Soil biodiversity forms the foundation of ecosystem functioning by supporting an intricate assemblage of organisms responsible for decomposition, nutrient cycling, trophic regulation, and the modification of soil physical structure. Classifying these organisms into three functional groups, chemical engineers, biological engineers, and ecosystem engineers, provides a robust framework for evaluating their ecological contributions. Chemical engineers, primarily comprising bacteria and fungi, mediate key biochemical transformations and drive the breakdown of organic substrates, thereby enhancing the bioavailability of essential nutrients. Biological engineers, encompassing protists, nematodes, and diverse microarthropods, regulate population dynamics within the soil food web and exert cascading influences on nutrient turnover and ecosystem stability through their predatory and grazing activities. Ecosystem engineers including earthworms, termites, ants, isopods, moles, and plant root systems reshape soil architecture by promoting aggregation, altering pore networks, and redistributing resources, ultimately improving aeration and hydrological functioning. Importantly, many soil organisms perform multiple ecological functions, integrating processes across trophic hierarchies and temporal scales. A functional understanding of these organismal roles provides critical insights into soil resilience, productivity, and the development of sustainable land management strategies, highlighting the primacy of functional diversity over mere taxonomic composition