1. Soil-Plant-Water Relationships

2. Modules - Soil Plant Water Relationships
Aim
To provide an understanding of:
the soil-plant-atmosphere continuum;
the factors affecting, the methods for measuring, the entry, retention and movement of water into and through soils;
the availability and importance of water and oxygen to plants;
atmospheric water demand and the transport of water through plants. The unit is relevant to courses in water management and irrigation.

3. Module outline
The soil-plant-atmosphere continuum: the function of water in plants; transport of water from the soil through the plant to the atmosphere in relation to water potential
Plant water relationships: the functins of water in plants, mechanisms by which plants are supplied with water.
Water in the soil: the physical properties of water, soil water potential and its components, the soil water characteristic curve, methods of measuring soil water content and potential.

4. Module outline cont’d
Soil water movement: saturated flow, Darcy's Law, measurement of hydraulic conductivity, unsaturated flow, infiltration, saturated flow analysis.
Evapotranspiration: Factors influencing rates of evaporation and transpiration, energy balance and aerodynamic approaches, alternative ways of estimating evaporation, advection, crop factors.
Four laboratory sessions covering: soil water characteristics, soil water content measurement, water flow in soils, and plant water relations.

5. Reading List- textbooks in library
David L Rowell (1994). Soil Science, Methods and Applications, Pearson Education, Edinburgh.
Nyle C Brady and Ray R. Weil (2008). The Nature and Properties of Soils, Prentice Hall.
Henry, D Foth (2003). Fundumentals of Soil Science 8th edition, Wiley.
Robert, E. White (2006). Principles and Practice of Soil Science: The Soil as a Natural Resource. 4th ed. Blackwell Publishing.
L Donahue, Raymond R Miller and shickluna ( ). Soils-An introduction to soils and plant growth. 5th Edition. Prentice Hall.
Panda SC (2003). Principles and practices of water management.
Agribios India.
Various FAO irrigation papers and publications as appropriate

6. Soil is a three phase system
- heterogeneous, multiphase, disperse, porous system.
Soil phases ( solids, liquids and gases)
Soil solid particles
Mainly alumino-silicates --weathered rocks and organic matter -- decayed vegetation.
Organic matter usually in top layers of the soil.
The soil particles provide rigid support for plants
Soil Water
contains dissolved minerals .
Almost all nutrients obtained from soil solution.

7. Soil Air provides O2 for root resprn & microbial activity.
Irrigation and drainage maintain optimum balance between soil water and soil air.
Too much water :
Shallow roots; Roots may rot; Anaerobic reactions
Production of toxic byproducts, growth reduced
Too littlewater :
Limited nutrient supply, retarded growth; Wilting ; reduced yields; and plant death if extreme
Some plants adapted to extreme moisture condns.
Due to efficient mechanisms for supplying oxygen or for conserving water.

8. Soil Properties Texture Structure
Rel. proptns of various sizes of individual soil particles ie sand, silt and clay
USDA classification system
Sand: – 2.0 mm
Silt: mm
Clay: <0.002 mm
international system also used
Textural triangle: defines textural Classes
Coarse vs. Fine, Light vs. Heavy
Affects water movement and storage
Structure
How soil particles are grouped/arranged into aggregates
Affects root penetration and water intake and movement

9. Soil textural classes

11. Mass volume relationships
Bulk Density (b)
b = soil bulk density, g/cm3
Ms = mass of dry soil, g
Vt= volume of soil sample, cm3
Typical values: g/cm3
Particle Density (p)
s = soil particle density, g/cm3
Vs = volume of solids, cm3
Typical values: g/cm3

12. Porosity ()
Porosity is the proportion of the soil volume not occupied by solids expressed as a percentage of the total soil volume
Typical values: %
Sands have large and continuous pores
Clays are dominated by very fine pores
Clays have a higher porosity than sands but

13. The Void Ratio e note that
The void ration is a measure that indicates the total volume change of soil with volume change of voids
Gives information on dynamics of pores due to external load or compaction forces
Important in engineering works

14. Water in Soils Soil water content Mass water content (m)
m = mass water content (fraction)
Mw = mass of water evaporated, g (24 105oC)
Ms = mass of dry soil, g

15. Volumetric water content (v)
V = volumetric water content (fraction)
Vw = volume of water
Vt = volume of soil sample
At saturation, V = 
V = As m
As = apparent soil specific gravity = b/w (w = density of water = 1 g/cm3)
As = b numerically when units of g/cm3 are used

16. Equivalent depth of water (de)
de = vol of water per unit area= (v A D)/ A = v D
de = equivalent depth of water in a soil layer
D = depth (thickness) of the soil layer
(cm3)
Equivalent Depth
(cm3)
(g)
(g)

17. Volumetric Water Content & Equivalent Depth Typical Values for Agricultural Soils
Soil Solids (Particles): 50%
0.50 m
1 m.
Very Large Pores: % (Gravitational Water)
0.15 m
Total Pore Space: %
Medium-sized Pores: 20% (Plant Available Water)
0.20 m.
Very Small Pores: % (Unavailable Water)
0.15 m.

