Although there was acidification of the compost to pH 4.8, which might be in response to the application of the ammonium nitrate treatment, this effect would have been much the same across all treatments. There did not appear to be any suggestion that either toxicity or deficiency, in regard to S or K, Ca and Mn, respectively, could explain limitations in further nitrogen-induced growth enhancement. We therefore conclude, with respect to growth, that nitrogen application at approximately 100 mg L21, and a leaf concentration of approximately 5.7% (w/w), was optimal for A. annua growth.