We compared the biostimulant effect of a novel chicken feather hydrolysate (FH) and a reference protein hydrolysate (RH) on barley and wheat in a pot experiment. Their interactions with arbuscular mycorrhizal fungi (AMF) and phosphorus (P) supply were also addressed. All experimental factors influenced barley growth. Shoot height and biomass of barley were increased by FH and reduced by RH. AMF decreased barley biomass at high P-supply. In wheat, the biomass was slightly reduced by AMF while other factors had no significant effect. In the parallel field experiment, RH but not FH increased yield and grain size of barley, while there was no significant effect of either hydrolysate on wheat. Application time had no effect on hydrolysate efficacy. Both hydrolysates promoted severity of net blotch (Pyrenophora teres maculata) on barley in the pot experiment, but reduced it in field. FH promoted wheat root colonisation by AMF under low-P supply. Our results show limited transferability of pot results to field conditions and manifest complex interactions between hydrolysates, soil phosphorus, and plant symbionts and pathogens.