4.5 Soil Composition and Properties

Soil Layers and Composition

Soil forms in horizontal layers called horizons (O, A, E, B, C, R), visible in a soil profile. The O horizon has organic matter/humus; the A horizon (topsoil) is rich in humus and minerals, best for plant growth; deeper horizons have less organic material. Soil itself is a mix of minerals, organic matter, water, and air.

Soil Texture and Porosity

Soil texture depends on particle size: sand (largest, well-drained, low nutrient retention), silt (medium), and clay (smallest, holds water/nutrients but drains poorly). Loam, a balanced mix, is ideal for agriculture. Porosity (pore space) affects water/air infiltration, while permeability describes how easily water flows through soil.

Soil Fertility and pH

Fertility depends on nutrient content, organic matter, and cation exchange capacity (CEC) — the soil's ability to hold and supply positively charged nutrients like Ca²⁺ and K⁺. Soil pH affects nutrient availability; most crops thrive near neutral pH. Human activities like erosion, over-farming, and pollution can degrade soil fertility over time.

Key terms

Soil horizon

A distinct horizontal layer of soil (O, A, E, B, C, R) with unique composition and properties.

Soil texture

Classification of soil based on particle size: sand, silt, and clay.

Loam

A balanced mixture of sand, silt, and clay that is ideal for agriculture.

Cation exchange capacity (CEC)

A soil's ability to hold and supply positively charged nutrients, such as Ca²⁺ and K⁺.

Soil porosity vs. permeability

Porosity is the amount of pore space in soil; permeability is how easily water flows through those pores.

Try an AP-style question

Which soil particle has the highest water-holding capacity?

1. Clay
2. Silt
3. Sand
4. Gravel