Water potential in soil - A laboratory demonstration

Abstract

The concept of water potential is often difficult to grasp for students in an introductory soil science course. To assist in understanding the idea, the student can construct a simple glass-capillary wedge and use it to physically demonstrate water potential and its role in retention and movement of soil water. Mathematical description, using a simplified version of the capillary rise equation for a wedge, is emphasized. The concepts of gravitational water, field capacity, plant available and unavailable water, water movement down a potential gradient, and removal of soil water by a plant root may be readily demonstrated. Contrast between the pore space in the glass capillary wedge and soil is explained. The students answer a series of questions by interpreting the physical phenomena occurring with water in the glass capillary. Step-by-step explanation and coordination by a laboratory instructor is desirable to maximize the effectiveness of the exercise.