Fertile soils are highly important for arable farming. To maintain soil fertility, crop rotation including legumes is applied, particularly in stockless farms. Yet, many commercial farms report yield reductions for legumes. The aim of the present study was to evaluate whether the causes for the observed soil fatigue can be narrowed down by a bioassay (modified differential diagnosis according to Bouhot) and whether this bioassay could be a useful tool to predict the suitability of a field to grow legumes.