18. Water-Holding Capacity of Soil Effect of Soil Texture
Coarse Sand Silty Clay Loam
Dry Soil
Gravitational Water
Water Holding Capacity
Available Water
Unavailable Water `

19. Soil Water Measurement
Gravimetric
Measures mass water content (m)
Take field samples  weigh  oven dry  weigh
Advantages: accurate; Multiple locations
Disadvantages: labor; Time delay
Feel and appearance
Take field samples & feel by hand
Advs: low cost; Multiple locations
Disadvs: exp rqd; Not v. accurate

20. Soil Water Measurement
Neutron scattering (attenuation)
Measures volumetric water content (v)
Attenuation of high-energy neutrons by hydrogen nucleus
Advantages:
samples a relatively large soil sphere
repeatedly sample same site and several depths
accurate
Disadvantages:
high cost instrument
radioactive licensing and safety
not reliable for shallow measurements near the soil surface

21. Dielectric constant
A soil’s dielectric constant is dependent on soil moisture
Time domain reflectometry (TDR)
Frequency domain reflectometry (FDR)
Primarily used for research purposes at this time

22. Neutron scattering probe

23. Assignment 1(Due in the next lesson)
1. An undisturbed soil core is 10 cm in diameter and 10 cm in length. The wet soil mass is 1350 g. After oven drying the core, the dry soil mass is 1130 g. The mineral density of the soil is 2.6 g cm-3.
Calculate:
a. Dry soil bulk density
b. Water content on a mass basis
c. Water content on a volume basis
d. Soil porosity
e. Equivalent depth of water (cm) contained in a 1 m soil profile, if the undisturbed core is representative of the 1 m soil depth
2.Consider a 1.2 m depth soil profile with 3 layers. The dry bulk density of each layer (top, center, bottom) is 1.20, 1.35, and 1.48 g/cm3. The top 30-cm layer has a water content of 0.12 g/g, the center 50-cm layer has a water content of 0.18 g/g, and the bottom 40 cm layer has a water content of 0.22 g/g.
a. What is the total amount of water in the whole profile in mm?
b. How much water (mm) do you need to apply to bring the 1.2 m soil profile to a volumetric water content of 0.35 cm3 / cm3 ?
3. Briefly describe and explain the meaning of the following terms:
a. porosity b. void ratio c. degree of saturation
d. air filled porosity e. equivalent depth of soil water

24. Ass 1b 1e and no 2 due next lecture
e. Equivalent depth of water (cm) contained in a 1 m soil profile, if the undisturbed core is representative of the 1 m soil depth
2.Consider a 1.2 m depth soil profile with 3 layers. The dry bulk density of each layer (top, center, bottom) is 1.20, 1.35, and 1.48 g/cm3. The top 30-cm layer has a water content of 0.12 g/g, the center 50-cm layer has a water content of 0.18 g/g, and the bottom 40 cm layer has a water content of 0.22 g/g.
a. What is the total amount of water in the whole profile in mm?
b. How much water (mm) do you need to apply to bring the 1.2 m soil profile to a volumetric water content of 0.35 cm3 / cm3 ?
3. Briefly describe and explain the meaning of the following terms:
a. porosity b. void ratio c. degree of saturation
d. air filled porosity e. equivalent depth of soil water

25. Diagram showing soil water classification  

26. Soil water properties and behavior in soils
Adhesion water
- strongly adsorbed and very immobile- unavailable to plants
- analogous to drop of water spread on oven dry soil- spreads as a very thin film
Gravitational Water
-moves under gravity
-lies beyond the sphere of influence of adhesive forces
-drains away quickly, not usually available to plants
-may actually be detrimental to plants- hampers root respiration
-present in saturated soils

