# Quantifying Nutrient Content in the Leaves of Cowpea Using Remote Sensing
## 2 Materials and Methods  
The trial was carried out in an experimental area on the Pici Campus of the Federal University of Ceara, (546920 E, 9586497 S) (Figure 1). According to [PERSON] [33], the climate in the region is classified as tropical rainy (Aw'), with a rainy season during the summer and a dry season during the winter, and average monthly temperatures greater than 18 \({}^{\circ}\)C. During the experiment, the minimum, mean and maximum temperatures were 25.9, 27.6 and 28.7 \({}^{\circ}\)C, respectively, while the minimum, medium and maximum values for relative humidity were 64.25%, 72% and 81.5%, respectively. The soil in the area has a sandy texture, and the main physical and chemical characteristics are shown in Table 1.  
The experimental plots consisted of five plant rows at a spacing of 1.0 m \(\times\) 0.3 m with a total area of 9.0 m\({}^{2}\). Two plants were kept per hole, giving a total population of approximately 67,000 plants ha\({}^{-1}\). The crop was drip-irrigated every two days, with irrigation suspended at the fruit maturation stage. According to [PERSON] et al. [34], the irrigation of _Vigna unguculata_ (L.) Walp should be suspended during the maturation of the pods. The normal evolution of the crop requires a lack or low availability of water during the final stage of the cycle to avoid the grain rotting or the appearance of diseases. To better explain the processes used in the methodology, a flowchart was created containing the steps adopted in the study (Figure 2). The crop was fertilised using a standard treatment, including N (urea), P (single superphosphate) and K (potassium chloride) in mineral form, which was defined based on the soil analysis and crop requirements, as per the manual of recommendations and liming for the state of Ceara [35]. In addition, the crop was fertilised with organic compost at doses of 100%, 200% and 300%, where the organic fertiliser was defined based on the nitrogen from the mineral treatment, i.e., 100% of the organic compost contained the same amount of nitrogen as the mineral treatment. Nutrient availability from the organic compost was determined using the equation proposed by [36] and modified by [37]. The crop cycle of _Vigna unguculata_ (L.) Walp is composed of ten phenological stages, five vegetative and five reproductive stages. The vegetative stages are: Start of germination (V0); Cotyledons level with the soil (V1); Expanded primary leaves (V2); First trifoliate leaf (V3); and Third trifoliate leaf (V4), respectively. The reproductive stages are: Flower bud formation (R5); Opening of the first flower (R6); Appearance of pods (R7); First full pods (R8) and Physiological maturity (R9), respectively, defined by [PERSON] and [PERSON] [39]. The leaf samples were collected at stages V4 (third fully open composite leaf), R6 (at least 50% of open flowers) and R9 (pod maturation). According to Embrapa [34], the most critical  
Figure 1: Location of the experimental area. The leaves were always collected at the same time (08:00 a.m. to 11:00 a.m.). The radiometric readings were taken at the Laboratory of Spectroradiometry of the Federal University of Ceara immediately after collection. The samples were then dried in an oven at 65 \({}^{\circ}\)C to constant weight and used for mineral analysis. To acquire the radiometric data, the FieldSpec 3 Hi-Res spectroradiometer was used (Figure 3). This has a field of view of 25\({}^{\circ}\) and operates in the region of 350 to 2500 nm, with a spectral resolution of 3 nm at 700 nm, 8.5 nm at 1400 nm, and 6.5 nm at 2100 nm. The sensor resamples the data to 1 nm, giving a total of 2151 bands. The readings were carried out in a darkroom, thereby eliminating interference from external radiation that might compromise the results. The sensor was positioned at the nadir, 7 cm away from the samples. As a light source, a 250 W halogen lamp was used, with a parabolic reflector, a beam collimated along the target plane and a 30\({}^{\circ}\) zenith angle of illumination.