The incorrect fertilization of the crops can cause problems in the environment and extra costs. A solution is to perform fertigation controlling the amount of fertilizer in the water. In this paper, we test different combinations of coils for determining the amount of fertilizer in the water. A coil is powered by a sine wave of 3.3 peak-to-peak Volts for inducing another coil. These sensors will be included in a smart irrigation tube as a part of a smart irrigation system based on the Internet of Things (IoT). The aim of this system is to detect different sorts of problems that can cause incorrect fertilization, which affects the sustainability of agriculture. This system can be used in different scenarios where tubes are used to irrigate. We present the performed test to evaluate the suitability of the created prototypes. At first, we test with different dilutions of NaCl (table salt) and, after it, we performed tests with nitromagnesium (a fertilizer). We checked that at the same salinity the induction value changes if it is found in water with NaCl or nitromagnesium. Of all the tested prototypes it is concluded that the prototype P2 is the most optimal g/L because there is a difference in the induced voltage between 0 and 45 g/L of nitromagnesium of 3.79 V with a good correlation coefficient. In addition, the average error in the different samples tested in the verification test is 2.15%.