Medidores portáteis de clorofila na estimativa da adubação nitrogenada em cobertura no feijoeiro

Abstract

Nitrogen (N) is the nutrient most demanded by common bean (Phaseolus vulgaris L.), however, nitrogen fertilization often occurs inappropriately through insufficient or excessive doses based on outdated recommendation methods. In this context, portable chlorophyll meters can be used to improve the efficiency of nitrogen fertilizer use in topdressing fertilizers. Thus, the objective of this study was to estimate nitrogen rates applied to coverage for the cultivars TAA Gol, Pearl, BRSMG Uai, and for the strain VR 20, using two different chlorophyll meters, Minolta SPAD-502 and ClorofiLOG CFL 1030. The project was developed in the municipality of Lavras-MG, in two stages, each consisting of eight experiments, corresponding to each of the different genotypes combined with the two used devices. In the first stage of the project, which was carried out in the 2017/18 crop, in each experiment, the treatments consisted of N applications related to four nitrogen sufficiency indices (ISN). A reference area was implanted for each of the evaluated genotypes, being this area common for the two chlorophyll meters used. After the establishment of the ISN, nitrogen fertilization was performed, associating the fertilization criterion to each one of them. For each 1% lower than the ISN of the treatment, 15 kg ha-1 of N. was applied. In the second stage of the project, in the 2018/19 harvest, in each experiment, the ISN that presented the best results in the first experimental stage was adopted. The treatments consisted of the fertilizer factors: 0; 5; 10 and 20 kg ha -1 N for each 1% below the ISN defined as appropriate in the previous step. At flowering, new readings were performed with the devices, and the leaves were detached and destined for leaf analysis to quantify N. At the end of the cycle of each genotype, the number of pods per plant, the number of beans per pod, 100 grain mass, grain yield and N content in the grains were evaluated. Data were submitted to analysis of variance and when significant differences were detected, regression analyzes and other tests were performed. The discrepancies between ISN obtained by the devices in the same genotype and by the same device in different genotypes did not allow a joint analysis between experiments. Therefore, it was observed the need for adjustments in the methodology of use of ISN to define the doses of N provided in coverage, according to the genotype cultivated and the apparatus that will be used. It was not possible to obtain precise adjustments for the chlorophyll usage methodology meters in the cultivar TAA Gol. The combination of the fertilization factor of 12.5 kg ha-1 of N every 1% below the 90% ISN proved to be efficient for the Pearl cultivar when the SPAD-502 chlorophyll meter was used. In genotypes with similar size and cycle characteristics, the cultivar BRSMG Uai and the strain VR 20, using a fertilization factor between 14 and 16 kg ha-1 of N, the 95% ISN should be reduced, aiming to increase the efficiency of nitrogen fertilization. Only the relative chlorophyll index obtained during flowering is not efficient to determine the nutritional status of the crop in relation to nitrogen.