In recent years, irrigation method using the negative pressure difference attracts attention from the point of view of water saving. In addition, it is proved that this technique is effective in upbringing of the plant as well as saving of water. By measuring water distribution of soil, active irrigation control will be performed In our previous study, we confirmed that the resonance frequency of a leaf is influenced by the water stress to the plant. Thus the vibration measurement of a leaf was expected to be a new inspection technique for monitoring the health condition of the plant.  In order to examine the relationship between the characteristic frequency and the water content of the leaf which is related to the water stress of the plant, we performed the following fundamental experiments. As a result of the fundamental experiment, it was confirmed that the characteristic frequency of an individual leaf which was picked from a plant decreased in proportion to the reduction of the amount of water of the leaf, and that the hysteresis of the relationship was very small.

However, we faced some problems on building an automatically measuring system for a long period of time, because we used an air gun for a vibration source. Therefore, for the automatic measuring, we attempted to use a loudspeaker, as well as for the frequency control of the vibration. But it was not successful to vibrate the leaf in low frequencies with usual speakers.

Fortunately, we found that it is possible not only to vibrate a leaf and a stalk very efficiently but also to control the frequency freely by using the acoustic radiation force generated by a high power ultrasonic speaker (i.e. parametric speaker). Therefore, the experiment about the ability of monitoring the health condition of the plant under cultivation by measuring the resonance frequency of the leaf using this acoustic radiation force was conducted in this study. We confirmed that a high power ultrasonic speaker could use as a non contact vibration source of vibration of a leaf. According to this fact, long-time vibration measurement of a leaf was attained. From the experimental result, it is shown that the reaction to water stress is clearly regarded as change of the resonance frequency. As a future task, the difference in the vibration characteristic of a leaf and a stalk will be considered, and also we are going to develop the most suitable irrigation control using this phenomenon.