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Colin S. Campbell, Gaylon S. Campbell, Douglas R. Cobos, and Brody Teare

Knowledge of soil water, fertilizer, and temperature is important when growing plants in any type of growing media. Although instruments to measure these properties have been available for several years, they are often expensive, failure-prone, and require different calibration for individual soil types. Recently, a low-cost sensor (Trade name: ECH2O-TE) was released that measures volumetric water content, electrical conductivity (EC), and temperature. The objective of this study was to determine how the probe performed in various soil and soilless media, as well as various salinities. We found the probe performed very well over a wide range of soil types and salinities. From the data, it appears that a single calibration can be used for all mineral soils and organic potting soils. A second calibration equation may be required for substrates such as rockwool. The output of the probe was not affected by the salinity (EC) of the soil from 0.1 to greater than 10 dS/m and showed considerable improvement in temperature sensitivity compared to existing technology. These results suggest the sensor provides a low-cost, reliable, easier-to-use alternative to other sensors of its kind.

Open access

Andrea Myers, Anthony Gunderman, Renee Threlfall, and Yue Chen

-made force-sensing apparatus attached to the thumb and fingers of a person was developed to quantify forces for harvesting blackberries and to identify appendages essential for harvesting ( Fig. 1 ). The apparatus was designed with resistive force sensors

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Ed Stover, James Salvatore, and Ferdinand Wirth

Sensor-actuated precision spray systems are designed to prevent pesticide delivery unless canopy is detected in the corresponding spray zone. Where frequent gaps are present in the tree row, using orchard sprayers with these systems is likely to lower pesticide costs and reduce off-target deposition. Pesticide savings from use of a sensor-actuated precision spray system were assessed in 27 grapefruit (Citrus paradisi) blocks selected without prior knowledge of grove characteristics, with nine blocks in each of three age categories: 5-6 years, 10 to 12 years and 20 years and older. The sprayer was optimized for each block by opening only those nozzles appropriate for tree size and furrow depth, so that no spray was delivered under or over the canopy of most trees. The same randomly selected 3.0 to 4.7 acre (1.2 to 1.9 ha) section was then sprayed in each block both with and without activation of the precision spray system. In each block, the precision spray system computer also calculated spray savings based on precision sprayer use with no operator nozzle adjustment. Mean savings in spray material from use of the precision sprayer was 6.6% of total conventional output when comparisons were made with optimal sprayer nozzling in each grove versus 18.6% with no operator adjustment of nozzles. In this study, optimizing nozzling provided a larger proportion of spray savings than use of the precision sprayer on 100% of groves 5 to 12 years old and 44% of groves greater than 20 years old. However, in 70% of groves tested, precision spray systems increased spray savings by more than 2% even when using optimal nozzling. Assignment of precision sprayers to groves with greatest potential for savings will likely provide greatest efficiency, while uniform groves forming hedgerow will offer so little potential savings that even the additional cost of weed management will probably not be recovered.

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Thomas R. Clarke

Irrigation scheduling can be improved by directly monitoring plant water status rather than depending solely on soil water content measurements or modeled evapotranspiration estimates. Plants receiving sufficient water through their roots have cooler leaves than those that are water stressed, leading to the development of the crop water stress index, which uses hand-held infrared thermometers as tools for scheduling irrigations. However, substantial error can occur in partial canopies when a downward-pointing infrared thermometer measures leaf temperature and the temperature of exposed, hot soil. To overcome this weakness, red and near-infrared images were combined mathematically as a vegetation index, which was used to provide a crop-specific measure of vegetative cover. Coupling the vegetation index with the paired radiant surface temperature from a thermal image, a trapezoidal two-dimensional index was empirically derived capable of detecting water stress even with a low percentage of canopy cover. Images acquired with airborne sensors over subsurface drip-irrigated muskmelon (Cucumis melo L.) fields demonstrated the method's ability to detect areas with clogged emitters, insufficient irrigation rate, and system water leaks. Although the procedure needs to be automated for faster image processing, the approach is an advance in irrigation scheduling and water stress detection technology.

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R. Nuñez-Elisea, B. Schaffer, M. Zekri, S.K. O'Hair, and J.H. Crane

Tropical fruit trees in southern Florida are grown in porous, oolitic limestone soil that has very low organic matter content and water-holding capacity. Thus, trees require frequent irrigation during dry periods. In these soils, a quantitative basis for monitoring soil water content to determine when and how much to irrigate has been lacking. Multi-sensor capacitance probes (EnviroSCAN™, Sentek, Australia) were installed in commercial carambola, lime, and avocado orchards to continuously monitor changes in soil water content at depths of 10, 20, 30, and 50 cm. Eight probes were installed per orchard. Volumetric soil water content was recorded at 15-min intervals with a solar-powered datalogger. Results were downloaded to a laptop computer twice a week. Monitoring the rate of soil water depletion (evapotranspiration) allowed irrigation before the onset of water stress. The time at which soil reached field capacity could be determined after each irrigation (or rain) event. Soil water tension was recorded periodically using low-tension (0–40 cbars) tensiometers placed adjacent to selected capacitance probes at 10- and 30-cm depths. Soil water tension was better correlated with volumetric soil water content at a 10-cm depth than at 30-cm depth. Using multi-sensor capacitance probes is a highly accurate, although relatively expensive, method of monitoring soil water content for scheduling irrigation in tropical fruit orchards. Whereas tensiometers require periodic maintenance, the multi-sensor capacitance probe system has been virtually maintenance free. The correlation between soil water content and soil water tension obtained in situ indicates that tensiometers are a less precise, but considerably cheaper, alternative for scheduling irrigation in tropical fruit orchards in southern Florida.

