1. Soil Water Tension
Department of Agricultural and Biological Engineering
University of Illinois at Urbana-Champaign

3. +h

4. -h

5. Soil Water Characteristic
(Release Curve) .
Matric Potential

6. Tensiometer for Measuring Soil Water Potential
Water Reservoir
Variable Tube Length (12 in- 48 in) Based on Root Zone Depth
Vacuum Gauge
Porous Ceramic Tip

7. Soil Water Potential Measure of the energy status of the soil water
Important because it reflects how hard plants must work to extract water
Units of measure are normally bars or atmospheres
Soil water potentials are negative pressures (tension or suction)
Water flows from a higher (less negative) potential to a lower (more negative) potential

8. Soil Water Potential Components t = total soil water potential
g = gravitational potential (force of gravity pulling on the water)
m = matric potential (force placed on the water by the soil matrix – soil water “tension”)
o = osmotic potential (due to the difference in salt concentration across a semi-permeable membrane, such as a plant root)
Matric potential, m, normally has the greatest effect on release of water from soil to plants

9. Water adheres to soil particles Water held in large pores
Hydroscopic
Water
Capillary
Water
Gravitational
Water
Water adheres to soil particles
Water held in large pores
Available for crop use
Water drains through soil profile
Wilting
Point
15 bars
Field
Capacity
1/3 bar

10. Water Movement in Plants
Illustration of the energy differentials which drive the water movement from the soil, into the roots, up the stalk, into the leaves and out into the atmosphere.
The water moves from a less negative soil moisture tension to a more negative tension in the atmosphere.

11. Yw~ -1.3 MPa Yw~ -1.0 MPa Yw~ -0.8 MPa Yw~ -0.75 MPa Yw~ -0.15 MPa
Leaf air spaces
Yw~ -1.0 MPa
Xylem
Yw~ -0.8 MPa
Yw~ MPa
Yw~ MPa
Ys~ MPa

12. Effect of Soil Texture Water Content Matric Potential Fine Textured
Coarse
Textured
Soil
Matric Potential

13. Matric Potential and Soil Texture
The tension or suction created by small capillary tubes (small soil pores) is greater that that created by large tubes (large soil pores). At any given matric potential coarse soils hold less water than fine-textured soils.
Height of capillary rise inversely related to tube diameter

14. Effect of Soil Structure
Aggregated
Soil
Water Content
Compacted
Soil
Matric Potential

15. Van Genuchten Representation
[1 + (a h)n]m
q = qr (qs - qr)
a = empirical parameter
h = soil water pressure head
n = empirical parameter that affects the shape of the curve
m = empirical parameter that affects the shape of the curve
(m = 1 - 1/n)
qr = residual water content (an extrapolated parameter)
qs = saturated water content
q = volumetric water content at head, h.

16. Brooks and Corey Representation
q = qr (qs - qr)
(hd/h)l
l = empirical parameter
hd = soil water pressure head
qr = residual water content (an extrapolated parameter)
qs = saturated water content
q = volumetric water content at head, h.

21. A moist sand sample has a volume of 456 cm3 in the natural state and a weight of 843 g. The dry weight is 763 g and the specific gravity of the soil particles is Determine the porosity, volumetric moisture content, and degree of saturation (water filled porosity/total porosity)

22. A soil sample with a volume V=100 cm3 was placed in a pressure plate apparatus and brought to apparent saturation. The following outflow volumes were measured at equilibrium for the given pressure settings.
After equilibrium was obtained at the 500 cm pressure, the sample was found to have a dry bulk density of 1.64 g/cm3, and a gravimetric water content (mass of water/mass of solids) of Evaluate and plot the soil water retention curve from these data
Pressure (cm) 10 20 30 40 50 60 70 80
100
150
200
500
Outflow Volume (cm3)
0.3
1.4
3.1
3.6
3.4
3.0
2.2
1.5
1.6
2.1

23. Van Genuchten Parameters for Tama
Determine the volume (depth) of water that is released from Tama Silt Loam when the water table falls from the soil surface to 1 m below the soil surface.
Depth (cm)
Van Genuchten Parameters for Tama qr qs a
(1/cm) n
0 - 18
0.052
0.39
0.0063
1.34
0.057
0.44
0.0100
1.30
0.091
0.47
0.0055
1.46
0.088
0.59
0.0057
1.53
0.081
0.0052
1.56

24. Drummer Silty Clay Loam
This poorly drained "prairie soil" formed in 40 to 60 inches of loess over Wisconsinan drift. It occupies nearly 1.6 million acres. It has a high management level productivity index (PI) of 150.