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 infruit yield and tree canopy size. Site-specific grove management by variable rate delivery of inputs such as fertilizers on atree size basis could improve horticultural profitability and environmental protection. Tree canopy sizes were measuredreal-time in a typical 17-ha Valencia grove with an automated ultrasonic sensor system equipped with Differential GlobalPositioning System (DGPS). Prescription maps for variable application of nitrogen fertilizer were generated fromultrasonically scanned tree sizes on a single tree basis using ArcView GIS and Midtech Fieldware. Leaf samples from treeswith different canopy sizes, which had been fertilized at a conventional uniform rate of 270 kg N/ha/y, were analyzed fornitrogen concentration. Analysis of 2980 tree spaces in the grove showed a skewed size distribution, with 62% in the 0- to100-m3/tree volume classes and a median volume of 79 m3/tree. The tree volumes ranged from 0 to 240 m3/tree. Regressionanalysis showed that trees with excess leaf nitrogen (>3%) had canopies less than 100 m3. These trees receiving excessnitrogen are likely to have lower fruit yields and quality, and wasted fertilizer nitrates may leach beyond the root zone togroundwater. In order to rectify the excess fertilization of smaller trees, a granular fertilizer spreader with hydraulicallypowered split-chain outputs controlled with a MidTech Legacy 6000 controller was used for variable rate application ofnitrogen in one-half of the grove. A 38% to 40% saving in granular fertilizer cost was achieved for this grove when variableN rates were implemented on a per-tree basis ranging from 135 to 270 kg N/ha/y.