Soil Water
Background Information
Read Plaster (1997), chapter 4
Important terms for this lab:
- soil water content
- oven-dry soil
- volumetric soil water content
- water depth
- gravitational potential
- matric potential
- osmotic potential
- soil water retention
- soil bulk density
- adhesion
- cohesion
- capillary
- saturation
- field capacity
Link to soil water drainage boxes . Examples from experiment.
Water - without it there would not be life (as we know it). It is common, essential, and peculiar. Many of water's properties are anomolous, such as its high specific heat and its density (solid phase less dense than liquid).
Water is one of the main constituants of soil - its presence and/or abundance there controls the soil's chemical, physical, and biological properties.
Water is the single most limiting factor to plant growth worldwide.
Soil water properties are important for agriculture, environmental quality, construction, recreation, forestry...and much more.
Read the background information included in the Study guide before you come to lab. Also, look at the study questions at the end of the lab write-up.
UNITS!!
A common problem with this lab is the use of incorrect units. REMEMBER
- Water content is expressed as mass of water per mass of dry soil (g/g, kg/kg, lb/lb etc.), or may be shown as unitless. DO NOT express this as a percent (%).
- Volumetric water content is volume per volume (cm3/cm3...). It may be expressed as a %
- Water depth is a unit of length, that is cm, in, ft etc.
Be sure you have these units correct before you leave the lab!
Lab Study Questions
1. What is the difference between soil water content and volumetric water content? Give examples of the most common units used to express these terms.
2. What is the difference between water retention and water content?
3. Explain the gravimetric method for calculating soil water content.
4. Assume that the sponge used in Procedure C had a total porosity of 0.80. Calculate the amount of pore space filled with air for each of the three orientations.
volumetric air content = total porosity - volumetric water content
a. Which of the three soils had the most air in them?
b. Which of the three soils is most like an ideal topsoil, which has half of the pore space filled with air, half with water?
5. Which soil textural class would you expect to retain the most water?
6. Which soil textural class would you expect to retain the most plant-available water?
7. If you were installing tile drains in a field to drain excess water away from the plant roots, where would you place the drain – at a shallow depth (~30 cm) or deep (~2 meters)?
Study Questions Answers
1. Soil water content: Mass water per mass oven dry soil. Units: mass per mass (g/g) or decimal (unitless).
Volumetric water content: Volume soil water relative to bulk soil volume. Units: volume per volume (cm3/cm3), %, or decimal (unitless).
2. Water retention: ability of a soil to retain water.
Water content: the water retained by the soil.
3. Mass of moist soil minus the mass of same soil after oven drying gives the mass of water. Divide this by the mass of the oven dry soil.
4. volumetric air content = total porosity-volumetric water content.
a. Since the total porosity is the same for all three orientations, the sponge with the least water would have had the most air.
b. The orientation with about 50% volumetric water content.
5. Fine textured soils - clays.
6. Loams or silt loams usually have the most plant-available water. Sandy soils do not hold much water, clay soils hold a lot of water, but much of it is held too tightly for plants to use.
7. Seep placement of tile drains would create a deep soil column which would produce a good balance of air and water-filled pores. Shallow placement would create a shallow soil column that would be saturated with water, drowning plant roots.
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Exam Questions from Labs
1. A soil sample contains 56.9 g of water and 692 g of oven dry soil. The moisture content of the sample is
a. 56.9
b. 56.9/692
c. 692/56.9
d. 56.9/(56.9 + 692)
e. 56.9/(692 - 56.9)
2. A 18.0 cm depth of soil has a volumetric water content of 0.297. What depth of water does the soil contain?
a. 18/0.297
b. (18) (0.297)
c. (18) (0.297) (100)
d. 18.0
e. 0.297
3. A soil has a gravimetric water content of 0.216 and bulk density of 1.45 g/cm3. What depth of water is contained in a 50-cm depth of this soil?
a. (0.216) (1.45) (50)
b. (0.216) (1.45) / 50
c. (21.6) (1.45) / 50
d. (21.6) (1.45) (50)
Click here for the Answers to these questions.
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This page last modified on January 20, 2003
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