5.4 Weathering and Soil Formation

Weathering is essential for soil formation, which is crucial for sustaining life on Earth.
Definitions:
- Common definition: Any loose material on Earth's surface.
- Geologist definition: Soil contains organic matter, lies within top few centimetres of the surface, and sustains plant growth.

Soil is a complex mixture composed of:
- Minerals: Approximately 45%
- Organic Matter: Approximately 5%
- Empty Space: Approximately 50% (filled with air and water)
Dominant minerals in soil include clay minerals and quartz, with minor amounts of feldspar and small rock fragments.

Weathering significantly influences soil composition and texture.
In warm climates, where chemical weathering dominates, soils are typically richer in clay.
Soil texture is described based on the proportions of sand, silt, and clay.
The U.S. Department of Agriculture soil texture diagram illustrates this classification.

Soil forms through the accumulation and decay of organic matter and through mechanical and chemical weathering processes.

Soil development is influenced by:
- Temperature and precipitation
- Moderate conditions support optimal soil formation (not too wet or dry).
- Warm conditions enhance chemical weathering and plant growth.
Excessive water may lead to nutrient leaching, causing acidic soils.
In humid regions, swampy conditions can develop organic-rich soils.
Insufficient water results in limited chemical transport and salt accumulation, typical in deserts.

Types of Parent Materials:
- Bedrock and unconsolidated sediments (e.g., glacial or stream deposits).
- Soils are classified as:
- Residual Soils: Develop on bedrock.
- Transported Soils: Develop on unconsolidated material.
Parent materials affect soil nutrient content:
- Quartz-rich materials (e.g., granite, sandstone) create sandy soils.
- Quartz-poor materials (e.g., shale, basalt) yield soils with less sand.
- Calcium-phosphate mineral apatite in granite provides phosphorus, essential for plant growth.
- Basaltic materials produce fertile soils due to phosphorus, iron, magnesium, and calcium content.

Soil can develop only where surface materials remain stable (not frequently moved by erosion).
Steep slopes:
- Rate of soil formation < Rate of erosion → little or no soil development.

Ideal conditions are required for soil formation, which can take thousands of years.
Regions recently impacted by glaciation (e.g., southern Canada) exhibit relatively young, undeveloped soils.
Newly created surfaces (e.g., deltas or sandbars) also have young soils.

Soil formation results in the development of distinct layers known as soil horizons:
- O horizon: Layer of organic matter.
- A horizon: Partially decayed organic matter mixed with minerals.
- E horizon: Eluviated layer, leached of some clay and iron, often sandier.
- B horizon: Accumulation layer for clay, iron, and other elements.
- C horizon: Layer of partially weathered parent material.
In hot arid regions, a special horizon known as caliche can form:
- Precipitation of calcite from calcium ions results in a concrete-like layer.

Soil erosion occurs naturally but is intensified by human activities (e.g., forestry, agriculture).
Vegetation serves as protection against erosion; its removal results in exposed soil prone to erosion.
Primary agents of soil erosion:
- Water: Fast-flowing water has greater eroding power.
- Raindrops disaggregate soil particles; Sheetwash carries suspended materials away.
- Wind: Erosion exacerbated by removal of trees and agricultural practices exposing bare soil.
- Tillage can also contribute to slope erosion.

Soil formation processes can take thousands of years under optimal environmental conditions.

U.S. Department of Agriculture soil texture diagram (Figure 5.14).
Example of poorly developed soil on wind-blown silt in northeastern Washington State (Figure 5.15).
Diagram of soil horizons (Figure 5.16).
Visuals of soil erosion due to rainfall and wind (Figures 5.17 and 5.18).