Soil Formation & Properties

You’ll be able to

Order and describe the major soil horizons in a soil profile
Use the soil texture triangle to classify a soil from its sand, silt, and clay content
Relate soil texture to porosity, permeability, and water-holding capacity

How soil forms

Soil forms slowly as weathering breaks parent rock into mineral particles, which mix with organic matter from decaying plants and animals. Physical (mechanical) weathering — freezing water, abrasion, root growth — breaks rock into smaller pieces without changing its chemistry. Chemical weathering — acid, oxidation, dissolving — alters the minerals themselves. Add decomposers, water, and time, and the result is soil: a mix of minerals, organic matter (humus), air, and water. Forming a few centimeters of fertile topsoil can take hundreds to thousands of years, which is why soil is treated as a slowly renewable resource.

The soil profile: horizons

A vertical slice of soil reveals distinct layers, or horizons. The O horizon on top is organic litter — fallen leaves and humus. Below it, the A horizon (topsoil) blends humus with mineral particles and holds most of the biological activity and plant roots. The B horizon (subsoil) collects minerals and nutrients leached (illuviated) down from above — a process called leaching when water carries dissolved nutrients downward. The C horizon is partially weathered parent material, grading into solid bedrock below. Topsoil (the A horizon) is the layer agriculture depends on.

Texture controls how soil behaves

Soil texture is set by the proportions of three particle sizes: sand (largest), silt (medium), and clay (smallest). Texture drives two key properties. Porosity is the fraction of pore space; permeability is how easily water flows through. Sandy soils have large pores — high permeability, water drains fast, but they hold little water or nutrients. Clay soils have tiny pores — low permeability, so water and nutrients are held tightly but drainage is poor and they can waterlog. Loam, a balanced mix of all three, has the ideal blend of drainage, aeration, and water retention, which is why it is prized for agriculture.

Texture, drainage, and retention

sand → high permeability, low water-holding · clay → low permeability, high water-holding · loam → balanced

Larger particles (sand) leave larger gaps, so water drains fast but little is retained. Smaller particles (clay) pack tightly, holding water but draining slowly.

Worked example

A soil sample is 40% sand, 40% silt, and 20% clay. Use the soil texture triangle to classify it, and predict its suitability for growing crops.

1.Confirm the percentages sum to 100%: 40 + 40 + 20 = 100. Good.
2.On the soil texture triangle, follow the sand line to 40%, the silt line to 40%, and the clay line to 20% — the three intersect in the loam region.
3.This roughly equal blend of sand, silt, and clay is classified as a loam.
4.Loam balances drainage (from sand) with water and nutrient retention (from silt and clay), making it excellent for crops.

Answer: The soil is a loam. Its balanced texture drains well yet retains enough water and nutrients, so it is among the best soils for agriculture.

Checkpoint

A farmer’s field is almost pure sand. Compared with a loam soil, this sandy soil will most likely:

Watch out

Don’t confuse porosity with permeability. Clay can have high porosity (lots of tiny pore spaces) yet low permeability, because the pores are so small and poorly connected that water barely moves through. High porosity does not guarantee fast drainage.

Checkpoint

In a soil profile, which horizon is the topsoil that contains the most humus and is most important for plant growth?

On the exam

To read the soil texture triangle, follow all three axes (percent sand, silt, and clay) and find where they intersect — the percentages must add to 100%. Loam sits near the middle. Sandy soils plot toward the sand corner (fast drainage); clay soils toward the clay corner (slow drainage, high retention).

Answer the 2 checkpoints as you read.