.
For example, in this part of Texas, our soil is very high in clay which is great for retaining water, but doesn't do much for plant root growth since the soil is so compact. On the other hand, plants that live in areas with high sand content, such as a desert, would have the opposite problem. Sand allows for root growth, but drains water quickly.
The most productive soils are loam soils. Loam soils have just the right amount of each sand, silt, and clay in order to offer the best growing conditions for most plants. This doesn't mean that plants won't grow in soils where one of the particles far outweighs the others, but it does provide the best conditions for plant growth. Although the soil texture can't be changed (the soil is what it is), the arrangement of the particles can be changed, such as through processes like aeration. Changing the arrangement isn't a cure all, but it can provide better opportunity for plant growth in areas with less desirable soil textures.
Soil is Like Onions (and Ogres)
What do soil, onions, and famous ogres have in common, you ask? They all have layers! Soil is made of five distinct major layers, called horizons. Each horizon has different properties that can be readily observed, such as thickness, color, and texture, and other less obvious properties such as chemical and mineral content.
The first horizon is the "O" Horizon (aka humus), and is made of decomposing organic matter from plants and animals. This layer is thin compared to the other layers and is very dark and rich in nutrients. Directly underneath is the "A" Horizon, also called the topsoil. This layer also has decomposing organic matter and nutrients so it's darker than the layers below it. In addition, the "A" Horizon is where most soil organisms live (i.e. earthworms, fungi, bacteria, etc.) and is the layer in which plants germinate and take root.
Below the "A" Horizon is the subsoil, or "B" Horizon. The "B" Horizon has more mineral deposits and clay, with less organic matter than the above layers. It is also a lighter color. Eluviation, or the downward movement of nutrients and soil, occurs between the "A" and "B" horizons, and is occasionally called the "E" horizon, although it is a process between layers and not a layer itself.
Just below "B" is Horizon "C", or the substratum. This layer is considered the "parent layer" because the layers above it developed from it. The "C" Horizon is mostly made of rock and very little organic matter, so the roots of plants aren't found in this layer. The deepest layer is the "R" Horizon, or bedrock. Bedrock is not actually soil, but is a thick layer of rock that is the foundation of the soil above it.
Many people use the terms "soil" and "dirt" interchangeably, but they are actually two different things. The easiest way to distinguish between the two is that dirt is dead and soil is alive. Dirt is the particles like sand, silt, clay, and gravel. Even potting soil you buy at the gardening center is usually "soil-less" and is simply dirt. This is because soil is dirt plus that fifth component of soil, humus, which has all the nutrients and organic organisms that live or once lived. Soil is its own ecosystem full of living micro and macroorganisms which influence the health and productivity of the earth. In fact, soil is so full of life that there could be more than 5,000 pounds of living organisms in the top six inches of soil on one acre of land. That's a lot of living creatures!
Soil contains billions of microorganisms, too small to see with the naked eye. These organisms, like bacteria, fungi, actinmyocetes, algae, and protozoans, all add to the fertility and health of the soil. Macroorganisms, such as earthworms, nematodes, mites, millipedes, pill bugs, and many others, also contribute to soil quality. Together, these macro and microorganisms play important roles in nutrient cycles, such as the carbon and nitrogen cycles.
Without healthy soils, plants are unable to flourish. Soil rich in organic matter, organisms, and proper amounts of nutrients (such as nitrogen, phosphorus, and potassium) is essential for plant growth, development, and function.
Many people, especially farmers and ranch owners, test their soils for these and other elements regularly. Testing frequently ensures that their fields are healthy for the crops that they grow. Just like different animals have different food needs, plants do too. Not all plants use the same amount of nutrients, so it's necessary to test the soil regularly to know what the soil might be lacking (or have too much of). This ensures that farmers can add the proper amount of nutrients and rotate their crops in a way that will keep the soil healthy and productive.
One way that they keep their soil healthy is through the practice of minimum till or no till farming (above left). This method reduces soil disturbance and as a by product ground cover remains fairly constant. By continually having plant cover on farmland, it keeps the soil rich in nutrients through the decomposing organic material. These practices also reduce erosion. Like we discussed last month, erosion happens when water (or wind) picks up soil and deposits it elsewhere. Having constant plant cover reduces the movement of soil from place to place, and keeps it exactly where it needs to be.
