2026/11/13
The primary challenge in granulating bio-organic fertilizer is not merely achieving granule formation, but ensuring the survival of the live microbial cultures throughout the process. While the production of conventional organic fertilizers prioritizes granule roundness and mechanical strength, the production of bio-organic fertilizers requires a careful balance between granule quality and microbial viability.
Temperature. The activity of most functional microorganisms declines rapidly when the material temperature exceeds 50°C. Although temperature increases caused by mechanical friction during granulation are generally manageable, the drying stage poses a greater risk. High-temperature drying can dry out the granule surface while simultaneously killing most of the microorganisms within the core. It is recommended to use natural air-drying or low-temperature fluidized-bed drying, keeping the material temperature below 45°C.
Timing of Microbial Inoculation. Microbial agents should not be mixed with the raw powder for extended periods prior to granulation, nor should they be introduced to high-temperature materials. A more reliable approach is to granulate the raw material first, allow it to cool, and then spray the microbial suspension onto the surface of the granules, using a binder to create an outer coating. This method avoids subjecting the microorganisms to the mechanical compression and friction of the fertilizer granulator, resulting in significantly higher survival rates compared to mixing the microbes into the raw material before granulation. If internal mixing is unavoidable, the mixing duration should be minimized, and the temperature rise within the equipment must be strictly controlled.
Moisture Content. Moisture levels must be balanced to facilitate both granule formation and microbial survival. For disc granulators, the optimal moisture content for granulation is typically 15%–25%; however, fermented organic materials are often excessively moist. High moisture levels can cause granules to stick to the equipment walls and increase the drying load, while low moisture levels lead to poor granulation rates and excessive dust. Before granulation, materials should undergo semi-wet pulverization and conditioning (aging) to stabilize the moisture content within an appropriate range, thereby avoiding the need for repeated water addition during the granulation process.
Granule Strength vs. Microbial Count. Pursuing high mechanical strength often necessitates greater compression and a denser internal structure, both of which are detrimental to microorganisms. For bio-organic fertilizers, the compressive strength of the granules only needs to be sufficient for storage and transportation; there is no need to match the standards of inorganic compound fertilizers. Reducing the forming pressure—thereby preserving micropores within the granules—actually supports the survival of the microorganisms and facilitates their subsequent release.
In short, the core logic of bio-organic fertilizer granulation is to "make way for the microbes." By striking the right balance among these factors, you can produce granules that are both commercially presentable and capable of passing tests for viable microbial counts.
