## 1 Introduction  
The world population is growing very fast. The number of people will continue to rise, growing at an average of 1.1% a year up to 2030. Agriculture is becoming very important to supply the required food necessities, according to the Food and Agriculture Organization (FAO) between 1900 and 2000 year,in a report about the evolution of food, agriculture, and other reliable crop production [1]. According to agricultural indexes, agriculture showed some stability between 1960 and 1980. Nonetheless, after 1980, the values decreased very fast. Thus, agriculture production is not able to sustain the requirements of world food demand, which is becoming a huge problem nowadays. Agriculture depends entirely on soil for obtaining the highest quality and quantity of food. Soil is a big ecosystem formed by a high amount of microorganisms, such as bacteria and fungi, which takes part in 90% of the decomposition of organic material (OM) [2]. Moreover, soil has different kinds of minerals and nutrients. The three main nutrients are nitrogen (N), phosphorus (P), and potassium (K). Furthermore, other important nutrients are calcium, magnesium, and sulphur. Additionally, plants need small quantities of other elements, known as trace elements [3]. Among all these minerals, one of the most important is nitrogen. Nitrogen interacts with carbon (C) in C/N relation, which is necessary to fix the nitrogen, and to obtain good crop production [4]. In this context, to increase production, farmers are using fertilisers. These compounds are necessary to provide the required nutrients to the soil so that the plants can have all the nutrients they need. This increment in nutrients causes higher production, increasing the harvest [5]. Fertilisers can be divided into three different classes: (i) simple or multi nutrient fertilisers, depending on if they are composed of one or more nutrients; (ii) organic or inorganic; and (iii) fast or low release. Generally, the use of multi nutrient fertilisers is recommended more. Otherwise, the increase of a single nutrient ends up creating new limiting nutrients. Moreover, the use of organic fertilizer (OF) can carry disadvantages if it is not appropriately controlled. The soil must contain oxygen for the correct function of the bacteria. The decrease in oxygen levels is nearly related to the increase of OM [6]. Furthermore, when OF is misused, the runoff occurs, and the OM contaminates the water bodies, such as rivers, lakes, or aquifers [7]. One of the best forms to control the necessities of the crops is the monitoring of irrigation water quality [8]. The monitoring of water has decisive importance because it could incorporate nutrients or compounds into the land. In the case of fertigation systems, irrigation systems, which include fertiliser, the most relevant parameters to control the fertiliser quantity, are the conductivity, turbidity, and, in some cases, the pH. The fertilisers have a percentage of minerals that increase the conductivity of the water. In addition, the OFs have a dark colour and, in solution with water, provokes an increment of the turbidity. Finally, some fertilisers could cause pH variations in the irrigation water. The standard form to monitor the water quality is the extraction of an irrigation water sample, and measuring in the laboratory. The use of a conductivity meter in agriculture is another method for controlling the quality of water. Nevertheless, this type of analyses is not useful since it is not possible to measure the water in situ and obtain continuous measurements. Moreover, this type of methodology has a high economic cost because it is needed for a person to take measures, specialised equipment, and reagents. Nowadays, the use of sensors for monitoring agriculture parameters is increasing. In this context, different works are being developed. The application of a Wireless Sensor Network (WSN) for monitoring the irrigation for crops is one of the examples of the use of a sensors network in agriculture [9].