Not that long ago, the United States suffered substantially as a result of soil and nutrient loss. The Dust Bowl was caused by the perfect storm of drought, over farming, and removal of native grasses, and the winds literally blew the topsoil away. High winds occur all the time, especially on flat prairies, and the nutrient rich topsoil is essential. Wind breaks are a common device used to protect soil from wind erosion. Wind breaks (above right) are rows of trees and shrubs which slow down the wind and keep it from picking up soil as it blows across.
Ideally though, native grasses are the best way to reduce erosion, and its impacts. Plant root systems are pros at holding onto soil, so areas with high plant growth (especially with native plant species) experience less erosion. This weekend I transplanted a lavender plant, and I noticed that as I moved it to the larger pot, most of the soil came out with the plant and stayed in its square shape. Have you seen that before? That happened because the roots of the plant had hold on the soil - they were keeping it all together. Ultimately, healthy soil produces more plants, and more plants reduce the rate at which erosion occurs, which leaves more soil on the ground. Just like many things, soil is a limited resource so we need to make an effort to conserve it. Healthy soils and native plants play an important role in soil conservation.
Soil as Art
Not only is soil teeming with living things and an important part of plant development, but it has also been used in cultures throughout history in art and architecture. The earliest artworks and records of history were done using soil on cave walls. Animals, hunts, fantastic creatures, history were communicated through these drawings.
In addition to artwork, many people used (and still use) soil for pottery and architecture. Clay is widely available in soils all over the world, so pottery was invented by many cultures independently throughout history. The oldest dated pottery was discovered in central and western Europe. Because of the prevalence of clay and so many cultures discovered its uses, pottery is the most frequently found artifact type by archaeologists globally. In addition to pottery, clay is also used, along with sand, water, sometimes gravel, and grass or straw, to make adobe - a type of brick, which is not only the most common building material known to man, but also the oldest.
Just like many other resources, soil is in limited supply and should be cared for accordingly. Rich in organisms, nutrients, and history, soil plays an important role in nature, history, and humanity. As much as people (including myself) use dirt and soil interchangeably, once you do some digging it's pretty clear that they are very different things. So the next time you hear soil called "dirt", you can share the dirt on soil with them and help them to do a little digging of their own.
By: Jennifer Cheesman, with contributions from Scott Lightle
References & Resources
1. Soils & Organisms : USDA ARS, www.ars.usda.gov/plains-area/sidney-mt/northern-plains-agricultural-research-laboratory/nparl-docs/just-for-kids/soils-organisms/.
2. “Cave Art History.” RSC Education, 11 Aug. 2015, edu.rsc.org/resources/cave-art-history/1528.article.
3. “FOUR LAYERS OF SOIL.” Eco Kids, www.cleanair.pima.gov/more/FourLayersSoil.html.
4. History.com Editors. “Dust Bowl.” History.com, A&E Television Networks, 27 Oct. 2009, www.history.com/topics/great-depression/dust-bowl.
5. Hoorman, James J. “Understanding Soil Microbes and Nutrient Recycling.” Ohioline, 7 Sept. 2010, ohioline.osu.edu/factsheet/sag-16.
6. “Living Soils: The Role of Microorganisms in Soil Health.” Future Directions International, 22 June 2017, www.futuredirections.org.au/publication/living-soils-role-microorganisms-soil-health/.
7. “Natural Resources Conservation Service.” A Soil Profile | NRCS Soils, www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/edu/?cid=nrcs142p2_054308.
8. “Natural Resources Conservation Service.” Soil Bacteria | NRCS Soils, www.nrcs.usda.gov/wps/portal/nrcs/detailfull/soils/health/biology/?cid=nrcs142p2_053862.
9. “Preservation Brief 5: Preservation of Historic Adobe Buildings.” National Parks Service, U.S. Department of the Interior, www.nps.gov/tps/how-to-preserve/briefs/5-adobe-buildings.htm#adobe.
10. Tabor. “Rooting Around in the Spring.” Room Without Walls, 1 Jan. 1970, taborsyard.blogspot.com/2015/04/rooting-around-in-spring.html.
11. User, Super. “Culture, History and Arts: Soils Topics Offering a Range of Soil Resources.” Design Bites, uksoils.org/topic/culture-history-and-the-arts.
12. Violatti, Cristian. “Pottery in Antiquity.” World History Encyclopedia, World History Encyclopedia, 20 Nov. 2021, www.worldhistory.org/pottery/.