A Simple Moisture-Gradient Experiment

Abstract

E veryone knows that a) plant species vary widely in adaptation to or conditions, and that b) moisture availability to plants varies with slope and elevation. A very simple and inexpensive way of providing an experimental moisture gradient is described here, and it can be adapted to demonstrate not only the basic principles but several ramifications and implications of soil moisture levels and plant responses. First, line a regular flat at least 22 long by 3 deep with sheet plastic to make it waterproof or use an undrained plastic tray of similar dimensions. Fill with regular potting mix and bury two 3 clay pots in the corners at one end for adding water. Then plant three or four rows lengthwise of different species, using equal cuttings of such common species as Coleus, Kalanchoe, etc., and/or seeds of bean, pea, corn, oats, etc. Moisten thoroughly and allow four days to a week for root establishment and germination, then prop the end farthest from the pots on a 12 pot or other firm support so that the flat slopes at an angle of 400 (see Fig. 1.) Fill the pots in the low end with water and keep them filled daily so that the low end is continually saturated and capillary action will provide a gradient from very wet to very dry. After three to four weeks observe, measure and record the growth and general appearance of the plants of each kind at each moisture level. One can divide the length of the flat into four or five equal zones, or better still, plot the height of each plant on graph paper using the relative wet to dry scale as the base. An example of data recorded by zones is given in Table 1. Note that, while all three species showed maximum growth and survival in the moist situation and no survival in the very dry end, Coleus showed the greatest decrease in the moderately dry zone as compared to the moist zone. This exercise is particularly useful in showing the differences in response of hydrophytes, mesophytes, and xerophytes. Many species may be used but it is best to choose species of about equal size and fairly rapid growth rates; true xerophytes and cacti are mostly too slow in growth response, but such relatively drougth-resistant things as oats or wheat, Kalanchoe species, or dune or prairie plants will show different curves of response than corn, beans, Coleus, etc. Emergent hydrophytes such as Sagittaria, water buttercup, or Proserpinaca will do well at the wet end. Plotting data from the three types of plants on graph paper will illustrate the basic principle of the Laws of Tolerance, showing curves of minimum, optimum and maximum range for each species as well as differences between species as to optimum level and breadth of range. The more species and the more flats for replication the better, and groups or teams of students might each set up, water and record their own, then compare results and get averages, if sufficient space is available. Greenhouse space which is not kept too humid is of course best, but a flat or two should show good response just kept in a sunny lab window, so that any lab class might use this experiment. For additional applications of this basic set up one might add the determination of actual soil moisture percent at different levels by the standard method of weight loss after oven drying. Also one could set up replicate flats of the same species on different soils to see how soil type affects moisture