27. Classification of soil water (contd)
Cohesion water
-Intermediate in properties between cohesion and gravitational water
-Important for plant growth, operates beyond the sphere of attraction of soil particles for water molecules
-held as a thick layer of water around soil particles in soil micro pores (fine pores)
molecular layers of water can be adsorbed to soil particles in this manner
-attractive forces of water molecules at the surface of soil particles decrease logarithmically with increasing distance from soil particles
-cohesion water is mobile and available for plant uptake

28. Structure and related properties of water
Water properties
--a simple molecule with 2 hydrogen atoms bonded to one oxygen atom
-- bonded covalently, each hydrogen atom sharing its electron with the oxygen atom
The H-O-H molecule is v shaped with bonds forming an anlge of 1050
charge distribution is not even making it polar and asymmetrical

29. Properties and behaviour contd
The water molecule exhibits hydrogen-bonding
Water molecules are attracted to each other through electrostatic forces resulting in weak H—O bonds between different water molecules
→water tend to polymerize due to attraction between water molecules. This accounts for:
High boiling point point of water (1000C ) effective as a coolant and affects the temperature of the soil-plant environment
High specific heat of vaporization
High viscosity also
liquid at room temp

30. Water is attracted to charged ions through hydration
attracted to electrostatically charged ions and colloidal surfaces:
Cations ..H+, Na+, K+, and Ca2+ exist in hydrated state (in a shell of water molecules)
→ good solvent for ionic substances
attract attracted to charged clay surfaces
→ this results in a lower energy state for water molecules when compared to water molecules in pure water

31. Water molecule is good solvent (contd)
The relatively small size of water molecules typically allows many water molecules to surround one molecule of solute
, ionic and polar substances such as acids, alcohols and salts are relatively soluble in water, and non-polar substances such as fats and oils are not.
Non-polar molecules stay together in water because it is energetically more favorable for the water molecules to hydrogen bond to each other than to engage in van der Waals interactions with non-polar molecules.

32. Cohesion and adhesion
cohesion i.e. the attraction of water molecules for each other due to hydrogen bonding
adhesion or adsorption -attraction to other materials i.e. to solid surfaces.
Adhesion and cohesion usually work together
Responsible for:
Water retention by soils
movement of water in soils and other media
plasticity of clays when wet—particles tend to stick together
movement of water In biological materials
A film of water between surfaces makes the clay particles slip and slide on one when a force is applied, but then hold to their new position.
To remove the strongly held layers of water of hydration—requires doing substantial work against these forces, called hydration forces.
In biological cells and organelles, water is in contact with membrane and protein surfaces that are hydrophilic; that is, surfaces that have a strong attraction to water.

33. Surface Tension
energy required to stretch a unit change in surface area. units are N * m m-2 = N/m.
The surface tension is due to the unbalanced force experienced by molecules at the surface of a liquid.
-a drop of liquid tends to form a sphere, minimize surface area according to Laplace’s law
low surface tension  high tendency to form films.
Water has high surface tension  mN/m at room T0
--in terms of energy. A molecule in contact with a neighbor is in a lower state of energy than if it no neighbors :
When detergent is added to water, it lowers the surface tension. Blowing soap water with a straw form bubbles, due to the low surface tension.
to minimize energy state, the number of higher energy boundary molecules must be minimized leading to minimized surface area.
As a result of surface area minimization, a surface will assume the smoothest shape it can
mlcles at surface lack other neighbors, pulled inwards.
higher energy boundary molecules minimized & hence surface area minimized

34. Viscosity
measure of a liquid’s inability to flow units are N s m-2 (SI Units) or poise (P) or centipoise (cP).       1 P = 0.1 N s m-2       1 cP = N s m-2
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress.
In everyday terms (and for fluids only), viscosity is "thickness" or "internal friction".
water is "thin", (lower viscosity), honey is "thick", (higher viscosity).
Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction.
-caused by attraction between fluid molecules thereby resisting motion from one laminar fluid layer to another
Pressure forces causing fluid flow is counteracted by these drag forces

