The ‘home-field advantage’ (HFA) hypothesis predicts that litter decomposition is accelerated in its home environment (i.e. in conspecific soil). Soil organisms play a key role in driving such HFA effects. Soil biota have a large range of body sizes, referred to as size fractions, which may influence their roles in the decomposition process and in the generation of HFA effects. However, how HFA effects depend on the different size fractions of the soil biotic community is unknown. We conducted a microcosm decomposition experiment to examine how size fractions of the soil biotic community affected litter decomposition and HFA effects. In a semi-natural grassland in the Netherlands, we collected leaf litter and soil from two abundant forbs: Tanacetum vulgare and Jacobaea vulgaris. Watery extracts of the soils were sieved through differently-meshed sieves (ranging from 850 μm to 6 μm) to obtain soil communities of different size fractions. Microcosms were inoculated with these different size fractions of the soil biotic community and we examined their effects on microbial composition, litter mass loss and HFA effects. Three months after inoculation, the diversity of the fungal community in the inoculated pots decreased with decreasing size fractions of the soil biotic community. Similarly, litter mass loss also decreased with decreasing soil biotic community size. In contrast, the HFA effect increased with decreasing size fractions of the soil biotic community, but these differences disappeared after six months of decomposition. Our results indicate that soil microorganisms, mainly the smallest size fractions, are specialized to decompose specific resources and thus promote HFA effects, but that their effect is only apparent during specific stages of litter decomposition.