Concept 1. Soil is formed under processes and conditions that have existed for a
long time (SLIDE 1).
The dissection of this plateau into its present features has taken from 10 to 50
million years, through the erosion of the West Branch of the Susquehanna River in northern
Pennsylvania. The weathered material that gives rise to the present-day soils was
disturbed by premafrost and glacial action, so they are not quite all that old, but are at
least 8,000 to 10,000 years old. Most are older. The formation of a complete soil cannot
be observed in one human lifetime.
Concept 2. Soil exists as the interface between the atmosphere and the lithosphere (SLIDE 2).
The lithosphere with its upper weathered mantle is shown. We are not generally conscious of soil and what occurs there as we walk over the landscape. This interface is the locus of the exchange of energy, gasses, and water (see figure 1.1 in the lab manual). During daylight hours, incoming solar radiation is converted to heat and is absorbed by the soil; some is reflected. At night heat is lost to the atmosphere in the form of long-wave radiation; hence, soil temperature fluctuations are greatest at the surface. Precipitation infiltrates into the soil (overland flow is rare in forest soils) and percolates through the profile. Some water is removed from the soil as evaporation or by transpiration, collectively referred to as evapotranspiration (ET). Some water is lost to the groundwater table by deep percolation. The greatest fluctuations in soil moisture also occur near the surface. Thus, the deeper portion of the soil may remain moist for a longer period of time.
Gaseous diffusion occurs between the atmosphere and the porous soil, with the greatest rate at the surface, creating optimum conditions for soil-inhabiting organisms. Vegetation introduces dynamic processes such as the carbon cycle (organic matter decomposition and transformation) and related nutrient cycles. Production of organic acids during these cycles enhances weathering of inorganic materials. Weathering is most intense at the surface. The gradient in transfer on energy, gasses and water results in the formation of the soil profile. As soil forms, weathering of the regolith is reduced so that the soil acts as a partially protective cover to the regolith.
Concept 3. Soil is a dynamic system consisting of physical, chemical and
biological processes.
The exposed root system of a white pine illustrates that these processes occur in all
soils where plants are present, their intensities and significance varying according to
environmental factors (SLIDE 3). These interrelated complex
processes characterize a functioning soil ecosystem.
Concept 4. The process of clay formation is unique to soil development.
Clay is a secondary alumino-silicate mineral, with some very significant characteristics (SLIDE 4). Clay formation is illustrated in the slide by the relatively bright colors and appearance of this subsoil.
Concept 5. Soil physical, chemical and biological properties differ due to differences in soil formation processes.
These properties lend to the soil certain capabilities and limitations for plant growth and use SLIDE 5). . Some of these properties can be manipulated or modified by man, others cannot. The modifications may be detrimental or beneficial. A roadside planting in disturbed soil where the fertile topsoil has been stripped away differs from the intact soil profile in the background. The contrast in the growth of the loblolly pine is obvious, even though the smaller trees may be younger. The foliage is chloritic and the needles are shorter.
Concept 6. Environmental factors such as climate, vegetation, and parent material as modified by topography, all integrated over time, influence the soil development process.
A soils map of the United States (p 68 in the text) shows that different soils are found in different localities (SLIDE 6). One of the main considerations of a resource manager or forest biologist is to recognize and appreciate these differences, and make use of them in management decisions. We will discuss survey and mapping of soils later in the course.
Concept 7. Once the soil has formed, the rate of change decreases significantly because of greater diversity in processes, substances, organisms, and end products; soil tends toward equilibrium.
Other physical conditions, including protection against weathering, are also involved (SLIDE 7). The general trend from unstable to stable state of organization occurs during soil development. The soil in its later stages of development is more resistant to change. However, equilibrium is rarely attained in nature, although a dynamic equilibrium may be achieved.
The Soil Profile
A slice through the soil exposing the cross-section is referred to as the soil profile. The soil is layered in character as the result of the interface effect; it is an isotropic in nature. On a small scale it varies more in the vertical axis than it does horizontally. These layers, termed horizons, are used to describe and classify the soil. Horizon characteristics influence the capabilities and limitations of the soil in question.
Although differences among the profiles shown in SLIDE 8 are not clear in this slide, real differences in the growth of upland oak occur among these soils (SLIDE 9).
Here are four soils derived from the same sandstone parent material, but the profiles are quite different (SLIDE 10). Tree growth differences are also significant. Can you pick out the soil which would have the best growth and which would have the worst? If you picked the Laidig as the best and the soil on the far left as the worst, you are correct on both counts. The knowledge that you will develop during this course will enable you to make a more accurate assessment of productivity.
The two soil monoliths from the norteastern US are different (SLIDE 11). The Maine soil supports a stand of spruce and fir trees while the PA soil is from an agricultural field.
Horizontal differences illustrated in the drawing (SLIDE 12) should be readily apparent. Soil formation, or pedogenesis is a complex combination of processes with one or several being dominant depending upon the influence of environmental factors.
