Summarizing the Soil

Summary

Plants obtain mineral nutrients from the soil. Soil is the outer loose layer that covers the surface of Earth. Soil quality depends on the chemical composition of the soil, the topography, the presence of living organisms, the climate, and time. Agricultural practice and history may also modify the characteristics and fertility of soil. Soil consists of four major components: 1) inorganic mineral matter, 2) organic matter, 3) water and air, and 4) living matter. The organic material of soil is made of humus, which improves soil structure and provides water and minerals. Soil inorganic material consists of rock slowly broken down into smaller particles that vary in size, such as sand, silt, and loam.

Soil formation results from a combination of biological, physical, and chemical processes. Soil is not homogenous because its formation results in the production of layers called a soil profile. Factors that affect soil formation include: parent material, climate, topography, biological factors, and time. Soils are classified based on their horizons, soil particle size, and proportions. Most soils have four distinct horizons: O, A, B, and C.

Glossary

A horizon

consists of a mixture of organic material with inorganic products of weathering

B horizon

soil layer that is an accumulation of mostly fine material that has moved downward

bedrock

solid rock that lies beneath the soil

C horizon

layer of soil that contains the parent material, and the organic and inorganic material that is broken down to form soil; also known as the soil base

clay

soil particles that are less than 0.002 mm in diameter

horizon

soil layer with distinct physical and chemical properties, which differs from other layers depending on how and when it was formed

humus

organic material of soil; made up of microorganisms, dead animals and plants in varying stages of decay

loam

soil that has no dominant particle size

mineral soil

type of soil that is formed from the weathering of rocks and inorganic material; composed primarily of sand, silt, and clay

O horizon

layer of soil with humus at the surface and decomposed vegetation at the base

organic soil

type of soil that is formed from sedimentation; composed primarily of organic material

parent material

organic and inorganic material in which soils form

rhizosphere

area of soil affected by root secretions and microorganisms

sand

soil particles between 0.1–2 mm in diameter

silt

soil particles between 0.002 and 0.1 mm in diameter

soil profile

vertical section of a soil

soil

outer loose layer that covers the surface of Earth