When cotton was grown in Hula peat soil following alfalfa, the germination, growth and yield were lower than when it followed wheat or bare soil. Results indicated that the formation of N-NO3 in the soil was reduced by alfalfa crop growth, or when alfalfa root leachate was applied to bare soil. The inhibition of nitrification appeared to be due to substances originating in the plant and released by the roots. It is suggested that saponine, which is known to be released by alfalfa roots, may be responsible for this effect. © 1978 Martinus Nijhoff.