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Aluminum and hydrogen ions comprise the main parts of the soil solution and exchange cations of an acid soil, disregarding those derived from organic matter, i.e. an acid soil is a mixed Al-H clay system. In an strongly acid soil, the acidity is made up of exchangeable Al and H ions. Aluminum ion contributes to the acidity by its hydrolysis. In order to understand the contribution of aluminum ion to the soil acidity, five acid soils were selected, their soil acidity were determined by Daikuhara''s procedure, using various concentration of KC1 solutions and a soil: solution ratio of 1:2.5. The soil suspension was shaken for 2 hours, and after being left standing for 22 hours, a half volume of the supernatant solution was withdrawn for analyses of extracted H, Al, Ca, and Mg ions. To the remaining soil solution mixture a fresh original extracting solution was replenished to the original volume, and the extraction procedure repeated 10 times altogether. The amounts of H and Al in each extract were determined by titration with a standard NaOH solution, those of Ca and Mg were determined by atomic absorption spectrometry. The cation exchange capacities of the soils were determined by four different methods, the results were compared to understand the acid characteristics of the acid soils of Taiwan.
By comparing the effective acidity and those extacted in the 10 successive extractions, it is found that the total acidity extracted by 1M KC1 solution corresponded to the effective acidity (94-109%), but the acidity extracted decreased with the decrease in the concentration of the extracting solution. According to Daikuhara''s, the total acidity present in the soil (S) should equal to S+2｛y1+a1/(1-K)｝, in which y1 is a half of the acidity determined in the first extract, a1=y2-y1/2, and K=a2/a1=a3/a2=…and it should not be dependent on the concentration of the extracting solution. If the soil: solution ratio was 1:10, the total acidity obtained was 82-92% of the value which was obtained with a soil: solution ratio of 1:2.5. When the extracting solution was 0.1M KC1, the total acidity determined was 50-70% of that obtained with 1M KC1. When the extracting solution was 0.01M KC1, the total acidity obtained d was even less, being 20-30% of that obtained with 1M KC1. The K values were in the range of 0.8 to 0.9, which were close to those obtained by Daikuhara''s. The total Ca and Mg extracted with 1M KC1and 0.1M KC1 were the same with the total exchangeable Ca and Mg, respectively.
The results show that extractable Al comprised about 80% of total soil acidity of Pingchen and Laopi series, while for Chiwulan and Tanshui series, the extractable Al comprised 71 and 76%, respectively. For the Potu series, the total extractable Al comprised only 53% of the total acidity ,probably due to its relatively higher pH. With 0.1M KC1, the proportions of extractable Al to the total acidity were nearly the same with those extracted by 1M KC1, except that of Chiwulan series, in which the proportion reduced to 45%. With 0.01M KC1, the proportion of extractable Al reduced markedly for all soils studied. The amounts of Al extracted into the solution were remarkably affected by the concentration of the extracting solution.
The logarithms of the ionic product, log[(Al3+)(OH)3], of all solutions were in the range of -34.5 to -33.0, which were quite close to the value of gibbsite, -33.96. It indicates that the presence of aluminum hydroxide mineral is controlling the aluminum ion activities of the equilibrium solutions.
From the experimental result we may deduce when acid soils reacted with extraction solution, which Ca, Mg ionic equilibrium only by ion exchange but that the aluminum ionic equilibrium included the ion exchange reaction and the dissolution of a form of aluminum hydroxide mineral present in the acid soil.
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