Fertility Studies on Soils of Okinawa in Relation to Sugarcane Production : 1. Nitrogen absorption of sugarcane on a mature and an immature fields(Department of Agricultural Chemistry)

稲嶺統土壌の熟畑と風化国頭礫層に造成された未熟畑に栽培される株出2回目のサトウキビ(共に1976∿1977年作物)について窒素吸収量と原料茎収量との関係を調べた。併せてりん酸とカリの吸収についても調べた。稲嶺統土壌熟畑においては原料茎数150,000本/haで, 収量は135t/haであった。風化国頭礫層未熟畑では原料茎数94,000本/haで収量は98t/haであった。この場合原料茎1本平均重は両圃場の間で大差がないので, 原料茎収量の多少は単位面積当たりの茎数によって決まることが明らかになった。稲嶺統土壌熟畑では窒素地力が高いこと, および窒素施用量が378kg/haあったことが相まってサトウキビの生育中期(7月∿11月初旬)の有効茎数確保が充分に行なわれた。これに対し風化国頭礫層未熟畑では窒素地力が未だ十分に高まっておらず, そのため窒素を7月以前に303kg/haと7月下旬に76kg/ha合計379kg/ha施用したにもかかわらず生育中期には葉身中の窒素濃度が低くなり, また有効茎の割合が低くなった。このようなことからこの未熟畑ではサトウキビの生育初期と中期に窒素が不足したものと推定した。サトウキビのりん酸吸収については稲嶺統土壌の熟畑においては135kg/haのりん酸施用量に対して68kg/haをサトウキビが吸収した。このことからこの熟畑がりん酸地力に富んでいると解釈された。一方の風化国頭礫層未熟畑では493kg/haのりん酸施用量に対する吸収量は36kg/haであったが, 特にサトウキビ生育中期のりん酸吸収割合(施肥量に対する)が低かった。サトウキビのこの様なりん酸吸収状態からみてこの未熟畑のりん酸地力は著しく低く, これが原料茎収量を高める場合に対策が必要な障害の一つであると解釈した。今回の研究から抽出される今後の研究課題は(i)窒素地力の低い造成未熟畑における窒素施用法の確立, (ii)造成未熟畑はりん酸地力も低いのでりん酸資材により土壌改良を行なった場合に, サトウキビのりん酸吸収がどのように改善されるかを明らかにすること, (iii)熟畑および造成未熟畑においてサトウキビの機械化栽培が行なわれる場合に, 機械に規制される植付畦巾・株間などが生育茎数の確保におよぼす影響を明らかにすること, などである。Nitrogen absorption of sugarcane was studied in relation to the cane yield on a mature and an immature fields. Immature cane fields are expected to expand in Okinawa, because weathered rocks or mother materials become exposed by leveling of hilly lands which is requested to some extent for future mechanization of the sugarcane cultiviation. It is considered important for the cane production to clarify the difference in nutrient status between the mature fields and the immature fields. In the present study, an university farm of Inamine series soil was used as a mature cane field and an experiment farm at Nago, which was prepared by leveling a hilly topography of weathered Kunigami gravel in 1973,was used as an immature cane field. The second ratoon crop (1976-1977 crop) of sugarcane was studied on the both farms. The cane yields of the 12 month crop were 135t/ha and 98t/ha on the mature field of Inamine soil and the immature field of weathered Kunigami gravel, respectively. The difference in the yield was accounted for a greater number of cane stand on the mature field (150,000/ha) and a lesser on the immature field (94,000/ha), because no significant differences were found in the mean weight of the cane stand between the respective fields. The sugarcane crop on the mature field of Inamine series soil absorbed 305kg of N/ha as received 378kg of nitrogen during the growth period. On the contrary the cane crop on the immature field of weathered Kunigami gravel was found to absorb only 184kg of N/ha as received 379kg of nitrogen. This finding suggested that the mature field has a higher potential to supply the cane crop with soil nitrogen than the immature field. Rather poor recovery of nitrogen by the cane crop of the immature field, specifically at the middle stage of growth, suggested that the cane crop resulted in ineffective tillering which led to a lesser number of the cane stand and a lower yield of the raw cane than that of the mature field. Absorption of phosphorus and potassium by the cane crop was also investigated to some extent on the two different fields, and found was the poorer recovery of phosphorus by the cane crop on the immature field than the mature field. The immature field of weathered Kunigami gravel studied here seemed to be less fertile in phosphorus than the mature field. Future necessities suggested were to study practical effects of improving phosphorus fertility of the immature field on the phosphorus absorption and yield of sugarcane