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.
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.
The size of the mineral soil particles determines the soil texture
Is determined using sieving and sedimentation
Soil textural classes according to USDA
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.
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
A soil sample of volume V The same sample with each phase ‘packed’ together. V denotes volume and M mass.Definition
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?
Some examples of field capacity and wilting points for different soil textures
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)
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)
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
Water movement is driven by total potential gradients
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.