Lab 9: Lab 9: BuildingBuilding a Soil Moisture a Soil Moisture Characteristic Curve or Moisture Characteristic Curve or Moisture Release CurveRelease Curve

A plot of water content, , vs. soil tension, or versus pressure, .

We are Going to Create an Approximate We are Going to Create an Approximate Soil Moisture Characteristic CurveSoil 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)

Saturation

Field Capacity

Air Dry

Oven Dry

Water Content CalculationsWater Content Calculations

Gravimetric WaterGravimetric Water ContentContent: : ◦ = Mass water / Mass Oven dry soil (g/g)

Volumetric Water Content:Volumetric Water Content:◦ = Volume water / Volume soil (cm3/cm3)

◦ g (g/g) x BD (g/cm3) = (cm3/cm3)

Water Depth:Water Depth:◦ = v x Ds (Soil Depth)

Calculating Plant Available WaterCalculating Plant Available Water PAW = PAW = v v FCFC - - vvWPWP

◦ 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 Saturated Water Content (0 bars)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

Field Capacity (0.1 bars)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)

Air-Dry and Oven-Dry 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

Estimate Pore Radius of Different Estimate Pore Radius of Different Sands Using Capillary RiseSands Using Capillary Rise

The The wickingwicking effect caused by small effect caused by small porespores◦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:The height of rise is higher in:◦clays than silts◦silts than sands◦sands than gravels

Soil TensiometerSoil TensiometerUsed 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 Time Domain ReflectometerReflectometerUsed 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