Lecture 14 Soil Water (1)Soil Properties

• Basic Soil Properties

• Soil Water Storage

• Soil Water Forces (Potential)

General Comments• Only 0.17% of all fresh water in the world, but very important for plant growth and food production. • Short residence time of about 3 months, that effectively quadruples its annual contribution to water cycle.

Hydrological Processes in Soil

• Capillary Rise• Infiltration• Percolation• Runoff

Basic Soil Physical Properties

Soil system: consists of three phases: solid phase comprising the mineral and organic particles of the porous medium gaseous phase of soil air in the poresliquid phase of soil water

A. Wild, 1995. Soils and the Environment: An Introduction. Cambridge University Press

Soil Texture

Basic Soil Physical Properties

Texture: size distribution of the mineral particles in the soil. Three basic soil textural types are: clay (small particles), silt (medium particles), sand (large particles) (Figure 2.1 AW)

A. Wild, 1995. Soils and the Environment: An Introduction. Cambridge University Press

Terms to Remember

Basic Soil Physical Properties

Texture: size distribution of the mineral particles in the soil. Three basic soil textural types are: clay (small particles), silt (medium particles) and sand (large particles) (Figure 2.1 AW)

Structure: the aggregation of these particles

Porosity: the percentage of pore space occupied by either air of water in the soil. It is determined mostly by soil texture and by structure to some extent. So it can be separated into textural porosity and structural porosity

Permeability: connectedness of pores

Specific surface: particle area per unit volume. Clays have the largest, and sands have the smallest specific area (Table 6.1 WR)

Specific Surface Area

Basic Soil Physical Properties

Soil Water StorageTerms to Remember

Soil water content: percentage of volume occupied by water in the soil, usually in units of cm3/cm3

Field capacity: the capacity of soil to hold water under gravity

Wilting point: minimum water content of the soil at which plants can extract water. It depends on texture. Clays have the highest wilting point while sands have the lowest.

Available water capacity: field capacity less wilting point

Water table: depth of soil at which the soil is saturated (Figure 3.1 Jones)

Water Table in a Hydrological System

Soil Water ForcesCapillarity: surface tension at the interface between soil air and soil water

Adsorption: electrostatic forces in which the polar water molecules are attached to charged faces of the solids. They occur only very close to the surface of solids.

Osmosis: forces due to solutes in soil water. Saline water, for example, can have large osmosis that prevents plant roots from extracting the water.

Gravity: vertical forces exerted to water by gravity of the Earth.

Pressure: forces that increase below water table

Soil water potential The potential energy of soil water relative to that of water in a standard state (elevation, purity, atmospheric pressure, temperature)

opg is the total water potential g is gravitational potentialp is pressure potentialo is osmotic potential

Water PotentialWhat is the physical meaning of water potential?It is energy per unit volume.It has the same unit as pressure (bar, mb, Pascal)

Unit of mass: kg (or g)Units of forces: kg m/s2

Units of pressure, i.e., force per unit area: (kg m/s2)/m2= kg/(ms2)=Pascal

Units of energy (equivalent to force times distance): J=kg m/s2 m = kg m2/s2

Units of energy per unit volume (pressure): (kg m2/s2)/m3=kg/(ms2)