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Dig In: A Quick Lesson on Soil Science

Hi all, I hope you have been enjoying the winter months and recent solstice. I for one really enjoy the winter as a night person who works outside. But, I know for others it can be difficult to get outside as often when the days feel shorter and the weather isn’t as welcoming. One thing I really miss in the winter is gardening and getting my hands in the soil. I hope this quick and dirty overview of soil science can help you feel connected and grounded.


This blog post will be discussing soil with a focus on earth science rather than gardening or fertility. We will go over what soil is and isn’t, how soil is made / formed, and what the major components of soil are.


Close up of kaolinite clay in Jurassic sandstone, UK North Sea, clay type confirmed by X-ray diffraction. Width of image: 20 microns across (50 images side by side would measure 1mm). Photo: Evelyne Delbos. Courtesy of The James Hutton Institute.

What are soil and dirt?

So, what are soil and dirt? The answer is complicated and depends on who you ask, as are many generalized science questions. For soil scientists, soil is a mixture of organic matter and minerals near the earth’s surface that support life, while dirt is a soil-like material that does not have/support life. These labels are mostly semantics, unless you start to delve deeper into scientific literature. Some people are very invested in these differences-- definitely a faux pas to ask soil scientist agronomist professors what dirt they garden with.


The factors that influence the formation of soil are: CLimate (CL), Organisms (O), Relief (R), Parent material (P), and Time (T). You can remember this with the easy acronym CLORPT. Each of these categories often overlap and influence each other. Soil is the recorded history of influence and changes of our world.  


Weathering and Soil via https://www.geologyin.com/2015/01/weathering-and-soil.html

Climate shapes soil formation mainly through precipitation (or lack of) and temperature. Think of the difference between the sand dune deserts and a Redwood forest. Then, the difference between the Alaskan tundra and a Floridian swamp. Rain very strongly influences what plants can grow (CLimate and Organisms). While Alaska and Florida may both be abundant in water, when that water is locked in as ice vs. a constant spring, the soil is either spending lots of time biologically inactive and solid or loose and active. 


Organisms are all the living things, from tiny little single-celled microbes to vast elk herds. From mighty biocrust and interconnected mycelium to swaying forest. Most definitely you, me, and all humans. Nearly all ecosystems depend on soil as the foundation of life, excluding some oceanic food chains and others. But not all organisms strongly influence the formation of soils. The way people use soil is probably the easiest to see and understand. Removal and mixing from mining, dramatic changes in nutrient and plants for farming, and the compaction paving over urban areas. Of course, others also play key roles. Examples include earthworms, prairie dogs, pigs rooting, birds spreading seeds, and salmon bringing back nutrients from the sea, all these are influencing the development and characterization of soil. 


Relief is a geological term referring to  the general shape of the landscape. Imagine a mountain. The top is bare, rocky, and steep. Then down slope it can gradually become a gentler slope with more soil and plants. At the base, lots of soil accumulates from erosion and gravity. All those factors are in some ways related to erosion of soil. Water is a huge component of erosion, The ways that rivers move and deposit sediment is also a relief soil-forming factor.


Parent Material refers to the original rocks that were weathered (eroded/broken down) to make soil. Most soil is formed in place, slowly increasing in depth from the top down. Utah's red sandstone gives rise to sandy red soils. Hawaii’s volcanic soils are also deeply red, but the ash fall over lava flows make hydrous silty clay loam soil. 


Finally, time; did you know it can take anywhere from 300-1300 years for an inch of topsoil to form? There is such a wide range of times because there is a wide, complicated set of CLORPT factors forming soil. At its most basic, the more time you have, the more things that happen to the soil, if the soil isn’t lost. 


Robert W. Christopherson and Charlie Thomsen Chapter 18 “The Geography of Soils”

Major Components of Soil

There are three major components of soil: minerals, organic matter, and pore space for air and water. The minerals are divided into three class sizes: sand, silt, and clays. The mineral size class goes from big to really small. I hope you all know what sand is and have an idea of the size; if not, think table salt. Silt is the medium sized particles. When dry, it often is described as feeling like bread flour. It is easy to find large amounts of silt in lake beds. Clays are the itty bitty minerals (and as such are extra special) and feel very smooth and sticky. Clay particles can’t be seen by the naked eye, an electron microscope is needed. Individual clay particles can be only a few atoms thick, have the most surface area of all the particles, and, the best part, is some expand and contract when they get wet, which makes all sorts of stuff happen.


You might be thinking ‘hey hold on, everyone always talks about loamy soils, where's that explanation?’ Well most soils are a mixture of all, some, or none of each mineral size class. Loam is the trifecta balance of sand, silt and clay. People who have loamy garden soil are very lucky. Organic matter (OM) is plant and animal material that is starting to decompose into the soil. It starts as materials that haven't broken down at all, like freshly fallen leaves or scat. All the way to a the black mush that has no recognizable individual parts left. OM is very important for soil fertility and stability, it is both chemically and physically sticky.


Last, but not least, is the often forgotten pore space. Pore space is the amount of room between all these minerals and organic matter that the soil has. This space isn’t empty, but filled with either air or water, like a sponge. This space is also where many microorganisms live. Most healthy soils need to have some pore space to exchange gasses for plants (breath) and to allow water to percolate into the aquifers (drink).


I hope this quick soil introduction was helpful and not clear as mud. If there were any parts that weren’t clear… well, it was a dirty explanation and I’ll let you do your own research. Or you can look out for TMPF’s Science Talks for an upcoming soil topic.


If you have any questions on soil science, feel free to contact Cory at cory@tmparksfoundation.org.


 

About the Author

Cory is a graduate of Cal Poly Humboldt (formerly Humboldt State University) with a dual B.S. in Rangeland Resource Management and Soils and a Minor in Watersheds. He grew up in the Durango, Colorado, Four Corners area. He has held a variety of jobs within the Federal Government, including the Forest Service, Bureau of Land Management (BLM), and the Natural Resources Conservation Services (NRCS). This is his second time serving with AmeriCorps; previously he was down in Las Cruces, New Mexico in 2020 with the Southwest Conservation Corps. Cory is very excited to be in the Reno area experiencing city life.

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Sandy Denmon
Sandy Denmon
Jan 16
Rated 5 out of 5 stars.

I have a better than average understanding of the various components of soil, but your article was so thoroughly helpful and interesting; good molecular info ;)

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Guest
Jan 16
Rated 5 out of 5 stars.

very informative, thank you :)

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