W. M. Miller

Investigation of Laser and Ultrasonic Ranging Sensors for Measurements of Citrus Canopy Volume

This study compared ultrasonic and laser measurements of citrus canopy volume with manual measurement methods. Fifteen trees with different canopy heights and volumes were used. Manual and ultrasonic measurements provided dimensions for computing the canopy volume whereas laser measurements gave information that could be used to compute a ‘laser canopy volume index.’ Ultrasonic and laser methods agreed with manual methods (R 2 > 0.85, RMSE < 2.15 m 3 ). Laser showed better prediction of canopy volume than the ultrasonic system because of the higher resolution. Ultrasonic or laser sensors can be used for automatic mapping and quantification of the canopy volumes of citrus trees.

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.

Low-cost automatic yield mapping in hand-harvested citrus

A simple system has been developed to generate yield maps of hand-harvested citrus in a reliable, low-cost manner using a commercial GPS recorder. Individual 0.7-m3 container locations are mapped to indicate yield variations within a citrus block. Further future refinements are discussed. The system is applicable to other manually harvested crops and harvesting systems.

PRECISION FARMING APPLICATIONS IN FLORIDA CITRUS

A cooperative effort between researchers, manufacturers, and growers has been investigating precision farming applications in Florida citrus. Citrus yields, based on the location of volume-based containers, were mapped using a conventional fruit-loading truck, manual harvesters (> 99% of Florida citrus is manually harvested), and GIS/GPS components. These maps were overlaid on geo-referenced aerial photographs of the tree canopies. Two fruit weighing systems were mounted on a fruit-loading truck and integrated with the GIS/GPS components to investigate mapping weight-based yields. Results to date indicate the truck-mounted weighing systems were within 1 to 6% of certified scale weights on 20 t loads of fruit. Electronically recording harvester identity is being integrated with yield mapping to make the entire system more reliable and attractive to harvesters and growers.

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.

A DGPS YIELD MONITORING SYSTEM FOR FLORIDA CITRUS

A yield monitoring system was developed based on a differential global positioning system (DGPS) and three fruit weighing systems. The DGPS unit eliminated post processing of the data and provided accurate location of fruit containers used in conventional manual harvesting. Three DGPS units did not vary significantly when comparing position accuracy. The weighing systems consisted of a pressure transducer mounted in the pressure line of the truck bed lifting cylinder, a system with four load cells under the four corners of the truck bed, and a single load cell in the loader boom. On trailer loads of fruit, the most to least accurate system was the lift cylinder (~1% error), the loader boom (<2% error), and the load cells (~5% error). On individual pallet bin loads of fruit, however, the coefficients of variation for the lift cylinder, loader boom, and load cells were 15, 6, and 25%, respectively.

Energy storage via desiccants for food/agricultural applications

Abstract Energy storage has become an important consideration in adopting solar energy or providing energy during interruptable service of fossil fuels. Desiccants are a potential media especially suited for drying applications. Both liquid and solid dessicants are manufactured and could be used in small or large scale drying for crop or food product drying. This report reviews the applications of combining solar energy and desiccant energy storage. Various physical and thermal properties have been complied. A cost comparison revealed that energy storage via desiccants was competitive with phase-change materials, rock-bed storages and water systems.

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%.

DEVELOPMENT AND TESTING OF A CITRUS YIELD MONITOR

Site–specific monitoring of citrus yield is important in understanding yield variability and its causes. A microcontroller–based system was designed to automatically monitor citrus yield harvested into individual tubs. The microcontroller was interfaced with a differential global positioning system (DGPS), a flash memory key, two limit switches, a buzzer, and a counter, which provided position information, storage, triggering, acknowledgement, and counting, respectively. The microcontroller system was mounted on a special truck (goat) that handles fruit and fruit tubs in the groves. The system was designed such that whenever the fruit was dumped into the truck bed, DGPS information was acquired and the counter and buzzer were triggered. The position information was then stored into a flash key and later downloaded to a personal computer for data analysis and mapping. This article details the design and evaluation of the citrus yield monitoring system. The evaluation phase showed that the system was able to detect the dumping events and store the DGPS locations, 100 and 98% of the time, respectively.

Physical Properties Data for Postharvest Handling of Florida Citrus

Physical properties of common Florida citrus varieties relevant to materials handling were analyzed over a two-season period. The following properties were measured: dimensional size, mass, surface roughness, mechanical strength (compressive and puncture) and coefficient of friction. Mechanical parameters and density were monitored after freezes (>4h@<— 2 °C) that occurred each season. The decline in density known to occur via desiccation after freezing was highly correlated with compressive and puncture properties other than maximum compression force.