This literature review explores the assessment of biochar quality and its impact on soil properties using diffuse reflectance spectroscopy. Biochar, a product of biomass pyrolysis, is recognized for its positive effects on soil fertility and carbon sequestration. This review emphasizes the need for systematic research on biochar stability and highlights the potential of diffuse reflectance spectroscopy for analyzing soil–biochar interactions. Biochar acts as a soil conditioner, improving physical, chemical, and biological properties and enhancing soil fertility and crop yield. Furthermore, it aids in mitigating climate change by sequestering carbon dioxide. However, the long-term behavior of biochar and its interactions with various factors require further field research for optimal utilization, as the aging process of biochar in soil is complex, involving physical, chemical, and biological interactions that influence its impact on the agroecosystem. This review also emphasizes the importance of studying the interaction between biochar and soil microbes, as it plays a crucial role in enhancing soil fertility and plant resistance to pathogens. However, research on this interaction is limited. VIS-NIR spectroscopy is a valuable tool for monitoring biochar application to soil. Nevertheless, controversial results highlight the intricate interactions between biochar, soil, and environmental conditions.