This paper presents an embedded sensing system for agricultural machines to collect information about plants and also to control the application of fertilizer with variable rate in corn crop. The Crop Circle reflectance sensor was used with the aim to explore the spectral features of foliar pigment to use the plant as the indicator of availability of nitrogen in soil. The sensing system was built in Uniport 3000 NPK fertilizer applicator of Jacto that presents a distribution system of solid fertilizer which is granulated by means of high uniformity air-flux. The Uniport also presents an electronic controller of fertilization that can be started with manual or automatic control mode through RS-232 serial communication port. Furthermore, the system has the capacity to inform the machine displacement speed and the fertilizer application rate. Two dedicated softwares were developed: one was responsible for reading and storing the sensor data; the other one was responsible for sending commands to system controller with desired fertilizer application rate. In order for the development of the software that sends commands to the system of fertilizer application controller, the machine displacement speed and the system mechanical response time were taken into account to determine the resolution of fertilizer application rate. The results shows that the proposed system is able to collect the desired information and achieves the control of fertilizer application with variable rate. Moreover, the proposed system also provides practical observations of requirements for embedded system development in agricultural applications.