Plant Physiology (Biology 327)  - Dr. Stephen G. Saupe;  College of St. Benedict/ St. John's University;  Biology Department; Collegeville, MN  56321; (320) 363 - 2782; (320) 363 - 3202, fax;    ssaupe@csbsju.edu

Spuds McSaupe Plays with Sponges

General:  The purpose of today's visit by Spuds McSaupe is to study soil water relations.  A cellulose sponge is a reasonable model for soil.  We will use the sponge to demonstrate saturated soil, field capacity, gravitational water, capillary water, and permanent wilting point (percentage).

Exercise 1.  Lessons from a Wet Sponge.  Soak a sponge in water.  Then, place the sponge on a rack and allow it to drain.  Squeeze out the excess water in the sponge.

• The soaking wet sponge is analogous to soil that is (saturated / at field capacity / at the PWP).
• The water potential of the water in this sponge is approximately  _________ MPa.
• After sitting on the drainage rack, excess water drains from the sponge.  The water that drains from the sponge is analogous to (capillary water / gravitational) water.
• After the excess water has drained away, the moist sponge is analogous to soil that is (saturated / at field capacity / at the PWP).
• The water remaining in the sponge is (capillary water / gravitational water).
• The water potential of the water in this sponge is approximately  _________ MPa.
• Squeezing out the water in the sponge is analogous to the soil drying out.  This results in a sponge that is barely damp.  The sponge is analogous to soil that is (saturated / at field capacity / at the PWP).
• The water potential of the water in this sponge is approximately  _________ MPa.

Exercise 2.  Container Size and Plant Watering.  Thoroughly saturate two large sponges.  On a rack, lay one flat and stand the other on end.  After several minutes, squeeze the sponges into a funnel into beaker. 

• Which sponge, upright or flat, do you hypothesize will have the greatest water remaining?  Explain.
• Do your data match your predictions? 
• What implications does this demonstration have for pot size?  For example, what are the advantages and disadvantages of a shallow versus a deep pot? Which will dry out more readily?  Which will require more frequent watering?  Which will you be more prone to over-water?  In which container would it be best to include more coarse-textured materials?

Exercise 3.  Distribution of Water in Soil.  One frequent cause of houseplant death is over watering.  This exercise with use sponges to model the distribution of water in the soil.  Soak 10 sponges in water until they are completely saturated.  Remove and then stack them on a rack to drain.  After several minutes, remove each sponge from the stack in sequence and squeeze out the excess water into funnel in a large test tube in a rack.  Repeat with each sponge.  Note the amount of water obtained from each sponge and the distribution in the the column of sponges.

• Which sponge do you hypothesize will have the greatest amount of excess water - the (top / middle / bottom) sponge in the column?
• In which sponge do you hypothesize will there be the greatest amount of pore (air) space - he (top / middle / bottom) sponge in the column?
• Did your observations match your predictions?  Explain.
• Explain how this demonstration applies to watering a houseplant and explain why it is easy to over-water plants.
• What is the relationship between soil depth and water (or pore space) content?

Applications:

1. Why do plants usually die when they are over-watered?
2. Some plants such as water lilies and rice spend their lives in water.  How do they avoid the problems associated with over-watering houseplants?
3. Riparian plants often must deal with flooding.  How do they manage?
4. Why do gardeners sometimes place pebbles or broken flower pot shards in the bottom of a pot?

Last updated:  02/16/2009     � Copyright  by SG Saupe