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J. Ryan Stewart and Roger Kjelgren

Infrared sensors were used to quantify canopy temperature and thus detect differences in incipient water stress between a cool-season grass [Kentucky bluegrass (KBG) (Poa pratensis)] and a warm-season grass [buffalograss (BG) (Buchloe dactyloides)]. The infrared sensors, connected to a datalogger, measured average hourly leaf–air temperatures (TL–TA) 1 m above eight replicate plots of Kentucky bluegrass and eight replicate plots of buffalograss. Air temperature and relative humidity from a nearby weather station were used to calculate the average hourly vapor pressure deficit (VPD). In late July, we ceased irrigating and measured TL–TA and soil water content while allowing the turf to dry down for 5 weeks. Soil water content was measured with a neutron probe. Both species exhibited a significant relationship between TL–TA and VPD. As the VPD increased, TL–TA decreased in both species (KBG r 2 = 0.73, BG r 2 = 0.71) on the 2nd day after an irrigation during well-watered conditions. An artifact was created on the first day after an irrigation as a result of excessive surface evaporation. KBG and BG were similar under well-watered conditions. KBG had a higher TL–TA after 4 to 5 days without irrigation. By contrast, BG did not have a higher TL–TA until 25 to 30 days without irrigation. Part of BG's drought avoidance was extraction of soil water down to 0.9 m vs. 0.45 m for KBG.

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Willis Omondi Owino, Ryohei Nakano, Yasutaka Kubo, and Akitsugu Inaba

We investigated the differential regulation of two 1-aminocyclopropane-1-carboxylate synthase (ACS) genes, one 1-aminocyclopropane-1-carboxylate oxidase (ACO) gene and one ethylene response sensor (ERS1) ortholog during ripening and in response to wounding in avocados (Persea americana Mill. `Bacon'). The 1-aminocyclopropane-1-carboxylate (ACC) content, ACS activity and detectable expression of PA-ACS1 mRNA increased and reached a maximum prior to the climacteric peak, whereas ACO activity and the PA-ACO mRNA levels increased markedly only at the upsurge of ripening ethylene. A basal level of PA-ERS1 transcript was detected as from harvest, however, PA-ERS1 transcript was hyper-induced at the climacteric peak of ethylene production. 1-Methylcyclopropene (1-MCP) application at thepreclimacteric and the onset of climacteric stages inhibited the ACS and ACO activities, the transcription of PA-ACS1 and suppressed PA-ACO and PA-ERS1 mRNAs to trace levels. Discontinuation of 1-MCP treatment led to super-induction of PA-ACS1, PA-ACO, and PA-ERS1 transcripts. Wound induced ethylene biosynthesis and wound-induced PA-ACS2 mRNA accumulation were enhanced by 1-MCP, whereas wound-induced PA-ACO mRNA accumulation was unaffected by 1-MCP. These results indicate positive feedback regulation of the PA-ACS1 gene and negative feedback regulation of the PA-ACS2 gene by ethylene, while PA-ACO exhibits positive feedback regulation by ethylene and is also induced by wounding. The hyper-induction of PA-ERS1 mRNA at relatively high concentrations of ethylene may be a mechanism of avocados to regulate the ethylene responsiveness of the tissues by dissipation of the gas.

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Thomas R. Clarke and M. Susan Moran

Water application efficiency can be improved by directly monitoring plant water status rather than depending on soil moisture measurements or modeled ET estimates. Plants receiving sufficient water through their roots have cooler leaves than those that are water-stressed, leading to the development of the Crop Water Stress Index based on hand-held infrared thermometry. Substantial error can occur in partial canopies, however, as exposed hot soil contributes to deceptively warm temperature readings. Mathematically comparing red and near-infrared reflectances provides a measure of vegetative cover, and this information was combined with thermal radiance to give a two-dimensional index capable of detecting water stress even with a low percentage of canopy cover. Thermal, red, and near-infrared images acquired over subsurface drip-irrigated cantaloupe fields demonstrated the method's ability to detect areas with clogged emitters, insufficient irrigation rate, and system water leaks.

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J.M. Tarara and J.C. Ferguson

Management strategies like “deficit irrigation” in wine grapes require accurate, reliable information on vine water use, making direct measurements of vine transpiration highly desirable. The heat-balance sap flow method has the advantages of being non-invasive and requiring no other calibration beyond a zero-flow set. Potential violations of the method's assumptions were dealt with and the heat balance method successfully applied to mature grape vines under conditions of extremely high sap flow. Greenhouse studies suggested that vines transpire at night, up to 9.5% of the total 24-h water loss, thus violating the zero-flow assumption for setting the gauge constant. Using a predetermined gauge constant caused smaller errors than using daily, pre-dawn constants set in situ. The steady-state assumption was violated only in early and late hours of the day, and the inclusion of a term to account for the change in heat stored by the stem only marginally improved daily estimates of water use. The assumption of radially uniform temperature across the heated stem segment is violated at very high flows (e.g., >700 g·h–1), but can be corrected for by using wider heaters and adjusting the placement of thermocouples. For a mature, potted vine in the greenhouse, the maximum absolute error in cumulative daytime water use between a sap gauge and a precision load cell was about –10%, with the gauge almost exclusively underestimating water loss. A custom-built, 20-gauge system was run continuously in the field for 90 days. Vine-to-vine variability in water use was not accounted for by normalizing sap flow by leaf area, suggesting that it is critical to include in any field study the largest number of gauges that are technically feasible.

Open access

Rhuanito Soranz Ferrarezi and Tzu Wei Peng

while protecting the environment ( Payero et al., 2017 ). According to a study with large-scale commercial corn ( Zea mays ) in Nebraska, irrigation scheduling based on sensors reduced water application rates by 33%. It also reduced pumping cost by $28