THE USE OF SOIL MOISTURE PROBES FOR IMPROVED UNIFORMITY AND IRRIGATION CONTROL IN GREENHOUSES

Abstract

There is wide variety of soil moisture sensors available for use in greenhouse production and research applications. Such sensors can play a valuable role in improving uniformity of substrate water content in greenhouses, as well as in automating irrigation based on plant water use. Quantification of spatial variability can be used to improve the design of irrigation systems to better match plant water use. The use of soil moisture sensors for irrigation control is promising, because it can greatly reduce temporal variability in substrate water content by watering based on actual crop water use. This not only results in better temporal uniformity, but can also greatly reduce water use. Understanding the spatial distribution of substrate water content and root water uptake within a container is important in determining the optimal sensor location within a container. At the same time, the properties of different sensors need to be considered when choosing the optimal sensor for a particular application. In small containers, it may be possible to measure most of the substrate with a single sensor, while in larger containers sensors ideally would be placed in that part of the substrate where most of the water uptake occurs. INTRODUCTION High quality irrigation water is becoming increasingly scarce in many areas of the world. Population growth, increased urbanization, and drought conditions can result in a decrease in the quantity of water available for irrigation, while salt water infiltration into fresh water aquifers can affect water quality in coastal areas. Efficient irrigation practices for agricultural production, including greenhouse production, will become increasingly important. Not only is greenhouse irrigation directly affected by the quality of the available irrigation water, greenhouse irrigation practices also can affect water quality. Soilless substrates generally have a low anion exchange capacity, and greenhouse growers typically use water-soluble fertilizers. This combination can result in leaching of water and dissolved nutrients, especially nitrate and phosphate. If this leachate is not collected and recycled, runoff from greenhouses can result in significant non-point source pollution (Lea-Cox et al., 2001). Increasingly strict environmental regulations put pressure on greenhouse growers to irrigate more efficiently in order to conserve water and reduce the risk of negative environmental impacts of their facilities. To improve irrigation practices, a detailed understanding of the spatial and temporal dynamics of the water in the root zone is required. Many commercial greenhouses use timers to turn irrigation on and off. Timers do not take into account that crop water use changes on a day-to-day basis, based on environmental conditions such as solar radiation, temperature, humidity, and air circulation. For example, daily water use of vinca (Catharanthus roseus) throughout a 40day period is shown in Figure 1, along with the daily light integral (DLI, total daily a mvanier@uga.edu b sburnett@maine.edu Proc. IS on High Technology for Greenhouse Systems GreenSys2009