Organic Fertilizer Enriched with Humic Acids and Plant Growth-Promoting Bacteria Applied in a Mandarin Orchard

The expansion of large-scale organic fruit production in Brazil is constrained by the scarcity of biological inputs. One potential solution is to enrich organic fertilizers with humic acids and plant growth-promoting bacteria, which could increase plant nutrition efficiency. A compost produced on an industrial scale from pruning residues of urban trees and cattle slaughterhouse waste was enriched with humic acids extracted from the organic compost itself and inoculated with a microbial consortium formed by the bacteria Herbaspirillum seropedicae strain HRC54, Bacillus safensis strain 77, Bacillus pumilus strain J1.1,and Gluconacetobacter diazotrophicus strain PR2. Chemical and spectroscopic characterization of the organic fertilizer was performed, and the resulting product was applied to three plots with a total of 200 g of nitrogen per plant per year. Solid-state 13C nuclear magnetic resonance (NMR) analysis revealed the presence of lignocellulosic materials and smaller but significant amounts of hydrophobic alkyl molecules. Gas chromatography coupled to mass spectrometry of the pyrolysis products of the compound indicated the presence of nitrogen, lignin, carbohydrates, and lipids. There were no significant differences in biometric parameters, such as plant height and stem diameter, among the different mandarin trees. However, plants treated with the enriched fertilizer showed significantly higher productivity in the first year of production, with fruitweights ranging from 3.8 to 6.2 kg per plant, compared to plants receiving the unenriched fertilizer, which produced between 2.2 and 3.8 kg of fruit per plant. Biological enrichment is a viable alternative to increase the efficiency of organicfertilizers.