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19. And speaking of caliche, here's a lovely roadcut on the way from Casablanca to Rabat, Morocco, taken back in 1976. This Aridisol exposure contains caliche (calcium carbonate) deposits precipitated from the evaporation of water moving through in the soil.
23. Another young soil, apparently an inceptisol, developing on top of Silurian dolostone of the Byron Dolomite Formation. There is a well-defined topsoil layer (A horizon) and a C horizon that contains pieces of weathered rock material as well as loose, cobble-sized stones. One is tempted to also use the term rendzina to describe it, because the rich brown topsoil has formed over a highly calcareous substrate. This roadcut is along Route 42, Ephraim, Door County, Wisconsin.
22. A soil profile revealed by excavation in Blackhawk Forest Preserve, near South Elgin, Illinois. At first glace, the thick, dark topsoil layer close to the modern bank of the Fox River suggests a Mollisol. But if so, where are the lower, lighter horizons? The topsoil sits directly atop unaltered glacial outwash. Was this site originally a wetland that formed a Histosol? Given the evidence, that seems to be the case. The only other option would be that a very hefty amount of rich topsoil was spread over the outwash in recent times by forest-preserve personnel.
1. Two classic parent materials for soils of the glaciated Midwest and Northeast. The bluish gray deposits toward the bottom are till, an unsorted, crazy-quilt mixture of clay, pebbles, and larger rocks that was deposited under or directly in front of the latest continental glacier to reach these parts during the modern ice age. The lighter and somewhat younger material above is outwash -- a deposit of water-sorted sand, gravel, and small cobbles deposited by glacial meltwater. This shot was taken at the Fox Ridge Stone Company gravel pit, Oswego, Illinois. The Mollisol (prairie soil) profile that originally capped the outwash has here been stripped away.
PARENT MATERIALS OF SOILS
Soils are the living skin of the Earth. All true soils have three basic ingredients: nonorganic mineral matter, living organisms, and humus -- the residual organic matter of former organ-isms. In themselves, parent materials are not soils, but they are the raw material from which soils evolve.
2. A thick deposit of loess (in the U.S., pronounced "luss") blanketing the western bluff of the Illinois and Mississippi Rivers, at Pere Marquette State Park, Illinois. This calcium-rich silt was deposited in river valleys by glacial meltwater, and was subsequently carried by the wind to upland areas. When exposed by road excavation or erosion, loess characteristically forms vertical faces like this one. Thinner deposits of loess in farm areas have done much to ensure the good tilth and general quality of the Midwest's most fertile agricultural soils. Note the thin soil profile (dark horizons, or layers) developing here on top of the loess.
3. In northeastern Illinois, the draining of a lagoon at the Chicago Botanic Garden reveals still-water deposits laid down in a vanished embayment of an earlier, higher version of Lake Michigan. The buff-colored parent material that lies atop the stony, steel-gray Wadsworth Formation till is probably oxidized Grayslake Peat. To the south, within the city limits of Chicago, the parent material is often gloppy and poorly drained lakebed clay--a fact much regretted by Windy City gardeners and landscapers.
4. A closer look at the Grayslake Peat deposit at the Chicago Botanic Garden. As the freshly dug hole shows, this mucky material is originally as black as charcoal, but it quickly oxidizes to a buff or ash-gray color when exposed to the air. This sediment contains herbaceous plant stems, bivalve shells, and sand grains, all suggestive of a marsh environment not far removed from a sandy shore or feeder stream. Also note the polygonal cracking in this rather fine-grained material as it dries out.
6. The weathered derivative of bedrock can be a parent material, too. At Buffalo Rock State Park, Illinois, glacial drift is absent, perhaps because it was scoured away 15,500 years ago by the great flood of meltwater geologists call the Kankakee Torrent. Here trees cling to a thin mantle of soil that lies directly on top of the Pennsylvanian-Period Colchester Coal (the jet-black seam at upper center). In other words, a modern woodland now winds its roots around the remains of a 300-million-year-old swamp forest.
SOIL-FORMING PROCESSES
8. The bravest of the brave. On the surf- and wind-lashed shore of Schoodic Point, in Maine's Acadia National Park, these remarkably enduring lichens (the yellow Psilolechia lucida and at least one gray foliose species as well) form the first wave of colonizers on bare rock. Actually sym-biotic associations of fungi and algae or cyanobacteria, lichens contribute to organic weathering by secreting acids that slowly break solid rock down into uncon-solidated parent material. The boulder at right is also an example of physical weathering. Its transverse crack is probably the result of either thermal stress or frost wedging. In this harsh climate, water trapped in crevices freezes, expands, and pries rock apart.
