Desenvolvimento de um instrumento espectral portátil aplicado à determinação de nitrogênio foliar na cultura do milho

Abstract

In agriculture, the study of electromagnetic radiation flux in plants has contributed to a more in-depth knowledge about their conditions, allowing the observation from the chlorophyll content, nutritional status or water stress. In this perspective, the work aimed to develop a portable spectral equipment to estimate the nitrogen content in corn plants. Data acquisition by the equipment occurred through the AS7265x spectral sensor connected to an Arduino UNO and ported to other electronic components for data transmission and communication with the operator. The structure of the equipment was elaborated by 3D printing, containing a compartment dedicated to the sensor. Then, an application was developed for communication, via Bluetooth, of the equipment developed with a smartphone. For the calibration of the developed equipment, called MSPAT, the experiment was carried out in agricultural soil and in pots using the hybrid corn culture AG-1051 with different doses of nitrogen fertilizer (0,30, 60, 120, 180 and 240 kg. ha-1). The activities took place in the experimental area of the Agricultural Electronics and Mechanics Laboratory (LEMA) located at the Federal University of Ceará (UFC). Morphological parameters were measured: plant height, number of leaves, stem thickness and LAI. Operation with SPAD-502 optical equipment in the field and FieldSpec Pro FR 3 in a spectroradiometric laboratory were performed for comparison with the indices generated with MSPAT. Chemical analysis in the laboratory to determine the concentration of nitrogen in the corn leaf tissue was performed at the Soil Management Laboratory of the UFC Soil Department. The analysis of variance and test of means were carried out using the SISVAR 4.2 software, while the elaboration of the models was carried out in the MATLAB 7.9 software. The indices generated with MSPAT for correlation with nitrogen content were the 18 bands of the sensor individually and all possibilities of ratio between the bands. The individual bands that presented the most relevant coefficients of determination were over the green and red region due to the biophysical characteristics of the peppers of the plants. As for the band ratio, the indices that presented the most satisfactory results correspond to the use of the 535 nm band with the infrared bands (900 and 940 nm) characterizing the effect of chlorophyll and the condition of the leaf tissue structure. With the spectroradiometer dataset, the indices that were most relevant were the ratios 410 and 535 nm (R² = 0.93), 410 and 560 nm (R² = 0.927), 410 and 585 nm (R² = 0.929), being associated the sensitivity of these wavelengths to the concentration of chlorophyll a and b. Evaluating the estimated values of nitrogen content between the MSPAT and the spectroradiometer, an R² of 0.657 was observed, while comparing the values of the spectroradiometer with the SPAD it was 0.685. However, it is possible to observe that MSPAT presented a satisfactory performance, similar to SPAD-502, demonstrating its potential to estimate the concentration of nitrogen in corn leaves.