Plants & Human Affairs (BIOL106)  -  Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe

Lessons from a Sponge

    The purpose of these exercises is to examine the impact of container size & shape on soil water content.  We will use a sponge to represent a container with filled with soil.  A sponge is a good model for the soil matrix since it is a mixture of solid particles with pore spaces. 

Exercise 1:  Soil Water Content and Container Shape. 

Question:  How is the ability of a saturated sponge to hold water affected by the orientation of the sponge?

Hypotheses:  If a saturated sponge is placed on edge, then......

Protocol:

1. Place the sponge flat (this models a shallow container) on a screen.
2. Pour water on the sponge until it freely flows through (e.g., is saturated). 
3. Let the sponge stand until it no longer drains. 
4. Now, stand on its side (the long edge, this models a container of medium depth).  Record your observations.
5. Finally, stand the sponge on end (this models a deep container) and record your observations. 

 Analysis Conclusions:

1. What does this exercise tell you about container size and soil water content?
2. Which shape/size container will dry out most readily?
3. Plants in which shape/size container will require more frequent watering?
4. Which shape/size container is it easiest to over-water?
5. In which container would it be best to include more coarse-textured materials?

Exercise 2.  Distribution of Water in a Container 
    The purpose of this exercise it to determine how water is distributed in a plant growth container.  We will stack 10 sponges in a column and then saturate the sponges.

Question:  After the sponges are allowed to drain, which sponge (top, middle or bottom one in the stack) will hold the greatest amount of water?

Hypothesis: 

Protocol:

1. Stack 10 similar-sized sponges in a vertical column on a screen. 
2. Measure the height of each sponge (in cm) in the stack and record your data in column #2.
3. Pour water on the top of the column until it drains from the bottom sponge.
4. After no more drainage is observed from the bottom sponge, remove each sponge in sequence and squeeze out all the water into a container and measure the volume with a graduate cylinder.  Record your data in column #3
5. Calculate the volume (l x w x h) of one sponge.  Record your data in table 3 below.
6. Calculate the volume of sponge solids by subtracting the water at saturation (which equals the volume of water squeezed from the bottom sponge) from the sponge volume
7. Calculate the sponge pore volume (mL) by subtracting the solids volume from the sponge volume.
8. Calculate the percent of the sponge pore space filled with water (column #4) by dividing the water squeezed from the sponge (column #3) by the sponge pore volume
9. Calculate the percent of the sponge pore space filled with air (column #5) by subtracting % of the pore space filled with water (column #4) from 100.
10. Plot Column height (cm) vs. water squeezed from sponge (mL)
11. Plot Column height (cm) vs. % pore space filled with water.
12. Plot column height (cm) vs. % pore space filled with air.

Conclusions:

1. What is the relationship between soil depth and aeration?
2. What is the relationship between soil depth and water content?
3. How does this exercise apply to your houseplants?
4. Why is it relatively easy to over-water a plant?
5. Explain why is it bad to over-water a plant with reference to our results.

References:

• Spomer, LA (1974) Two classroom exercise demonstrating the pattern of container soil water distribution.  HortScience 9: 152 � 153.
• Spomer, LA & DR Hershey (1990) Container soil-water relations. Journal of College Science Teaching. September/October; pp 44 � 45.

Last updated:  02/09/2005 / � Copyright  by SG Saupe / URL:http://www.employees.csbsju.edu/ssaupe/index.html