10. Nothing succeeds like success. Once humbler organisms create pockets of rudimentary soil on an otherwise barren surface, more substantial plants arrive. Here, on the coarse-grained crystalline rock of New Hampshire's Mount Cardigan, there are small islands of surprisingly complex plant communities that resemble embattled platoons huddling under heavy fire. In the first ranks, lying prone, are crowberry and mountain cranberry; behind them stand Labrador tea, sheep laurel, mountain ash, and dwarfed red spruce. Whether these islands represent the slow expansion of life on an originally barren surface, or are the only surviving segments of a once-larger plant community, they are now contributing to the development of soil.
9. Another sort of lichen, called reindeer moss (Cladina species), growing on the sand of a fixed dune, Whitefish Dunes State Park, Wisconsin. Its soil-making proclivities are here shared with some sand-loving plants. The round-ed form of reindeer moss resembles shrubs in miniature -- hence its use, when spray-painted green, in dioramas and model-train layouts.
URBAN AND SUBURBAN SOILS (OR A LACK THEREOF)
If soils could scream, they'd do most of their screaming in our cities and suburban-sprawl zones. (Maybe it's time someone established a Soil Abuse Hotline.) While all of my urban street tree pictures shown below were taken when I lived in New York City, no indictment of that locale is intended. The same scenes can be found in practically any other town in the world. In fact, New York is generations ahead of most other cities in matters of enlightened urban forestry.
OTHER TYPES OF HUMAN IMPACT ON SOILS
THE SOIL ORDERS
Soil orders are the equivalent of the biologist's kingdoms of life. Accordingly, they represent the highest and most generalized level of soil classification. Their names contain the suffix -sol, from solum, a Latin term for "ground" or "soil." (Note: I here us the U.S. NRCA Soil Taxon-omy system.)
11. This farm field is in rural Attica, north of Athens, Greece. The limestone outcrops above the field are typical of much of the Mediterranean landscape. As limestone is weathered and dissolved, it leaves behind a rusty-hued, iron-rich clay compound called terra rossa. (In Italian, this means "red earth"; the term is often misspelled terra rosa--which means "pink earth.") The reddish topsoil here shows the strong influence that terra rossa has on soils of this region.
5. Sand is another common parent material, especially near aggradational shorelines where currents, wave action, and the wind are busy building up, rather than eroding, the coastline. These low dunes are on the western shore of Lake Michigan, at Illinois Beach State Park, Illinois.
20. An Entisol, the youngest of soils. Eventually, this soil will turn into a Spodosol (a sandy soil with more developed horizons), but now it has only the thinnest topsoil on top of the sand parent material. On the southern shore of Lake Michigan, at Indiana Dunes State Park, Indiana.
24. Illinois' version of Black Gold. This exposed epipedon (topsoil or A horizon) is in the prairie-reconstruction area of the Fermilab complex, Batavia, Illinois. While this land was intensively farmed for several generations, it was first part of the once-vast Midwestern grassland -- as evidenced by its very dark, Mollisol topsoil. Each year, a portion of the extensive, fibrous root systems of prairie grasses dies back and rots, giving the surrounding soil the welcome gift of added organic matter.
21. There is one soil order with a higher organic content. In fact, it is virtually all organic content. Histosols form in wetland environments--bogs, fens, swamps, and marshes. They are basically one continuous horizon, made of peat. Here, at Volo Bog, in northeastern Illinois, is the perfect Histosol environment: a floating mat of peat moss, poison sumac, tamarack trees, and many other fascinating peatland species that form a strange but vibrant community living in the confines of a glacial kettle.
12. An Alfisol epipedon near Belle Rive, in southern Illinois' Mount Vernon Hill Country. The light brown topsoil is characteristic of a forest soil. Most nonfarmers assume that such soils must be the most agriculturally desirable, given the abundant leaf litter that once blanketed their surface. In fact, little of this organic material ever mkes it into the soil. Instead, it is consumed with impressive efficiency by surface-dwelling fungi, bacteria, and small animals that together ultimately consume about 95% of all plant material. While Alfisols are often quite fertile for other reasons, their topsoil is thinner than that of the prairie Mollisols found to the north, and in modern times they are not as favored for maize (corn) and soybean production. In the early nineteenth century the situation was reversed -- once the trees were cleared away, alfisols were much easier to plow. An early farmer would rather do battle with a dense forest than a field of tough prairie grasses.
