A study on the methods of determination of exchangeable bases and cation exchange capacity in calcareous soils in Okinawa region (Department of Agricultural Chemistry)

沖縄地方に分布する中性∿弱アルカリ性の石灰質土壌について, 交換性塩基量および陽イオン交換容量を測定するために, 先ず80%メタノールの塩化アンモニウム緩衝液(pH8.2)による測定法を検討した。この測定法は, 試料中の遊離の炭酸塩の溶出をかなり抑制し, 石灰質の土壌や試料の塩基飽和度をいずれも100%以下に抑制した。しかし, 交換性塩基類を原子吸光光度法および炎光分析法で測定する時, その分析装置の試料液を吸引する毛細管部分に好ましくない現象が, 経時的に見られた。一方は毛細管中の流速が減少して不安定になり, 他方は毛細管先端部に白色結晶化合物(恐らくメチレートの錯体生成物)が生成した。そのために多数の試料を連続測定するには不適当であった。次に, 80%メタノールの塩化アンモニウム緩衝液(pH8.2)による測定法に炎光分析法を導入するため, 交換浸出液を検討した。80%メタノールの塩化ストロンチウム(pH8.2)液による交換性塩基量は, 原法に比べて大略4me/100g程度大きい傾向を示した。また陽イオン交換容量は, 原法に比べ石灰質土壌では約4me/100g程度大きく, 酸性土壌では近似する傾向をそれぞれ示した。そして, 原法と同様に試料中の遊離の炭酸塩の溶出をかなり抑制し, 石灰質の土壌や試料の塩基飽和度をいずれも100%以下に抑制した。新しく提案する測定法の概略と操作手順は, Fig.5に示した。新しく提案する測定法では, 石灰質のジャーガルやその母材のクチヤおよび島尻マージの交換性塩基類は, Caが約70∿95%を占め, それらの陽イオン交換容量は大略22∿30me/100g, 塩基飽和度は80∿99%をそれぞれ示した。また, 酸性の国頭マージの交換性Caは1me/100g前後, 陽イオン交換容量は10∿18me/100g, 塩基飽和度は18∿26%をそれぞれ示した。The calcareous soils distribute extensively in Okinawa region and these reaction is newtral to weak alkaline. It is objectives of the present paper to determine exchangeable bases and cation exchange capacity of calcareous soils. First, for that purpose a method ofNH_4-EC in 80% methanol buffer (pH 8.2) systemcited in a manual edited by M. L. Jackson (1973) was studied. This method kept considerably under control in dissolution of a free carbonates contained in soil specimens and was less than 100 percentage in base saturation of their calcareous. soils. When exchangeable bases extracted from soils by using this method, however, were determined by means of atomicadsorption and flame photometry, the unsuitable phenomena to bring about error as following appeared in quantitative analyses of the bases with time. One is slow down in a current speed in a capillary tube which was equipped that photometer. The other is to form a something white colored compounds (it probably is a complex compound of methyrate) at the top of that tube attached with sample solution. Therefore. it seems to be inadequate for a continuous determination of bases on a large number of samples. Second, in order to introduce a flame photometry to the above mentioned original method, it was studied on the salt exchange solution utilizing for NH_4Cl solution. As a result of investigation on it, the original method was modified and developed to a new proposed method ofSr-EC in 80% methanol (pH 8.2) systemas showing in a flow chart (Fig. 5) of its method in the present paper. A new method by means of SrCl_2 in 80% methanol solution (pH 8.2) had a tendency that total of the exchangeable bases was more about 4me/100g than it of the original method in the calcareous soils and was very similar to it in the acidic soils, respectively. And moreover similarly to the properties of the original method, a new proposed method kept under control in dissolution of a carbonates and was less than 100 persentage in base saturation, too. According to a new proposed method, in the calcareous soils which were calledJaagaru, their parent materials ofKuchaandShimajiri-maajiby an Okinawa dialect, exchangeable bases were 18 to 29me/100g in the total, of which 70 to 95 percentage was occupied by cation exchange capacity and base saturation percentage ranged from 22 to 30me/100g and from 80 to 99,respectively. In the acidic soils which were calledKunigami-maajiby an Okinawa dialect, the total of exchangeable bases was about 3me/100g and cation exchange capacity and base saturation percentage ranged from 10 to 18me/100g and from 18 to 26,respectively