The use of potential biostimulants is of broad interest in plant science for improving yields. The application of a humic derivative called fulvic acid (FA) may improve forage crop production. FA is an uncharacterised mixture of chemicals, and although it has been reported to increase growth parameters in many species including legumes, its mode of action remains unclear. Previous studies of the action of FA have lacked appropriate controls, and few have included field trials. Here we report yield increases due to FA application in three European Medicago sativa cultivars, in studies which include the appropriate nutritional controls and hitherto unused. No significant growth stimulation was seen after FA treatment in grass species in this study at the treatment rate tested. Direct application to bacteria increased Rhizobium growth and in Medicago sativa trials root nodulation was stimulated. RNA transcriptional analysis of FA-treated plants revealed upregulation of many important early nodulation-signalling genes after only three days. Experiments in plate, glasshouse, and field environments showed yield increases, providing substantial evidence for the use of FA to benefit Medicago sativa forage production.