Expected Outcomes

• Intuitive and obvious water holding properties of sponges are easily related to similar but less intuitive properties in soils.
• Students derive formula for "Soil Water Content" when faced with the challenge of finding which sponge holds the most water.

What to do and how to do it

• Ask the questions:
  1. How do you standardize your balance?
  2. How much water will your sponge hold?
  3. How will you compare your results with another group?
• Have each group experiment with different sponges, balance and water
• After 8-10 minutes, Ask the question: What procedure did you follow to answer these questions?
• Using simple notation, make a table of each groups response (see below)
• Ask the question: How is a sponge like a soil?
• Discuss:
  • pore size, water holding capacity?
  • what process is modeled by squeezing?
  • why does some water leak out immediately while the rest is held?

Table of Procedures

Group    Formula/Method                          Comments

  1


  2


  3


  4


  5


Illustrative Answers:

  1      (wet weight - dry weight)        only gives water weight
  2      (water weight) / (sample volume) better - stick with weights?
  3      (water weight) / (wet weight)    better - a logical response
  4      (water weight) / (dry weight)   Yea! This is what we will use

Premise

• This definition makes SWC independent of the size and shape of the sample.
• The dry weight normalization is a scientific convention - and is why we report SWC rather than percent SWC.

Extensions

• Repeat the exercise after air drying the sponges