Adjustable-Rate Fertigation System for Container Nurseries

Abstract. Site-specific fertigation can limit fertilizer waste and loss to the environment, but is not easily implemented when there are many small hydrozones, each demanding a unique rate of fertilizer application. With a simple and inexpensive fertilizer injection system, a separate injector could be installed at each zone to provide a unique fertilizer delivery rate, even when those zones are in simultaneous operation. Integration with a wireless control network would allow automated adjustment of the fertilizer delivery rate for each hydrozone and aid installation in existing facilities. A variable-rate injector was developed using a simple venturi injector, solenoid valve, electrical conductivity (EC) sensor, and small computer board (controller). EC was related to fertilizer concentration expressed as parts-per-million (ppm) nitrogen (N). Fertilizer injection rate was controlled by opening and closing a solenoid valve connected to the suction line of the venturi injector. The rate was adjusted by changing the valve duty cycle (percentage of time that the valve was open). For initial testing with a 2000 ppm N stock solution, the duty cycle was manually adjusted from 0 to 100% to deliver an applied fertilizer concentration of 0 to 200 ppm N at an injection ratio of about 1:10. For closed-loop control, the controller was programmed to monitor EC and automatically increase or decrease the valve duty cycle in order to achieve the target fertilizer concentration. The root mean square percentage error for the closed-loop control injection system was 4.8% in laboratory tests over a target range of 0 to 200 ppm N. A mobile injector station was assembled and integrated with a wireless sensor and control network in order to test adjustable-rate fertigation in container nurseries. Field trials to deliver specified nitrogen concentrations to several different nursery beds gave a root mean square percentage error of 2.2% over a target range of 50 to 150 ppm N. The fertilizer concentrations matched that specified by the user, indicating that the system did a good job of adjusting the duty cycle to meet the target rate, and the irrigation and fertilizer injection durations were correct. Furthermore, the wireless network performed well and exhibited acceptable range.