35. Capillary action
Due to an interplay of the forces of adhesion and surface tension, water exhibits capillary action
water rises into a narrow tube against the force of gravity.
Water adheres to the inside wall of the tube and surface tension tends to straighten the surface causing a surface rise and more water is pulled up through cohesion.
This happens if a tube is sufficiently narrow and the liquid adhesion to its walls is sufficiently strong
The process continues as the water flows up the tube until there is enough water such that gravity balances the adhesive force.
Surface tension and capillary action are important in biology.
when water is carried through xylem up stems in plants, the strong intermolecular attractions (cohesion) hold the water column together and adhesive properties maintain the water attachment to the xylem and prevent tension rupture caused by transpiration pull.
An old style mercury barometer consists of a vertical glass tube about 1 cm in diameter partially filled with mercury, and with a vacuum (called Torricelli's vacuum) in the unfilled volume (see diagram).
Diagram of a mercury barometer entire cross section of the tube.
the mercury level at the center of the tube is higher than at the edges, making the upper surface of the mercury dome-shaped. The center of mass of the entire column of mercury would be slightly lower if the top surface of the mercury were flat
But the dome-shaped top gives slightly less surface area to the entire mass of mercury→the two effects combine to minimize the total potential energy.

36. Capillary and height of rise in soils
For any curved air-water interface radius r:
surface tension is balanced by a pressure drop, ∆P across the air-water interface
pressure is greater on concave side of air-water interface
for hemispherical interface, force balance gives
∆P= 2σ/r = Pair-Pwater
r= radius of curvature and
σ =surface tension

37. The contact angle θ
interface to the liquid ais msrd from the liquid-solid ir interface through the liquid
For a small θ(<900), liquid is preferentially attracted to the solid surface by adhesion and wets the solid
When cohesion forces of the liquid are stronger than the attractive force to the solid, the liquid repels from the solid surface, θ is large
θ adjusts to attain equilibrium between various surface tension forces

38. Capillary rise in tube capillary tube inserted in water
a concave meniscus forms
net surface tension forces pull meniscus along the capillary wall
water will rise in capillary if diameter small enough
At equilibrium (when water stops rising and remains in balance), surface tension force is equal to the downward force by the weight of the water in capillary tube

39. θ is the angle of contact described above
θ is the angle of contact described above. If is greater than 90°, as with mercury in a glass container, the liquid will be depressed rather than lifted.
Usually attraction is so strong that θ = 0
and cosθ = 1
The value of h reduces to
and reduces further to
For a water-filled glass tube in air at standard laboratory conditions, σ = N/m at 20 °C, ρ is 1000 kg/m3, and g = 9.8 m/s2. For these values, the height of the water column is 

40. Height of rise in soils Surface tension: Upward force = σ.2πr.Cos θ
Weight of liquid in tube: Downward force = ρghπr2
σ.2πr.Cos θ = ρghπr2
thus the height the column is lifted to is given by
Where
h is the height the liquid is lifted,
σ is the liquid-air surface tension,
ρ is the density of the liquid,
r is the radius of the capillary,
g is the acceleration due to gravity,

41. θ is the angle of contact described above. If is greater than 90°, as with mercury in a glass container, the liquid will be depressed rather than lifted. Usually attraction is so strong that θ = 0 and cosθ = 1 The value of h reduces to and reduces further to Where σ is the surface tension, ρ is the density of water and r is the pore radius For a water-filled glass tube in air at standard laboratory conditions, σ = N/m at 20 °C, ρ is 1000 kg/m3, and g = 9.8 m/s2. For these values, the height of the water column is

42. h = 2*0.0728/(1000kgm-3x9.8m/s2)
2πrσ Cosθ σ r θ mg

43. Water retention by soils
Evidenced and explained by:
water remaining in soil-
- drainage ceases in wet soils, gravitational force is still acting on the water… it is therefore balanced by a force holding the water in soil
At the PWP, water remaining in the soil cannot be extracted by the suction exerted by the root surface it is held back by ‘forces’ exerted by the soil

44. Mechanisms of water retention
Water retention is due to:
Hydrogen bonding
Van der waals forces
Hydration of cations
Soil water is under tension due to attraction of water molecules for each other and also for surfaces of soil particles
This is termed the soil water suction

45. Moisture retention (contd)
Removal of water from the soil requires the application of suction greater than soil water suction
The suction at which various pores empty is dependent on pore size or pore radius
Soil Water Release Curve
Curve of matric potential (tension) vs. water content
Less water  more tension
At a given tension, finer- textured soils retain more water (larger number of small pores)
Moisture retention curve
Moisture characteristic curves

46. Relationship between suction and pore size
Pore size (µm)
critical suction (KPa)
Equiv head mH20
Comments
20 000
0.015
0.002
2 cm crack
4000
0.075
0.008
Earth worm channel
300
1.0
0.1
The diameter of a cereal root
60-30
5-10
Soil water suction at FC 2
150 15
The size of a bacterial cell
0.2
1500
Water suction at WP
0.003
10 000
Water suction in air –dry soil

