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Soil physics. Magnus Persson. What is soil?. You have 3 minutes to develop a group consensus definition. What is soil?. What’s the difference between soil and dirt? Dirt is what you find under your fingernails. Soil is what you find under your feet.

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soil physics

Soil physics

Magnus Persson

what is soil
What is soil?
  • You have 3 minutes to develop a group consensus definition
what is soil1
What is soil?
  • What’s the difference between soil and dirt? Dirt is what you find under your fingernails. Soil is what you find under your feet.
  • Soil is a complex mixture of organic and inorganic compounds
  • Definition, Soil Science Glossary, SSSA
    • (i) The unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants.
why soil science
Why soil science?
  • Geology, the soil material
  • Agriculture, soil-plant interaction
  • Engineering, soil mechanics
  • Hydrology, soil-water interaction
soil science disciplines
Soil science disciplines
  • Pedology (the study of soils in their natural environment)
    • Pedogenesis (how soils are created)
    • soil morphology (attributes of the soil within the various soil horizons)
    • soil classification (taxonomy)
  • Soil fertility
  • Hydrogeology
  • Soil physics
  • Soil chemistry
  • Soil biology
soil taxonomy
Soil taxonomy

A soil horizon is a specific layer in the soil parallel to the soil surface and possesses physical characteristics which differ from the layers above and beneath.

soil taxonomy2
Soil taxonomy
  • Food and Agriculture Organization of the United Nations (FAO)
  • United States Department of Agriculture (USDA)
unsaturated and saturated zones
Unsaturated and saturated zones
  • The unsaturated zone
    • lies between the soil surface and groundwater table
    • The soil pores are not completely filled with water
    • Mainly vertical water movement
  • The saturated zone
    • All pores are completely filled with water
    • Exists in aquifers
    • Mainly horizontal water movement
soil properties
Soil properties
  • Soil properties depends on
    • Formation processes
    • Parent material
    • Climate/time
    • Texture
    • Structure
particle size
Particle size

The size of the mineral soil particles determines the soil texture

Is determined using sieving and sedimentation

soil textural classes
Soil textural classes

Soil textural classes according to USDA

soil structure
Soil structure

Describes how individual soil granules clump or bind together and aggregate and arrangement of soil pores between them.

The five major classes of structure seen in soils are; platy, prismatic, columnar, granular, and blocky. There are also structureless soils.

specific surface
Specific surface

The specific surface is the total surface area per unit of mass or bulk volume. Generally it increases with decreasing particle size. It also depends on the particle shape

soil water




Soil water

Soils can contain water. Water is retained in the soil by capillary or adsorptive forces

Remember that the water molecule is a dipole


Vair, Mair

Vwater, Mwater

Vsolid, Msolid

A soil sample of volume V The same sample with each phase ‘packed’ together. V denotes volume and M mass.


b = Msolid/V (bulk density) 1.2-1.7 g/cm3

s = Msolid/Vsolid (particle density) 2.65 g/cm3

v = Vwater/V (volumetric water content)

g = Mwater/Msolid (gravimetric water content)

n = (Vwater + Vair)/V (porosity) 0.2 – 0.7 m3/m3


How do you convert g to v?

water content
Water content
  • With all pores completely filled with water, you have saturated conditions θs.
  • If you let a saturated sample drain until drainage stops you have field capacity θfc.
  • When there is so little water that a plant can not suck any water you have the wilting point θwp
  • Effective porosity θs- θfc
  • Plant available water θfc- θwp
  • Soil moisture deficit (θfc- θwp)*root depth
water content1
Water content

Some examples of field capacity and wilting points for different soil textures

surface tension




2·R·cos q





Surface tension

Due to surface tension water can be held at negative pressure in capillary tubes. (P1<P2 = Patm)

The smaller the diameter of the tube, the higher capillary rise.

An useful analogy is that the soil can be considered to act like a bundle of capillary tubes with different diameters (representing the range of pore sizes)

pf curve
pF curve

The soil moisture potential, or soil water suction, is sometimes given in pF = log(-pressure in cm H2O).

The water retention curve, soil moisture characteristic, or pF curve, is the relationship between the water content, θ, and the soil water potential, ψ. This curve is characteristic for different types of soil

(1 bar = 100 kPa = 1000 cm H2O)

pf curve1
pF curve

Several different models describing the pF curves exist, one of the most commonly used was developed by van Genuchten in 1980

where θs and θr are the saturated and residual water content, respectively, α, n, and m are empirical soil specific parameters


Wetting and drying curves are different

soil water potential
Soil water potential
  • The total potential consists of the moisture potential (synonyms; pore water tension, soil water suction)  and the elevation potential, z.
  • Normally the groundwater surface is used as a reference level (z = 0)
water movement
Water movement

Water movement is driven by total potential gradients

pedotransfer function
Pedotransfer function

A pedotransfer function is a predictive function of certain soil properties from other more available, easily, routinely, or cheaply measured properties.

For example, the ROSETTA model estimates the soil water retention curve from soil texture.

literature and links
Literature and links
  • (models for download)
  • (glossary of soil science terms)
  • (soil taxonomy FAO)
  • (soil taxonomy USDA)
  • (online textbook)