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iron, and aluminum, while the sand in a soil originating from weathered granite
supports a texture that allows the soil to drain. The erosion, transport, and deposition of weathered granite bedrock from the Canadian Shield to the Midwestern
USA by glaciers 10,000–25,000 years ago is one of the reasons there is a high concentration of fertile well-drained soils there.
In contrast to recently glaciated soils, highly weathered surface soils—which
may have been in place for hundreds of thousands of years—typically have lost
many nutrients and their structure and chemistry have changed due to the weathering process. These changes follow predictable chemical patterns based on parent
materials and climate, with the oldest soils containing only relatively resistant primary minerals like quartz, silicate clays, and aluminum and iron oxide clays—the
latter of which may be observed as a pronounced reddish soil color. In warm climates with high rainfall such as the southeastern USA the weathering-induced loss
of nutrient adsorption sites on the soil’s clay particles may allow substantially more
fertilizer to be lost from the root zone than would occur on a Midwestern US soil
(the associated nitrogen loss to groundwater supplies can have significant human
health effects). In warm dry climates like the southwestern USA where evaporation
often exceeds precipitation, areas with a high groundwater table may experience the
concentration of salts and calcium carbonate in a concrete-like layer at the soil surface that discourages plant growth. Both the history of the soil’s formation and the
current climate therefore have considerable influence on the vegetation that can be
supported on the land.
Landscapes containing older soils may also contain newer soil formations due to
the activity of volcanoes, wind, and flooding, resulting in specific characteristics
depending on landscape features, geologic characteristics, climate, and the amount
of time soil formation processes have been at work. For example, volcanic ash is a
unique material in its light weight and ability to hold water and be compacted; eruptions may deposit it widely across the landscape or concentrate it thickly in lower
areas, changing the growing characteristics in those areas. Silt, clay and fine sand
particles also may be transported long distances by wind and the resulting soil,
termed loess, typically has high potential for fertility under the right climatic conditions due to the nutrient-holding capacity associated with its fine particles. Transport
of fine particles via the sedimentation process is one of the reasons for the richness
of soils formed in the alluvium of river floodplains.
The biogeochemical cycle and other soil formation processes work together to
form different soil structures—also called pedoliths or peds—over geologic timeframes, starting with unconsolidated deposits or parent materials. In the case of
loess, its chemical composition results in particulate alignment that allow the soil to
cleave off in near-vertical bluffs. More typically, agriculturally productive soil contains small blocky peds that allow for good aeration and drainage of plant roots and
ample nutrient retention. In contrast an overly cultivated soil may have had its peds
broken down to a size that negatively affects aeration, drainage and plant growth,
through the mechanical mixing and compaction of particles, chemical modification,
and loss of soil microorganisms due to plowing and irrigation.
N. Matthews et al.
iron, and aluminum, while the sand in a soil originating from weathered granite
supports a texture that allows the soil to drain. The erosion, transport, and deposition of weathered granite bedrock from the Canadian Shield to the Midwestern
USA by glaciers 10,000–25,000 years ago is one of the reasons there is a high concentration of fertile well-drained soils there.
In contrast to recently glaciated soils, highly weathered surface soils—which
may have been in place for hundreds of thousands of years—typically have lost
many nutrients and their structure and chemistry have changed due to the weathering process. These changes follow predictable chemical patterns based on parent
materials and climate, with the oldest soils containing only relatively resistant primary minerals like quartz, silicate clays, and aluminum and iron oxide clays—the
latter of which may be observed as a pronounced reddish soil color. In warm climates with high rainfall such as the southeastern USA the weathering-induced loss
of nutrient adsorption sites on the soil’s clay particles may allow substantially more
fertilizer to be lost from the root zone than would occur on a Midwestern US soil
(the associated nitrogen loss to groundwater supplies can have significant human
health effects). In warm dry climates like the southwestern USA where evaporation
often exceeds precipitation, areas with a high groundwater table may experience the
concentration of salts and calcium carbonate in a concrete-like layer at the soil surface that discourages plant growth. Both the history of the soil’s formation and the
current climate therefore have considerable influence on the vegetation that can be
supported on the land.
Landscapes containing older soils may also contain newer soil formations due to
the activity of volcanoes, wind, and flooding, resulting in specific characteristics
depending on landscape features, geologic characteristics, climate, and the amount
of time soil formation processes have been at work. For example, volcanic ash is a
unique material in its light weight and ability to hold water and be compacted; eruptions may deposit it widely across the landscape or concentrate it thickly in lower
areas, changing the growing characteristics in those areas. Silt, clay and fine sand
particles also may be transported long distances by wind and the resulting soil,
termed loess, typically has high potential for fertility under the right climatic conditions due to the nutrient-holding capacity associated with its fine particles. Transport
of fine particles via the sedimentation process is one of the reasons for the richness
of soils formed in the alluvium of river floodplains.
The biogeochemical cycle and other soil formation processes work together to
form different soil structures—also called pedoliths or peds—over geologic timeframes, starting with unconsolidated deposits or parent materials. In the case of
loess, its chemical composition results in particulate alignment that allow the soil to
cleave off in near-vertical bluffs. More typically, agriculturally productive soil contains small blocky peds that allow for good aeration and drainage of plant roots and
ample nutrient retention. In contrast an overly cultivated soil may have had its peds
broken down to a size that negatively affects aeration, drainage and plant growth,
through the mechanical mixing and compaction of particles, chemical modification,
and loss of soil microorganisms due to plowing and irrigation.
N. Matthews et al.