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既報において，ポット条件下ではカリ肥料の違いによりサトウキビの品質に与える影響が異なり，塩化カリ（KCl）が過剰に施肥された場合のみ糖度低下が確認され，硫酸カリ（K_2SO_4）施肥区ではそのような傾向は見られなかった．そこで，圃場条件下での再現性の有無や収量への影響を明らかにするためカリ肥料の種類および施肥量を変更した圃場試験を行った．試験1では施肥量は変えずカリ肥料にKClおよびK_2SO_4を用いた区を設定した．搾汁液中K^+Cl^−含有量と糖度との間には有意な負の相関関係が認められたが処理による違いは見られなかった．試験2ではKCl，K_2SO_4を0，6，18および60 kg 10a^施肥した区を設けた．KCl施肥量の増加にともない搾汁液および土壌中K^+，Cl^−含有量は増加する傾向が見られた．また，試験1と比べ明確ではなかったものの搾汁液中K^+，Cl^−含有量と糖度との間に負の相関関係が確認された．KCl 60 kg 10a^区は甘蔗糖度，原料茎重，可製糖量が最低であった．以上より，圃場条件下でもK^+，Cl^−は糖度を低下させ，K^+，Cl^−が過剰に蓄積している圃場では収量と品質の両方に悪影響を及ぼすことが明らかになった．その様な圃場に対してはKCl減肥によって糖度増加が可能であると考えられたが，施肥管理の面からだけでなく栽培環境全体のK^+，Cl^−を減らす工夫も必要である．, From the previous reports, it was revealed that, under pot conditions, different potassium fertilizers had different effects on sugarcane quality and sugar concentration was reduced only with increasing potassium chloride (KCl) levels, while potassium sulfate (K_2SO_4) had no adverse effect. To investigate the effects on sugarcane yield and quality under field conditions, two field experiments with different types and/or levels of potassium fertilizer were conducted. Three plots were set up applying the same amount of potassium as the conventional plot but with different kinds of fertilizer, KCl and potassium sulfate (K_2SO_4) in the first experiment. In all the plots, juice K^+ and Cl^− concentrations had significantly negative correlations with sugar concentration. The second experiment was consisted of seven plots changing types (KCl and K_2SO_4) and levels (0, 6, 18 and 60 kg 10a^) of potassium. Juice and soil K^+ and Cl^− concentrations were increased by increasing KCl doses. The relationships were not as clear as the first experiment, however, pol in cane decreased with an increase of juice K^+ and Cl^− concentrations. The plot treated with the highest KCl amount had the lowest pol in cane and millable cane yield, resulting in the lowest sugar yield. From these, it was concluded that K+ and Cl− could be harmful factors to sugar content under field conditions as well and both sugarcane yield and quality are negatively influenced in fields where K^+ and Cl^− are excessively accumulated. Less KCl should be applied in such areas for the quality improvement; and besides, we need to lower K^+ and Cl^− existing in the environments by other ways as well as fertilizer management practices., 論文