Soil quality and crop yield responses to application methods and rates of coffee husk biochar

Abstract

The use of biochar as a soil amendment has been widely recognized as a promising strategy to improve soil physicochemical and biological attributes, particularly in tropical agricultural systems. Despite the reported benefits, important gaps remain regarding the definition of agronomically efficient application rates and placement strategies that maximize positive effects while minimizing risks associated with improper management, such as localized salinization and nutrient imbalances. In this context, this study aimed to evaluate the effects of coffee husk biochar rates, applied using different localized placement methods, on soil quality and grain yield of common bean and maize over three successive cropping cycles (bean–maize– bean) under field conditions. The experiment was conducted in a 5 x 2 factorial design with four replications, testing five biochar rates (0, 0.6, 0.9, 1.2, and 1.8 Mg ha⁻¹) applied either on the soil surface or incorporated into the planting furrow. The biochar was produced from coffee husk in an industrial pyrolysis reactor at 600 °C. Applications were performed before the first bean crop and reapplied before maize cultivation, whereas only residual effects were assessed in the second bean cycle. Throughout the experiment, soil chemical, physical, and biological attributes, plant nutritional status, and grain yield were evaluated. After the first bean crop, soil K availability increased significantly with increasing biochar rates, reflecting the high K content of the material and resulting in yield gains of up to 50%. In contrast, maize yield decreased linearly with increasing rates, reaching an approximate 25% reduction at the highest rate. This effect was predominantly attributed to excessive K accumulation in the soil, which likely induced salt stress and nutritional imbalances. Nevertheless, incorporated application outperformed surface application, resulting in higher soil pH, greater P availability, and higher maize yield. In the second bean crop, a positive residual effect of biochar was observed, with yield increases of up to 28%, associated with increased soil pH and P availability. These findings indicate that biochar acts gradually in the soil, promoting medium- to long-term effects related to slow nutrient release and enhanced ion exchange capacity. By the end of the study, a significant reduction in soil penetration resistance in the surface layer was observed, indicating improvement in soil physical quality. Overall, coffee husk biochar improved soil chemical, biological, and physical attributes and increased bean productivity in tropical agricultural systems. However, localized and successive applications require careful management of K content, crop type, and reapplication interval to avoid salt accumulation and nutrient imbalances. These findings highlight the importance of rational biochar use as a tool for sustainable soil management and enhanced agricultural productivity. Keywords: K dynamics; nutrient availability; residual effect; tropical agriculture; grain production.