{This is an Earth Cache – there is no container to find} In order to claim a find here, email or message me the answers to the questions at the end of this lesson. You will need a 6 ft measuring tape and paper towels/rags and water to clean your hands. At this location, you will learn about weathering and soil formation.
Introduction
Soil is formed from the weathering of a parent material, such as bedrock or aeolian (wind-blown), river or glacial deposits. Weathering can include the mechanical breakup of rocks and the chemical weathering of minerals.
Most soils develop horizons, or layers, that are parallel to the ground surface and contain physical properties such as texture, color, structure and composition that are different from the layers directly above and below it. These layers form as water dissolves and moves minerals and clay down through the soil column. The soil horizons are labeled with capital letters (as shown below) and may also include lowercase letters or numbers to differentiate similar horizons that have different prominent features.
- O - layer of organic matter (not present in all soils)
- A - layer of organic matter mixed with mineral material (topsoil)
- E - layer of leaching (some clay and iron have been removed), lighter in color (not present in all soils)
- B - layer of accumulation of clay, iron, and other elements from the overlying soil
- C - Parent material, or layer of incomplete weathering
Physical properties of soil:
Texture – the relative percentages of the particles sand, silt and clay. Sandy soils will feel gritty; silty soils will feel smooth; and clayey soils may feel smooth, hard or sticky. A soil texture of loam refers to equal parts of sand, silt, and clay. Soil layers may contain pebbles or gravel.
Color – the shade of yellow, red, orange, gray, brown, or black the soil appears to the eye. Color is primarily influenced by the mineral or chemical composition of the soil. Dark brown soils are rich in organic materials. Red or orange soils are rich in iron.
Structure – the shape of the soil clumps. Common textures are granular (small groupings of particles), blocky (larger, chunky clumps), columnar (vertical columns), and massive (appears as one giant mass).
Composition – the type and amounts of elements or minerals present. Iron, clay minerals, aluminum oxides, and quartz are common components of soil. Some weathered soils may include layers where clay or iron has accumulated and hardened. Highly weathered soils may include plinthite (see below).
Weathering Features
Argillic horizons – layers of accumulated clay that were formed from the downward movement of water in the soil.
Iron Oxides – iron may become dissolved from minerals in soil and combine with oxygen to form deposits of iron oxides, which appear as a rusty coating on the soil particles.
Leaching – minerals dissolve in groundwater and are flushed out or redeposited lower in the soil column, leaving behind a lighter, grayer layer of soil near the upper portion of the soil column.
Mottling – a spotted (or “blotchy”) appearance of darker and lighter soil material that can indicate a high water table or seasonally fluctuating depths to water.
Plinthite - a nodule or layer of hardened iron mixed with clay and quartz that appears as small, hard, red pebbles, or as a hard, discontinuous layer. Plinthite forms when the soil is exposed to repeated wetting and drying, as well as sunlight.
Redoximorphic (Redox) features – mottling that is primarily blotches of gray and reddish-brown soil colors occurring at the same depth, related to the movement of iron. Redox features are typically lower in the soil column and often found along root channels.
Ultisols
Ultisols are “old” soils that form over thousands of years of continuous weathering in humid environments. They are the “ultimate” product in terms of weathering in a humid environment and are very common in the southeastern United States. Most of the weatherable or dissolvable minerals have been leached from the upper soil layers and deposited/accumulated in the lower soil layers. They may be red, orange or yellow due to the presence of iron oxides.
This soil profile, visible from the roadway, is an example of an Ultisol. The uppermost layer (A Horizon) is dark from organic material and relatively thin compared to the rest of the soil layers. The middle layer (B1 Horizon) is lighter and thicker than the uppermost layer. The bottom layer (B2 Horizon) is a different color and the thickest layer. Each layer has a different color, texture, and weathering features that distinguish it from the other layers. The parent material (C Horizon) is not visible in this location.
Questions (email or message the answers to me)
- For the 3 main soil horizons, describe the color, texture and thickness for each layer, from top to bottom (A, B1, B2).
- Which weathering features (listed above) do you think are present in this soil outcropping?
- Describe the size, location/depth, and appearance of any plinthite. (Note, plinthite is present in more than 1 location/depth).
- Describe the material in the little trench between the roadbed and soil outcropping (you may be standing on it).
- Do you think the material in the trench eroded from the soil profile or the adjacent road? Why you think this?
References
Soils: Genesis and Geomorphology – Schaetzl, 2005
Soil and Water Chemistry: An Integrative Approach – Essington, 2003
Soil Science and Management – Plaster, 1996
https://websoilsurvey.nrcs.usda.gov/app/
https://archive.org/details/usda-soil-survey-of-baldwin-county-alabama/page/102/mode/2up