Arnold W. Schumann

VARIABLE RATE NITROGEN APPLICATION IN FLORIDA CITRUS BASED ON ULTRASONICALLY-SENSED TREE SIZE

Most Florida citrus groves are still managed as large contiguous uniform blocks, despite significant variation in fruit yield and tree canopy size. Site-specific grove management by variable rate delivery of inputs such as fertilizers on a tree size basis could improve horticultural profitability and environmental protection. Tree canopy sizes were measured real-time in a typical 17-ha Valencia grove with an automated ultrasonic sensor system equipped with Differential Global Positioning System (DGPS). Prescription maps for variable application of nitrogen fertilizer were generated from ultrasonically scanned tree sizes on a single tree basis using ArcView GIS and Midtech Fieldware. Leaf samples from trees with different canopy sizes, which had been fertilized at a conventional uniform rate of 270 kg N/ha/y, were analyzed for nitrogen concentration. Analysis of 2980 tree spaces in the grove showed a skewed size distribution, with 62% in the 0- to 100-m3/tree volume classes and a median volume of 79 m3/tree. The tree volumes ranged from 0 to 240 m3/tree. Regression analysis showed that trees with excess leaf nitrogen (>3%) had canopies less than 100 m3. These trees receiving excess nitrogen are likely to have lower fruit yields and quality, and wasted fertilizer nitrates may leach beyond the root zone to groundwater. In order to rectify the excess fertilization of smaller trees, a granular fertilizer spreader with hydraulically powered split-chain outputs controlled with a MidTech Legacy 6000 controller was used for variable rate application of nitrogen in one-half of the grove. A 38% to 40% saving in granular fertilizer cost was achieved for this grove when variable N rates were implemented on a per-tree basis ranging from 135 to 270 kg N/ha/y.

Estimation of Citrus Fruit Yield Using Ultrasonically-Sensed Tree Size

Tree canopy mapping with an automated ultrasonic system is inexpensive, fairly straightforward and could be used to estimate fruit yield within a grove to plan site-specific management practices. Tree canopy volumes and fruit yield were measured and mapped in a 17.0-ha ‘Valencia’ grove with an automated ultrasonic system and a sensor-based automatic yield monitoring system, respectively. The spatial data were divided into 40 equal sized plots using ArcView GIS software to relate tree sizes and fruit yield. A linear calibration model with half the data showed that ultrasonically-sensed tree sizes correlated significantly (R2 = 0.80) with fruit yield. The correlation between actual and predicted fruit yield with the remaining data was used for validation and was also significant (R2 = 0.42). The average prediction accuracy was 90.6% while the standard error of prediction and root mean square error were 4.25 and 4.16 Mg/ha, respectively. The ultrasonically-sensed tree canopy volumes of plots ranged from 7421 to 20,900 m3/ha;fruit yield within the grove was also variable, ranging from 21 to 45 Mg/ha. Tree size and yield maps produced similar spatial patterns within the grove, as high-yielding areas were associated with large tree canopies. Therefore, tree canopy size could be used to estimate fruit yield within the grove. This information is valuable to forecast yields, to plan harvest schedules and to generate prescription maps for site-specific management practices on an individual tree basis.

Estimation of Wild Blueberry Fruit Yield Using Digital Color Photography

The wild blueberry industry of North America may benefit significantly from precision agriculture technology. Currently, crop management practices are implemented on an average basis without considering the substantial variation in soil properties, bare spots, topographic features, and fruit yield in blueberry fields. Yield maps along with fertility, weed, and topographic maps can be used to generate prescription maps for site-specific application of agrochemicals (e.g., fertilizer or pesticide). Two wild blueberry fields in central Nova Scotia were selected to evaluate a photographic method for fruit yield estimation. A 10-megapixel 24-bit digital color camera was mounted on a tripod and pointed downwards to take photographs of the blueberry crop from a height of approximately 1 m. At harvest time, blueberry crop images were collected in each field at 30 different sample locations displaying a range in yield. Actual fruit yield was sampled from the same locations by hand-harvesting out of a 0.5 × 0.5 m quadrat using a commercial blueberry rake. Custom image processing software was developed to count the blue pixels of ripe fruit in the quadrat region of each image and express it as a percentage of total quadrat pixels. Linear regression was used to calibrate the fruit yield with percentage blue pixels separately in each field, and then the calibration equation of field 1 was used to predict fruit yield in field 2 for validation of the method. Percentage blue pixels correlated highly significantly with hand-harvested fruit yield in field 1 (R2 = 0.98; P < 0.001; n = 30) and field 2 (R2 = 0.99; P < 0.001; n = 30). The correlation between actual and predicted fruit yield in field 2 (validation) was also highly significant (R2 = 0.99; P < 0.001; n = 30; RMSE = 277 kg/ha). Non-significance of the t-test for actual versus predicted yield indicated that there was no significant bias in the yield estimation and that the predicted yield was accurate. Based on these results, an automated yield monitoring system consisting of a digital camera, computer, and DGPS will be developed and incorporated into a harvester to monitor and map blueberry fruit yield in real time.

