Regenerative Farming with Organic Fertilizer and Biologics: A New Approach to Enhancing Soybean Yield and Soil Chemical Quality

Composted sewage sludge (CSS) applications and the use of plant growth-promoting bacteria (PGPB) are emerging as sustainable alternatives in tropical agriculture. However, no studies have validated the combined use of these practices. This study aimed to evaluate the residual effect of three CSS applications on soil fertility (0.0–0.2 m and 0.2–0.4 m layer), plant nutrition, morphological and yield components, and grain yield and quality of soybean, with and without co-inoculation of Bradyrhizobium japonicum combined with Azospirillum brasilense, under a no-tillage system (NTS) in the Cerrado region. The field experiment was conducted over a six-year period in Selvíria, Mato Grosso do Sul, Brazil. This research was evaluated during the 2022/23 first cropping season. The experimental design was a randomized complete block with four replicates, arranged in a 5 × 2 + 1 factorial scheme, consisting of five cumulative CSS rates (0.0, 15.0, 22.5, 30.0, and 37.5 Mg ha−1, wet basis), with and without co-inoculation of A. brasilense, plus an additional control treatment with conventional mineral fertilization (CMF). The residual effect of the cumulative CSS rates improved soil fertility in both layers, similarly to CMF, regardless of co-inoculation. Co-inoculation with Bradyrhizobium japonicum and Azospirillum brasilense did not influence the soybean variables assessed. We found that the 24.7 Mg ha−1 CSS accumulated rate yielded the highest soybean yield (4990 kg ha−1). CSS can be used as an organic fertilizer in soybean cultivation, helping to improve the efficiency of mineral fertilizers while ensuring environmentally friendly disposal of municipal sewage sludge.