Explain how soil forms from Ohio's glacial and bedrock parent materials, and analyze mass wasting, erosion, sediment contamination and desertification

Learning goal: Explain how soil forms and describe soil horizons and texture; relate Ohio's soils to glacial and bedrock parent materials; identify the types of mass wasting and the factors of slope, water content and soil type that control them; explain erosion and how it is managed at construction sites; explain how sediment carries contamination; and describe the steps of desertification.

What soil is and how it forms. Soil is the loose layer of weathered mineral grains, organic matter, water and air at the surface that can support plants. It is not simply dirt; it is a product of processes acting over time. Five factors control what soil forms: parent material (the rock or sediment weathered), climate (water and temperature drive chemical weathering and leaching), organisms (roots, burrowers, fungi and bacteria mix soil and add organic matter), topography (steep slopes lose soil and stay thin; low, wet places collect water and organic matter) and time. Because the same factors can be read in any landscape, soil maps are, in effect, maps of geology, climate and history.

Soil horizons. As soil matures it separates into layers called horizons. The O horizon is fallen leaves and decomposing organic matter. The A horizon, topsoil, is dark with humus mixed into mineral grains and is where most roots live. In many forest soils an E horizon lies beneath it: pale, because water moving down has leached out clay, iron and organic matter (eluviation). The B horizon, subsoil, is where those materials accumulate (illuviation), so it is often denser, more clay-rich and red or brown. The C horizon is partly weathered parent material, and R is unweathered bedrock. Not every soil has every horizon; young soils on recent sediment may have only an A over a C.

Soil texture. Texture is the proportion of mineral grains of three sizes: sand (2 to 0.05 mm), silt (0.05 to 0.002 mm) and clay (smaller than 0.002 mm). Sandy soils drain fast and hold few nutrients; clay soils hold water and nutrients but drain slowly and can become sticky and weak when wet; loams, mixtures of all three, are generally best for crops. Texture is found by settling: sand settles in under a minute, silt within hours, and clay may stay suspended for days.

Ohio's soils and their parent materials. Most of Ohio's soils are young, because they formed only since the last glaciers retreated from the state, roughly 14,000 to 20,000 years ago. In western and central Ohio, soils formed in glacial till, an unsorted mixture of clay, silt, sand and stones that is often rich in lime from the limestone the ice ground up; these soils are among the most fertile in the country, though many are poorly drained. The Miamian series, formed in till, is Ohio's designated state soil. In northwestern Ohio, soils formed on the flat clay and silt of former glacial lake beds; this was the Great Black Swamp until it was drained with ditches and tile in the late 1800s. Wind-blown silt called loess caps some areas. In unglaciated southeastern Ohio, soils formed from weathered sandstone and shale on steep slopes; they are thinner, more acidic and less fertile.

Mass wasting. Mass wasting is the downslope movement of soil and rock by gravity, without a transporting medium such as flowing water or ice. Types differ in speed and style. Creep is very slow, millimeters to centimeters per year, and shows as tilted fence posts and trees with curved trunks. A slump is a block that slides along a curved surface and rotates backward, leaving a scarp at the top and a bulging toe. A slide moves a mass along a flat plane such as a bedding surface. A rockfall is free fall of rock from a cliff. Flows, such as earthflows, mudflows and debris flows, move as a mixed fluid mass. Three factors matter most. Slope: the steeper the slope, the larger the part of gravity pulling material downhill. Water content: a little water holds grains together by surface tension, as in a sandcastle, but saturation adds weight and raises the water pressure in pores, pushing grains apart and cutting friction. Soil type: sand drains fast and fails as loose grains; clay swells and loses strength when saturated and fails in slumps and flows. Undercutting by streams or road cuts, and removal of vegetation, also trigger failures. Ohio has abundant examples: slumps in clay-rich shale and colluvium on the hillsides around Cincinnati and in southeastern Ohio, rockfalls from sandstone cliffs, and bluffs along Lake Erie retreating as waves undercut them.

Erosion and construction mud. Erosion is the removal and transport of soil by water, wind or ice. Water erosion begins with raindrop splash, then thin sheet flow, then small channels called rills, then gullies. A construction site stripped of vegetation can lose soil many times faster than the same land under grass or forest. Controls work by keeping soil in place or slowing water: keeping vegetation until it is needed, covering bare soil with straw or mulch, silt fences of fabric that trap sediment, sediment basins that let muddy water stand while particles settle, and storm-water designs that slow runoff.

Sediment and contamination. Sediment is itself a pollutant: it makes water turbid, blocks light, clogs fish gills and buries stream-bed habitat. It is also a carrier. Fine silt, clay and organic particles have large surface areas and charged surfaces that bind phosphorus, heavy metals such as lead and mercury, and organic chemicals such as PCBs. Contaminants settle with the sediment in slow water, in harbors and behind dams, and can be released again when the sediment is stirred up or dredged. Several Ohio rivers that enter Lake Erie were listed as Areas of Concern partly because of contaminated sediment, and managing dredged harbor sediment is a continuing Ohio issue.

