Abstract
Nitrogen application methods were studied, based on growth rate, dry matter production and nitrogen absorption properties for autumn harvesting Japanese bunching onion (Allium fistulosum L.) grown under greenhouse conditions for 3 years. The following results were obtained: l) Growth stages of Japanese bunching onion were divided into three stages based on growth rate by measuring dry weight : [stage 1] initial growth period : from transplant to about 30 d after transplant, [stage 2] maximum growth period : about 30 to 80 d after transplant, and [stage 3] blanching culture period : about 80 d after transplant to harvest time. As basal fertilizer, 100 kg nitrogen ha^<-1> was appropriate to maintain high crop growth rate during initial growth periods, and also to obtain high early growths and high yields. To apply 100 kg nitrogen ha^<-1> as topdressing about 30 to 60 d after transplant led to higher yields, since more gain of dry matter was observed when topdressing was applied during maximum growth periods. Gain of weight of leaf blades during maximum growth periods led to gain of weight of leaf sheaths during blanching culture periods, and resulted in high yields. 2) Japanese bunching onion absorbed nitrogen of 255 kg ha^<-1> when it yielded fresh matter of 158 Mg ha^<-1> or dry matter of 14.1 Mg ha^<-1>. Proper nitrogen application rates amounted to 200 kg ha^<-1>. 3) Proper relations between soil nitrate nitrogen contents before applying basal fertilizer and the amount of nitrogen fertilizer as basal and topdressing were as follows : under soil nitrate nitrogen contents of less than 50 mg kg^<-1>, basal fertilizer of 100 kg ha^<-1> plus topdressing of 100 kg ha^<-1> were appropriate ; under soil nitrate nitrogen contents from 50 to 100 mg kg^<-1>, 50 kg ha^<-1> plus 100 kg ha^<-1> ; under soil nitrate nitrogen contents from 100 to 200 mg kg^<-1>, no basal fertilizer plus topdressing of 100 kg ha^<-1>, under soil nitrate nitrogen contents of more than 200 mg kg^<-1>, only topdressing of 50 kg ha^<-1> was appropriate. 4) Soil nitrate nitrogen contents in 39% of the greenhouses that were surveyed in this study where Japanese bunching onions have been cultivated, were more than 200 mg kg^<-1> in typical production areas in southern Hokkaido. In addition, applied nitrogen amounts were in excess there. Therefore, these nitrogen application methods enable the reduction of soil nitrate nitrogen contents in most greenhouses and maintain low levels of it in the long run.
