Soil exists as a thin layer on top of land. It is composed of minerals from rocks and organic materials from living and decomposing plant and animal matter that have been broken down through weathering, along with pockets of open space that are filled with air and water.

Healthy soil capable of fostering plant life is typically composed of 45% minerals, 25% water, 25% air, and 5% organic matter. The development of soil depends on the parent rock that it was weathered from, the precipitation and temperature of the surrounding environment, the presence of good decomposers like bacteria and fungi, and the geological characteristics of the location in which it is forming.

Weathering of rocks and organic matter can occur in three different ways:

Physical changes to the rock due to wind and water
Chemical changes to the rock due to reactions between the rock and other substances, such as atmospheric oxygen or acidic solutions
Changes caused by the activity of living organisms

Depending on the amount of weathering that has taken place and the interactions between the biotic and abiotic components of the soil, the particle size of soil can vary. There are three classifications of rock grains found in soil: sand, silt, and clay. Grains that have the smallest particle size, less than 0.002 mm, are known as clay. Silt is the classification of grains with particle sizes from 0.002 to 0.05 mm. Sand grains feel gritty with the largest particle size, from 0.05 to 2.0 mm.

Like the rocks that make up aquifers, soil can be described in terms of its porosity and permeability (see Explain III: Water Resources and Use). Soil porosity is especially important because it affects the amount of water and air that can be stored in the soil. Course grains, like sand, have larger pores that hold more oxygen, while fine grains, like clay, have smaller pores and retain water better. As a result, permeability decreases as particle size decreases.

The composition of clay, silt, and sand determines the way soil is classified. The diagram below shows the percentages of the grains in different classes of soil. For example, a typical loam is categorized by about 20% clay, 40% sand, and 40% silt. If the percentage of sand is a little higher, the soil is classified as sandy loam. Loams tend to be the most fertile classification of soil. Soils with more clay do not drain well.

Soil types by clay, silt, and sand composition

(Image Source: By Zephyris at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=13490204)

Soils that have had time to mature form layers or horizons. There are four horizons in typical soil. Each layer has its own unique composition and characteristics:

Horizon or Layer	Composition	Characteristics
O: Organic	Leaves, biological waste, live and decomposed organisms	Dark colored, crumbly-texture, thickness varying depending on climate Protects the layers below from erosion and water loss through evaporation
A: Topsoil	Weathered rock and organic material that has moved downward	Also dark colored Most fertile layer Provides anchor for plants
B: Subsoil	Accumulated metals such as aluminum and iron, inorganic rock materials such as sand, silt, and clay	Yellowish, rich in leeched nutrients Not as fertile, provides anchorage
C: Parent Rock	Larger weathered rock fragments Lacks organic material	Transition zone between soil and bedrock May be saturated with groundwater due to impermeable bedrock below

(Image Source: http://soils.usda.gov/education/resources/lessons/profile/profile.jpg)