Soil Profiles, Soil Horizons, and Soil Texture
A soil profile is a vertical slice through the soil from the surface down to the parent rock, made of distinct layers called soil horizons, and soil texture describes the proportions of sand, silt and clay particles in the soil. In brief: a typical profile has an O horizon (organic litter), A horizon (topsoil), E horizon (leached layer), B horizon (subsoil), C horizon (weathered parent material) and R (bedrock). Sand particles are 0.05–2 mm, silt 0.002–0.05 mm and clay under 0.002 mm, and a loam has a balanced mix of all three.
Soil is a thin layer that supports almost all land plants and filters water. This guide explains how a soil profile develops, the horizons in detail, soil texture and how to find it, compares the horizons in a table, works through texture and porosity calculations, outlines the history of soil science and corrects common mistakes.
How does soil form?
Soil forms slowly from weathered rock mixed with organic matter, air and water. Five factors control its development: parent material (the rock below), climate (temperature and rainfall), organisms (plants, animals, microbes), topography (slope and drainage) and time. Forming one centimeter of soil can take hundreds of years or more, which is why soil is treated as a non-renewable resource over human lifetimes.
Water moving down the profile carries dissolved minerals and fine particles from the upper layers to the deeper ones, which is the reason the layers differ in color and composition.
What are soil horizons?
Soil scientists describe a profile by horizons, labeled with letters. Not every soil has all of them.
- O horizon: loose litter and partly decomposed organic matter (humus) on the surface, dark in color. It is best developed in forests.
- A horizon (topsoil): a mix of mineral particles and humus. It is dark, fertile, full of roots and organisms, and it is where most plant growth occurs.
- E horizon (eluviated layer): pale, because water has washed out (leached) clay, iron and organic matter. It is found mostly under forests in cool, wet climates.
- B horizon (subsoil): receives the minerals and clay washed down from above (illuviation). It is denser, lighter in organic matter and often more brown or red.
- C horizon: partly weathered parent material with little biological activity.
- R: solid bedrock.
The sharpness and thickness of horizons depend on climate. In wet tropical soils, intense leaching removes silica and nutrients, leaving red soil rich in iron oxides, and nutrients are held mainly in the plants. Grassland soils (such as chernozems) have a thick, dark A horizon rich in humus.
What is soil texture?
Texture is the proportion of three mineral particle sizes. Sand (0.05–2 mm) feels gritty, silt (0.002–0.05 mm) feels smooth like flour and clay (smaller than 0.002 mm) feels sticky when wet and can be rolled into a ribbon. Gravel, larger than 2 mm, is not counted as part of the fine soil.
The soil texture triangle places a soil in a class such as sandy loam, silt loam, clay loam or clay according to the percentages of the three fractions, which always add up to 100%. In the field, texture is estimated by the feel method: rubbing moist soil between finger and thumb, and trying to roll it into a ball and then a ribbon. A lab method is sieving and sedimentation, in which larger particles settle faster than finer ones in a jar of water.
How do texture and structure affect soil?
Texture affects how soil handles water and nutrients. Sandy soils have large pores, drain quickly, warm up early and are easy to work, but they hold few nutrients and dry out. Clay soils have tiny pores, hold large amounts of water and nutrients but drain poorly, become waterlogged and are heavy to cultivate. Loam, a balance of sand, silt and clay, usually combines good drainage with good water and nutrient retention and is generally best for farming.
Soil structure is how particles clump into aggregates (crumbs, blocks, plates). Humus and earthworms improve crumb structure, which increases pore space for air and water. Soil pH (acidity), organic matter content and living organisms also influence fertility.
Soil horizons compared
| Horizon | Common name | Main content | Typical color |
|---|---|---|---|
| O | Organic layer | Litter and humus | Dark brown to black |
| A | Topsoil | Minerals and humus | Dark |
| E | Leached layer | Sand and silt, few nutrients | Pale gray |
| B | Subsoil | Clay, iron and minerals from above | Brown to red |
| C | Parent material | Broken, weathered rock | Varies |
| R | Bedrock | Solid rock | Varies |
Worked example: texture and water
- Texture percentages: a sample of dry soil weighs 100 g and contains 40 g sand and 20 g clay. The silt is 100 − 40 − 20 = 40 g, so the soil is 40% sand, 40% silt and 20% clay, which falls in the loam class of the texture triangle.
- Sedimentation test: after shaking a soil in a jar of water, the sand settles in about 1 minute, silt over several hours and clay remains suspended for days. If the settled layers are 3 cm sand, 2 cm silt and 1 cm clay, the total is 6 cm, so sand is 3 ÷ 6 = 50%, silt 33% and clay 17% (approximate, by volume).
- Moisture content: a moist sample weighs 120 g and weighs 100 g after oven drying. The water lost is 20 g, so the moisture content is 20 ÷ 100 = 20% by dry mass (or 20 ÷ 120 = 16.7% of the wet mass).
- Porosity: if soil particles have a density of 2.65 g/cm3 and the dry bulk density of the soil is 1.325 g/cm3, the porosity is 1 − (1.325 ÷ 2.65) = 1 − 0.5 = 0.5, or 50% of the volume is pore space.
- Soil formation rate: if 1 cm of soil forms in 200 years, 30 cm of topsoil would need 30 × 200 = 6,000 years under the same conditions.
How did soil science develop?
The Russian scientist Vasily Dokuchaev, working in the 1880s, showed that soils are natural bodies formed by climate, organisms and parent material, and he described the Russian chernozem, the black earth of the steppe. This laid the foundation of modern soil science (pedology). Hans Jenny later formalized the factors of soil formation in 1941. Classification systems such as USDA Soil Taxonomy and the World Reference Base now group soils into orders by their horizons and properties. The Dust Bowl in the 1930s raised public attention to the importance of soil.
Which mistakes are common?
Mistake 1: Thinking that all soils have the same layers
Profiles vary with climate and parent material. Many soils lack an E horizon or an O horizon.
Mistake 2: Confusing texture with structure
Texture is the size mix of the particles. Structure is how the particles are arranged into clumps.
Mistake 3: Thinking that a clay soil is mostly clay
A soil is classed as clay once it contains roughly 40% clay particles or more, because even a moderate share of such tiny particles dominates how the soil behaves. It does not need to be 100% clay.
Mistake 4: Believing that the darkest soil is always the most fertile
Dark color often indicates humus, but fertility also depends on nutrients, pH, drainage and climate.