1. Chapter 3 Soil Water Properties Pages 63 – 95
Landscape Design Basics

2. Water potential (WP) . . .
Water moves from an area of high water potential (low negative number)
To an area of high water potential (high negative number)

3. Water potential (WP) . . . Is measured in kilopascals (kPa)
System of measuring the amount of ‘work’ required to move water through soil . . .
Or to a ‘zero’ reference state

4. Water potential (WP) . . .
Is the tendency of water to react, move, enter a plant, or change phase
Water in a soil is always at a negative potential
The drier the soil the more negative the water potential
Ø kPa is called “free water” or saturation

5. Osmotic potential . . . Or solute potential
Is effect of dissolved substances on the ability of water to do work
Osmotic potential is always negative

6. Osmotic potential . . .
Water is drawn to an area of high osmotic potential (lower negative water potential)
From an area of low osmotic potential (higher water potential)

7. Matric potential . . .
Is measure of the adhesive and cohesive forces of water
Matric potential values are always negative
Matric potential is not a factor in saturated soils

8. Matric potential . . .
Effects of water held by adsorption to soil particles or capillary pores
Measures forces required to move water from areas of high potential to areas of low potential via capillary action

9. And then . . . There’s the term Pressure potential . . .
It’s usually disregarded

10. Water potential (WP) = osmotic potential + matric potential
+ pressure potential
WP = osmotic potential + matric potential + pressure potential

11. An example
We’re going to plant a pot full of pansies . . .

13. We’re going to add a layer of gravel at the bottom of the pot . . .

15. Good idea Bad idea?

16. Saturation must occur approaching Ø kPa to overcome the adhesive properties of water to the soil particles.

17. Water adsorbed to soil particles or held in capillary pores by hydrogen bonding (cohesion) is too tightly held at an extremely low (negative) water potential to break free into the large spaces in the gravel

18. So . . .
It’s really a bad idea!
It actually impedes drainage

19. States of Soil Moisture

20. Saturation Liquid state
Soil pores filled to capacity, puddling and runoff occur
Energy state: Ø kPa
Unavailable to plants because gravitational force is stronger than the matric attraction

21. Gravitational Water (GW)
Liquid state
Water loosely held between soil particles
Energy state: > -33kPa
Not available to plants
Considered > -10 kPa in sandy soils

22. Field Capacity (FC) Liquid state
Energy state: = -33 kPa (-10 kPa for sands)
Held tightly enough to resist gravity
Available to plants

23. Plant Available Water (PAW)
Liquid state
Between gravitational water and permanent wilting point (PWP)
Energy state: range between -33 kPa and kPa
Capillary water movement occurs
Available to plants

24. Permanent Wilting Point (PWP)
Liquid state and vapor state
Energy state: ≤ kPa
Capillary water movement still occurs
Unavailable to plants

25. Hygroscopic Moisture Water vapor state Energy state: ≤ -3100 kPa
Unavailable to plants

26. Air Dried Soil Water vapor state
Energy state: ≤ -100,000kPa (-100 MPa)
About the same level of moisture as humidity in air
Unavailable to plants

27. Oven Dried Soil Water vapor state
Energy state: ≤ -1,000,000 kPa (-1000 MPa)
Unavailable to plants

28. Capillary Water Movement
Ranges from field capacity through hygroscopic water
Liquid water through water vapor state
Energy state: -33 kPa through kPa

29. Capillary Water Movement
occurs from field capacity through hygroscopic moisture
availability to plants determined by current moisture state

31. Adhesion vs. Cohesion Adhesion – water binding to soil particles
Cohesion – water molecules binding to water molecules

32. Potting soils . . .
Can hold more than 100% moisture by volume

33. Measuring water in soil
Water loss in soil can be measured in height/time
Mass moisture % = H2O weight (g)
Dry soil weight (g)

34. Measuring water in soil
Water content (inches water/inch of soil)
= mass moisture % x bulk density
Water content (inches water/inch of soil) = mass moisture % x bulk density

35. Measuring water in soil
Available water
= water FC – water PWP

36. Measuring water in soil
Irrigation amount in inches
= (H2O FC – current H2O content) x depth of H2O penetration
Irrig. amt. in inches = (H2O FC – current H2O content) x depth of H2O penetration

37. Soil moisture measuring devices
Soil tensiometers
instrument used to measure water potential
measures the sucking or negative pressure
requires continual maintenance and calibration

38. Soil moisture measuring devices
Conductivity meters
can be fooled by salty irrigation water or salty soils
can not calibrate

39. Soil moisture measuring devices
Neither tensiometers or conductivity meters are practical

40. Evaporation
The loss of water form the surface of the soil

41. Transpiration
Loss of water from leaf surfaces via the stomata

42. Evapotranspiration (ET)
Combined term to measure water loss from evaporation and transpiration

43. Reference Evapotranspiration (ETO)
Measures the ET of a standard reference plant
For turf well ― watered Kentucky bluegrass at a height of 4” – 6” is used

44. Weather stations
California counties have weather stations capable of providing weather data to a single source, including:
temperature
wind speed
wind direction

45. Water deficit management
Calculates the use of water by plants
Affected by:
plant species
light
relative humidity
wind speed

46. Water in soils Measure of water content in soils
Mass of water = (wet soil – dry soil)
Mass moisture % = H2O weight (g)
Dry soil weight (g)
(isolated number with limited value)

47. Water in soils Mass moisture % x BD = Volume of Moisture %
Volume of Moisture % compares volume of water to the volume of soil

48. Water in soils
Volume of Moisture % → converts to inches water/inch of soil
Inches of water/inch of soil is usually less than 0.33 in/in

49. Water in soils
Reduction of water availability to plants causes stress to plants
Losses include:
loss of vigor . . .
loss of flower and . . .
loss of fruit production

50. Water in soils
Container plants are very susceptible to damage caused by permanent wilting point (PWP)

51. Water in soils
Irrigation must occur before moisture state reaches Permanent Wilting Point (PWP)

52. MAD
Irrigation requirements are based on Management Allowable Depletion tables (MAD)

53. Remember . . . Clays hold water and don’t release it
Sands release water and don’t hold it

54. MAD
Default MAD is 50% in non-sand and non-clay soils

55. MAD
MAD levels vary with soil types
with sands – 70%
with clays – 30%

56. MAD
MAD can drop lower in sandy soils because sands give up water more freely
The opposite is true with clay soils

57. MAD MAD = (FC – PWP)/2 x 100% (provides a median value)
Percent of ETO is dependent on plant types

58. MAD Above 50% - irrigation can be withheld
Below 50% - irrigation is required

59. The measurement of water use in plants is more accurately determined by daily weather

60. CIMIS
California Irrigation Management Information System

61. CIMIS series of weather stations through the state
measures and sends weather data results to Sacramento for dissemination
produces ETO numbers based upon reference bluegrass needs

62. CIMIS measures water use by the reference plant on a daily basis
less accurate where microclimates are concerned
ETO for the South Bay is ≈ 45”/year

63. Smart controllers link via satellites

64. WUCOLS Water Use Classification of Landscape Species
Provides references available online

65. Example ETO for August for mixed, fairly drought-tolerant shrubs
KCL = 50% (crop coefficient)
Irrigation system puts out 1”/hour
Therefore . . .
Irrigation should run 3 hours

66. Infiltration Movement of water into the upper layer of soil
Infiltration rates change with soil moisture

67. Percolation
Movement of water through wetted soil