VARIABLE RATE GRANULAR FERTILIZATION OF CITRUS GROVES: SPREADER PERFORMANCE WITH SINGLE-TREE PRESCRIPTION ZONES

Commercial variable rate technology (VRT) fertilizer spreaders for citrus are currently being implemented in Florida groves to improve profitability and reduce nitrate contamination of groundwater. Although VRT spreaders incorporate proven embedded controllers and tree sensors which permit changing fertilizer rates according to tree size, there is currently limited information on their performance characteristics in spatially variable groves. This study investigated the performance characteristics of a split-chain, spinner-type VRT spreader during fertilization of a commercial citrus grove. Six nitrogen rates (0, 134, 168, 202, 236, and 270 kg ha-1 y-1) were varied according to a prescription map developed from ultrasonically measured tree size information. Missing trees and one-year-old reset trees were not fertilized with the spreader. Target fertilizer rates for 1490 trees in an 8.1-ha experimental area were compared with actual fertilizer rates calculated from geartooth speed sensors monitoring the conveyor chain speed. Through classification and regression analysis, spreader performance and response times during transitions from zero or low fertilizer rates to high rates and vice versa were compared. In this grove, 73.1% of the fertilizer rate changes were required between a single-tree space of 5.3-m linear row distance, taking about 4 s to drive at 1.34 m s-1. The spreader had an average on-off response time of . 3 s, and an average rate changing response time of 2 to 5 s. Based on these data, the spreader design is not suitable for rapid fertilizer rate changes between single tree spaces, but could be greatly improved by substituting its hydraulic servo control valves with faster devices.

Water stress and root injury from simulated flooding and Diaprepes abbreviatus root weevil larval feeding in citrus

Environmental stress from flooding can occur simultaneously with root weevil infestation to damage plant root systems. We conducted two factorial studies of flooding duration and Diaprepes abbreviatus (L.) root weevil larval feeding injury on citrus in the greenhouse in 2002 and 2003. Our objectives were to investigate the effect of soil anoxia by simulated flooding on plant water stress and the impact of prior flooding on root susceptibility to subsequent larval weevil feeding. The treatments consisted of two rootstock varieties, Swingle citrumelo [SWI; Citrus paradisi Macfad × Poncirus trifoliata (L.) Raf.] and Smooth Flat Seville (SFS; Citrus aurantium L.), flooding durations of 0, 10, 20, 30, or 40 days, and Diaprepes larval infestations of 0 and 5 neonates per seedling for 40 days. We used a Candler sand with 8 replicates in Experiment I and a Floridana loam with 15 replicates in Experiment II. Treatments were arranged in a completely ramdomized design. Plants were flooded, drained for a week, and then 1-day-old neonate larvae were introduced onto the soil surface of each seedling. Flooding significantly reduced soil redox potential (Eh), leaf stomatal conductance (gs), and shoot growth (P < 0.05). Soil Eh decreased from +220 to −0;100 mV within 1-3 days after flooding, and leaf gs declined from 260 to 80 mmol m−2 s−1 within 20 days of flooding. Flood-injured and larval-injured roots had little growth. With equal previous flooding durations (20 days), the larval survival was on average 25% higher in sandy soil than in loamy soil. Twenty-day prior flooded roots were more water stressed and also more susceptible to Diaprepes larval feeding injury. It is suggested that limited soil waterlogging and early root weevil larval control would be useful for plant protection.

Soil and Diaprepes abbreviatus root weevil spatial variability in a poorly drained citrus grove

Soil and water variability in space and time could be important for management of the citrus root weevil, Diaprepes abbreviatus (L.). We conducted a study of soil, tree, and root weevil relationships in a poorly drained grove of Hamlin orange on Swingle citrumelo rootstock (Citrus paradisi Macfad. x Poncirus trifoliata (L) Raf.). in central Florida in 2002. We hypothesized that spatial soil and water variability might influence tree health and root weevil patterns. The objectives were to assess the spatial variability of soil, water, tree health, and Diaprepes root weevil (DRW) and to determine DRW management zones based on spatial correlations. Adult weevils were monitored using Tedders traps arranged in a 34 × 25-m grid across the grove. Soil electrical conductivity (EC) was assessed using EM38, and water table, soil texture, water content, organic matter, pH, P, K, Ca, Mg, B, Zn, Mn, Fe, and Cu were measured at each trap. The weevil population peaked in June (P < 0.001), and weevil density was high in areas that were low in Mg and Ca concentrations (P < 0.05). Semivariograms, a spatial structure function, for DRW, Mg, Ca, and EC, ranged within 75 to 100 m, which matched the limits of DRW management zones delineated using DRW and EC underlying patterns. Soil EC, Mg, Ca, and Fe were correlated, and tree decline was associated with high levels of Fe and soil flooding because plants were more water stressed in flooded areas than in non-flooded areas (P < 0.01). We suggest that a management unit approach might be an option for DRW control, and that flooding events and soil Fe, Mg and Ca levels might be related to tree decline and DRW distribution patterns. (Soil Science 2004; 169:650–662)

