USE OF TENSIOMETERS IN MEASURING AVAILABILITY OF WATER TO PLANTS

Efforts by several investigators to measure availability of water to plants by vapor pressure (2,12) and other methods (10,14) have indicated that the potential of water at the permanent wilting percentage is approximately 16 x 10e ergs per gram, whereas others (1, 11) have obtained results which indicate somewhat lower values. A great many experiments (3, 4, 5, 6, 13) have demonstrated that soil moisture seems to be equally available to plants at all times, as measured by the response of plants, as the moisture content is decreased from a high value to about the permanent wilting percentage. This conclusion seems to be general for all plants which have been studied. Evidently plants do not respond markedly enough to show any change in availability of water until the soil moisture is reduced to about the permanent wilting percentage. On the other hand, physical measurements show that the energy required to remove water from the soil, changes materially as the soil-moisture content decreases. It was surprising therefore, when Rogers (9) found, while working with strawberry plants in pots, that the plants wilted severely at a tension of 60 cm. of mercury (potential of 0.8 ? 10e ergs per gram). Furthermore, his illustration shows plants wilted slightly at 47 centimeters (0.6 x 10e ergs per gram). The maximum potential measurable by tensiometers is less than 1 x 10e ergs per gram. The work reported herein was undertaken largely in an effort to explain this apparent discrepancy.