Static calibration of a system for automatic control of fertilizer variable dosage
In general, fertilizer seeder machines are designed for a nominal rate range that may vary depending on the type of fertilizer to use, which requires its calibration for each specific type of application. In this experiment a system of automatic control of variable-rate fertilization was implemented and evaluated in the laboratory. The system consists of a microcontroller with liquid crystal display (LCD), a proportional electro-valve of hydraulic flow control, a hydraulic motor mounted on the dispenser shaft of the seeder machine and a speed sensor of rotation of the shaft dispenser. The control software of this system operates with prescription map of fertilizer rate, based on site-specific fertility of soil previously determined. The system was calibrated for rates from 70 to 130 kg ha-1 for a controlled-release fertilizer (FLC) called Basacote Starter (BS) of uniform granulometry, and a rate from 190 to 420 kg ha-1 for a physical mixture (MF) of fertilizer (urea 29 % + diammonium phosphate 71 %). The calibration constants calculated were 259.6 cm3 rev-1 for FLC and 270.2 cm3 rev-1 for MF, which were charged to the microcomputer for a static-automatic calibration. The final automatic calibration indicated that the coefficient of variation in the rate of the automatic system varies from 0.37 % for a target rate of 70 kg ha-1 with Starter Basacote at 3.74 % to a target rate of 420 kg ha-1 in MF. In addition, the expected error of rating of the automatic system in the field can vary from 0.19 kg ha-1 for a target rate of 70 kg ha-1 of Basacote Starter at 29.26 kg ha-1 for a target rate of 420 kg ha-1 of MF.