Submitted to: Applied Engineering in Agriculture
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/6/2006
Publication Date: 8/20/2006
Citation: Hansen, R.C., Christman, J.C., Derksen, R.C. 2006. Statistical Evaluation of Instruments Designed to Measure Volumetric Water of Soilless Container Media. Applied Engineering in Agriculture. 22(5):753-763. Interpretive Summary: Information on soil moisture can significantly improve how nursery and greenhouse growers manage water input. Many commercially available, moisture-sensing devices, are available for measuring volumetric water content (VWC) of naturally occurring soil. Growers do not have information on the accuracy and reliability of VWC sensors for use in soilless container media being used in frequently in the production of ornamental plants. The measurement capability of three moisture sensors, the HydroSense, ThetaProbe, and WET Sensor, were evaluated in this research project. Each sensor was used to measure VWC in soilless media prepared at different moisture levels and with different levels of nutrients to study the impact of high electroconductivity on the measurements. In addition, the impact of using different human appraisers to probe the soil with the sensors was also evaluated. Overall, based on specified repeatability and reproducibility measurement procedures, measurement error for all sensors was less than 2.5% for all sensors. All three sensors were able to distinguish the potting medium moisture contents of prepared mixes from 5 to 50% VWC. Differences in how human appraisers used the instruments led to 10% higher readings for the ThetaProbe. Nursery and greenhouse growers using soilless potting mediums can use these results to gain greater confidence in the reliability, repeatability, and capability of VWC sensors and improve their ability to manage water which will reduce overuse and minimize wasteful discharge to the surrounding environment while maximizing plant production.
Technical Abstract: Information on the moisture content of soilless mediums could enhance production of container grower crops through improving water management programs. Many moisture sensing devices are commercially available for monitoring the volumetric water content (VWC) of naturally occurring field soils. Little information is available on the accuracy of these same sensors when used in soilless container mediums. A series of experiments were designed to evaluate the measurement capability of three moisture sensors to detect VWC of potting mediums used on nursery and greenhouse production systems. The three sensors were the HydroSense, ThetaProbe, and WET Sensor. Tests were conducted in commercial pots filled with known amounts of soilless media with known amounts of moisture and nutrient amendments. Three statistical properties were used to characterize the quality of data for this study: 1) bias, 2) variance, and 3) measurement system discrimination. Equipment variation, appraiser variation, measurement error, part variation, and total variation were calculated for all three sensors. Estimated stand deviation based on standardized repeatability and reproducibility measure produces for determining variance were 2.11%, 1.21%, and 1.43% VWC for the WET Sensor, HydroSense, and ThetaProbe, respectively. All three sensors were able to discriminate at least 10 distinct categories of VWC ranging from 5 to 50%. All readings were found to be affected in some way by the operator technique. All three sensors could be reliably used by producers for aiding in making irrigation management decisions which will optimize plant production and minimize resource use and wasteful discharge.