Terrestrial plants depend on the soil for water supply. Thus, plant water relations and plant growth and productivity are tightly linked with water status in the soil. The soil can be viewed as a reservoir from which the plants draw the water but, water behavior in the soil is not simple. The unique physical properties of the soil particles determine the forces which govern soil water holding capacity and their uptake by the plants. This article presents a visual aid which helps teaching the physical phenomena related to water status and water movement in the soil. These phenomena are relevant for the studies of plant physiology, ecology, and agriculture. However, many teachers are reluctant to teach them because certain basic concepts related to soil-water physics which operate at the microscopic and molecular levels are difficult to explain to students at the high-school and college levels. Most notable among these concepts are matric water potential, diffusion path tortuosity, and void space partitioning between gas and liquid phases. A full mathematical formulation of the concepts in question is, in many cases, beyond the scope of basic courses in plant physiology. On the other hand, verbal description of these concepts and their role in determining the dynamics of water movement through the soil, falls short of the explanation necessary to convey them to students. I am presenting here a visual aid, which is easy to construct and use, that helped me in demonstrating these concepts to undergraduate students. It is designed to generate an image of water movement among soil particles on an overhead projector, which is easy for all students in the class to view at the same time. Darcian flow of water in the soil, capillary rise of water through dry or partially dry soil, and gradual displacement of air by water from inter-particle spaces can be demonstrated easily. Unlike microscopic observations which students perform individually, with this demonstration, the teacher and students all see the same image. This makes all explanations and teacher-student communication much more effective.
A schematic diagram, is presented in Fig. 1. The base plate is made of a thin sheet of transparent plastic. A regular overhead transparency serves well for that purpose. The image of magnified soil particles is generated by pieces of similar opaque plastic sheet cut into irregular shapes, glued onto the base plate in a pattern which resembles a microscopic view of dispersed soil. Photocopying the upper part of Fig. 1 onto a transparency and cutting out the transparent areas can help you create a similar design. The gaps between the "particles" should be uneven, representing large and small air-spaces. Two strips of the same kind of plastic should be glued along the sides to prevent water from running out of bounds. A plate of glass is laid on top, and can be removed to allow blotting the water away between demonstrations. Clear window glass of any thickness or an old TLC (Thin Layer Chromatography) plate may serve that purpose well. Use a colored aqueous solution to represent the water flowing between the soil particles. Food coloring are suitable and unharmful, but any other colorful water soluble chemical from the school's laboratory in high enough concentration will produce the desired effect. Prepare a concentrated solution with a dark color, since it will form a very thin layer between the base sheet and the glass plate that will not absorb much light. Put a few milliliters of the dye solution on the base plate by the edge of the glass cover using a pipette or a dropper and watch how the liquid is sucked in among the “soil particles” filling the air spaces. Explain to the students that the force driving the liquid flow is not gravity, but capillary forces similar to matric forces which are responsible for Darcian water flow in the soil. If you will point your demonstration so that the liquid will flow toward the class, to the students it will look like the water is flowing from the bottom toward the top of the screen. This will help them remember that matric suction forces can make water rise through soil capillaries to upper soil layers from an underground water- table. Once most of the spaces are filled with liquid, you can demonstrate water suction from the soil by using a piece of filter paper to represent a plant root. Use the paper the same way it is used to draw excess water from a microscope slide. The paper will draw the water because the gaps between its fibers are much smaller than the spaces in the model. The students will be able to see that not all spaces drain at the same time and the “soil” ends up with water and air occupying different parts of the space volume. Tortuosity of the diffusion path through the liquid phase from one point in the soil to another will also be obvious. I must caution here that this model should not be taken to represent the processes quantitatively. The dimension that determines the capillary forces in this model is the distance between the base sheet and the top glass plate, not the distance between the particles. Because of the limited magnification power of the overhead projector (ca. x 10), it will be much more difficult to build a model which will demonstrate accurately the differences between fine and coarse matrices. The model presented here will help the students visualize the processes which take place at the microscopic level. However, no microscopic preparation can show that as easily, because particle movement will occur whenever a drop of water is put onto a dry sample. I recommend beginning the class with this demonstration and then leaving the model image on the screen. With this image in the background, the teacher may continue with a more detailed explanation of the suction forces relationship to meniscus radii, and other related topics, referring to it in the course of the lecture.