In the drawing you'll note that the first profile labeled (a) is covered by grass, whereas the remaining three have forest cover. This infers differences in climate: the first is under a rainfall regime that is insufficient to support tree growth. Evaporation exceeds precipitation, but the soil profile is moist at some time during the year. Ca is adsorbed by plant roots and stored. When the plant dies soil fauna and flora decompose the tissue, recycling the Ca in soluble form. Biomass turnover is rapid and soil Ca supply is maintained. Little translocation takes place in the profile and the soil remains fertile. This soil-forming process is termed calcification. In (b) a soil profile formed as the result of podzolization is shown. This process is common in regions characterized by cool temperatures, high humidity supporting forest vegetation. Because of the high rainfall, leaching of salts from the profile occurs. Few bases are absorbed by tree roots and the soil becomes acidic from the production of orgainc acids during the decomposition of organic matter. Iron, aluminum and other inorganic materials become soluble in the upper profile and are translocated to a lower horizon. Infertile soils result.
Laterization is illustrated in profile (c). The soil is occupied by forest vegetation (at least originally). However, the climate is warm to hot, humid with high rainfall and no dry season. Leaching of bases is intense, so that hydrolysis of minerals is rapid. These are absorbed by the rapidly growing vegetation. The dead vegetation or forest floor litter is decomposed rapidly. Nutrient cycling is therefore rapid, but the amounts are small. Silica is leached from the soil leaving behind concentrations of iron and aluminum.
The soil may lose fertility rapidly if the forest cover is removed. In (d) of the drawing, we again have forest vegetation under moist climate which may be cool or warm. The soil is waterlogged for one reason or another, and low evaporation occurs. Reduction of iron, organic matter, and silicate compounds occurs under the waterlogged conditions. Gray or bluish colors in the profile result. Few bases are recycled and accumulate in unavailable form in the undecomposed organic matter on the soil surface. These are usually bog or swamp soils.
Soil formation is a continuous process. When we utilize a soil, we are making use of a dynamic system in a particular stage of development. When we use the soil for certain activities, such as agriculture, the pedogenic process is modified to a degree that is related to the magnitude of disturbance.
Slide 15 An example of the initiation of soil development on
igneous rock at Crane Mountain, Johnsburg, New York. The rock becomes progressively
covered with lichens, mosses, and after a rudimentary organic layer is established,
grasses and sedges invade the surface. Physical and chemical weathering begin to
disentegrate and decompose the rock surface, and small rock fragments accumulate. This
condition illustrates 'time zero' or birth of the soil profile. Soil formation is slow
because the solid rock must be broken down before profile development can proceed rapidly.
Thus, rate of weathering is directly related to the amount of surface area exposed to the
forces of weathering. In the background is unconsolidated glacial till in which a soil
profile has already formed. Thus, the forested area is in a later stage than the bedrock
area. The time sequence, or life cycle, of soil profile development is illustrated in
Figure 1.4.
Slide 16A roadcut exposing a soil profile in the early part of
(c) stage of Figure 1.4. The original soil profile was partially destroyed by glaciation
and the present one has developed since Pleistocene (Ice Age) time. Note the bedrock and
the fact that there is very little C horizon here
Slide 17 A roadcut exposing bedrock, parent material and solum
of a steeply sloping soil in the Ridge and Valley Province of Pennsylvania. Note that
bedrock, a sandstone, is tilted, thus controlling the degree of slope.
Slide 18A profile of the Leetonia series of the same landscape
as in Slide 17. This soil is derived from acid-gray quartzite, and is normally shallow and
excessively drained, occurring on high slopes and ridgetops. Note the well developed E
horizon and the very light yellowish-brown B horizon beneath. This soil lacks a discrete,
continuous horizon, because of the concentration of stones on the surface. The stones have
accumulated as the result of a combination of partial glaciation and gravity with
long-term weathering of a fractured, but resistant, bedrock.
Slide 19 A profile of the Lakewood series of the Lower Coastal
Plain of North Carolina. This soil has formed on unconsolidated marine sediments or old
beach deposits. Contrast this profile with that of the Leetonia series (Slide 18) in
regards to the degree of E horizon development and also the coarse fragment contents. This
soil is quite sandy with very little silt and clay present.
Slide 20 A profile of the Palmyra series derived from gravel
deposits of glaical lakes near South Onondaga, New York. Note the rounded gravel
throughout the profile, the wavy boundary or tongues of the solum penetrating the parent
material. Because of their high fertility (the gravel is a mixture of limestone and other
rock types) and good drainage, these soils are highly productive. Note also the upper
plowed layer or Ap horizon.
Slide 21 The Hagerstown series of the limestone valleys of
Pennsylvania. Notice how deep this soil is compared to the previous profiles, as well as
the sparseness of coarse fragments. Under humid conditions, limestone weathers deeply to
form fine-textured, well drained fertile soils with very few, if any, limitations for use:
a major reason why the Pennsylvania Dutch (actually from Germany and Switzerland) are
prosperous farmers and can maintain their frugal way of life.
Slide 22 This soil profile also exhibits little horizon
differentiation and is the Bowie series of the Middle Coastal Plain of southern Alabama.
It contains more silt and clay than the Lakewood series (Slide 19). Note that both the
Lakewood and Bowie series support stands of longleaf pine.
Slide 23 A soil derived from shaly glacial till, quite thin
with a distinct Ap horizon and a fragipan in the B horizon. Note the grayish color due to
restricted aeration in the cracks between the peds and the presence of roots there. This
is the Aurora series of the Allegheny Plateau near Jamesville, New York