1. SOIL WATER
CHAPTER 7

2. SOIL WATER Functions: plant cells 50-90% water keeps turgor
seed germination transpiration
photosynthesis moves products
nutrients available lowers soil strength
chemical reactions microbial activity

3. Water Stress Initially, decreased photosynthesis . . . Continued . . .
temporary wilting point
further . . .
permanent wilting point

4. Forces on Soil Water Gravitational – pull of gravity downward
Adhesion – attraction of water to soil
Cohesion – attraction of water to water
adhesion and cohesion result from shape of water molecule and sharing of electrons in oxygen-hydrogen covalent bonds

5. Polarity of Water

6. Polarity of Water Effects of Water Molecule Polarity:
Hydrogen of one molecule attracted to oxygen of another molecule in a hydrogen bond accounts for cohesion
Hydrogen bond between hydrogen of water and oxygen of silica (SiO2) accounts for adhesion
Adhesion water is very tightly held!!!
Cohesion water can move and is available for use

7. Capillarity Additive force of adhesion and cohesion
- can move against force of gravity
- small pores conduct capillary water

8. Soil Water Potential Work water can do Potential energy
Tendency of water to flow/move freely in soil
Water will always try to move from a state of high energy to a low-energy state
The lower the soil water potential the more tightly water is adsorbed to soil particles

9. Refers to the ability of water to move in soil
Water POTENTIAL
Refers to the ability of water to move in soil
More water in soil = More water potential
At saturation, potential is near 0 (zero)
As soil dries, values become more negative
Water is held more tightly by soil!!

10. WATER FILM – WATER POTENTIAL

11. Three Forces of Water Potential
Gravitational – potential energy due to gravity positive
Matric – most common force; effect of soil on water
negative
Osmotic – special case of salty soils
Total water potential is sum of three forces

12. Units of Potential
Official unit is the Pascal (Pa), kilopascal (kPa), or Megapascal (MPa)
- common usage of older unit bar
- equivalent to 0.1 MPa or 100 kPa
Soil water potential is usually negative because of negative matric potential

13. TYPES OF SOIL WATER
Gravitational – at saturation, will drain from larger pores within 24 to 48 hours in well-drained soils
Available – can be absorbed by plants; held between gravitational water and wilting point
Cohesion – held between gravitational and adhesion (hygroscopic) water
Hygroscopic – held tightly by soil particles; air dry

14. REFRERENCE POINTS RELATED TO SOIL WATER

15. FOUR CATEGORIES OF SOIL MOISTURE
Chemically combined unavailable
Hygroscopic unavailable
Gravitational moves downward by gravity
Capillary taken up by plants

16. WATER RETENTION
Total water-holding capacity and available water-holding capacity are based on soil texture

17. WATER RETENTION
Medium-textured soils have the highest available water-holding capacity e.g. Silt Loam
Organic matter influences water-holding capacity
Increases amount of available water

18. WATER MOVEMENT Gravitational flow – moves by gravity
occurs only under saturated conditions
rapid in course soils – large pores
usually percolation through soil profile

19. SATURATED SOILS Sandy soil: Clay loam:
gravitational water moves rapidly downward
Clay loam:
gravitational water retained 2-3 days afterward

20. Once soils lose gravitational water (drain) movement is by . . .
Capillarity – movement due to attraction between water molecules and soil particles
Rapid in sandy soils but limited in distance
Slow in clay soils but may move great distances

21. WATER MOVEMENT Unsaturated flow – lateral movement; capillary flow
depends on unbroken films of water spreading through connected capillary pores
moves from moist to dry soil
can move in any direction

22. WETTING FRONT A distinct “line” where water is moving in soil –
Wet behind, Dry ahead
Soils must be nearly saturated in order for the front to advance; Why?
Dry soil cannot “pull” the water deeper
All the soil must be wet in order for the front to advance

23. CAPILLARY RISE
Upward movement of water from higher to lower potentials
Explains evaporation of water from soil to atmosphere
Continuation of capillary rise when entire soil column dries
Boundary in soil serves to protect from further losses
Unsaturated flow only moves over short distances
Saturated soil near the surface encourages capillary rise
Responsible for accumulation of salts at surface of soils in dry climates and in potted plants

24. Effect of Soil Horizons
water flows differently in different textures . . .
stratified layers will slow percolation

25. Vapor Flow
occurs when water vapor moves from moist to drier soil . . .
- condenses on cooler soil particles
- very slow
- minimal water moved

26. Preferential Flow
Saturated soil conditions . . .
water enters biopores or other soil channels
Increases infiltration and percolation
May also move pollutants!!!

27. How Roots Gather Water Governed by Soil Water Potential
Root hairs draw from higher potential regions
Capillary flow moves water

28. Soil – Plant – Atmosphere continuum
Plants create “unbroken” column of water
Driven by plant transpiration

29. Patterns of Water Removal
Plants will use water near the surface first
Oxygen is highest Respiration drives uptake
As surface dries, plant roots grow deeper . . .
absorption shifts downward
If surface is rewetted, absorption shifts upward

30. Measuring Soil Water Four methods: - gravimetric measurements
- potentiometers
- resistance blocks
- neutron probes (mainly research)

31. Gravimetric measures soil water content by weight
water content = moist wt – dry wt dry wt
Example: soil sample at field capacity 162 grams dry sample 135 grams
water content = 162g – 135g = .20
135g

32. Volume Basis More useful – utilizes gravimetric water content
volumetric water content =
gravimetric water content x soil bulk density
water density
From previous gravimetric example . . .

33. If bulk density of soil is 1
If bulk density of soil is 1.4 grams per cubic cm, and we know density of water is 1.0 g/cc
Volumetric water content =
.20 x 1.4g/cc = .28
1.0 g/cc

34. Soil Depth Basis Measures “inches of water” per foot of soil
- Uses volumetric water content
- Simple calculation . . .
Inches water per foot =
12 inches x volumetric water content
Continue from previous example . . .

35. Inches water per foot soil =
12 inches x = 3.36
Or simply stated Each foot of soil depth contains 3.36 inches of water assuming constant soil conditions

36. Practical Measuring Devices
Gravimetric method not very practical management
More useful and practical are . . .
Potentiometers (tensiometers)
Resistance Blocks (gypsum blocks)

37. Potentiometers Measure soil moisture potential at given levels
Water exiting tube creates vacuum
Measured by gauge/instrument
Function best at higher potentials

38. Resistance Blocks
Measure resistance of electrical flow between two electrodes embedded in block buried in soil
- moist soil with ions of salts in solution carry electrical flow
- resistance blocks designed to buffer salt effects (gypsum accomplishes this)
- works well between field capacity and WP