The curse of the black box

Background Soil is a foremost provider of (agro-)ecosystem services, making plant-soil interactions pivotal in agriculture research. The functioning of soils entails complex interactions between soil biota and the abiotic soil environment and is therefore often considered as a ‘black box’. The study of Verbruggen et al. (this volume) tries to crack the black box open by examining the role of soil microbial communities from conventional and organic farming fields for the growth of Zea mays and phosphorous retention in the soil. Scope In this commentary on the paper of Verbruggen et al. (2011) we use the study to illustrate that investigating soils, and specifically the role of soil biota in ecosystem functioning, is not straightforward, given the overwhelming soil biodiversity and the complexity of soil as a habitat. We discuss the key elements that need to be considered in order to translate results of highly controlled experiments with inoculated soil biota to their functioning in the field. Conclusions Verbruggen et al. contribute to our understanding of the functional role of AMF in agro-ecosystems. Yet the results only allow us to merely speculate about the realized functional role of AMF communities in the field, a very interesting avenue for future research.