## 1 Introduction
The use of technology for monitoring agricultural parameters is known as Precision Agriculture (PA). PA combines the WSN, information systems, enhanced machinery, and informed management to optimise production, by accounting for variability and uncertainties within the agricultural system. This strategy is applied to several agroecosystems [10]. One way to control the amount of fertilisers is by monitoring the soil parameters using different kinds of devices. [PERSON]. [PERSON] [11] developed a grid of micro-mechanical (MEMS) sensors based in a gas detector, which will be able to detect the ammonia presented in fertilisers. The gathered data are provided using the deflection and voltage output from the piezoelectric layer. This is useful for informing the farmers about soil conditions in real-time. Another type of monitoring system is presented by [PERSON] et al. [12]. The system that they present is based on the monitoring and modelling of nitrogen leaching caused by over-fertilisation. They use a Hydrus- One dimensional (1D) simulation model to obtain data for optimal fertiliser management practices. Moreover, the application of smart fertilisers is another alternative to control or reduce the amount of nutrient in agricultural soil. [PERSON] et al. [PERSON] et al. (2018) proposed a controlled-slow release fertiliser. This \"smart\" fertiliser is based on polymer brushes of poly (\(N\),\(N\)-dimethyl aminoethyl methacrylate). [PERSON] et al. [PERSON] et al. (2018) proposed a similar solution based on slow-release fertilisers. They studied a fertilised with natural atapulgite, clay, ethyl cellulose, film, and sodium carboxymethyl cellulose/hydroxylethyl cellulose hydrogel formulation. [PERSON] et al. [PERSON] et al. (2018) developed fertilisers based on biodegradable substances to obtain more sustainable crops. The created fertiliser is composed of biodegradable polymers that release water and nutrients gradually to the environment, without leaving residue. The improvement of fertilisation techniques is crucial in order to have proper management of fertilisers in agricultural lands. In this context, [PERSON] et al. [PERSON] et al. (2018) described a Global System for Mobile Communications (GSM)-based irrigation system with control of soil moisture and water level. They propose the monitoring and control of water and fertiliser with a liquid level detector and different control schemes and monitoring methods. Their system was implemented using the micro-controller 89552 and Programmable Integrated Circuited (PIC) 18F4550. Finally, [PERSON], [PERSON] et al. [PERSON] et al. (2018) proposed a system based on the Internet of Things and other technologies for real-time monitoring. This system will able to predict and forecast the water requirement of crops in the different growth periods and make the decision concerning automatic irrigation and fertilisation. The decision-making is possible by using a model based in Big Data. When selecting a sensor to be part of a WSN for monitoring the presence of OFs in the fertigation systems, it is recommended to use physical devices that do not need maintenance. Chemical sensors--such as the ones used for monitoring the pH--are not the best options to be applied in the field, since they need periodic maintenance to replace some membranes and electrodes or clean the device. Therefore, a turbidity sensor, which is composed of optical devices, is one of the alternatives [PERSON] et al. (2018). The last alternative is the salinity sensor, which could detect changes in the conductivity of the water due to the presence of fertiliser [PERSON] et al. (2018). Regarding conductivity sensors, there are two types of sensors: ones composed of electrodes and ones composed of inductive coils. The latter are preferred due to the absence of electrodes and the possibility of isolating the sensor. The capability of inductive coils for detection of inorganic fertiliser is already demonstrated.