47. Soil suction
The strength of adsorption + cohesion forces ie surface tension = capillary forces
This force equals soil suction or soil moisture tension
Strength of the soil suction is dependent on mc and the pore size distribution within soil
↠it is therefore possible to find pore size distribution of soil …
….↠by looking at how soil mc changes at a given suction
This derives the moisture-suction or soil moisture characteristic curve
A suction-moisture curve may be derived for a soil using a pressure plate apparatus

48. Soil suction (continued)
pressure membrane apparatus-uses pressure chamber to raise air pressure around soil sample
Forces water out of pores through a ceramic plate at its base
When no more water can be forced out, the soil suction (capilllary forces) = air pressure
Sample can be weighed to msre θ (ie mc)
By steadly increasing the air pressure btwn soil mc measurements a suction-mc curve can be derived
Can be interpreted to give information on:
Pore size distribution,
unsaturated hydraulic conductivity
for a given soil mc (Klute, 1986)

49. Moisture retention curve

50. The Energy state of water
Total energy state of water defined by its equivalent potential energy as determined by the various forces acting on the water per unit quantity
In general flow rates of water in soils is too small to consider kinetic
The energy state of soil water is defined by its equivalent potential energy ie the energy the water possesses by virtue of its position in a force field

51. Forces acting on soil water
Capillary forces
Adsorptive forces
→ capillary and adsorptive forces together create matric potential
Gravitational forces
Drag or shear forces at soil surface-water interface
Water in soil flows from points with high soil water potential to points of lower potential energy

52. Forces acting on soil water (contd)
Driving force for flow is the change in potential energy with distance –soil water potential gradient
The above forces determine-
Direction and magnitude of water flow
Plant water extraction rate
Upward water movt– capillary rise
Soil temp changes
Solute contaminant transport rates

53. Soil water potential Description
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, Pascals
Soil water potentials are negative pressures (tension or suction)
Water flows from a higher (less negative) potential to a lower (more negative) potential

54. Definition of soil water potential
Quantifying potential energy of water requires a ref state
The ref state is defined as: the potential energy of pure water with no external force acting on it, at reference pressure (atmc), reference temperature and reference elevation
Soil water potential- determined as potential energy per unit quantity of water, rel to the reference potential of zero
Soil water has various forces acting on it, differing from pt to pt →thus its potential energy is variable

55. Free energy definiton of soil water potential
As more water is added to dry soil it is held by progressively weaker forces
When saturated, as more water is added, the energy state approaches that of pure water (no forces acting on it, no solutes)
The reference water must be ‘free’ … not affected by forces other than gravity
The energy state of soil water is defined as diff in free energy btwn 1 mole of water in the soil and 1 mole of pure, free water at std T, P and elevation.this is the chemical potential energy of soil water

56. Chemical Potential of soil water
Potential of soil water Ψ = Ψw (soil) – Ψ0 w (std state) = RT ln(e/e 0 )
Where e/e 0 is the ratio of vapour pressure of soil water to vapour pressure of pure water
R is the universal gas constant
T is the temp in degrees Kelvin

57. Components of soil water potential
Ψt = Ψp (or m) + Ψg + Ψo
t = total soil water potential
g = gravitational potential (force of gravity pulling on the water) responsible for dainage of excess water from the soil--- gravitational water
p (m) = por m ie pressure or matric potential
p is pressure or submerged potential
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

58. Soil water potential Units
Potential per unit mass
Potential per unit volume
Denoted by µ =
µ = mgl/m
µ = gl
Units are thus….J/kg
= Nm/kg
denoted by Ψ=
Ψ = ρWVgl/ V
Ψ = ρWgl
Units are thus N/m
Potential per unit weight
denoted by H = Potential/weight
Ψ p = mgl /mg
Ψp = l
Units are thus given as head and this is In metres

59. Soil water potential units
So there is no need to compute soil water potential directly by computing the amount of work needed … thus
We measure soil water potential indirectly from pressure or weight or height measurements!!!

