1. Lab 9: Building a Soil Moisture Characteristic Curve or Moisture Release Curve
A plot of water content, , vs. soil tension, or versus pressure, .

2. We are Going to Create an Approximate Soil Moisture Characteristic Curve
A Moisture Characteristic Curve is a graph of the water content of the soil vs. the water tension in the soil.
We will estimate the soil moisture content at:
Saturation
Field Capacity
Air Dry
Oven Dry (which is equal to 0)

3. Oven Dry
Air Dry
Field Capacity
Saturation

4. Water Content Calculations
Gravimetric Water Content:
= Mass water / Mass Oven dry soil (g/g)
Volumetric Water Content:
= Volume water / Volume soil (cm3/cm3)
g (g/g) x BD (g/cm3) = (cm3/cm3)
Water Depth:
= v x Ds (Soil Depth)

5. Calculating Plant Available Water
PAW = v FC - vWP
The water in the soil between field capacity,  = 0.1 bar, and the wilting point,  = 15 bars:
The water bound less tightly than the field capacity is termed gravitational water because gravity easily drains this water before the plants can get it.
Water bound beyond the wilting point is unavailable, because plant roots can not pull hard enough to overcome absorption of the water to the soil

6. Saturated Water Content (0 bars)
Weigh cup and glass stir rod
Add 25g soil (nearest 0.01g)
Add water slowly
Reweigh
Gravimetric content at Sat.
Sat = Mass water/ mass dry soil

7. Field Capacity (0.1 bars) 100 mL in graduated cylinder
60 mL put a straw in
BD = g soil / cm3
Pipette 10 mL water slowly
Parafilm/ 20 min
Mass soil wetted(g) = Vol. w. soil (cm3) x BD (g/cm3)
FC(g/g) = Mass water (g)/ Mass soil wetted(g)

8. Air-Dry and Oven-Dry Air-Dry mass on the beaker
AD water % = (AD – OD)/ OD
Plot AD (1,000)
Plot OD (10,000): H2O content = 0

9. Estimate Pore Radius of Different Sands Using Capillary Rise
The wicking effect caused by small pores
h = 0.15 / r
h is the height of rise in tube, cm
r is the radius of pore size, cm
You are solving for “r”
The height of rise is higher in:
clays than silts
silts than sands
sands than gravels

10. Time Domain Reflectometer
Soil Tensiometer Used to measure soil tension (pressure) A pressure gage is connected to a water column A porous ceramic cup lets water move, but not air
Time Domain Reflectometer
Used to measure soil moisture
An electrical pulse is sent down the rod
The pulse bounces off the end and returns to the source
The wetter the soil, the longer the delay in returning