Composition of Soils - Mass/volume relationship

Module Topics

What is Soil Physics?
    Why do we study soil physics?
     Learning objectives
     History of soil physics
Composition of Soils
    Mass/volume relationship
        Soil separates
        Textural triangle (1)
        Textural triangle (2)
        Textural triangle (3)
        Soil characteristics

Soil Structure
    Description of soil structure
    Soil density

Transport Processes
    Potential energy
    Soil and water
        Water retention curve
Water movement
Hysteresis (1)
        Hysteresis (2)

        Infiltration and preferential flow
    Soil and air
    Heat Flow and Soil Temperature
        Long- and short-wave radiation
        Soil profile temperatures
        How water affects soil temperature

The soil is a porous medium, composed of solid phase intermingled with pore space. The graphs in this diagram represent the four components of a soil: mineral and organic matter (solid phase), and water and air occupying the pore space. These components have different densities which cause the mass and volume relationships to be different. The graph on the left represents the volume relationship, and the graph on the right represents the mass relationship between components. The negligible density of air is the cause of the absence of this component in the right graph.


The soil depicted in the graphs has 50% porespace, which is an ideal proportion between solids and porespace. The porespace can contain air and water in variable amounts. The situation with half of the porespace filled with water is ideal in terms of biological activity.

As the soil dries, the amount of water in the porespace decreases, down to a minimum amount, which represents water strongly, adsorbed to the soil particles. As the soil water content increases, the air content decreases. Field soils usually will not reach complete saturation because of air entrapped in pore space.