1. Soil physics
Magnus Persson

2. What is soil?
You have 3 minutes to develop a group consensus definition

3. 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.

4. Why soil science? Geology, the soil material
Agriculture, soil-plant interaction
Engineering, soil mechanics
Hydrology, soil-water interaction

5. 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

6. 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.

7. Soil taxonomy

8. Soil taxonomy
Food and Agriculture Organization of the United Nations (FAO)
United States Department of Agriculture (USDA)

9. The Hydrological cycle

10. 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

11. Soil properties Soil properties depends on Formation processes
Parent material
Climate/time
Texture
Structure

12. Particle size
The size of the mineral soil particles determines the soil texture
Is determined using sieving and sedimentation

13. Soil textural classes
Soil textural classes according to USDA

14. 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.

15. Soil structure

16. 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 type
Specific surface (m2 g-1)
Sand
<10
Sandy loam
5-20
Clay
>25
Organic matter
Bentonite

17. Soil water
Soils can contain water. Water is retained in the soil by capillary or adsorptive forces
O-- H+
Remember that the water molecule is a dipole

18. Definition b = Msolid/V (bulk density) 1.2-1.7 g/cm3
A soil sample of volume V The same sample with each phase ‘packed’ together. V denotes volume and M mass.
Vair, Mair
Vwater, Mwater
Vsolid, Msolid
b = Msolid/V (bulk density) g/cm3
s = Msolid/Vsolid (particle density) g/cm3
v = Vwater/V (volumetric water content)
g = Mwater/Msolid (gravimetric water content)
n = (Vwater + Vair)/V (porosity) – 0.7 m3/m3

19. Exercise
How do you convert g to v?

20. 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

21. Water content
Some examples of field capacity and wilting points for different soil textures
Textural class
Wilting point (m3/m3)
Field capacity (m3/m3)
Clay
0.25
0.40
Silt
0.15
0.35
Loam
0.10
0.30
Sand
0.05

22. 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)
p-2q q
2·R·cos q R
2·r P1 P2 z

23. 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)

24. 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

25. Hysteresis
Wetting and drying curves are different

26. 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)

27. Water movement
Water movement is driven by total potential gradients

28. 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.

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