Variable rate spreader for real-time spot-application of granular fertilizer in wild blueberry

The commercial variable rate (VR) fertilizer spreader can change fertilizer rates in different management zones using global positioning system (GPS) guided prescription maps. The VR fertilizer spreader allows the independent control only on half of the fertilizer boom (3.66m; 6 nozzles). This existing VR fertilizer spreader fertilizes unevenly distributed small sized bare spots/weed patches which are less than half boom length in wild blueberry fields. Therefore, the existing VR fertilizer spreader was modified to control each pair of nozzles for spot-application of fertilizer only in plant areas. The automated sensing and control system consisting of 6μEye color cameras, solenoid valves, pneumatic cylinders, programmable logic controller (PLC), 8-channel VR controller (VRC) interfaced to a pocket PC (PPC) using wireless Bluetooth® radio with Windows Mobile® and a custom software was developed. The μEye color cameras were mounted on a separate boom in front of the tractor at a height of 1.2m. The modified VR spreader was capable of using prescription maps and automated sensing and control system simultaneously to detect foliage/bare spots in real-time to avoid fertilization in bare spots/weed patches. The ACCU-RATE controller calibration and response time calculations were performed prior to field experiment.The results of calibration tests for the ACCU-RATE controller suggested <5% deviation when compared with the manually measured application rate. The results of response time revealed that the maximum of 2.38s and 2.25s were taken to dispense clay filler and fertilizer respectively, after detection of the target. In order to assess the real-time performance accuracy of modified variable rate granular (MVRG) fertilizer spreader grass and wild blueberry fields were selected. Artificial bare spots were made using orange colored tarps in the selected grass field and equal numbers of weighed rat catchers were placed on artificial bare spots and grass. The MVRG fertilizer spreader was operated on VR mode (detection and no application in artificial bare spots) and the rat catchers were collected and re-weighed. The same procedure was repeated for uniform (UN) application mode of the system (application on both grass and artificial bare spots). This test was repeated for wild blueberry field to detect bare spots and plants for spot-application of fertilizer only in plant areas. The results of paired t-tests showed significant difference (P-value<0.0001) between VR and UN treatments when the clay filler weight collected from bare spots were compared for both fields. The results showed non-significant difference (P-value>0.05) between VR and UN treatments in grass/plant areas. These results suggested that the MVRG fertilizer spreader was efficient and accurate for spot-application of fertilizer to increase farm profitability and reduce environmental risks.