Earthworms effect on plant growth is mediated by their dejections or “casts”, a complex mixture of organic matter, minerals and microbes. In casts, different processes such as organic matter mineralization and signal molecule production follow a complex temporal dynamics. An adaptation of root morphology to cast dynamics could allow an efficient nitrogen capture by the plant. The plant Brachypodium distachyon was grown in a laboratory experiment with different proportions of casts of increasing ages. Casts were labelled with 15N to quantify the plant N uptake from the casts. Plant biomass and morphology, especially root system structure, were analysed. The age of casts had an effect on fine root length, highlighting the importance of the dynamics of cast maturation in root adaptation. Plant biomass production was affected by the interaction between the age and proportion of casts. A positive correlation between the 15N proportion in plant tissues and plant biomasses indicated that plants were more efficient in foraging N in casts than in the bulk soil. Our results suggested that both a timely adaptation of the root system structure and a significant proportion of casts are necessary to observe a positive effect of casts on plant growth.