# Total Soil Water Potential

Soil and Water Ch 5 Continued evaporation

Created by Dr. Michael Pidwirny, Department of

Geography, Okanagan University College, BC, CA

Soil Water Energy Concept

 Retention and movement of water in soils

 Uptake and translocation in plants

 Loss to atmosphere

Potential m * g * h

Kinetic

0.5*m*v 2

Free Energy

Free energy: All forms of energy available to do work

Water movement in a soil is from a zone where free energy of water is high to one where free energy is low

Total water potential tells us which direction water moves

Forces Affecting Free Energy

 Matric Force: Adhesion and cohesion

 Osmotic Force: Attraction of ions and other solutes

 Gravity Force: Causes water to flow

+

Free energy of pure water higher elevation

Free energy of pure water

0

-

Decrease due to osmotic effects

Decrease due to matric effects

Free energy of soil water

Gravitational potential

Osmotic potential

Matric potential

Note: Both osmotic and matric potential are negative and are referred to as suction or tension also

Atmospheric potential

The matric potential is always negative and

The submergence or pressure potential is positive

Total Soil Water Potential

The difference in free energy from one contiguous site to another is of greater practical significance. This difference is termed as total soil water potential , which ultimately determines the soil water behavior.

Definition :

The amount of work that must be done per unit quantity of pure water in order to transport reversibly and isothermally an infinitesimal quantity of water from a pool of pure water at a specified elevation at atmospheric pressure to the soil water.

Soil water potential is the difference between energy state of soil water and that of pure free water .

Total Soil Water Potential = Gravitational + matric +

Osmotic

Y

T

=

Y g

+

Y m

+

Y o

+ ……

Gravitational Potential : The force of gravity acts on soil water exactly as it does on any other body, the attraction being towards the center of earth.

Y g

= g * h or h

What direction does water move?

http://www.maf.govt.nz/mafnet/schools/activities/swi/swi-04.htm

Soil Water Content Soil Moisture Content

Water that may be evaporated from soil by heating at 105 0 C to a constant weight mass of water evaporated (g)

Gravimetric moisture content (w) = mass of dry soil (g)

Volumetric moisture content ( q

) = volume of water evaporated (cm 3 ) volume of soil (cm 3 ) q

= w * bulk density of soil density of water

Bulk density of soil (Db) = cm

3 cm 3

 g g g cm

3 g cm 3

 g g g cm 3 cm

3 g mass of dry soil (g) volume of soil (cm 3 )

Example: A soil is sampled by a cylinder measuring 7.6 cm in diameter and 7.6 cm length. Calculate gravimetric and volumetric water contents, and bulk density using the following data:

1. Weight of empty cylinder = 300 g

2. Weight of cylinder + wet soil = 1000 g

3. Weight of cylinder + oven dry (105 0 C) soil = 860 g

Volume of cylinder = p

*r 2 *h = 3.14*(7.6/2) 2 *7.6 = 345 cm 3

Weight of wet soil = 1000 – 300 = 700 g

Weight of dry soil = 860 – 300 = 560 g

Bulk density = 560 g / 345 cm 3 = 1.62 g cm -3

Gravimetric moisture content = (700-560g)/560g = 0.25 or 25%

Volumetric moisture content = (Db x w) / Dw =

(1.62 g/cc x 0.25)/1.0 g/cc = 0.41 or 41%

### Calculating dry soil weight for analytical samples

1. Weigh an empty drying pan

2. Weigh a soil subsample + pan

3. Oven dry the subsample at 105

C for 24 hr.

4. Weigh the dried soil + pan

5. Calculate the moisture content (w): w = (g moist soil – g dry soil)/(g dry soil – pan)

6. Rearrange the eqn to solve for dry soil wt.

Dry soil wt = g moist soil / (1 + w)

## Derivation of dry soil wt calc’n

Define moisture content as: w = (g moist soil – g dry soil) / (g dry soil)

(= g water evaporated)

( separate your numerators) w = (g moist soil / g dry soil) – (g dry soil / g dry soil)

= g moist soil / g dry soil – 1

1 + w = g moist soil / g dry soil

(invert both sides)

1 / (1 + w) = g dry soil / g moist soil

(multiply both sides by g moist soil) g moist soil / (1 + w) = g dry soil

Don’t oven-dry all of your soil!

## Example for lab analyses

You’ve collected some soil, air-dried it, and sieved to <2-mm.

Calculate the moisture content (w) of the airdry soil (e.g., 2% or 0.02)

E.g., Determine the dry weight basis of 5.00 g of air-dry (or moist) soil:

Dry soil wt = g moist soil / (1 + w)

Dry soil wt = 5.00 g / (1 + 0.02)

Dry soil wt = 4.90 g