13. Not all Alfisols have light-brown topsoil. The forest soils of northeastern Wisconsin are often pink, orange, or even maroon. Such reddish hues are indicative of iron-oxide content, but in this case it's not caused by the presence of terra rossa. Rather, the parent material is glacial till--unsorted rock debris transported by the most recent continetal ice sheet from the iron ranges of Michigan's Upper Peninsula. This farm field is located in Manitowoc County.
14. A closeup of the site shown in Photo 13. Even after many decades of farming, this soil reveals its close ties to its parent material and the environment in which the soil itself formed.
25. A scene from the rolling moraine country of Kane County, Illinois, west of the Fox River. Is the farm soil across the road a Mollisol, an Alfisol, or both? In fact, Mollisols predominate in this area. What's shown here is a good example of the concept called catena -- the expression of how one basic type of soil varies with the changing lay of the land and other environmental factors. Lighter topsoil on the swells and darker topsoil in the swales is one of the most common sights in Illinois farm country . The low areas are natural collection sites for moisture and organic matter washed downslope.
26. In terms of its climate, its human culture, and its natural history, the unglaciated southern tip of Illinois resembles America's Old South more than it resembles the rest of its own state. At Shetlerville, in a ravine running down to the bank of the Ohio River, one can find soil profiles that resemble the ancient, nutrient-poor Ultisols of Cotton Country. Here the gooey subsoil (B horizon) is reminiscent of Georgia's famous red clay.
27. This street-tree pit in Brooklyn, New York shows a common problem encountered in urban and suburban environments. Trees are planted at great expense, as part of highly publicized civic-improvement projects, but then the most crucial factor of all, the soil they grow in, is neglected. Here a shockingly humus-poor soil has been trampled into a concretelike consistency. Crucial under-ground pore space has been eliminated. The tree's roots are robbed of the oxygen they need, and the soil is almost completely impervious to water.
28. Another instructive scene from the Park Slope section of Brooklyn. Whatever that toxic black glop in the gold can really is, the odds are ten to one that it will end up in this London Plane Tree's already-miserable soil. Also note the attractive scenic backdrop of the dumpster pushed against the tree.
29. It seems that some pit bull resented this sign. Most urban pet owners choose to believe that their dogs' toilet habits help to fertilize trees. If so, perhaps Rex and Fifi should be trained to visit their owners' houseplants. In fact, "dog wastes" acidify the soil and are high in toxic salt content.
30. Getting no respect. Some urban trees are so ashamed of their soil that they take special effort to hide it.
31. And then there's suburbia. Try estimating the per-centage of soil here that hasn't been paved over, in this DuPage County neighborhood west of Chicago. Also, try to imagine what happens to a landscape where almost all of the rainwater and snowmelt drains into storm sewers instead of seeping into the soil to replenish the ground-water. What do you think the ramifications of this might be? Here's a hint: those ramifications often extend hundreds of miles downstream.
32. Ah, progress. Welcome to Phase I of Bluebird Meadow Estates. It's going to be a fine example of gracious countryside living. There's just one problem. The soil is gone. It has been scraped off the naked glacial till you see here -- try growing a garden in that stuff -- and has been dumped in a pile on the edge of the property. There its precious living community of microorganisms and inver-tebrates will bake in the sun and freeze in the winter winds until it is no more. Eventually, the "developer," if not completely unscrupulous, will spread part of this lifeless heap of soil back on the houselots. But in doing so the crucial soil structure is lost, as the future inmates of this compound will learn when their Crimson King maples and Family Mix turf begin to languish. But, as I tell my suburban soil-science students, there is a solution. Just keep the property in your family for the next fourteen thousand years, and the soil will heal itself.
33. It's hard for Americans to imagine, but there are quite a few places in the world that have been in constant cultivation for thousands of years. This red-tinted soil, a terra-rossa type, lies in a farm field near Gaeta, in the southern part of Italy's Latium district. It could be that the famed Roman orator Marcus Tullius Cicero paused here, as he fled Marc Antony's henchmen in 43 B.C. After all, his tomb lies nearby; he did not escape his pursuers. Most terra-rossa soils are classified as Alfisols.
