Soils: processes, types and environmental concerns

Lecture notes for Environmental Geology course

Content:

Photo to right of Kansas grassland at the Smoky Valley Ranch preserve of the Nature Conservancy. The vegetation is supported by and contributes to the thin veneer of soil that mantles this landscape. In the background are badlands where erosion through the soil cover has removed the soil and one can see the underlying Niobrara Chalks which the soil developed in and on top of.


Introduction

Motivation for addressing soils in this course?

Pedology is the traditional science of studying soils. This is now morphing into critical zone science and there is a Critical Zone Observatory. A definition for what the critical zone from there web site is as follows: " It is a living, breathing, constantly evolving boundary layer where rock, soil, water, air, and living organisms interact. These complex interactions regulate the natural habitat and determine the availability of life-sustaining resources, including our food production and water quality."


Soil forming processes and features

What is a soil?

What is in soil? Minerals, rock fragments, organic compounds (humus), microbes, larger organisms, water, air.

Types of minerals in soils?

Scanning electron microscope image of two different types of clay minerals. Note their sheet-like character. Scale bar is 4 micrometers. Image source USGS report: 3-D Reservoir Characterization of the House Creek Oil Field, Powder River Basin, Wyoming, V1.00 : http://pubs.usgs.gov/dds/dds-033/USGS_3D/ssx_txt/figure.htm , accessed 2/26/2015.

Rock fragments: some common rock types (represent source material for sediment or soil) are as follows:

Soil microbes:

Image of cryptobiotic soil. Note the corrugate micro-topography in the soil. This micro-topography is held together by the microbial communities. You can also note a slight greyish tinge from the microbial mats. When it is dry the cryptobiotic mat can be destroyed if you walk on it, making the loose sediment below prone to wind erosion. The importance of these mats has only been recognized relatively recently. The image is from a Wikipedia site but is originally a USGS image.

Image of filamentous cyanobacteria holding together sand grains. From USGS site on cryptobiotic soils.

Weathering as a soil forming process:

View of weathering pattern in a shallow Tertiary intrusive from the foot of the Chisos Mountains in Big Bend, Texas. Note the rectangular fracture pattern that outline weathered blocks. The rock was originally all uniform in color and the banding represents oscillatory iron staining and reflects weathering that worked from the fracture margins into the interior of the blocks. Water moved along the fractures and induced the weathering. The middle of the blocks are less weathered than the fracture margins. This is the same geometry and process that gives rise to spheroidal boulders when the more weathered material is eroded away.


Types of soils

Above are three examples of very different soil profiles. A standard approach to studying soils is to dig a pit and a vertical wall to profile the vertical zonation in the soil. The vertical zonation is key to understanding soils and recognizing soil horizons. Image source: Photos courtesy of USDA NRCS.

Some examples give an idea of how soils are classified.

NRCS site on soil types with photos and maps.

What are the factors that influence the type of soil?


Soil Erosion

Erosion by wind:

Image of dust storm moving out into the Atlantic from West Africa. The SW corner of spain is up in the upper right corner of the image. This satellite image nicely demonstrates the ability of wind to erode, carry and deposit soil material. Image modified from http://coastal.er.usgs.gov/african_dust/satellite.html.

Fire can be an event that promotes erosion in some cases. A USGS site studying this process can be found at http://fresc.usgs.gov/research/researchPage.aspx?Research_Page_ID=109 .

By water runoff:

Photo of erosional rills that developed on a burned slope. This helps demonstrate the importance of plant matter in holding the soil in place and in soaking up rain so that there is less runoff. Image source: http://wwwbrr.cr.usgs.gov/projects/Burned_Watersheds/. Photo by John Moody.

Image from USDA site showing gully erosion into a field. WIth time the gullies will continue to grown and branch up slope. Image source: http://www.ars.usda.gov/AboutUs/aboutUs.htm?modecode=60-60-05-05

Sediment from soil erosion that enters streams can:

Mitigation measures for soil erosion:

Image of an Iowa farm where terraces, contour plowing, and a buffer strip of maintained vegetation along the stream all serve to prevent soil erosion. The buffer strip also helps improve surface water quality. Image source: originally the USDA, downloaded from: https://commons.wikimedia.org/wiki/File:TerracesBuffers.JPG .

