Field testing of an automatic control system for variable rate fertilizer application

An automatic system to control variable rate of fertilizer application was implemented and tested in an experimental spring wheat crop. The system included a micro-controller, a LCD display, a proportional flow control electro-valve, a rotational speed sensor and a hydraulic motor mounted on the shaft of the fertilizer dispenser of a drilling machine. The control software used a fertilizer rate prescription map previously determined from a map of soil fertility. The system was tested in a small field of 0.6 ha using an experimental design of four randomized treatments and three repetitions. In order to develop the application rate map, 21 samples were previously taken for analysis of soil fertility. The treatments tested the application of a constant rate and variable rate of physical mixture (PM) of 16% of N and 42% of P, and a constant rate and variable rate of Basacote Starter® (BS), which is a controlled release fertilizer. The amount of fertilizer used in the field was measured and compared in each case with the prescribed rate for spatial resolutions of 1 m, 2 m, 3 m, 4 m and 5 m and a width of 3.5 m. The error of application rate was between 20% and 29% at 1 m resolution and decreased to values between 12% and 14% at 5 m resolution for the whole set of treatments.