Soil is Essential in the Hydrologic, Carbon, and Nutrient Cycles

A large proportion of precipitation falling onto Earth's land
surface area falls directly or indirectly onto soil. Depending
soil factors such as soil moisture status, texture, organic
matter content, and soil structure, precipitation may infiltrate
into soil where it may be stored for plant use. Additionally,
some of the water may percolate downward through the soil
or run over the soil surface (surface runoff). Percolation
plays an important role in recharging groundwater aquifers,
whereas, surface runoff often flows directly into surface
water bodies (rivers, streams, and lakes) where it may be
used to support aquatic ecosystems and provide sustainable
water supplies.

Soil is an integral component of carbon cycling on Earth. Soil
provides a medium for plant growth, thus permitting plants to
assimilate carbon into their biomass through photosynthesis.
Upon plant or animal death, soil organisms decompose organism
remains and incorporate a portion of carbon stored within these
organisms into soil to form soil organic matter or humus. The
portion of carbon not incorporated into soil is released to the
atmosphere as carbon dioxide (CO2) via decomposer respiration.
Soils have a great capacity to store soil carbon in the form of
soil organic matter. Therefore, proper soil management can be
used to increase soil organic matter content thus reducing
atmospheric CO2 concentrations. This is very beneficial because
CO2 is a known greenhouse gas that contributes to global warming.

Soils have a large capacity to retain and provide nutrients for
plant growth in natural or managed ecosystems. Nutrients in
soils may be derived from atmospheric deposition, plant
decomposition, and rock and soil mineral weathering. These
nutrients may then be retained on clay surfaces or organic
matter where they can be slowly released into soil water.
Plants and other organisms can then uptake nutrients from soil
water to sustain and enhance growth. However, a portion of these
nutrients may be lost from soils or the plant rooting zone
through a process called leaching.

The Soil Body

Soil is a collection of living and non-living matter that forms
a three-dimensional body (soil body) that covers the surface of
the earth. The soil body begins where the atmosphere meets the
soil surface and ends when bedrock or unweathered parent material
is encountered.

In order to study soil, scientists examine the soil pedon. A pedon is
the smallest unit than can be called a soil. A pedon is a three-dimensional
sample that extends from the soil surface to the deepest roots or genetic
soil horizons. The area covered by a pedon varies from 10 - 100 square feet,
depending on changes in soil properties. Groups of pedons with very similar
characteristics that are closely associated in the landscape are called
polypedons. Polypedons that have a common set of characteristics that fall
within a particular range are referred to as a soil series (Brady and Weil,
1999). Soil series are the basic units used by the U.S.D.A Natural Resources
Conservation Service to map soils within the United States.

Soil Composition

Soil consists of solids, liquids, and gases. The solid material consists of
minerals, organic materials, and pores voids) between solid materials. Pores
hold liquids and gases that are essential for plant and microorganism growth,
and solid materials hold or provide nutrients that can become accessible for
organism growth.

During a precipitation event or snowmelt, water infiltrates into soil through
the pores space and pores become filled. As the soil begins to drain or dry,
air replaces water in the pores. On average, soil contains 50% pore space
which can be evenly divided between air and water.

Soil is formed slowly through the weathering of geologic materials called parent
material, and parent material is defined as bedrock or unconsolidated mineral and
organic matter from which soils develop. As bedrock erodes into smaller particles
near the earth's surface, organic matter decays and mixes with inorganic material
(rock fragments, soil minerals, water, and gases) to form soil.

The processes of weathering include both physical and chemical weathering reactions.

Physical weathering includes the effects of physical factors (temperature,
water, and wind) on rock weathering or breakdown. For example, frost-wedging occurs when
water freezes and expands in a crack within rock. When water enters into a rock and freezes
it can expand up to 10% and effectively pry the rock apart. With sufficient time this
process can cause the rock to fracture and break into smaller pieces. In addition to daily
freeze and thaw of water in the winter, and heating and cooling of rock throughout the year
also fractures rock to generate smaller particles.