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Matthew Chappell, Sue K. Dove, Marc W. van Iersel, Paul A. Thomas, and John Ruter

calibration of sensors required to calculate an irrigation event to one, the capacitance-based soil moisture probe; 3) it uses on-farm data to determine soil moisture and therefore increases the precision and accuracy of environmental measurements compared

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Bruce L. Dunn and Carla Goad

( Wang et al., 2004 ). Use of optical sensors allows for an increased number of plants and leaves to be analyzed and results are immediately available ( Mielke et al., 2012 ). The SPAD (SPAD-502; Konica Minolta, Japan) is widely accepted in the agronomic

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Yun-wen Wang, Bruce L. Dunn, Daryl B. Arnall, and Pei-sheng Mao

sensor with high-intensity light-emitting diodes that emit light at 660 nm (red) and 780 nm [near-infrared (NIR)] wavelengths. The magnitude of the light reflected from the target is measured by a photodiode detector. NDVI values are then generated from

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John C. Majsztrik, Elizabeth W. Price, and Dennis M. King

al., 2013 ). WSIN involve a fusion of soil/soilless moisture sensor networks, specialized hardware/software interfaces, and decision-support tools that have many potential applications. In ornamental crop applications, they have been shown to allow

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Amanda Bayer, Imran Mahbub, Matthew Chappell, John Ruter, and Marc W. van Iersel

more sophisticated soil moisture sensor-based irrigation systems that can irrigate based on plant water use, has the potential to reduce the amount of water needed for irrigation and the amount of runoff produced during and immediately after an

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Marco Bittelli

theory and applications. Since there are a range of techniques based on different physical principles and a variety of available sensors, this review is not intended to provide an exhaustive scientific description of each method, but rather to provide

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O. Monje, G.D. Goins, H.G. Levine, and G.W. Stutte

Tight control of growth media moisture content is needed when plant growth systems employ shallow root zones or for cultivating fast-growing plants (i.e., crops). Poor control of moisture can affect both growth rate and plant quality by either excessive watering (waterlogging) or drought events. We evaluated the performance of two types of moisture sensors: tensiometers and heat-pulse moisture sensors. The output from each sensor type was evaluated as a function of volumetric moisture content in 1 to 2 mm Turface. The tensiometers were more sensitive between 30% and 60% volumetric moisture content, and their output was nonlinear because they measure water potential directly. In contrast, both the sensitivity and the output of the heat-pulse moisture sensors, as a function of volumetric moisture content, were linear. The heat-pulse moisture sensors were used to control moisture content in a shallow root zone, whereby water was added or removed from the media through a porous tube using peristaltic pumps. Moisture content in the media could be maintained within ±2% of setpoint for moisture contents ranging from 20% to 100% volumetric moisture content. The heat-pulse sensors were better suited for controlling media moisture because of their linear output and because of their constant sensitivity as a function of volumetric moisture content.

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Krishna Nemali* and Marc van Iersel

Monitoring moisture status of the growing medium is essential as growth and quality of greenhouse crops largely depend on the amount of available water. Recently, two new types of moisture sensors have been developed (ECH2 O, Decagon devices, Inc., Pullman, Wash.; Theta probe ML2X, Delta -T devices Ltd., Burwell, Cambridge, U.K.). We studied the performance of these sensors for measuring the volumetric water content (VWC) of a soilless growing medium. We also tested the sensitivity of these sensors to temperature and electrical conductivity (EC) of the growing medium. Our results indicate that these sensors can be calibrated and used effectively for measuring a wide range of moisture contents in the growing medium; however media specific calibration may be required. Regression analysis indicated that the output of ECH2 O probes was affected by changes in the EC and temperature of the growing medium. Effects of EC were too small to be of practical significance, while the measured VWC increased by 0.003 m3/m3 for each °C increase in temperature. The output from the Theta Probe was not affected by changes in the EC or temperature of the growing medium. In a comparison study, both probes were found to give similar estimates of the VWC of the growing medium within the common range seen under greenhouse production.

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Kristof Vermeulen, Kathy Steppe, Katrien Janssen, Peter Bleyaert, Jan Dekock, Jean-Marie Aerts, Daniel Berckmans, and Raoul Lemeur

, D. 2005 Unravelling the relationship between stem temperature and air temperature to correct for errors in sap-flow calculations using stem heat balance sensors Funct. Plant Biol. 32 599 609 Steinberg, S.L. van Bavel, C.H.M. McFarland, M.J. 1989 A

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Rafael Muñoz-Carpena, Yuncong C. Li, Waldemar Klassen, and Michael D. Dukes

A low-volume/high frequency (LVHF) soil moisture-based drip irrigation system was tested on a shallow sandy soil at a commercial tomato (Lycopersicon esculentum) farm in southern Florida. Six LVHF irrigation treatments were compared with the standard commercial practice on the farm (control), where a portable pump was used for manual drip irrigation twice each week. In the six LVHF treatments the system was continuously pressurized by means of an electrical pump and a pressure tank, and controlled by an irrigation timer set to irrigate a maximum of five times per day with the irrigation time (i.e., volume) set according to historical evapotranspiration (ET) demands in the area. Two treatments were based on timer schedules, one to supply 100% of the maximum recommended crop water needs in the area based on historical ET (ET-100%), and the other to supply 150% of those needs (ET-150%). The other four treatments were created by interfacing two types of soil moisture sensors (switching tensiometers and granular matrix sensors with control modules) set at two moisture points (wet = 10 kPa, optimal = 15 kPa) in a closed control loop with the irrigation timer programmed at the ET-100% schedule. Results showed that the six LVHF treatments reduced water use while not significantly affecting tomato yields. Switching tensiometers at the 15 kPa set point performed the best (up to 73% reduction in water use when compared to the control, 50% with respect to ET-100%). The results show that water use below historical ET levels can be obtained without sacrificing yield by keeping the root zone moisture at controlled levels with the soil-moisture based system. Routine maintenance was critical for reliable operation of the switching tensiometers. Granular matrix sensor based irrigation behaved erratically, and did not improve water savings compared to ET-100%, indicating that this system was not effective under the conditions of the area due to the sensor's slow response to frequent wetting-rewetting cycles and characteristics of the interface.