Soil Science Exam 2- soil water

Surface Tension

- Due to cohesion (water molecules sticking together) but adhesive (Water sticking to other molecules) forces are also present

-Water surface tries to contract due to unequal attractive forces, and the uppermost molecules behave like an elastic membrane

Surface wettability

Adsorption (adhesion) vs surface tension

- When liquid is out on a non-wettable surface, surface tension dominated the attractive forces on the surface

- When attractive forces to surface exceed surface tension, the liquid wets the surface

Capillarity

Result of cohesion and adhesion

Capillary movement in soils

- Soil contains many capillary pores having a range of sizes

- Upward capillary movement of water through rubes of different bore and soils with different pore sizes

- capillary rise in smaller pores is greater but slower due to friction/viscosity

Why does water move from one location to another?

Moves to where it's energy state will be lower

Adhesion_______the energy state of water

lowers (it is less "free")

Total water potential (Ψt) is comprised of...

1. Pressure potential

- Matric potential

- Hydrostatic Potential

2. Osmotic potential

3. Gravitational Potential

Matric Potential (Ψm)

- ≤0

- present in all non-saturated soils

- Varies widely, ranging from 0 kPa in completely saturated soils to -1500 kPa in very dry soils

- results from capillarity

Capillary Fringe

the capillary fringe is directly related to matric potential; it is the zone directly above the water table where pore water pressure is negative due to capillary forces, a state described by matric potential. At the top of the capillary fringe, the matric potential is equal to the air-entry potential (or bubbling pressure), the point at which air begins to invade the pores.

Osmotic Potential

Significant only in soils containing considering dissolved salts

- if the soil solution is salty, the plant water uptake is reduced. A semi permeable membrane must be present

Gravitational Potential

Significant only under saturated or nearly saturated soil conditions

- water moved downward in larger pores under force of gravity

Field Capacity

Soil moisture content after all of the gravitational water has drained away

Permanent Wilting point

the soil moisture content at which a wilted plant cannot regain turgidity when placed in the dark at 100% relative humidity

or... soil moisture content at which plants can no longer extract water

Available Water Holding Capacity

The water held in the soil between field capacity and permanent wilting point

Plant Available Water

Water available to plants at a given point in time

- portion of available water holding capacity

Commonly used approximation of FC is soil water content at...

-10 kPa

Commonly used approximation of PWP is soil water content at...

-1500 kPa water potential

Pores larger than 30 um ...

Cannot hold water against the force of gravity (gravitational force vs capillary water)

Pores smaller than .2 um ....

- Hold water too tightly for plants to use (plant-unavailable capillary water)

- Associated potentials less than -1500 kPa

Saturated soil

All pores water-filled

Field capacity

Larger pores have emptied

wilting coefficient

Water remaining is not plant available

T/F : Water films on soils become thinner as soil dries

True

T/F : Potentials that decrease nearer the soil particle due to a greater influence of adhesive forces

True

texture affects...

Pore size, surface area, and water retention

Silt loams have more pores ___ to ___ um in size = more AWHC

0.2 to 30

Affect of soil organic matter on available water holding capacity

As SOM increases, increases in FC are ty[ically greater than increases in PWP

- More SOM = more AWHC

AWC of soil derived from volcanic ash

1.25 cm / 30 cm soil

AWC of clay loam soil

5 cm / 30 cm soil

Soil water potential curves

- At any given water potential, finer textured soils contain more water than coarse-textured soils

- At any given soil water content, finer-textured soils have lower (more negative) SWPs

Saturated hydraulic conductivity

The readiness in which water flows through a saturated soil in response to a potential gradient.

- moderately permeable soils conduct 2 cm of water per hour when saturated

Preferential Flow

- the rapid movement of water through distinct pathways in soil, such as macropores or cracks, rather than through the entire soil matrix. This bypass flow occurs when water infiltration rates exceed the soil's ability to absorb it, leading to fast, channelized movement

- requires that the macropore be open at the soil surface

- tillage destroys these -> slower infiltration -> more runoff -> more erosion

- can be a beneficial thing

Infiltration

- The potential rate of water entry into the soil, or infiltration capacity, can be measured by recording the drop in water level in a double ring infiltrometer

- Changes in the infiltration rate of three soils during a period water application by rainfall or irrigation are shown

Unsaturated water flow in soil

- Water still moves from higher potential to lower potential

- takes place largely via water films on soil particles. Also involves differences in sizes of adjacent pores

- thicker films -> thinner films

- Larger filled pores -> smaller pores

- In the absence of high water potential, water in smaller pore will not fill an adjacent larger empty pore

- It is the primary means by which water moved toward plant roots

- It is slower than saturated flow, due to higher viscosity of water near soil particles and presence of discontinuous water films

T/F : Layers with contrasting textures do not impede soil drainage

FALSE

ex. Water in a relatively small pore cannot move into the adjacent larger empty pore because it is held to tightly

- Water WILL move into the large pore only if gravitational forces increase the total water potential to match the potential at which the large pore fills