Soil Basics
Soil is comprised of mineral and organic components, as well as voids. The mineral portion of soil includes three size classes of particles: sand, silt, and clay. For the average homeowner and gardener, we discuss these size classes based on the United States Department of Agriculture (USDA) classification system. In that system, mineral soil is defined as particles less than 2.0 millimeters in diameter. Sand sized particles fall between 2.0-0.05 millimeters, silt ranges from 0.05-0.002 millimeters, and clay sized particles are less than 0.002 millimeters. Anything greater than 2.0 millimeters is defined as a rock fragment.
Organic matter typically comprises are relatively small volume of a soil, and is concentrated in the near-surface zone. The dark black and brown colors we associate with topsoil are a result of organic matter present in the soil. Topsoil is actually a general term used to describe organically enriched surface soil horizons (A horizons), but is not a term used in soil science directly.
The rest of a volume of soil is comprised of void spaces (pores) which contain either gases (atmosphere) or liquids (water). Although we sometimes overlook this portion of the soil, it is an absolutely critical component that has a tremendous effect on the development and properties of a particular soil.
Soil is comprised of mineral and organic components, as well as voids. The mineral portion of soil includes three size classes of particles: sand, silt, and clay. For the average homeowner and gardener, we discuss these size classes based on the United States Department of Agriculture (USDA) classification system. In that system, mineral soil is defined as particles less than 2.0 millimeters in diameter. Sand sized particles fall between 2.0-0.05 millimeters, silt ranges from 0.05-0.002 millimeters, and clay sized particles are less than 0.002 millimeters. Anything greater than 2.0 millimeters is defined as a rock fragment.
Organic matter typically comprises are relatively small volume of a soil, and is concentrated in the near-surface zone. The dark black and brown colors we associate with topsoil are a result of organic matter present in the soil. Topsoil is actually a general term used to describe organically enriched surface soil horizons (A horizons), but is not a term used in soil science directly.
The rest of a volume of soil is comprised of void spaces (pores) which contain either gases (atmosphere) or liquids (water). Although we sometimes overlook this portion of the soil, it is an absolutely critical component that has a tremendous effect on the development and properties of a particular soil.