4.2 Soil Formation

Soil Composition and Functions

• Mix of geological (rock) and organic (living) components: sand, silt, clay, humus (main organic part), nutrients (ammonium, phosphates, nitrates), water, air, and living organisms.

• Supports plant roots, provides water, shelter, and nutrients for growth (N, P, K, Mg).

• Filters rainwater and runoff, trapping pollutants, allowing clean water to enter groundwater and aquifers.

• Habitat for organisms like earthworms, fungi, and bacteria, contributing to nutrient recycling by decomposing organic matter.

Weathering and Erosion

• Weathering breaks down rocks into smaller pieces through:

  • Physical: wind, rain, freezing/thawing.

  • Biological: roots of trees crack rocks.

  • Chemical: acid rain, acids from moss/lichen.

• Erosion transports weathered rock fragments by wind and rain, leading to deposition in new locations.

Soil Formation Factors

• Influenced by:

  • Parent Material: affects soil pH and nutrient content.

  • Topography: steep slopes lead to more erosion; flatter areas promote deposition.

  • Climate: warmer conditions result in faster breakdown of organic matter and increased weathering.

  • Organisms: soil organisms like bacteria, fungi, and worms break down organic matter, enriching the soil.

Soil Horizons

• O-Horizon: layer of organic matter (plant roots, dead leaves, animal waste) on top of soil, providing nutrients and limiting water loss.

• A-Horizon (Topsoil): composed of humus and minerals, has high biological activity.

• B-Horizon (Subsoil): lighter layer mostly of minerals, contains some nutrients, with little organic matter.

• C-Horizon: least weathered soil, closest to the parent material, sometimes referred to as bedrock.

Soil Degradation

• Loss of Topsoil: caused by tilling and vegetation loss, which disturb soil, leading to increased erosion, drying out soil, and nutrient loss.

• Compaction: from machinery and grazing reduces the ability of soil to hold moisture, leading to erosion.

• Nutrient Depletion: repeated cropping removes essential nutrients (N, P, K, Na, Mg), reducing future agricultural productivity.