Soil and Water

These demonstrations use ball bearings of different diameters to show where water exists in soil, and what happens when plant roots extract water from the soil.
Brief video clips are available in Quicktime (QT) or RealPlayer (RP) formats. If neither is available on your computer, click the name to download it.
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This is a picture of ball bearings in a petri dish.  There is no water in the dish.  The balls are randomly distributed by diameter in the dish.
Water has been added to the dish.  The water appears pink in the picture because reverse lighting was used.  Notice the water fills the spaces (called voids or pores) between the balls.
Click to see a video of water being added: QT, RP.
Watch closely to see that the contact points and narrow pores fill first, followed by the larger pores.
The line at the bottom of the picture is a pipette. It is removing water from the dish much as a plant root extracts water from the soil. Notice all the pores near the pipette are still full while some large pores near the top of the picture are empty.
This video begins the process of water extraction, until three large pores are emptied: QT, RP.

Water is moving from the large pores near the top to the pipette at the bottom. Water moves through other water-filled pores.
As more water is removed, more large pores are emptied.
Most of the water is now gone from the pores, especially near the point of removal.  The water near the top is no longer connected to the water at the bottom by any large water-filled pores.  These dry spots between pockets of water limit water movement. When this condition occurs in soil, a root is not able to get much water.

These video clips show water being removed from the bottom, then from the top of the picture, as if two separate roots were at work.
     Bottom: QT, RP                      Top: QT, RP

Even though the pipette can extract no more water, there is still water in some of the smaller pores (QT, RP: top right, bottom right, bottom left). There is also water at every place two balls touch (QT, RP). This water is held by adhesion (attraction of unlike objects) to the balls, and by cohesion (attraction of like objects) to other water molecules.  These forces act like a weak glue holding the balls in contact. (See clip: QT, RP.)
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Updated 06-28-2005, CAR, Copyright 2005. Clay Robinson, Ph.D., as to all resources: Materials may not be reproduced without Dr. Robinson's written consent. Students are prohibited from selling (or being paid for taking) notes or webpages during this course to or by any person or commercial firm without the express written permission of the developer of these pages.