Soil texture refers to the relative proportions of different-sized soil particles, and soils are considered to be clay,
sand, silt, or a mixture of two or more of these types, depending on their composition. Soil texture plays a major
role in the soil’s water-holding capacity and is rated from
fine to coarse. Fine soils with high clay content hold water for much longer than sandy, well-drained soils. Fine
soils have much smaller pore spaces, which impacts the
movement of air and water and decreases the ability
of plant roots to penetrate. Soil depth can vary widely
across a landscape, depending on parent material, slope,
and vegetation type. Mountaintops tend to have shallow,
rocky soils, and level ground at the base of slopes generally has much more soil accumulation.
While a soil in a given area originates from the same
parent material, over time it begins to become diversified
with depth from the soil surface, becoming layered with
the downward movement of organic material through
the soil. We call this vertical layering the soil profile,
and each layer within the profile is referred to as a horizon. The surface layer, or O horizon, consists of organic
material that accumulates from decomposing plant materials such as needles, twigs, and leaves. Just below this
is the A horizon, more commonly referred to as the topsoil. While this layer is made up primarily of weathered
parent material, it is relatively rich in organic materials
due to leaching from the O horizon above it. The B horizon, sometimes called the subsoil, has limited organic
matter and often has accumulated mineral particles due
to leaching from the topsoil. This horizon is usually quite
dense, making it difficult for many plants to extend roots
down into this layer. Finally, the C horizon lies beneath
the subsoil and is comprised of unconsolidated materials.
There is low biological activity in this layer, and it retains
many characteristics of the parent material. Below the
C horizon is the bedrock or parent material (sometimes
called the R horizon). Very different soils arise from regional differences in parent material, climate, and vegetation communities. Soils are classified into twelve orders, each with distinctive characteristics.
Water and Light
Water is a critical component for all organisms, as its
presence in biotic tissues is needed to facilitate all physiological functioning. The amount of water present in an
ecosystem is an important determinant of what types
of plants and animals can live there. As we saw earlier,
ocean and atmospheric currents largely drive large-scale
precipitation patterns, with regional influences by topography. In addition, the moisture-holding capacity of
an area’s soils impacts how much of this precipitation is
effectively stored in the ecosystem for use by biological
organisms. Water availability is also influenced by surface runoff (overland flow of excess water), downward
percolation through pores in the soil, evapotranspiration (the sum of the water lost from evaporation plus
transpiration—the movement of water through a plant),
and groundwater flows. These processes are the major
Figure 1.16
Chlorophyll reflects green wavelengths from the sun, making
plants look green.
The conversion of sunlight into carbon compounds
by photosynthesis drives the production of energy
for use by biological organisms.
pathways by which water flows through an ecosystem.
Extreme conditions of very low or very high water availability create unique challenges for organisms that inhabit these areas, and many have evolved unique strategies to persist in these harsh environments.
The conversion of sunlight into carbon compounds by
photosynthesis drives the production of energy for use
by biological organisms. Photosynthesis has two major
steps: the first is harvesting energy from sunlight and the
second is fixing carbon to generate carbohydrates. The
rate of photosynthesis determines the supply of energy
available to organisms, which impacts individual growth
rates, reproductive success, and ultimately the geographic range of a species. The availability of light, then, is an
important driver of ecosystem productivity.
In aquatic ecosystems, there is a strong vertical gradient of light, with the greatest light availability at the
water surface and a rapid decline with depth due to the
absorption of light by water. In terrestrial environments,
light is absorbed by the plants themselves, influencing
the amount of light that can penetrate to the Earth’s surface. In a dense forest, much of the light entering the
ecosystem is absorbed by the highest layers of the forest
USAD Science Resource Guide • 2015-2016 • Revised Page
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