Transcript Chapter 9

Lab 9: Building a Soil Moisture Characteristic
Curve or Moisture Release Curve
A plot of water content, , vs. soil tension, or versus pressure, .
We are Going to Create an Approximate Soil
Moisture Characteristic Curve
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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:
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Saturation
Field Capacity
Air Dry
Oven Dry (which is equal to 0)
Oven Dry
Air Dry
Field Capacity
Saturation
Water Content Calculations
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Gravimetric Water Content:
◦ = Mass water / Mass Oven dry soil (g/g)
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Volumetric Water Content:
◦ = Volume water / Volume soil (cm3/cm3)
◦ g (g/g) x BD (g/cm3) = (cm3/cm3)
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Water Depth:
◦ = v x Ds (Soil Depth)
Calculating Plant Available Water
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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
Saturated Water Content (0 bars)
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Weigh cup and glass stir rod
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Add 25g soil (nearest 0.01g)
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Add water slowly
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Reweigh
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Gravimetric content at Sat.
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Sat = Mass water/ mass dry soil
Field Capacity (0.1 bars)
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100 mL in graduated cylinder
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60 mL put a straw in
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BD = g soil / cm3
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Pipette 10 mL water slowly
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Parafilm/ 20 min
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Mass soil wetted(g) = Vol. w. soil (cm3) x BD (g/cm3)
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FC(g/g) = Mass water (g)/ Mass soil wetted(g)
Air-Dry and Oven-Dry
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Air-Dry mass on the beaker
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AD water % = (AD – OD)/ OD
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Plot AD (1,000)
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Plot OD (10,000): H2O content = 0
Estimate Pore Radius of Different Sands
Using Capillary Rise
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The wicking effect caused by small pores
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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
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