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.

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?

Geology, the soil material Agriculture, soil-plant interaction Engineering, soil mechanics Hydrology, soil-water interaction

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 taxonomyO1 Undecomposed litterO Horizon

Organic PlantResidues O2 Partly decomposed debris

A1 Zone of humus accumulation

A2 Zone of strongest leaching

A HorizonZone ofeluviaiton(leaching) A3 Transition to B horizon

B1 Transition to A horizon

B2 Zone of strongest depositionSol

um, T

rue

Soi

l

B HorizonZone ofilluviation(deposition) B3 Transition to C horizonR

egol

ith,

Wea

ther

ed M

ater

ial

C HorizonParent Material

C Unconsolidated rock

R Layer - Bedrock R Consolidated rock

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 taxonomy

Soil taxonomy

Food and Agriculture Organization of the United Nations (FAO)

United States Department of Agriculture (USDA)

The Hydrological cycle

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 depends on Formation processes Parent material Climate/time Texture Structure

Particle sizeThe size of the mineral soil particles determines the soil texture

Is determined using sieving and sedimentation

Soil textural classes

Soil textural classes according to USDA

Soil structureDescribes 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.

Soil structure

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 shapeSoil type Specific surface (m2

g-1)

Sand <10

Sandy loam 5-20

Clay >25

Organic matter 500-800

Bentonite 100-200

Soil water

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

O--

H+

H+Remember that the water molecule is a dipole

Definition

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) 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

Exercise

How do you convert g to v?

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 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 0.10

Surface tension

2·R·cos R

2·r

P1

P2

z

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 curveThe 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 curve

mnrs

r ))(1()(

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

HysteresisWetting and drying curves are different

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 is driven by total potential gradients

Pedotransfer functionA 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

http://www.ars.usda.gov/Services/docs.htm?docid=15992 (models for download)

https://www.soils.org/sssagloss/ (glossary of soil science terms)

http://www.fao.org/docrep/W8594E/w8594e00.HTM (soil taxonomy FAO)

http://soils.usda.gov/technical/classification/taxonomy/ (soil taxonomy USDA)

http://wwwbrr.cr.usgs.gov/projects/GW_Unsat/Unsat_Zone_Book/index.html (online textbook)