Painting by Thomas Benton (1940) in the permanent exhibit of Joslyn Art Musuem in Omaha called "The Hailstorm". Benton focused in part on struggling common folk during the depression era, a time of massive soil erosion, and a lesson learned in the importance of soil prevention measures. Here the storm that is about to unleash itself will not only bring the farmer much needed rain (the depression was a time of drought), but potetially also soil erosion and gully formation. Note how the plow furrows run straight. Where they run down the hill, they can concentrate the surface flow of water and initiate gully formation. A solution is contour plowing.


Soil Salinization

Cycles of irrigation and some numbers to play with.

Diagram from USGS site: http://pubs.usgs.gov/fs/fs-170-98/ that shows a model for how irrigation waters and the contained salts influences the soils. Note the numbers that are quantifying the transfer rates.

Extent of concern?

Image from US Department of Agriculture's Agricultural Research Service of severe salinization in San Joaquin valley in CA. The white is salt right at the surface. Image source: http://www.ars.usda.gov/is/graphics/photos/sep04/k4500-12.htm .

Possible solutions:


Engineering concerns related to soils

Part of geotechnical engineering.

Differential compaction (settling):

Image to right is from the island of Burano, near Venice Italy, where the houses are noted for being so colorful. Note the very visibly leaning leaning church tower. They didn't build it this way. However, the underlying substrate is deltaic sediment, rich in compactable mud. Either because there was more of a load on one side than the other of the tower, or because there was more mud under one side than the other, the church tower started to lean. Once it leaned just a little bit that leaning shifted the center of weight and caused even more loading and compaction on the leaning side and less on the side it leaned away from, and so a bit of feedback cycle can initiate in these cases. More than one church tower leans in this area, but this is the most obvious one. Venice is famous for being the sinking city, so that plazas can be flooded during high tides, and that is a result of large scale compaction going on at a deeper level.

Swelling clays:

Image to right is of surface texture in part of the Big Badlands National Park. The sedimentary rocks here (part of the White River Group) are particularly rich in volcanic ash and its companion clay smectite. In the presence of water volcanic ash weathers or alters readily to produce smectite (and other products). Smectite is a clay mineral family known as swelling clays, because their lattice structure can expand significantly to accommodate water, and then can dry out, causing very large volume changes. When it dries out it produces a suite of intense fractures (mud cracking in extreme) that breaks the weathered material into small kernels such as depicted here to the right, and hence this surface weathering texture is known as 'popcorn'. When it rains the clays will incorporate the water, swell, and the cracks and the popcorn texture will disappear, only to appear again as the surface drys out.

Soil creep (already discussed in mass wasting lecture- function of slope).

Piping:

Seismic response and liquefaction potential (will be discussed in earthquake lecture).

Drainage characteristics of soils.

Permafrost: covered in section on cold climates and glaciers.


Soil, carbon cycle and carbon sequestration

Through the process of photosynthesis plants and blue green algae make organic compounds from water, sunlight and the carbon dioxide in our atmosphere. Other organisms eat the plans, and as they respire carbon dioxide is returned to the atmosphere. All these organisms die and the carbon rich organic compounds are used as food by decay organisms such as fungi and bacteria, and those organisms also return carbon dioxide to the atmosphere. Fundamentally there is the constant cycling of carbon exchange between the atmosphere and life, and that is the short term organic carbon cycle.

Soil is such a thin cover, but it is also very wide spread, and teaming with life from burrowers, to plants, to a great assortment of microbes. Because of this it is a significant part of the carbon cycle, and so when trying to understand the carbon cycle and related climate change dynamics, the soil is an important component to understand. Some refer to it as the living skin or our planet.

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Above image is from the USGS web site: http://pubs.usgs.gov/03financial/html/forward.html , and addresses how soil carbon is linked to the short term carbon cycle. The right image from USGS site addressing soil as a possible carbon sequestration reservoir. Image and additional information source: http://ca.water.usgs.gov/Carbon_Farm/

Some estimates suggest over half of the active carbon cycling in the short term organic carbon cycle moves through soil. Figuring out these dynamics is a very active research program in the geosciences at present.