60. Formal Defn- soil water potential
Total soil water potential is:
the amount of work per unit quantity of pure water that must be done by external forces to transfer reversibly and isothermally an infinitesimal amount of water from the standard state to the soil at the point under consideration

61. Ψp - Pressure potential
(either hydraustatic or matric potentials)
This is the energy per unit volume of water rqd to transfer an infinitesimal qtty of water from a ref pool of water at the elevation of the soil to the point of interest in the soil at a ref air pressure and temp
Pressure potential can be positive or negative
If the soil is saturated , Ψp is positive and also denoted by hydraustatic pressure potential

62. Ψm - Matric potential
If the soil is unsaturated, Ψp is negative and denoted by matric potential Ψm
For a water column, Ψ= Ψp ( can be both < 0 and > 0) as opposed to water in a soil matrix
Hydraustatic pressure is given by ..ρgh
P=F/A thus F = PA
µp = Fl/m = PAl/m = PV/m = P/ρw = Ψp/ρw
Ψp= µp ρw

63. Osmotic Potential - Ψo
Also called solute potential.. Attributable to salts in the soil soln
Solutes decrease the free energy of soil water
Osmotic potential is numerically equal to the osmotic pressure of the soil solution defined as the hydraustatic pressure necessary to just stop the flow of water when the solution is seperated from pure water by a semi permeable membrane
Osmotic Potential Ψo = - RTC
Where R = universal gas constant = J/degree kelvin/mol
T absolute temperature
C is the solute concentration in mol/m3
Only important when there is a semi permeable membrane- → not important in soils
As water molecules hydrate ions, energy is reduced (less free to move)
-major effect of Ψo is on uptake of water by plants

64. Osmotic potential & salinity
Soil solutions contain varied quantities and compositions of dissolved salts
In soils high in soluble salts, Ψosoil< Ψoroot cells
This impedes plant water uptake in saline soils
↠low soil water osmotic potential may cause young seedlings to collapse (plasmolyze) in extremely salty soils
Water moves from root cells to soil
The EC of the soil solution at saturation provides a useful approximation for estimating Ψo
Ψo = ~ 36 EC
Ψo controls the humidity of soil air phase-fewer molecules escape into air as vapour
affects movt of water vapour in soils
vapour pressure is lower in salty than in pure water

65. Gravitational Potential Ψg or Ψz
Ψg is the potential per unit weight
Force of gravity acts on water attracting it to the centre of the earth
May be expressed as Ψg = gh may be expressed as ‘gz’
h = Ψg/ρwg
h or z is the height of soil water above a reference elevation
Zsoil is +ve above the ref level and –ve below the reference level
Ref potential usually chosen within the soil profile or at its lower boundary
The gravitational potential of water will always be +ve
Important after heavy rains- it drains or removes excess water from upper soil horizons

66. Measurement of soil water potential
Tensiometers are commonly used
Operate in such a way that they illustrate how equivalent hydraulic head is a measure of soil suction
Diagram
Tensiometer consisting of a reservoir of water in a manometer (u-tube) connected to soil via a porous cup
Meniscus initially at A same height as the cup
Depending on the soil water suction water will be drawn into the soil thru the porous wall of the cup
Meniscus is consequently lowered to B
Pressure in the cup is reduced until at eqm it equals that in the soil water given by P= hwgρw

67. Tensiometers 0 to 80KPa
Tenacity of water atrraction to soil → expression of matric potential
Tensiometers msre this attraction or tension or suction
basically – water filled tube + ceramic cup+ seal (air tight)
water in tensiometer moves thru porous cup into soil up to eqm
At eqm water potential in soil = water potential in tensiometer
As water is drawn out of tensiometer, vacuum develops under the top seal ↠ Vacuum is measured by a gauge or transducer
Automation possible by connecting tensiometer to solenoid switch to turn irrigation on as soil dries
→range of potentials measured is kPa approx = 50% of soil water storage for most soils (this can be a limitation on medium- and fine-textured soils)
Tensiometers are thus ideal for irrigation management in most soils

68. Porous Ceramic Tip
Vacuum Gauge (0-100 centibar)
Water Reservoir
Variable Tube Length (12 in- 48 in) Based on Root Zone Depth

69. Filter paper method A filter paper – porous material, similar to soil
Moisture characteristic curve of filter paper
A filter paper – porous material, similar to soil
Also has a soil moisture characteristic curve
determined for Whatman No.42 filter paper
Measurement of soil water suction involves equilibrating a filter paper with moist soil until their water suctions are equal
Water content of the paper is determined & from the moisture characterisitic curve suction is determined
water (Mpa) potential 10 1
0.1
0.01
0.001
Filter paper moisture content (g/g)

