Soils, types and evironmental concerns

Lecture notes for Environmental Geology course

Introduction

Motivation for addressing soils in this course?

• soil is a critical agricultural resource.
  
• soil is a common foundation material.
• soil erosion causes siltation and related environmental concerns.
• role of soil as carbon sink, and soil contributions to atmospheric green house gases.
• often under appreciated.

What is a soil?

• from an agricultural/biologic perspective.
• from an engineering perspective.

What is in soil? Minerals, rock fragments, organics, mircrobes, water, air.

Minerals: inorganic, crystalline, naturally occurring.

Types of minerals in soils?

• silicates: most common type, (SiO4)-4 the common ionic group, includes quartz, feldspars, micas.
• oxides: form in presence of oxygen, by weathering. Hematite as nature's pigment. Common in some soils because communication with the atmosphere produces an oxidizing environment. Other soils reducing.
• salts: precipitate out of water.
• sulfides: form often in oxygen poor environments.
• carbonates: soluble so relatively mobile.
• clays: a special mineral group important in soils.
  
  • submicroscopic, and very variable and complex.
  • usually dominated by sheet silicates that provides them with unique traits (shrink-swell behavior, adsorption capabilities, low strength).

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

• igneous: granite, basalt, andesite.
• sedimentary: sandstone, shale, limestone.
• metamorphic: schist, gneiss, slate.

Microbial mats - demonstrate the importance biologic and especially microbial activity to soils.

Cryptobiotic crusts in arid environments.

• hold soil together, especially in dry desert environments.
• complex community of microbes
• crucial for desert soils and plants

Image of cryptobiotic soil. Note the corrugate microtopography in the soil. This microtopography 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 cyanobacterial holding together sand grains. From USGS site on cryptobiotic soils.

Weathering as a soil forming process:

Physical weathering versus chemical weathering.

Hydration reactions: silicate mineral (e.g. feldspar) + water -> clay + ions in solution + more acidic water.

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.

• aridisols - soils that develop in arid areas.
• histosols - soils that are dominantly organic (such as found in bogs and swamps).
• oxisols - soils that form by intense chemical weathering in tropical environments

NRCS site on soil types with photos and maps.

Soil Erosion

By wind:

• selectively effects crucial topsoil.
• most crucial in semi-arid, grassland areas.
• drought, fire, poor agricultural practices can all increase.

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.

By water:

• by sheetwash.
• formation of rills and gullies.
• reduce by contouring, minimum tillage, maintaining vegetative cover, small sediment traps.

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.

Soil Salinization

Cycles of irrigation and some numbers to play with.

• salinity of seawater: circa 35,000 ppm
• salinity of input irrigation waters 350 ppm.
• >2000 to 3000 ppm toxicity level for many crops
  
• critical question: if the climate is arid enough that water evaporates out of soil where does the salt go?

Extent of concern?

• San Joaquin valley in California,
• Pakistan,
• "half of the world's irrigated land .... suffers from reduction of crop yields due to salinization."

Possible solutions:

• desalinize irrigation water (very expensive).
• direct trickle irrigation (water usage reduction).
• tile fields: can flush salts through soil, run off waters high in Selenium in California, expensive.
• salt resistant crops (will only delay problem).

Engineering concerns related to soils

Differential compaction (settling).

• good local examples on campus: library, parking garage.
• are sediments naturally compacted or not: soil tests are critical.
• organic rich soils an extreme of 20' to 1' compaction (contributes to subsidence problems).
• muds can easily compact from several feet to 1'. Sand compacts a little.

Swelling clays.

• change volumes when add water, causing soil shifting.
• $2-7 billion annual damages in U.S. (Krohn & Slassna, 1980)
• Dallas area alone $10 million a year.
• common where volcanic ash a major component in soils or where have marine clays (Denver and Houston).
• USGS map of swelling clay potentil in midcontinent USA.

Soil creep.

Piping.

Seismic response and liquefaction potential.

Drainage characteristics.

Permafrost: will talk about in detail in future lecture.