Compare soil horizons using properties and measured depth

Learning goal: Explain how soil develops from mineral and organic materials under environmental influences, observe differences among horizons and compare profiles using measured properties. Soil is more than loose rock powder, and a horizon is more than a colored band in a memorized picture.

Soil contains mineral material, organic matter, water and air, with organisms contributing to its development and functioning. Weathering supplies or changes mineral material. Plant residues and other organic material enter the system, while organisms mix and transform materials. Water can move substances within or out of the profile. These additions, losses and changes help create properties different from the starting parent material. Parent material is the material in which a soil forms, such as weathered bedrock or sediment carried in from elsewhere.

Scientists use the word soil, and soil and dirt are not the same thing. Soil is the scientific term: the Soil Science Society of America describes it as the loose mineral or organic material at the surface of the land that serves as a natural medium for plant growth, a mixture of minerals, water, air, organic matter and living organisms. Dirt is an everyday word, not a scientific category; the Society uses it for soil that is out of place in our world, such as what is tracked indoors on shoes or gets on clothes. Moving soil on purpose to study it does not turn it into dirt: a sample you dig up, carry to class or mix in a jar is still a soil sample, even though it no longer shows its original horizon order; horizons are measured in a profile observed in place or in a carefully kept core or image of one. Use the term soil when you describe a profile, a sample or a test.

Formation depends on parent material, climate, organisms, landscape position and time. A wet low area and a nearby well-drained slope may develop different profiles even within the same region. Erosion can remove surface material, while deposition can add new material. A deeper dark layer does not automatically prove that one site is older; several processes could explain the difference.

A soil survey map divides an area into mapped soil units, each described by measured properties such as texture, drainage class and depth. The linked Web Soil Survey lets an adult look up the mapped soils for a school site. Combining a soil map with a geologic, aerial or topographic map can help explain why soils differ from place to place, for example between a wet low area and a well-drained slope, or over different bedrock.

A soil profile is a vertical sequence of horizons. Horizons are distinguished by properties such as color, texture, structure and thickness, with other characteristics requiring tests. Some profiles include an organic O horizon, a mineral surface A horizon influenced by organic matter, a B horizon showing subsurface development and a C layer relatively little altered by soil-forming processes. Hard bedrock, labeled R, is not soil.

Not every profile contains every letter in the same thickness or appearance. An E horizon can occur where materials have been removed from a subsurface layer. Some horizons are absent, mixed or buried. The letters organize observations; they should not make students invent layers that are not visible in a sample or source image.

Texture concerns the relative proportions of sand-, silt- and clay-sized mineral particles. Structure concerns how particles are grouped into aggregates. A clump can be much larger than its constituent grains, so clump size alone does not identify texture. Color also changes with moisture and composition. Compare samples under similar moisture and lighting conditions before attributing a color difference entirely to organic content.

Use a prepared profile photograph with a ruler, or observe an adult-approved shallow exposure from stable ground. Record depth downward from the surface, the boundary between contrasting zones and the properties used to identify it. A horizon from three to eighteen centimeters depth has thickness fifteen centimeters, not eighteen. Distinguish depth to a boundary from thickness of the layer above it.

The diagram shows two invented profiles on the same depth scale. Profile A has a three-centimeter organic surface layer (labeled O horizon), a fifteen-centimeter darker mineral layer (labeled A horizon) and a twenty-two-centimeter subsoil zone (labeled B horizon). Profile B has a one-centimeter organic layer, a nine-centimeter darker mineral layer and a thirty-centimeter subsoil zone, labeled the same way. Each displayed profile extends to forty centimeters before the pictured underlying parent material (labeled C layer) begins. The horizon letters A and B inside the columns are different from the profile names A and B above them.

These are model measurements, not a universal O/A/B thickness sequence or observations from a named Ohio site. They permit comparisons: the darker mineral layer is six centimeters thicker in A, and the subsoil zone is eight centimeters thicker in B. Those differences alone do not identify their causes. Ask about vegetation, slope, moisture, disturbance and parent material before proposing an explanation.

Loose soil in a jar does not preserve its original horizon order if it was collected as a mixed sample. A settling activity can help investigate particle-size behavior, but its settled bands are not newly formed natural soil horizons. The processes and timescales differ. Label a jar model according to what it actually demonstrates.

Soil can form slowly relative to the speed at which surface material is lost through erosion or removed during disturbance. Protecting existing structure and organic inputs can matter even though natural formation continues. Avoid promising one fixed number of years for every centimeter of soil; formation rates depend on conditions.

Worked thickness comparison: In Profile A, the darker mineral layer begins at three centimeters and ends at eighteen, so its thickness is fifteen. In B it begins at one and ends at ten, so its thickness is nine. Fifteen minus nine gives a six-centimeter difference.

Worked claim revision: “Profile B is younger because its surface layer is thinner.” Revised: “Profile B has a thinner surface layer in this model; age is one possible influence, but erosion, vegetation, moisture and disturbance also need evidence.” The revision preserves the observation without overstating its cause.

Supported profile investigation: Use prepared photographs, intact classroom soil cores or a safe adult-selected exposure. Do not enter pits or unstable road cuts. Mark the surface reference, measure boundaries and record color, visible roots, texture clues and structure without tasting or inhaling soil. Wash hands after handling approved samples.

Independent performance: Compare two profiles with a common scale, calculate layer thicknesses and explain the properties used to distinguish horizons. Identify missing or uncertain horizons honestly. Propose an environmental explanation with supporting evidence and an alternative to investigate. Explain why a settled mixed-soil jar does not reproduce natural horizon development.