Allelopathic effects of alfalfa.

Several experiments have been conducted at the University of Illinois, Urbana, Illinois, to determine the role of allelopathic (autotoxicity) effects of alfalfa (Medicago sativa L.) on the establishment of alfalfa. By means of correcting any soil fertility differences and protecting the alfalfa seedlings from fungal attack, results suggest that there is a release of phytotoxic factors from the previous crop on reestablishing alfalfa. Field results indicate that the best preceding crop for alfalfa establishment is corn (Zea mays L.), followed by various small grains, soybeans (Glycine max L.), and the worst preceding crop is alfalfa. Results indicate there were no major genetic differences among cultivars for resistance to autotoxicity. There was no evidence supporting saponins as being phytotoxic to alfalfa. Exudates of different saponin level cultivars indicated that saponin content is not the suspected phytotoxic factor.