Biochar-based fertilizers (BBFs) are attracting considerable interest due to their potential to improve soil properties and the nutrient use efficiency of plants. However, a sustainable agricultural system requires decreased dependency on chemical fertilizer for BBF production and further enhancement of the slow-release performance of BBFs. In this study, we propose a simple biochar-based slow-release fertilizer synthesis technique involving the co-pyrolysis of 10 to 25% (w/w) Ca-bentonite with chicken manure as the only nutrient source (N, P, K). To evaluate nutrient release in contrasting soil media, we mixed pristine and modified chicken manure biochars (CMB) with both quartz sand and clay loam soil and compared the release with that of the recommended fertilizer dose for sweet corn (Zea mays convar. saccharata). Fourier transform infrared spectroscopy and energy-dispersive X-ray spectroscopy revealed that Ca-bentonite reduced readily soluble orthophosphates by forming less-soluble Ca/Mg-phosphates. In addition, significantly slower K release in soil (on average ~ 22% slower than pristine CMB) was observed from biochar containing 25% Ca-bentonite, since K is strongly adsorbed in the exchange sites of crystalline bentonite during co-pyrolysis. Decomposable amides were unaltered and thus Ca-bentonite had no significant impact on N release. Comparison of nutrient release in different media indicated that on average P and K release from BBFs in coarse sand respectively was 38% and 24% higher than in clay loam, whereas N release was substantially greater (49%) in the latter, owing to significant microbial decomposition. Overall, Ca-bentonite-incorporated CMBs, without any additional fertilizer, can satisfy plant nutritional needs, and exhibit promising slow-release (P and K) performance. Further process modification is required to improve N-use efficiency after carefully considering the soil components.