34. At the Chicago Botanic Garden, in Glencoe, Illinois, there is an uneasy marriage between the ornamental plantings and the ancient sediments on which this horticultural showplace was constructed. Along the shores of the manmade lagoons the soil has slumped away in spots to reveal the underlying lakebed clay. Vigorous efforts have been made to correct or obscure this natural process, but the forces of nature are no repecters of human aesthetics.
15. Perched atop a vertical cliff of Neapolitan Yellow Tuff at Cuma, Italy is an Andisol profile that is host to a community of maquis (Mediterranean scrub) vegetation, including succulent agaves introduced from the New World. Derived from volcanic ash and rock, Andisol topsoils often have a high humus content that rivals that of Mollisols.
16. Looking southeast at the volcanic complex of the Campi Flegrei -- the Flaming Fields -- of antiquity, from Cuma. Here the heavy agricultural use attests to the natural fertility of Andisols. Incidentally, Cuma, or ancient Cumae, was the home of that legendary prophetess of the Graeco-Roman world, the Cumean Sibyll.
35. The upper reaches of Mount Monadnock, in southwestern New Hampshire. The rocky summit of this famous hiker's destination may seem to be above the tree line, but at the relatively modest altitude of 3,165 feet above mean sea level it isn't. Even in historical times it was covered with spruce trees and other vegetation. In 1800 a large fire killed the summit's plant community; two decades later, local farmers deliberately set another fire to drive wolves out of the charred remains of the forest. This removed all impediment to erosion, and the preexisting soil was quickly blown and washed away.
7. Volcanic ash and its derivatives can be excellent parent materials. This rock outcrop, at the famous archaeological site of Cuma, Italy, is made of the 12,000-year-old Neapolitan Yellow Tuff -- ash that has turned to stone. This tuff, which is by no means tough, is quite crumbly and has here weathered into fantastic shapes. The camera lens cap provides scale. (See also Mediterranean Geology Gallery, no. 12.)
PALEOSOLS (ANCIENT SOILS PRESERVED IN THE ROCK RECORD)
- Alfisols
- Andisols
- Entisols
- Inceptisols
- Histosols
- Ultisols
- Mollisols
- Aridisols
17. John Ruskin wrote that there is no such thing as bad weather -- only different types of good weather. The same goes with soils. Aridisols, the desert soils, may seem the very essence of a farmer's or gardener's nightmare, but they were never destined to support intensive farming or that dreary bane of modern American life, lawn turf. (Hear that, Scottsdale?) Instead, they are home to such marvellous organisms as this drought-resistant acacia. This Sinai Peninsula locale features one classic characteristic of Aridisols: the stony blanket known as desert pavement.
18. Half a world away from the preceding photo, at the Stillwell Ranch, Brewster County, Texas. This is a good Chihuahuan-Desert scene, and once again, there are drought-adapted plants (prickly pear cactus and xerophytic shrubs) and a covering of desert pavement. Beneath the soil surface there are, most likely, carbonate deposits, termed caliche, that form when mineral-rich rainwater is washed into the soil.
36. This ranch scene in Presidio County, western Texas shows the effects of overgrazing on what once were thriving desert grasslands and other drought-resistant plant communities. While the chief losers here are the native flora and fauna, the soil is affected, too.
37. A roadcut along Route 118, between Alpine and Big Bend National Park, Texas. An older basalt lava flow has been covered by a somewhat more recent and darker basalt flow. In between, in the middle of the photo, is an ancient soil -- probably a paleo-inceptisol -- that developed between the two eruptions. Along the upper edge of the topsoil (A horizon) there is a narrow "baked zone" where the younger lava cooked the soil as it rolled over it. Also, notice the subsoil (C) horizon, composed of bits of rock derived from weathering of the underlying basalt, in the red clay matrix.
38. A much older paleosol, dating to the late Proterozoic Eon (and therefore more than 600 million years old) is exposed along Route 501 in George Washington National Forest, Virginia. The upper portion of this outcrop is the low-grade metamorphic rock phyllite -- though it began as a clayey soil at a time where land plants and other important soil-dwelling soil organisms had not yet evolved.
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