70. Pressure membrane apparatus
Used to subject soils to matric potentials as low as Kpa
After application of a specific matric potential to a set of soil samples their mc is determined gravimetrically
An outside source of gas creates pressure inside the sealed chamber
Water is forced out through a porous plate into a cell at atmc pressure
The pressure applied when the outward flow of water ceases is equivalent to the water potential in the soil
It measures much lower moisture potentials than tensiometers and tension plates

71. Measuring soil water potential
Electrical resistance blocks
Measure soil water potential (tension)s
Principle: electrical conductivity decrease with decrease in soil moisture
Bouyoucos devped gypsum blocks with 2 electrodes inside them
mc in soil is proportional to resistance across electrodes.
work better at higher tensions (lower soil mc)
Affected by soil salinity- calibration necessary for each soil
Blocks may be made of fibreglass, nylon or other porous material

72. Methods of measuring soil water potential
Thermal dissipation blocks
Measure soil water potential (tension)
Require individual calibration

73. Electrical resistance blocks

74. WHC 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

75. soil water relationships
As a soil dries the water it contains as well as the soil itself undergoes gradual changes
For a saturated soil-
Ψg is more important, soil pores are water filled
water content equals porosity ie Θv = Ø
the matric potential, Ψm approximates to zero
Soil water potential changes happen along a continuum and not as discrete changes
As Θv decreases, behaviour of water is dictated more by the Ψm, and water retention happens against gravity

76. Soil water retention Maximum retentive capacity
This is the moisture content when the soil is saturated
Saturation is sustained only if infiltration continues-bcz water in macro pores continuously drains under gravity.
Maximum retentive capacity important in predicting the capacity of soil to temporarily store rain water
-determines the effectiveness of a soil in preventing down stream floods
When water infiltration stops, the soil continues to lose gravitational water gradually from the macro pores This continues until drainage stops1

77. ASSIGNMENT 3-(Individual: Due date 24/09/2012)
1. (a) Describe how gravimetric and volumetric content are related and in turn, how they are related to matric potential (Hint Give eqn as part of your answer) [9marks]
 (b) What are the forces associated with the potential of water in the soil and when is each important [6marks]
(a) Prove that Ψ = 0.15/r and state the appropriate units for ‘r’ and Ψ in this relationship [5marks]
(b) Calculate the radius of the largest water filled pores at :
(i) field capacity [3marks]
(ii)permanent wilting point [3 marks]
3. Write short notes on:
a) the soil moisture characteristic curve [5marks]
b) the capillary fringe [5marks]
c) hydraulic conductivity [5marks]
d) Air entry potential [5marks]

78. Field Capacity (FC or fc)
defined as the moisture content of a soil 1-3 days after rain or irrigation has stopped
It is the moisture content after water held in macropores has rapiidly drained downwards
Soil is not saturated but still in a very wet condition as micropores are still water filled
Traditionally defined as the water content corresponding to a soil water potential of -10 to -30 KPa
Represents the maximal amt of water useful to plants
It approximates to the soil’s lower plastic limit
Below field capacity the soil behaves as a crumbly semi-solid
Above FC soil behaves as a plastic putty
Approximates to the optimal wetness for ease of tillage/excavation

79. Permanent Wilting Point/coefficient (WP or wp)
After FC, further soil drying is gradual and is usu speeded up by plant uptake
Crop water uptake taps on water from the largest pores (rel high water potential)
As larger pores empty, water uptake progressively more difficult
 uptake rate falls below sufficient crop water needs
Initially plants begin to wilt during day time to conserve moisture, until they wilt even at night
at this pt roots are not able to generate sufficiently low water potentials to extract water from the soil
Soil water content beyond which plants cannot recover from water stress (they die)
Still some water in the soil but not enough to be of use to plants
Traditionally defined as the water content corresponding to -15 bars of SWP or -1500KPa ie Potential = -1500KPa for most plants
Beyond the PWP, plants die if water is not provided thru irrign or rain
Xerophytes may take up water between potentials = to-2000KPa

80. Plant Available Water Definition Available Water Capacity (AWC)
Water held in the soil between field capacity and permanent wilting point (ie -10 or -30KPa and -1500KPa )
“Available” for plant use
Soils high in OM may retain substantiial amts of water below the PWP
Available Water Capacity (AWC)
AWC = fc - wp
Units: depth of available water per unit depth of soil, “unitless” (in/in, or mm/mm)
Measured using field or laboratory methods
Hygroscopic Coeffcicient
Below PWP soil drying may continue thhru evaporation
Water remaining is tightly held (layers 4-5 molecules thick)
essentially soil is saturated with water vapour (98% RH)
Water potential at hygroscopic coefficient is Kpa
Hygroscopic water is not available to plants