Desertification. Desertification is the degradation of land in dry regions, arid, semi-arid and dry sub-humid, into less productive, desert-like land, caused by drought combined with human use. It proceeds in steps: vegetation is thinned by overgrazing, clearing or drought; bare soil is exposed; wind and flash floods strip the fertile topsoil and its organic matter; the surface crusts and absorbs less rain, so more runs off; less water means even less plant cover, a positive feedback. Examples include parts of the Sahel south of the Sahara and the 1930s Dust Bowl of the American Great Plains. On a globe, look for the transitions in the dry lands that border the great deserts, not in the deserts themselves: the Sahara, Kalahari, Gobi and central Australian deserts are dry because of climate, and they are not the end product of this process. The belts of climate on a globe are not stages of desertification either: semi-arid land is not degraded simply because it is semi-arid. Desertification is a change in the land's condition over time, and documented examples lie in those bordering dry lands: parts of the Sahel along the southern edge of the Sahara, with thinned vegetation and bare, crusted soil; the dry lands of northern China and Mongolia beside the Gobi, where drought brings large dust storms; the basin of the shrinking Aral Sea in central Asia, whose exposed bed is now a source of salty dust; and the American Great Plains, which passed through the same steps in the 1930s and partly recovered when farming practices changed and the rains returned. Because rainfall varies from year to year, a transition is identified by comparing dated maps or satellite images of plant cover and soil, not from the climate zone alone.

The misconception. Many think soil can be replaced quickly. Topsoil forms over centuries to millennia, while poor practice can remove centimeters in a few years, so on a human timescale lost soil is effectively nonrenewable.

Worked soil texture from a settling jar (model values): A jar of shaken soil settles into layers: sand 3.0 cm after 1 minute, silt 1.25 cm more after 2 hours, and clay 0.75 cm more after 2 days, total 5.0 cm. Percentages: sand 3.0 ÷ 5.0 = 60 percent, silt 1.25 ÷ 5.0 = 25 percent, clay 0.75 ÷ 5.0 = 15 percent; the three must sum to 100. On the soil texture triangle this is a sandy loam. A second sample of 20 percent sand, 65 percent silt and 15 percent clay plots as a silt loam, typical of soils with a loess cap.

Worked slope angle: A hypothetical hillside rises 3.0 m over a horizontal distance of 6.0 m. Slope = 3.0 ÷ 6.0 = 0.50, or 50 percent, and angle = tan⁻¹(0.50) = 26.6°. Dry sand typically stands at about 30 to 35°, so this slope is stable for dry sand, but a saturated clay soil can fail on a far gentler slope because water pressure and swelling reduce its strength. The skipped step is converting percent slope to degrees: a 100 percent slope is 45°, not 90°.

Worked sediment from a construction site (model values): A storm produces 500 m³ of runoff from a site, carrying 2,000 mg/L of suspended sediment. 500 m³ = 500,000 L; sediment = 500,000 × 2,000 mg = 1,000,000,000 mg = 1,000 kg. A silt fence and basin that trap 60 percent reduce the load leaving the site to 1,000 × 0.40 = 400 kg per storm.

Worked soil formation against erosion: Model values: a field forms soil at 0.5 metric tons per hectare per year and loses 10 metric tons per hectare per year to erosion. One centimeter of topsoil over a hectare is 10,000 m² × 0.01 m = 100 m³; at a bulk density of 1.3 metric tons per m³ that is 130 metric tons. Net loss = 10 − 0.5 = 9.5 metric tons per hectare per year, so a centimeter disappears in 130 ÷ 9.5 = about 14 years, far faster than it could re-form.

Representation task: Draw and label a soil profile from a local pit or road cut (or a teacher photograph), measuring each horizon in cm and noting color, texture by feel and root abundance. Then sketch four kinds of mass wasting (creep, slump, rockfall, debris flow) side by side, each with its evidence at the surface and an arrow for speed from slow to fast.

Safe investigation, landslide and construction-site models: (a) In a plastic tray hinged on a board, build 5 cm deep slopes of sand, a silt loam and a clay-rich soil. For each, set the slope at 20°, at 30° and at the steepest angle the dry material will hold (measure it; dry sand will not stand much beyond about 35°) with a protractor and add water with a spray bottle in fixed 50 mL steps. Record the water added when the slope fails, the failure style (grains trickling, slump, flow) and the runout distance. (b) Build a model construction site of bare soil on a 15° tray, then pour 1 L of water from a watering can at the top; collect the runoff and measure the sediment after settling. Repeat with a strip of fabric as a silt fence and with a straw mulch. Wear goggles, keep water away from outlets and wash hands. What the model cannot show: small trays exaggerate the effect of surface tension, and real slopes contain layered materials, roots and groundwater flow.

Soil contamination investigation: With the teacher, collect small soil samples (gloves on) beside a salted road or parking lot and from a lawn far from it. Stir 20 g of each into 100 mL of distilled water, let it settle and measure the conductivity of the water with a meter; higher conductivity near the road points to road-salt contamination. Record the data, compare the sites and recommend two ways to reduce the contamination.

Independent performance: Produce a soil or geologic map of your school grounds (or a teacher-supplied aerial map) and a 300-word analysis, and present it on a poster with the data table used to make it. Success criteria: the map shows at least three data points with location, texture and slope; it identifies any mass wasting or erosion evidence; it explains the parent material of the soil in terms of Ohio's glacial or bedrock history; it recommends one method to manage storm-water runoff and construction mud; and it explains one way sediment could carry a contaminant off the site.