1. BASIC SOIL PLANT WATER RELATIONS
N.L Mufute

2. 1. Soil Properties Texture Structure
Definition: relative proportions of various sizes of individual soil particles
USDA classifications
Sand: – 2.0 mm
Silt: mm
Clay: <0.002 mm
Textural triangle: USDA Textural Classes
Coarse vs. Fine, Light vs. Heavy
Affects water movement and storage
Structure
Definition: how soil particles are grouped or arranged
Affects root penetration and water intake and movement

3. Bulk Density (b) Typical values: 1.1 - 1.6 g/cm3
b = soil bulk density, g/cm3
Ms = mass of dry soil, g
Vb = volume of soil sample, cm3
Typical values: g/cm3
Particle Density (p)
P = soil particle density, g/cm3
Vs = volume of solids, cm3
Typical values: g/cm3

4. Porosity ()
Typical values: %

5. 2. SOIL WATER MEASUREMENT
Soil water content
Can be expressed as;
Mass water content (m) - The mass of water associated with a given mass of soil.
m = mass water content (fraction)
Mw = mass of water evaporated, g (24 105oC)
Ms = mass of dry soil, g
2.Volumetric water content (v) – The volume of water associated with a given volume of dry soil. (Usually expressed as m3 of water / m3 of soil

6. Either approach is acceptable but use of V is more common
V = volumetric water content (fraction)
Vw = volume of water
Vb = volume of soil sample
V = As m
As = apparent soil specific gravity = b/w (w = density of water = 1 g/cm3)
Either approach is acceptable but use of V is more common
Equivalent depth of water (d) or z
d = volume of water per unit land area = (v A L) / A = v L
d = equivalent depth of water in a soil layer
L = depth (thickness) of the soil layer

7. Volumetric Water Content & Equivalent Depth
(cm3)
Equivalent Depth
(cm3)
(g)
(g)

8. Volumetric Water Content & Equivalent Depth Typical Values for Agricultural Soils
Soil Solids (Particles): 50%
0.50 in.
1 in.
Very Large Pores: % (Gravitational Water)
0.15 in.
Total Pore Space: %
Medium-sized Pores: 20% (Plant Available Water)
0.20 in.
Very Small Pores: % (Unavailable Water)
0.15 in.
1 inch = (approx ) 2.54 cm

9. Soil Water Measurement :
Measuring soil water content and water potential.
A. Direct Methods
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 and feel them by hand
Advantages: low cost; Multiple locations
Disadvantages: experience required; Not highly accurate

10. Soil Water Measurement:
Indirect Methods
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
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

11. Soil Water Measurement Neutron Attenuation

12. Soil Water Measurement
Tensiometers
Measure soil water potential (tension)
Practical operating range is about 0 to 0.75 bar of tension (this can be a limitation on medium- and fine-textured soils)
Electrical resistance blocks
Tend to work better at higher tensions (lower water contents)
Thermal dissipation blocks
Require individual calibration

13. Tensiometer for Measuring Soil Water Potential
Water Reservoir
Variable Tube Length (12 in- 48 in) Based on Root Zone Depth
Porous Ceramic Tip
Vacuum Gauge (0-100 centibar)

14. Electrical Resistance Blocks & Meters

15. Water and Plant Growth:SOIL-WATER-PLANT RELATIONSHIPS.
Field Capacity (FC or fc)
Soil water content where gravity drainage becomes negligible
Soil is not saturated but still a very wet condition
Traditionally defined as the water content corresponding to a soil water potential of -1/10 to -1/3 bar
Permanent Wilting Point (WP or wp)
Soil water content beyond which plants cannot recover from water stress (dead)
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

16. Available Water Definition Available Water Capacity (AWC)
Water held in the soil between field capacity and permanent wilting point
“Available” for plant use
Available Water Capacity (AWC)
AWC = fc - wp
Units: depth of available water per unit depth of soil, “unitless” (in/in, or mm/mm)

17. Soil Hydraulic Properties and Soil Texture

18. Fraction 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)

19. Total Available Water (TAW or TAM)
TAW = (AWC) (RZD)
TAW = total available water capacity within the plant root zone, (mm)
AWC = available water capacity of the soil, (mm of H2O/mm of soil)
RZD = Root zone depth, (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, ( mm)
- 1, 2, N: subscripts represent each successive soil layer
RZD =L1+L2+…+LN

20. Management Allowable Depletion (MAD)
The growth and yield of plants or crops get severely hampered when the AWC is depleted below a certain level, i.e. as water content approaches wilting point.
Readily Available Moisture (RAM) refers to that part of AWC which the plant can easily extract from the root zone.
This varies with the root zone depth (RZD) of the plant.
So as someone involved in water management in Irrigation need to set an allowable or acceptable depletion of the AWC and then come in with an irrigation to replenish the lost water.

21. Management Allowable Depletion (Cont.)
This depletion level is what is called Management Allowable Depletion (MAD).
It actually means at that level of moisture depletion, crop growth and yield are not affected.
For most crops such as maize and wheat the MAD is 50% of the AWC.
For crops like cotton and sugarcane the MAD can go as far as 60% of the AWC and for vegetables and related crops, the MAD is limited to around 40% of the AWC.

22. 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

23. Depth of Soil Water Cont.
The concept of depth of soil water is very important in irrigation agronomy.
If the root zone depth of the soil is RZD or Rz, then the volume of soil per unit area of field surface is given by (Rz*1 ).
If d or z is the depth of the soil water available for plant use (assuming that all the soil water is brought together at one place), then the weight of water per unit area (Ww) is given by:
  Ww = (z*1)ρw
 Where ρw is the bulk density of water (standard at 1000kg/m3 or 1g/cm3).

24. Depth of Soil Water Cont.
The percentage water content, RAM (or Pw ) readily available for plant use i.e. that fraction of AWC that can safely be depleted without affecting plant performance is given by:
Pw = (z* ρ w/ ρ sRz)*100
Where ρs is the bulk density of the soil. Alternatively , the depth of soil water (z) can be expressed as
z= Pw *Rz*(ρs/ρw)/100
Note that z will have the same units as Rz.

25. Worked example
A soil has a FC of 22%, PWP of 10% and a bulk density of 1400kg/m3. The Rz depth is 100cm and the MAD is 60% of the AWC.
Given the above information, calculate or determine the AWC, percentage water content readily available for plant use (Pw) and the depth of soil water which can be safely depleted (z).

26. Worked example :Solution
AWC= FC-PW = (22-10)% = 12%,
Pw = 60% of AWC = 60/100*12% = 7.2%
z= Pw/100*(s/ w)* Rz =
7.2/100*1400/1000*1000mm= 100.8mm.

27. An Alternative Approach
In this approach (Which gives an approximate answer), the density of soil is not taken into consideration and hence;
z = MAD x AWC x Rz or Pw xRz
For the above example therefore
z= 60%x12%x1000mm =72mm
Or z = 7.2% x 1000mm = 72mm