81. Factors affecting plant available water capacity
Plant available water capacity of a soil is dependent on:
Soil texture or the water content-potential relationship ie
the water potential and amount of water held at FC and PWP
Organic matter content- organic matter has a high water holding capacity. The higher the OM content of a soil, the higher the water holding capacity
Osmotic potential
compaction
Soil depth and layering

82. Factors affecting plant available water contd
Effects of compaction on aeration
Compaction reduces available water
Macropores reduced into micropores
Increased BD limits root growth eg soil resistance beyond 2000KPa limits root growth
Total porosity and therefore water holding capacity is reduced
Less aeration near field capacity
Permanent wilting coefficient increases as fine pores ie fine micropores increase in proportion

83. Osmotic potential
Water uptake affected by presence of salts eg soluble salts from fertilizers
Total moisture stress in saline soils includes osmotic potential as well as matric potential
At the PWP, in saline soils, more water is retained in the soil than would be retained by the matric potential alone
OP effects are more important in dry arid regions than in wet humid region soils
—dry regions can accumulate salts through irrigation or by natural processes

84. Soil depth and layering
Total volume of water available in soil lies between FC and PWP
The vol of plant available water however depends on total volume of soil explored by roots ie this is dependent on:
Root restricting layers
Greatest rooting potential of plants or
Pot size for containerized plants
Rz is important in arid regions - perennials depend on rooting potential to tap into underground stored water
Soil stratification influences the mvmt of water in soil as well as root penetration

85. Least limiting water range is reduced
increase in proportion of fine pores, reduces the least limiting water range ie range mc for which soil conds do not severely restrict root growth
when air-filled porosity is less than 10%, soils are too wet for normal root growth-
oxygen becomes limiting to root growth at lower moisture levels than in loose soils - this happens at FC in loose well aerated soils
Compaction most damaging at low moisture potentials ie in dry soils when soil strength exceeds 2000kPa, (this is the pressure rqd to push a pointed root through soil) — soils are too dry for normal root growth
in loose well aerated soils this occurs at mc close to the PWP …but may occur at high mc in compacted soils

86. Total available water holding capacity of a soil profile (AWHC)
defn: the total amount of water available to a plant in a field soil
can be estimated from Rz (rooting depth)
AWC is calculated as the gravimetric mc at FC less the gravimetric mc at PWP
Gravimetric water content can be converted to volumetric water content by multiplying by the ratio of BD to density of water then multiplying by the depth of the horizon
AWHC = (θmFC-θmWP) * BD / ρw * L

87. Fraction of available water depleted (fd)
(fc - v) = soil water deficit (SWD)
v = current soil volumetric water content
Fraction available water remaining (fr)
(v - wp) = soil water balance (SWB)

88. Total Available Water (TAW)
TAW = (AWC) (Rd)
TAW = total available water capacity within the plant root zone, (mm or cm)
AWC = available water capacity of the soil, (mm of H2O/m of soil)
Rd = depth of the plant root zone, (mm)
If different soil layers have different AWC’s, need to sum up the layer-by-layer TAW’s
TAW = (AWC1) (L1) + (AWC2) (L2) (AWCN) (LN)
- L = thickness of soil layer, (inches)
- 1, 2, N: subscripts represent each successive soil layer

89. soil depth, cm
rel root length
soil depth increment, cm
soil BD, Mg/m3
Field Capacity, g/100g
wilting percent WP, g/100g
available water holding capacity (AWHC), cm
0-20
xxxxxxxxx 20
1.2 22 8
20-40
xxxx
1.4 16 7
40-75 xx 35
1.5 10
75-100 25 18
---
1.6 15 11
no roots
Total
=14.43

90. Potential expressed in different measurement units
Various soil and plant atmc conditions
Bars and atms.
kilopascals
Rel. energy potential
Saturated soil
high
Field Capacity
-0.33
-33
Medium
Plant available water
-0.33 to -15
-33 to -1500
Permanent wilting Point
At or below -15
At or below -1500
low
Air dried soil
-31
-3100
Oven dried soil
Below -31
Below -3100
Root tissue
-3 to -20
-300 to -2000
Leaf tissue
-15 to -20
-1500 to -3000
Atmosphere
-100 to -500 to
Very low
These figures are approximate due to differences in physical or chemical conditions in the soil, plants or atmosphere