Why does one soil look different than another? What
geological forces act upon soils and what are the effects of the action? The famous
geologist, Hans Jenny, answered these and other questions in his description of five
factors responsible for soil formation.
- Rocks and minerals change by the processes of weathering.
Typically weathering causes mineral materials to disintegrate into smaller parts. The
elements released as products of weathering may form new, secondary minerals. Weathering
products that are loose or unconsolidated are called soil. Weathering can be accomplished
by one or a combination of physical and chemical processes.
Physical weathering is most pronounced in cold and dry climates. Physical processes
include effects of temperature. Most notable is the force exerted by the expansion of
water as it freezes. Another physical process is abrasion caused by bombardment of
minerals by materials suspended in wind and flowing water. Finally, plant roots
established in the crack of a rocks often exert a force strong enough to cleave the rock.
Chemical weathering dominates in warm or moist climates. Worldwide, chemical weathering
processes tend to be more important in soil formation than physical forces. The chemical
processes are described in the textbook. The processes include: oxidation and reduction
(of great importance for iron-containing minerals), carbonation (dissolution of minerals
in water that has been made acidic by carbon dioxide), hydrolysis (when water splits into
hydrogen and hydroxide, and one or both components participate directly in the chemical
process), and hydration (when water is incorporated into the crystal structure of a
mineral, changing mineral properties). Minerals differ greatly in their resistance to
weathering (see Table 2-1 in the textbook).
- Soil Formation means the development of a particular soil
in a particular place. Weathering produces unconsolidated mineral material that one
would call soil. However the formation of any given unique soil includes process that
continue to operate long after this unconsolidated material is formed. The products of
weathering are the materials in which soils form. Soil materials are often transported
vertically within the soil profile (see Figure 2-2 in the textbook). Materials can be
added (such as humus from plants or sediments added by wind or water) or can be lost (such
as by erosion or by leaching through the soil profile). The unconsolidated materials in
which soils form is called parent material. The term, soil formation, implies the
formation of soil horizons and other features in the parent material.
- Parent material arrives on location through various
routes and mechanisms. The parent material in a landscape may have been transported to
the location (see Figure 2-9 in the textbook). Residual soils formed in place from
weathering of the underlying bedrock. Residual soils are common in the great plains of the
United States. Residual landforms include mesas, plateaus and plains.
Stream deposits are very common worldwide. Such deposits are called alluvium or alluvial.
Examples of alluvial soils are: flood plains (as along the Mississippi River), alluvial
fans (as in the piedmont region of the southeastern states), lacustrine deposits
(dispersed material deposited as a stream entered a lake), and marine deposits
(flocculated or aggregated material deposited as a stream entered the sea). In some
instances marine deposits form deltas.
Wind deposits are called eolian materials. Only small particles are readily carried by
wind. Large sands are too heavy to be carried by most winds. Clays also rarely blow
because they form aggregates with other clay particles producing large clods or peds.
Silts and fine sands are examples of material readily carried by wind. Silts are small
mineral fragments, barely visible to the naked eye. (A typical silt particle is about as
wide as the thickness of a sheet of paper.) Wild-blown silt deposits are important in
agriculture for the ease with which they are cultivated. Such deposits are called loess.
Sand dunes and landscapes made of ash are also eolian.
Glaciers have profoundly affected the landscape of the northern states. These deposits
from moving sheets of ice are called glacial materials or glacial till. During past 1.5
million years (Pleistocene--modern times) the world has experienced four ice ages. The
most recent was the Wisconsin ice age, ending 12,000 years ago. Much of the soil in the
midwest came from Canadian glaciers. Glaciers leave landforms such as: lateral moraines,
terminal moraines, ground moraines, and outwash plains.
Gravity deposits are colluvial material or colluvium. Such material results from
mass-wasting, mud flows, or the gradual movement of individual particles down a slope.
These deposits are found on a relatively small-scale such as mountain valleys, and are
usually coarse materials (see Figure 2-9 in the textbook again).
- According to the classic work by Hans Jenny, five
soil-forming factors account for the differences in soils. The five factors are:
climate, relief, organisms, parent material, and time. The acronym "CROPT" may
help you remember this.
- Parent Material is the only factor that can be considered
inherited as opposed to acquired. The effect of parent material on a soil include such
feature as soil texture, pH and mineral constituents.
- Climate is often considered the most powerful
soil-forming factor. Climate is expressed as both temperature effects and rainfall
effects. Temperature controls rates of chemical reactions. Many reactions proceed more
quickly as temperature increases. Warm-region soils are therefore normally more developed
or more mature than are cool-region soils. Rainfall affects leaching, pH and soil
aeration. In addition to direct effects of climate, climate also profoundly affects
vegetation which in turn also affects soil formation.
- Organisms (biota) affect and are affected by soil
formation. Man is perhaps now the most influential of all organisms. He affects the
soil by such activities as: plowing, irrigating, mining, clearing, disposing and leveling.
The effects of large animals other than man on the land are minor. The effects of
vegetation on soil formation are very profound. Different soils form in a grassland than
under a forest. Much of this difference is due to the rapid nutrient cycling in
grasslands. Vegetation effects extent of cover, thereby influencing runoff and erosion.
Vegetation type and amount directly influences the type and amount of organic matter
accumulation on the soil, and thereby influences such soil chemical properties as pH and
nutrient supply. Finally, vegetation is the food source for most microorganisms so the
vegetation exerts a strong influence on soil microbial populations.
- Relief (topography) modifies the effects of other
factors. Relief modifies climate by affecting the smoothness of the surface and also
the angle at which the soil surface orients towards the sun. A convoluted surface dilutes
solar energy over more surface area than does a smooth surface. In the northern hemisphere
a north-facing slope will be cooler than a south-facing slope. Relief also affects the
amount of rainfall that infiltrates a given parcel of soil. A steep slope will encourage
runoff. A soil in a sloping location will experience less effective rainfall than that
which one would measure in a rain gauge. Likewise, a low area may receive run-on water
beyond the actual rainfall. Also, relief influences erosion. Soil horizons form from the
top downwards. If the topsoil readily erodes away as it forms, the soil formation
processes appear to have halted.
- Soil formation is a function of time. Soil
development is a process, not an event. Soils change over time. Clays are secondary
minerals. They form in the soil, then change forms, and all the while they are moving
downward with leaching rainwater. Similarly, organic matter forms on the surface as it
moves downward with rainwater. It declines after reaching a maximum as old soils loose
their ability to produce vegetation fast enough to keep up with decomposition. These are
highly dynamic processes. Soils as viewed today are just snapshots in time. Soils looked
different in the past and will look different in the future.
Students are encouraged to look up the following vocabulary
words in the textbook glossary and to browse the following web site.
An elder statesman/soil scientist/artist/philospher opines
about soil. URL: www.soils.wisc.edu/soils/poets/hole1.htm