Automated, Low-Cost Yield Mapping of Wild Blueberry Fruit

The presence of weeds, bare spots, and variation in fruit yield within wild blueberry fields emphasizes the need for yield mapping for site-specific application of agrochemicals. An automated yield monitoring system (AYMS) consisting of a digital color camera, differential global positioning system, custom software, and a ruggedized laptop computer was developed and mounted on a specially designed Farm Motorized Vehicle (FMV) for real-time fruit yield mapping. Two wild blueberry fields were selected in central Nova Scotia to evaluate the performance of the AYMS. Calibration was carried out at 38 randomly selected data points, 19 in each field. The ripe fruit was hand-harvested out of a 0.5- × 0.5-m quadrant at each selected point and camera images were also taken from the same points to calculate the blue pixel ratio (fraction of blue pixels in the image). Linear regression was used to calibrate the actual fruit yield with percentage blue pixels. Real-time yield mapping was carried out with AYMS. Custom software was developed to acquire and process the images in real-time, and store the blue pixel ratio. The estimated yield per image along with geo-referenced coordinates was imported into ArcView 3.2 GIS software for mapping.

MAPPING WATER TABLE DEPTH BY ELECTROMAGNETIC INDUCTION

Currently most citrus groves are managed uniformly in large blocks, despite significant spatial variability in soil and water properties that influences the long-term viability of perennial citrus trees in Florida’s poorly drained Southern Flatwood soils. Detailed georeferenced water table maps would be useful to select suitable land for new citrus grove development, for drainage system design, or to identify and manage drainage problems in existing groves. The objective of this study was to develop a new precision agriculture application using ground conductivity measured with the EM38 electromagnetic soil profiler for the estimation and mapping of shallow water table depths in Florida’s citrus groves. Calibrations were developed and tested in five different months and two sites to evaluate the spatial and temporal accuracy of water table predictions. Three automated mobile surveys of water table depth were then conducted in the same groves using a differential global positioning system (DGPS) for georeferencing the ground conductivity data. The spatial variability of water table depths was mainly determined by soil type and the temporal variability was influenced strongly by the seasonal rainfall pattern. The vertical dipole (EMv) of the EM38 instrument was better than the horizontal dipole (EMh) for estimating water table depths because of its greater sensing depth. Accuracy, calculated as root mean square error (RMSE), ranged from 4.1 to 15.5 cm on a given day. Significant bias was however evident when comparing calibrations developed on different sites which were 12-km apart. Global calibrations incorporating a rainfall index as well as EMv were much better predictors of water table depth due to the added temporal information. The best results were obtained by using site-and-time-specific calibrations of <10 points for every survey, using a representative range of EMv values.

VARIABLE RATE APPLICATIONS OF GRANULAR FERTILIZER FOR CITRUS TEST PLOTS

A key component in precision agriculture is exact delivery of liquid and solid materials to site-specific locations. Addressing this concern, Florida growers are in the initial phase of implementing best management practices for various citrus growing areas. A 16.2-ha (40-acre) field trial was initiated with a grower in the Ridge growing section of the state. As part of that project, prescription maps were developed for multiple dry fertilizer applications throughout the season. A granular fertilizer unit with hydraulically powered dual split-chain outputs controlled from a MidTech Legacy 6000 controller was utilized. Data were collected on target and as-applied rates compensating for missing trees and resets. Additional mass balance data were obtained by weighing each load before and after spreader application. Application rates for each of 43 blocks (42 test, 1 conventional) were compared for eight tests conducted during the 2002-2003 and 2003-2004 fruit seasons. Prescription and as-applied rates exhibited a high correlation (r2 = 0.98) and linear relationship (slope = 0.98). Grand totals from vehicular weighing and controller discharge estimation produced an average absolute error of 7.7%.

Detecting Bare Spots in Wild Blueberry Fields Using Digital Color Photography

Wild blueberry fields are developed from native stands on deforested land by removing competing vegetation. The majority of fields are situated in naturally acidic and non-fertile soils that have high proportions of bare spots, weed patches, and gentle to severe topography. Producers presently apply agrochemicals uniformly without considering bare spots. The unnecessary or over-application of agrochemicals in bare spots may increase cost of production and environmental pollution. An automated cost-effective machine vision system using digital color photography was developed and tested to detect and map bare spots for site-specific application of agrochemicals within wild blueberry fields. The experiment was conducted at a 4-ha wild blueberry field in central Nova Scotia. The machine vision system consisting of a digital color camera, differential global positioning system, and notebook computer was mounted on a specialized farm vehicle. Custom software for grabbing and processing color images was developed in Delphi 5.0 and C++ programming languages. The images taken by the digital camera were stored in the notebook computer automatically and then processed in red, green, and blue (RGB), and hue, saturation, and value (HSV) color spaces to detect bare spots in real-time within blueberry fields. The best results were achieved in hue image color space with 99% accuracy and a processing speed of 661 ms per image. The results indicated that bare spots could be identified and mapped with this cost-effective digital photography technique in wild blueberry fields. This information is useful for site-specific application, and has the potential to reduce agrochemical usage and associated environmental impacts in the wild blueberry production system.