Characterization, nitrogen availability and agronomic efficiency of fermented composts in organic vegetable production

Abstract

Fermented composts obtained from a mixture of raw materials and a microbial inoculant, known as “bokashi,” are alternatives used by many farmers around the world. We evaluated the chemical composition, fermentation pathways, N availability, and agronomic efficiency of fermented compost obtained from different plant-based raw materials. The standard compost formulation composition was 60% wheat bran and 40% castor bean bran. From this formulation, wheat bran was gradually replaced by coffee husk, brewery residue, and elephant grass bran, and leguminous bran replaced castor bean bran. Incubation tests evaluated nutrient content (C, N, P, K, Ca, Mg), pH, electrical conductivity, and fermentation pathways (by the organic acids lactic, acetic, propanoic, butyric, and ethanol). A bioassay in greenhouse conditions accessed N availability. Additionally, a field experiment evaluated the agronomic efficiency of 5 formulations and 4 doses (0 to 400 kg N ha−¹) in successive vegetable production. The formulations with a balanced C/N ratio showed the potential to combine desirable fermentative and nutritional characteristics with good N availability and plant growth. Some formulations drastically changed the compost characteristics, especially the full replacement of wheat bran for coffee husk and elephant grass, which presented undesirable fermentation pathways. Leguminous bran maintained the fermentative quality and increased the soil’s biological activity but decreased the nutrient content, N availability, and vegetable productivity. The brewery residue showed the most prominent fermentation quality, nutrient content, and N availability. The addition of 30% coffee husk resulted in agronomic performance and nutrient accumulation similar to the standard compost.